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Hey guys!!

I hope it's OK to post this here as it is not a stompbox. I've been gathering info and having quite a bit of help from Slacker (Ian) to build his Rascal low voltage amp (schematic in the end). However, I kind of lost contact with him. Anyone knows if he is OK? I've been a bit worried as he has not been active in the forum nor contacted me for the past months... I hope he is fine though.

Anyhow, I have almost everything but I still need to order a few last components which I would like to to wisely. So I hope some of you could help me together with Slacker and while he is "away"

The components are the valves, output transformer and speaker. Based on my conversations with Slacker and also on old topics (like Sebsongs) I was planing into getting 12AU7 for the power and 12AT7 for the preamp.

For the transformer the equivalent to Fender 022921. Here there is a big doubt... any info I find on those transformers states that they have the 1ry impedance of 25000 and 2ry of 8ohms, but by the schematics and also Sebsongs info it should be 11.4kOhm -> 4 Ohm... so, what am I missing? Can I use this?

For the speaker, if I can use the transfomer above I was planning on a celestion 8inch 15w and 4Ohm... later I could upgrade... or a jensen... Does it sound about right?

I don't want to order anything wrong and be stuck with those, besides, I'm quite tight on money right now :P

Here is the schematic updated from Slacker to me: https://drive.google.com/open?id=0B45V-V1ycMU9dnhmcVhtNXlWbTg

I would like to thank Slacker very much for all his help!! And thank you guys in advance!!

Best,
Marcos

P.S. Happy new year!

Transformers really don't have pre-determined impedances, just ratios.  25k:8 = 12.5k:4, very much "close enough" for your purpose.  And going "a little higher" is usually a good thing.

The little Jensen champ speakers (8" 4ohm) sound really good, but take a while to break in.  They do "fizz out" at higher volumes, all 8" guitar speakers I've heard do that.

Thanks thermionix!

About the transformer, I was indeed thinking about the ratio. However, considering that it will lower a bit my 2ry impedance, right? Would that be a problem?

About the speaker, what do you mean by

Quotetake a while to break in

? I also though of 10 inch ones, but I'm not sure if this little amp will have power enough to drive those properly.

Thanks

No, that transformer connected to a 4 ohm speaker will have the output tube "see" a 12.5k primary impedence.  Well, that's how I look at it anyway.  But it will work fine for sure.

All speakers need to break in when new, some more than others.  The cone and surround need to flex a bunch before the speaker sounds "right" in most cases.  I've had both the Alnico and ceramic Jensen 8s and both needed many hours of breaking in before they souded as good as they can.  Not that they sound bad new, just not as good as they will.

Your amp can probably drive a 4x12.  Speaker size isn't the issue so much as efficiency.

Hmm...I think I understand what you mean. Sounds good about the transformer then.

About the speaker, considering all a 4Ohm would be the best fit right? Besides, do you guys prefer celestions or jensen? From the lower end of course (maybe someday I could get a greenback, but now I can't afford... lol )

What about the valves? Any thoughs?

Thanks again!

P.S. I don't have much experience in electronics besides building some pedals and other small stuff....so my knowledge is really limited as you can see. I was relying quite a lot in Slacker's help to go through this. Would you guys be able to help me through the end of it if it is the case? Otherwise I might have to put aside the project until I have more knowledge or until Slacker can help me back. However I'm really up to the challenge and I'm really motivated to it.

Sounds like 4 ohm would be best for the transformer you intend to use.  I like Celestions fine, but I don't think they have many low-priced options.  Their Chinese-made speakers are expensive.  For a speaker that doesn't need to handle much power, you might find something cheap in an old radio in a pawn shop or thrift store.

For 12AT7s and 12AU7s, you can still find American and European tubes at decent prices, at least in US you can.  The demand isn't as high as with 12AX7s, and there were a BUNCH of those tubes made for military equipment, high quality.  Other than that, I'd look at JJ brand, I think they make both types, and the new Russian tubes seem to have quality issues the last several years, grid leakage and such.  Chinese are usually okay, don't know if they make a 12AU7 though.

Thanks again!

For the speaker, initially I was intending to use one that I have from a solid state amp and then upgrade someday, but turns out it is 16ohms...

Celestions or Jensen 8 inches are not too expensive....around 30/40$. 10 inches are a bit more...though.....

For the valves, I was indeed thinking of JJ... But I]m still a bit worried of buying all and not having the expertise to solve issues along the way without Slacker for now....

Quote from: marcos_s_p on January 10, 2018, 08:57:51 PM
Celestions or Jensen 8 inches are not too expensive....around 30/40$. 10 inches are a bit more...though.....

I was thinking you could find something for $5 or $10.  Some of those old lightweight alnicos can sound really good, and if the cone is intact and DC resistance checks out, pretty much anything can handle 1 watt (1/2 watt?).  You might find a 10" or 12" super cheap, or even free, since people don't see much value in a really low power speaker.  But I guess it depends if you have those kinds of "junk" stores around where you live.

Quotestill a bit worried of buying all and not having the expertise to solve issues along the way without Slacker for now....

There are plenty of folks here that know WAY more than me about all electronic matters, tube amps included, somebody can help you if you get stuck.  And Slacker might show back up too, who knows.

You are running this on 24V ? ? ?

Then nothing is very critical. Nothing should burn-up. There may be an "optimum" load for best power, but in any case the power output will be so low that a little less is no disaster.

The output transformer can be a 120VAC 6VAC power transformer bigger than a golf ball. Or 220V:12V, or 220V:6V, etc; not critical. Raid the thrift-store or your cellar and collect a bunch to try. This is "wrong" for several technical reasons but WILL work if connected right.

QuoteI was thinking you could find something for $5 or $10.  Some of those old lightweight alnicos can sound really good, and if the cone is intact and DC resistance checks out, pretty much anything can handle 1 watt (1/2 watt?).  You might find a 10" or 12" super cheap, or even free, since people don't see much value in a really low power speaker.  But I guess it depends if you have those kinds of "junk" stores around where you live.

Well, that would be nice...i though about it before and I really like "recycling"old stuff... but here in freeezing Quebec city (lol) there aren't many of those....it's very and mean very difficult to find this kind of stuff....even new speakers the stores doesn't have in stock and I have probably to buy from US and pay shipping.... :/ But if some of you guys have something that you would like to get rid off, I'm here! :P

QuoteThere are plenty of folks here that know WAY more than me about all electronic matters, tube amps included, somebody can help you if you get stuck.  And Slacker might show back up too, who knows.

Sounds god then, my moral is elevated!! :D

PRR, it will run on 30-35V....it's rectified to that.....except the heaters that are lower...

QuoteThe output transformer can be a 120VAC 6VAC power transformer bigger than a golf ball. Or 220V:12V, or 220V:6V, etc; not critical. Raid the thrift-store or your cellar and collect a bunch to try. This is "wrong" for several technical reasons but WILL work if connected right.

I might consider, but this Fender replacement is not expesinve, around 14$...and it is properly for sound.... in the other tread Sebsongs and Slacker mention that it is a good idea to invest on this part a little.... ideally I would like to have an output that had two secondaries....for 4ohms and 8ohms....but I don't think there are transformers such as this that will fit....If someone know about one, please let me know.....

Many thanks for the responses again!!

Btw, here is the layout for the components....as I don't have much expertise I'm not sure if it looks ok... :P If someone have any opinion ,please let me know... ;)
https://drive.google.com/open?id=0B3RBntEXGMeNOC1lek1BTzlOUlU

There is one error though....the volume pots should be log... ;)

Quote from: marcos_s_p on January 11, 2018, 10:47:23 AM
Sounds god then, my moral is elevated!! :D

Duck, you taking that one, man?

Not sure if I follow [emoji14] lol

Sent from my LG-H831 using Tapatalk

Signature line (see below).

I know you're from Brazil, and your English is very good (I don't know ANY Portugese).  But you said "god" and "moral" instead of "good" and "morale", and the combination is pretty funny, because it sounds like you're talking about religion.

Just noticed lol hahahahah... I wrote in a hurry... Ops

Sent from my LG-H831 using Tapatalk

So, I'm planning in ordering the valves and transformer quite soon as I already have the rest of the components (except the speaker which I´ll wait a while yet).

As I'm negation in electronics (exaggerating a bit, lol, but kind of true) I cannot by reading the schematic predict which kind of output transformer is required. I know the adastra would fit as it is the info from Slacker and I also know that Sebsongs built this amp with just a few minor differences from the schematic presented here and he used the Fender 022921. As for the adastra I couldn't find out its specs, so I couldn't compare with the Fender.

Long story short, do you guys think I can use this Fender in here based on the schematics presented?

If so (if its 1ry and 2ry impedances fit to the schematic) I believe it will sound better than the adastra as it is specific for sound purposes.

As you guys are well more experienced than me, I'll wait for your input and just after order the parts ;)

Many thanks in advance!!

P.S.: Also if guys have good source (ebooks, etc) of readings on this topic I would appreciate as I'm really keen in learning :)

   

The 12AU7 plate is a several K Ohm source. One section at 200V, say 6K. Two sections 3K. Two sections but way down at 30V, maybe 6K-10K.

We want to load with a larger impedance. There will be some difference of power output and distortion. If you were building a Hi-Fi, or a Stadium Blaster, we would have to think pretty hard. But you already know this is a small amp with a colored sound. How small, how colored? Probably not important.

I approve of spending $14 for the small radio OT (which Fender used for reverb). But you said you are "quite tight" so I wanted to offer another path, which *may* be no-cost if you get lucky.

Thanks PRR

I appreciate the money concern! I'm really tight, but I guess the 14$ I can afford for now. I know it is a small practice amp and that's really the idea to have a nice tube coloured sounding amp for practice and home recordings. I want to move a bit from the solid state amps you see. Besides that, the idea is to have fun both building and playing! :D Which I love both....it just lacks me a bit of the knowledge.

I understood the idea behind the response but the physics is still a bit cloudy for me. Maybe there is somewhere I could read to understand better and learn, no?

For now, I'll trust your judgment and will order the Fender OT and the valves 12AT7 for the preamp and the 12AU7 for the power amp. I believe it should be a good match as the 12AU7 gives a cleaner output in the power according to the other threads.

So, tell me something, If I wanted a more coloured sound (of course this is for the future as I believe as it is is already enough), what could be done?

Many thanks! ;)

Hey guys, thanks again for the help!

Just ordered yesterday the valves (12AX7 and 12AU7) and the OT (Fender 22921).

The remaining of the components I have, so I plan to start soon tho solder the board and build the chassis, etc. So, I made a layout for wiring the board and components but I wonder if there is anything that I should worry that could influence in the success of the project? Or if the components are connected according to the schematics the position in the board is irrelevant?

Anyhow, if someone is willing to give a quick look on it it will be highly appreciated... it is in the beginning of the thread ;)

Thanks again!

I'm not too knowledgeable about tube amps myself, but I think that the 47K near the front end of the amp needs to be placed very close to the valve because it stops parasitic noise from being amplified.
Would you mind sharing the layout? I'm quite interested in this, might build one myself!

Hmm, interesting...well there are two 47k...the first is only for the input voltage to drop for the heaters...so, I imagine you're talking about the one near the guitar input jack... Right? For that one I could solder directly on the lug of the valve socket... Would that be better? What about the 1M5 and the 100nF cap on the input?

The schematic is up there... And there are other treads with good info around ;)

Sent from my LG-H831 using Tapatalk

Hey guys, any other thoughts? Depending on your input I'll adapt the schematic before I start ;)

Many thanks again!

Hey guys!

I did another option for the wiring based on a more point-to-point ish soldering. It looks a bit more messy, I guess...and it might be a bit more difficult to solder. Not sure though. Anyhow here it is.
https://drive.google.com/open?id=1oO3mspXBVNoIdqK2jyC0WtuXOJaqhmuA

What do you guys think? Is there any advantages into using the 1st version vs the second?

version 1:
https://drive.google.com/open?id=0B3RBntEXGMeNYkZaWWVXeW9FQnc

Also, Should I be careful with the placement of any component that I might have missed out?

Many thanks again!!

Hey guys,

me again! lol I had a rough year so unfortunately I had to put aside this project. However now I have everything and started build even the small cab! :) I have 2 questions though!

1- As I move a lot and and I have a set of two small speakers laying around I though of using those for starters (in the future I could replace if that's the case). However each one is rated at 0.8watt, which gives me roughly 1.5watt total. Would that be enough or the amp will blow them up? They are from a tiny practice amp that I had....

2- The grounds should be wired through the chassis and connected to the negative of my power supply, right? It is a ~30V supply from an old scanner that slacker said it should work fine :)

Thanks again for your patience and help!!
Best,
Marcos

QuoteWould that be enough or the amp will blow them up?

I don't think so, the 12au7 will give you 1W on the best conditions, like 200V at the plates, in PP. Without checking the loadline I would say this amplifier gives around 0.25w to 0.5w?
The 12AT7 is stronger, some companies claim it can give 5W in PP...
Since you are running this at 30V it won't be a problem. Unless the speaker company also overspecified it's wattage...

Quote2- The grounds should be wired through the chassis and connected to the negative of my power supply, right? It is a ~30V supply from an old scanner that slacker said it should work fine :)

Ground goes to negative side of the 30v PS, just like in a pedal. Normally in tube amps I would connect the circuit ground to the chassis through 2 points. One point is the first filter capacitor ground, and the second is at the in jack. So you have the shortest return for the power section and it is far away from the preamp section. Please, correct me if I'm wrong.

Hi Thomasha,

Thank you for your answer. About the ground, I think you have much more knowledge than I do on the subject. So, for curiosity, why shouldn't I ground near the preamp? But also, isn't this circuit so small that everything is close to the preamp? lol However I guess that applies more to amps that have a power transformer which would lead to ground loops that would interfere with the output transformer, etc, no? In my case, there isn't as it is in the power supply, outside the chassis.... so. I believe this interference wouldn´t happen, no?

In my wiring layout I envisioned 4 ground points just because of easier positioning for wires and components... Do you care to take a look and let me know if you would change it?

Many thanks again!!!
Marcos

There is a very good, and very short PDF from Merlin about grounding:

http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)

There you will find some information.

Even in smaller builds you still want the shortest path to the ground to avoid any loops. The chassis should not be the ground of the circuit, just connected to it. A star ground is always the best way to goo, try to reduce your 4 points. Is the layout the one from january?

Hmm, I see.

Here is the last layout: https://drive.google.com/file/d/1IGQzmuudnmOJICtt4Gm90WCK45JyYB_x/view?usp=drivesdk

I guess I could keep the input jack and then interconnect the others to the one where the eletrolitic cap following the input from the jack, no? And only connect to the chassis there. Right?

Thanks for the help!!!

QuoteI guess I could keep the input jack and then interconnect the others to the one where the eletrolitic cap following the input from the jack, no? And only connect to the chassis there. Right?

Yes, that should do.

Many many thanks!! :D

I do have one last question. There are 2 volume pots on it, and one is separated via a SPST footswitch. I need to bring the signal from the chassis to a stompbox where the switch will be. What cable would be appropriate? I guess I should use something that would shield the signal to not bring noise. But for that I would have two options. Using two mono guitar cables side by side and having the signal going in one and coming back in the other and connecting the shield to the ground. Or a stereo cable. But for the stereo there will be one signal coming and another going within the same shield which will create electromagnetic waves...not sure if that would be good. What do you think??

Thanks once again!!
Marcos

Wait, what?
You want to use the second one as a footswitch boost?
Is it the gain pot, (after the first stage) or a master volume?

I would use the stereo cable, because it's easier.
Another thing is, that when you use shielded cable for the gain pot you're trying to avoid that some stronger signal in the end of the chain couples to your cable creating some kind of feedback or noise.

Since both signal will be of the same magnitude, I don't think it will be a problem.

The idea is to have 2 set of settings to emulate two channels without having to add another pre-amp. So basically I can use to boost or to switch between a cleaner and dirtier sound with the footswitch :) . This is the grain/volume... There isn't a master on this amp... So the changes come from this. In this case from those as one is right after the other and is our not activated by the switch ;).

For the stereo... Wouldn't it be a problem that there will be signal in two directions in the same shield? Shouldn't they be shielded separately?

Thanks again!

QuoteFor the stereo... Wouldn't it be a problem that there will be signal in two directions in the same shield? Shouldn't they be shielded separately?

As I said before, I think that since both signals have a similar intensity one won't interfere with the other in a significant way. The problems we have in amps is high signal levels, as seen in later stages, creating a positive feedback with previous stages and resulting in squealing.

Here, there is no gain stage between the signals, in this case, if some signal is picked from the other line it won't create an ever increasing signal and squeal.

I was thinking that you could switch the grounds at the pots, but this way they would be kind of interactive. One pot would be grounded and be the acting voltage divider, while the other would be a resistance in parallel with the upper side of the voltage divider.
This will not work unfortunately.
If one gain is maxed ( the non-grounded) it would short the upper side of the second pot. Maybe with some changes... I'm missing something ...

> a set of two small speakers ... each one is rated at 0.8watt, which gives me roughly 1.5watt total. Would that be enough or the amp will blow them up

Are you still planning a tube at 24 Volts?

At such low voltages you will hardly have a few tenths of a Watt. You are fine.

Thomasha, about the stereo...I understand. And that's good as it's more practical.
About the pots, not sure if I understand the advantage. Anyhow, you said it didn't work... So it's better to keep as it is for now, no?

PRR, it will be fun at around 30V. Not much of a difference than 24V. I guess I'm fine :).

Thank you for your help :)
.

I'm halfway through it. Already built the chassis, soldered most of the components, and cut the wood for the cabinet :)

However,  thomasha you got me thinking. As I explained, the idea of the 2nd vol pot was to have a boost after....or to switch between cleaner and dirtier. Looking at the schematic, I'm not sure if that is the case.... is it? I'm still learning electronics...lol.... so I'm not completely sure how those 2 pots wired that way would behave....

Also, just to be sure....I should wire to the switch with the stereo...and the shield I just connect to the ground, right?

Many many thanks!!
Marcos

Hey guys,

So, I have all built up...but noticed a silly mistake.... the 2 speakers I have that I am recycling are 4Ohms 0.8watt each. So, for the OT that I am using which is the fender 022921 from the reverbs... ideally I should use a 4ohm speaker, right? So, of course I could use one of them with 0.8watt.....But this might be a bit borderline to the amp watts....

Anyhow, could I use them wired in series giving me 8ohms? Or even in parallel to 2Ohms..... Would my OT and amp be able to handle that.....or the speakers? What do you guys think is the best option here?

Thanks!!

Tube at 30V, you are likely well below 0.1 Watts. Also no possible "mis-load" will hurt the tube. Hook it up any/all ways and rock out. You might wake the baby. You probably won't call the cops in the next town over.

Thanks PRR,

Any advantage in hooking the 2 together? If so, better in parallel or series?

Thanks

:)

2 speakers is always better than one. Series or parallel, no clue, try both.

Hey guys,

I need some help... I finished it and plugged it...... but it is silent dead....no hum....no sound either.... I tried the speakers in parallel and series, nothing as well.... the valves light up, but very dim.

Any ideas? The only big difference from the schematic is that I had to use the BC549 transistor instead of the 2n5089...

Any help would be appreciated!

Thanks,
Marcos

Quote from: marcos_s_p on March 07, 2019, 09:04:19 PM
Any ideas? The only big difference from the schematic is that I had to use the BC549 transistor instead of the 2n5089...

Did you account for the different CBE vs. EBC pinouts?

Hey bluebunny,

Thanks for your answer! Yes I did. In this case it's a no-brainer as it's only a 180 turn of the component.  Could it be that the BC549 is not strong enough to drive the valve?

Another think I keep thinking. In this schematic there is an extra vol pot that was not in the original layout. Could it be the problem? It is wired with a connection from the ground (pin1) of vol pot 1 directly on the pin 3 of 2nd pot and through a switch to the middle pin. I wired the switch with a stereo cable and the shield I connected to ground in the input jack.

Any other ideas??

:)

Quote from: marcos_s_p on March 08, 2019, 09:48:30 AM
Thanks for your answer! Yes I did. In this case it's a no-brainer as it's only a 180 turn of the component.  Could it be that the BC549 is not strong enough to drive the valve?

Worth asking - been there, done that.   :)    I think the BC549 should be a reasonable sub: specs look to me to be in the same ballpark as the 2N5089.

Considering that, it shouldn't be the issue right? I could exchange the transistor... But I only have 2n5088... Which is lower in gain than the 89... :/

Aside of that, I have no idea where to start debugging nor how ..... Lol I'm not the best in electronics yet...

Any ideas? :\

Let's re-post the link to schematic-
https://www.diystompboxes.com/smfforum/index.php?topic=119475.msg1114788#msg1114788

> I have no idea where to start debugging

You have been here enough. You know the drill. VOLTAGE READINGS!!
https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Voltages are key here because the tubes are run FAR from "normal" conditions. They will also show if your BJT is backward or busted. At least we will have *something* to chew on.

PRR, I know and thanks, but as it is not a pedal should I give the readings of the transistor and each pin of the valves? Right?

How should I set the pots? Min, Max and middle?

As there is no ICs, or other stuff I guess this should be it... Should I take any extra care as I am dealing with valves?

Thanks!

Hi, readings at the valves help, both side of the output transformer (high voltage side).

- Test if the transformer is OK, if you have the 30V on both sides (plates and B+ node). Are the speakers good?

- Start with the output stage, check cathodes and anodes for voltages, grid should be at 0V but not grounded (volume must be set higher, I always set it in the middle just in case I wired it backwards). Tap at the grid pin, there should be something coming out. Even if the preamp is not working the power stage should be easy to fix.

From there work the audio path to the front of the amp. Checking the bias of each component, tube or transistor.

So, here it goes.

Project: Rascal mk1 from slacker
Schematic: https://drive.google.com/open?id=0B45V-V1ycMU9dnhmcVhtNXlWbTg
Layout: https://drive.google.com/open?id=1mCSB_WdmFk3OU46ZZ6xBH6R44bC6aPom
Picture of the chassis (a little messy I know): https://photos.app.goo.gl/i9AJVd4WSagEzkjr5

V+ from the power supply: 34.6V

Transistor BC549
C: 29.8
B: 23.3
E: 24.2

V1, Preamp - 12AT7
Pin1 - Anode: 22.9
2 - Grid: -0.6
3 - Cathode : 0
4 - Heater : 25.9
5 - Heater : 12.6
6 - Anode : 23.2
7 - Grid : -0.5
8 - Cathode : 0
9 - Heater center : 19.5 (pin not connected to anything)

V2, Power - 12AU7
Pin1 - Anode: 29.5 - To Output transfomer
2 - Grid: - 0
3 - Cathode : 0
4 - Heater : 12.7
5 - Heater : 0
6 - Anode : 29.5
7 - Grid : 0
8 - Cathode : 0
9 - Heater center : 6 (pin not connected to anything)

B+ to output transformer 33.2
Speakers are wired in parallel (but I tried in series). They should be fine as they were working in the previous amp that I'm recycling.

I hope that helps and thank you in advance!
Marcos

Your heater voltages are correct, though they can't really be wired as the schematic note says.

> Transistor BC549
> C: 29.8
> B: 23.3
> E: 24.2

We expect B to be 0.6V higher (more positive) than E. You show a 0.9V difference the wrong way. I'm tempted to say you read the legs wrong, but the difference is far larger than expected.

OTOH, you could not have 24.2V at the top of the 3k3 resistor(?) unless the transistor were working with normal gain.

Are you really dropping 5 volts across the output transformer primary? What part are you using there? (5V would be small for a 300V amp but looms large on a 35V amp.)

(https://i.postimg.cc/2q7CHx8m/marcos-s-p-1.gif) (https://postimg.cc/2q7CHx8m)

QuoteYour heater voltages are correct, though they can't really be wired as the schematic note says.

Well, I wired those as per Slacker instructions and he double checked it. Also this was all done by slacker and it was built and confirmed by him and sebsongs. So, I'm not sure if I follow... What is the issue? Could this be an alternative way of wiring them?

Quote> Transistor BC549
> C: 29.8
> B: 23.3
> E: 24.2

We expect B to be 0.6V higher (more positive) than E. You show a 0.9V difference the wrong way. I'm tempted to say you read the legs wrong, but the difference is far larger than expected.

I doble checked and it's definitely that way. The only other option would be EBC instead of CBE, but I'm pretty sure it's not....

QuoteAre you really dropping 5 volts across the output transformer primary? What part are you using there? (5V would be small for a 300V amp but looms large on a 35V amp.)

Using a Fender 0221921 used for reverbs...

What do you suggest then?

PS: My multi-meter is quite simple, so it might not be the most precise in terms of .0xxx (I have a better one but I need to fix its leads)

Thanks again!
Marcos

Quote from: PRR on March 10, 2019, 01:27:46 AM
Your heater voltages are correct, though they can't really be wired as the schematic note says.

It's been a while since we discussed it but I think he's running the heaters in series with each other and then a series resistor to drop the rest of the voltage from the supply.

BTW, the voltage after the diode to OT is 33.2... Still 3.4 V drop from the 1ry in the OT.

I'll try to fix my other multimeter that is better and re-do the readings, but It might take a while :/

About the heaters here is what we discussed: https://drive.google.com/open?id=0B45V-V1ycMU9dFAzblBEdWhJZm8

You can also see in the layout :)

> running the heaters in series

I was just wondering because the schematic I was looking at showed 12V to both. But the reported 25.9 - 12.6 - 12.7 - 0 is a fine series-string. The 3% difference from book-value is insignificant.

> it is silent dead....no hum....no sound either....

Utterly silent? Then I would suspect a bad connection from OT to speaker. Or a bad wire, OT, or 12AU7. (You say the speakers work on other systems.) A live 12AU7 and OT "should" make *some* hiss or buzz. Very faint, but some.

So the heaters are fine as it is?

QuoteUtterly silent? Then I would suspect a bad connection from OT to speaker. Or a bad wire, OT, or 12AU7. (You say the speakers work on other systems.) A live 12AU7 and OT "should" make *some* hiss or buzz. Very faint, but some.

If I put my ear glued to the speakers I can listen a very, but very faint hiss. But, plugging a guitar cable (that usually results in some noise) or adjusting the pots doesn't change anything nor make any noise. If I don´t put my ear there it's dead silent....

Valves and OT are brand new... could it be that the valves were not warm enough? I left quite a while on and tried again.... several min...almost 30m...

What about the other readings?

Thanks again 

Quote from: marcos_s_p on March 10, 2019, 07:07:25 PM
So the heaters are fine as it is?

Yeah the heaters are fine.

You could make an audio probe http://diy-fever.com/misc/audio-probe/ (http://diy-fever.com/misc/audio-probe/) to try and track down where the problem is. Start with the volume turned down low and have the volume low on the amp you're using to listen with, some of the signals should be loud. Start at the emitter of the transistor and if you have signal there the problem is with the power amp section. If you don't get signal on the emitter work back towards the input, you should have sound on both the anodes of V1, see if you can find where the signal goes missing.

I'll make the prob. It's been a while that I was planning to. :) As soon as I have news I'll let you know. It should take a some time though, maybe in the weekend :).

Should I avoid probing anywhere specifically in the circuit? Or should I have any special cares?

Thanks :)

Hi Guys,

It might be a dumb question, but as I'm still learning here it goes. Would that be the signal path? Or am I doing something very wrong....lol?

(https://i.postimg.cc/8kR1PPcK/Signal-path.jpg)

I'll likely check this over the weekend :)

Many thanks!
Marcos

Yeah, that's it. To hear anything after the volume pot the pot will have to be turned up a bit, sorry if that's obvious.

Thanks Ian

About pots, no worries about the comment...better to say than not in my opinion! However I do have a question about the behaviour of those volume pots... So, basically, if the switch is of only the V1 is controlling the volume and in the moment that the switch is on, V2 will add up to the signal of V1, right? Or am I missing something?

Thanks again for your patience!

Yeah that's right, when the switch is in the position shown in the schematic the second pot is engaged and increases the volume.

Hey Guys,

How are you on these stressing days? I hope you are all safe and well!

It's been a long time...too long. Sorry for not updating, but I had to focus on finishing my PhD last year which I finally did :) . But because of that I couldn't troubleshoot my amp yet.

So, how can I troubleshot it considering that right now I don't have a spare amp to plug an audio probe to it and then probe the built amp? I hope it was not too confusing lol. I already provided voltage readings previously, so hopefully that should give some clues to someone.

Do you have any ideas?

Many thanks!!
Marcos

Any ideas? :P

Thanks in advance!!

Quote from: marcos_s_p on April 08, 2020, 11:16:47 AM
So, how can I troubleshot it considering that right now I don't have a spare amp to plug an audio probe to it and then probe the built amp?

Build another amp just to use with the audio probe?

No, seriously. One LM386, any cap with value from 1nf to 100nf as the input cap, a 470uf as the output cap, any speaker you have on your parts bin (it could be any crappy one). Done.

Btw... I just saw you're using a 12AU7 as the output tube, and planning on running it at 24V. A 12AU7 will give you about 1W at 250V. At 24V, well... I'd bet your circuit is probably working, but the output is so low you can't hear anything at all.

A suggestion is to get rid of that 2N5088 and use a 555 smps to power it. So you can use a 12V power supply to power the heaters and the smps, and have about 200V-250V from the smps to power the amp.

Hey Marcos,

LOL I'm also from Brazil....but I live in NYC now.

Thanks for your reply. Well, I'm not sure if I have the components for the amp....I'll have to check....but I was wondering if there was another solution without building another thing.

About the power amp....I guess that's why there is a transistor there...to amplify the signal a bit before the valve. But this schematic was already tested and works with other people, so I guess it shouldn't be silent....there is definitively something going on. Maybe it has to do with the transistor that I used...I had to substitute from the original, could that be? Supposedly, it shouldn't... at least it was I was told by Slacker and other folks...

I appreciate any other opinions and ideas!

Thanks again,
M

I guessed you're brazilian by the name lol. Beleza, xará?

The transistor is part of the preamp. The power amp it's made of the 2u2 cap, 1K resistor, V2 and output transformer. The preamp part should work ok as is, but I'm not sure on the power amp. Even if you have a strong signal from the preamp, I don't know if such low voltage is enough to give you any sound out of the power amp.

That said, I have a tranformer here I made early this week to use with a 12AU7 and didn't tested yet. I can quickly breadboard this output stage and power it with around 20V to see how much sound I can get out of it (if my transformer works lol). Just need to find a simple preamp that runs at around 20V.

As for the audio probe amp, I suggest you to build a simple signal generator like this one (https://www.eleccircuit.com/simple-signal-injector-by-transistor/) and a simple LM386 based little amp. Connect the signal generator to the input of your circuit, connect the audio probe directly to the input of the LM386 amp and use any crappy speaker you have. You'll have a complete audio probe testing tool that won't need any external amp nor anybody playing the guitar while you do the audio probe.

So I breadboarded it, along with a simple preamp. Powering it with 20V, it was dead quiet. Then I powered the output stage with 150V and got a little buzz on the speaker. No guitar signal, because my preamp wasn't powered at all when I used 150V, but I could clearly hear a buzz. So probably the low voltage is getting you so low output you can't really heard it.

Wow, already?! lol thanks

That's interesting, but odd, no? I mean, considering that I built that in the specs that Slacker did and Sebsongs and for both it worked... not sure how to interpret that...

So, two things...is there a way for me to make it work as is? (more or less)

or, how can I mod it to work without much complications?

:P I'm not versed in High voltage stuff...hence I did the low voltage :P

Thanks,
Marcos

I can't even imagine how they got sound out of an 12AU7 with that low voltage. Maybe for an headphone amp it's doable, but for a regular amp I don't have a clue. Lowering from about 250V to 200V already results in a big wattage drop.

For higher voltage stuff, yeah they can be dangerous, but there are some stuff that's more safe to work. A big 100W all tube amp could possibly kill you, but a little 1/2W amp is almost as safe as a 9V circuit. You can use this smps to power circuits with 2 or 3 12AX7/12AU7 tubes: https://postimg.cc/k6d2Bg6V. As long as you don't tough the voltage output when the circuit is powered, you're good to go.

You can, for example, build a low watt P1 (https://ax84.com/archive/ax84.com/p1.html). If you open the "options" file, you'll see at the bottom of the page a low wattage option, very similar to the output stage of the amp you're building (but the resistor is 470r and there's a cap). The only thing that differs a 6N1P used on that doc and a 12AU7 is the heater wiring. Open the schematic, pick everything from the input until after the master volume pot (actually, keep that resistor), replace the output stage with the output stage from "options" doc, use that smps I posted above for the high voltage, use a 12V power supply to power the smps and heaters and you'll have a very nice low power amp with a full tone stack.

If you want to give it a go, I have a verified layout for that smps and also a verified layout for the P1 with all pots onboard using a different power tube, but it should be easily to adapt to use a 12AU7.

Interesting, but I'm still wondering that there is something to explain what's happening - because it was done and worked before. I wonder if anyone else might know why or what?

Also, I guess I would have to modify quite a bit the whole thing by applying your ideas, no? Do you think there will be any advantages on your layout in comparison to the one I did?

Thanks again! :)

The P1 is a different design from this one you're building. While P1 have a complete tonestack and a clean tone (that distorts a bit but don't get into mid/hi gain od), I believe this one may have a more jcm-like sound. You can use this design at high voltage if you remove the transistor stage and use 470uF and 22uF caps that can handle about 300V (you won't reach 300V, but it's better safe than sorry). But keep in mind the transistor stage is part of the sound, so it will sound a little bit different than the original purpose. And also you'll have to wire the heaters in a different way, but this is very easy to do.

If you want to try to make your amp work, the best thing is to contact somebody who built it sucessfuly. Slacker is one of them, and the last time he was online was on april 7, so he may be online again soon. But, as suggested, it's a great idea to have an audio probe, not just for this project but because it's a must have tool to debug non working audio circuits.

I see! And I agree about the audio probe...I have one....the thing is...I used to have a second amp...and now I don't :P And I don't have any spare parts to build a simple one.... lol

I contacted Slacker a few days ago, but haven't heard back yet. Let's see. Meanwhile if you guys have other ideas and suggestions I'm all ears!

Thanks again!!!

marcos_s_p - are you using some sort of computer to post your queries on this forum? and does your computer have a sound card, as we used to call them, of some description? most nearly all personal computers with sound capabillity these days will have a microphone input. feed your audio probe in and monitor on the comp speakers, if you are really desperate.