Author Topic: Rascal low voltage amp  (Read 1029 times)

marcos_s_p

Rascal low voltage amp
« on: January 10, 2018, 01:02:11 PM »
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!

thermionix

Re: Rascal low voltage amp
« Reply #1 on: January 10, 2018, 01:36:38 PM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #2 on: January 10, 2018, 02:31:40 PM »
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
Quote
take 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
« Last Edit: January 10, 2018, 02:47:12 PM by marcos_s_p »

thermionix

Re: Rascal low voltage amp
« Reply #3 on: January 10, 2018, 04:03:57 PM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #4 on: January 10, 2018, 05:00:11 PM »
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.

thermionix

Re: Rascal low voltage amp
« Reply #5 on: January 10, 2018, 08:51:16 PM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #6 on: January 10, 2018, 08:57:51 PM »
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....

thermionix

Re: Rascal low voltage amp
« Reply #7 on: January 11, 2018, 12:02:56 AM »
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.

Quote
still 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.

PRR

Re: Rascal low voltage amp
« Reply #8 on: January 11, 2018, 01:04:06 AM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #9 on: January 11, 2018, 10:47:23 AM »
Quote
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.

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

Quote
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.

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...

Quote
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.

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... ;)

thermionix

Re: Rascal low voltage amp
« Reply #10 on: January 11, 2018, 02:13:12 PM »
Sounds god then, my moral is elevated!! :D

Duck, you taking that one, man?

marcos_s_p

Re: Rascal low voltage amp
« Reply #11 on: January 11, 2018, 02:14:44 PM »
Not sure if I follow [emoji14] lol

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thermionix

Re: Rascal low voltage amp
« Reply #12 on: January 11, 2018, 02:29:36 PM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #13 on: January 11, 2018, 02:30:59 PM »
Just noticed lol hahahahah... I wrote in a hurry... Ops

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marcos_s_p

Re: Rascal low voltage amp
« Reply #14 on: January 11, 2018, 06:50:55 PM »
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 IŽll 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 :)

   
« Last Edit: January 12, 2018, 11:30:34 AM by marcos_s_p »

PRR

Re: Rascal low voltage amp
« Reply #15 on: January 12, 2018, 01:35:15 PM »
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.

marcos_s_p

Re: Rascal low voltage amp
« Reply #16 on: January 12, 2018, 01:49:10 PM »
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! ;)

marcos_s_p

Re: Rascal low voltage amp
« Reply #17 on: January 16, 2018, 10:42:43 AM »
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!

suryabeep

Re: Rascal low voltage amp
« Reply #18 on: January 16, 2018, 11:15:12 AM »
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!
Still in the process of learning, so bear with me if I ask dumb questions :P

marcos_s_p

Re: Rascal low voltage amp
« Reply #19 on: January 16, 2018, 11:33:22 AM »
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 ;)

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