Author Topic: Low voltage tube amp experiment  (Read 22794 times)

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slacker

Low voltage tube amp experiment
« on: September 16, 2008, 05:08:21 PM »
What with the current interest in tube projects on here I thought I'd share something I've been working on for the last month or so.
It's a simple low voltage tube amp using a couple of 12AT7s and a cheap transformer. I was originally going to build a Sopht amp but the tubes didn't seem that easy to get hold of in the UK so I thought I'd try and make something using "normal" tubes.

After a bit of experimenting here's what I came up with.



The preamp is basically just copied from the Fender Princeton and the output stage was inspired by the reverb driver on a Twin. I used a 100volt multitap line transformer because I've read about similar things being used for Sopht amps and they're cheap. Mine has taps for 1.9W, 3.75W, 7.5W, 15W and 30W on the primary side and 8 and 16 ohm taps on the secondary. I'm using the 1.9W and 8 ohm taps. The transistor buffer between the stages is needed because I found I could only get the "power amp" to work with a very low input impedance. The tubes in the preamp can't drive this so I added the buffer, this is directly coupled to the second preamp tube's plate, which I copied from Doug Hammond's Chime and Grind amp.
This means the signal path isn't 100% tube but if it's good enough for Doug it's good enough for me :)

Here's a quick soundclip. The beginning as about as clean as it gets and the last section is with the volume on max. I've no idea what the  wattage is  but through a 10" speaker it's loud enough as a bedroom practice amp.

I've still got a few issues to sort out, there's a bit of mains hum that seems to have become more noticeable since I boxed it up and I'm still working out the best way to provide power to the heaters.

Thre's a couple of pictures in my gallery if anyone's interested.

Any suggestions for improvements would be gratefully received.

frequencycentral

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Re: Low voltage tube amp experiment
« Reply #1 on: September 16, 2008, 05:19:43 PM »
Looks good Ian.  ;)

How are you powering it? Do you think it would be possible to use something like a MAX1044 or LT1054S to get the plate voltage from 9 or 12 volt wall wart?
« Last Edit: September 16, 2008, 06:10:16 PM by frequencycentral »

kurtlives

Re: Low voltage tube amp experiment
« Reply #2 on: September 16, 2008, 05:26:08 PM »
Looks and sounds great. Its got a nice crunch...it seems like it really responds to your feel/playing.

Did you build it on vero?

Have you tried dealing with the lead dress? Seperate the AC and DC wires and maybe shorten some wires up ???

Renegadrian

Re: Low voltage tube amp experiment
« Reply #3 on: September 16, 2008, 05:40:37 PM »
Looks good Ian.  ;)

How are you powering it? Do you think it would be possible to use something like a MAX1044 or LT1045S to get the plate voltage from 9 or 12 volt wall wart?

Rick, I was wondering if a charge pump could help us in our tube experiments, but i read you get very low mA, so I believe it wouldn't meet the higher mA requirements of the tubes...
Done an' workin'=Too many to mention - Tube addict!

slacker

Re: Low voltage tube amp experiment
« Reply #4 on: September 16, 2008, 05:57:57 PM »
Looks good Ian.  ;)

How are you powering it? Do you think it would be possible to use something like a MAX1044 or LT1045S to get the plate voltage from 9 or 12 volt wall wart?

At the minute I'm using a 24volt AC power supply which is then rectified to get 36 volts. The heaters are run off 12.6 volts using an LM317 this is the bit I'm still playing about with because the regulator needs a fairly big heat sink.
I'm not sure about using voltage multiplier chips it might be possible to do something with one. I'll have to measure it but I think the LT1054 could handle the current draw of the circuit.

Did you build it on vero?

Have you tried dealing with the lead dress? Seperate the AC and DC wires and maybe shorten some wires up ???

Yeah it's on vero. The cables are still a bit of a mess because I haven't figured out how the power is going to be wired up yet. Some better lead dress might help with the hum.

frequencycentral

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Re: Low voltage tube amp experiment
« Reply #5 on: September 16, 2008, 06:04:29 PM »
Looks good Ian.  ;)

How are you powering it? Do you think it would be possible to use something like a MAX1044 or LT1045S to get the plate voltage from 9 or 12 volt wall wart?

Rick, I was wondering if a charge pump could help us in our tube experiments, but i read you get very low mA, so I believe it wouldn't meet the higher mA requirements of the tubes...

From memory the MAX1044 is 10ma output, maximum input voltage 10.5 volts. The LT1054S is 100ma output and will handle 12 volts input. I've used MAX1044 with tubes in some experiments. I think it's the heaters that require the high ma, not the plates. Just curious if it would also handle the transformer.
« Last Edit: September 16, 2008, 06:10:45 PM by frequencycentral »

John Lyons

Re: Low voltage tube amp experiment
« Reply #6 on: September 16, 2008, 07:25:49 PM »
Looks and sounds cool Ian
The clip is a bit papery sounding but I'm sure it's just the laptop mic.
Does it hold it's own compared to a typical decent sounding amp?
(Not that it doesn't sound decent :icon_wink:

john

Basic Audio Pedals
www.basicaudio.net/

Br4d13y

Re: Low voltage tube amp experiment
« Reply #7 on: September 17, 2008, 01:00:38 AM »
do you have a vero layout for the newbs like me who can't read schematics well? ::)
freedom is the freedom to say 2+2=4

frank_p

Re: Low voltage tube amp experiment
« Reply #8 on: September 17, 2008, 01:45:24 AM »
do you have a vero layout for the newbs like me who can't read schematics well? ::)

Ask about symbols that you don't know !  A lot of guys (and girls: I wish there would be some more...),  here will inform you ( pleases don't be shy), and if other are condescendant in a bad way, outline it.    And after, you will be able to do more things: from the knowledge that your friends have given you.  And if you have more questions, ask again...   The more you will be specific, the more it will be profitable for you ( and also for the others) to be on this forum with us !!!

Yours !
F.H.P.
P.S. : Schematics reading is not so difficult !  There are not a lot of symbols to know about what you have to solder, aside from different ICs specific to some boxes.                             Again, ask specific questions (or general ones, and mention (and outline them) what is obscure to you) !



Be patient and kind, and you will get a lot of feedback (I assure you ! ) !

Slacker have posted a lot here (and his project is important to him, and I am sure that he is proud of it ) , and thus have contribued to the "friendship" athmosphere of this forum.

Welcome and be gentle here (and slowly learn), because everyone here want it this way.

A specific question is a contribution.
Yours again.

 :) ;)

And be assured that you are welcomed !  :D
Again: Ian have posted something that is not common since everybody on WWW is searching for new guitar effects, and his work is a contribution to the forum and persons that had been here for a little while.  Please cautious and kind !

PS.: Change your name: nobody will remember what it is...  Even if it is not your "real" name... We don't care. ( Well, I don't, as long as I (and others) can remember you )

F.H.P.

( AND BE SURE NOT NO BE SCARED BY ANYBODY, AS EVERYONE WILL BE WILLING TO HELP YOU ! STAY A WHILE AND YOU WILL GET MANY ANSWERS FRIENDS !




« Last Edit: September 17, 2008, 02:20:17 AM by frank_p »

Steben

Re: Low voltage tube amp experiment
« Reply #9 on: September 17, 2008, 03:17:54 AM »
Power stage: Is that class A in parallel?
Why not push-pull as in Firefly?
Rules apply only for those who are not allowed to break them

slacker

Re: Low voltage tube amp experiment
« Reply #10 on: September 17, 2008, 01:26:58 PM »
Looks and sounds cool Ian
The clip is a bit papery sounding but I'm sure it's just the laptop mic.
Does it hold it's own compared to a typical decent sounding amp?

Thanks John. In real life it sounds fuller than in the clip, the laptop mic loses some of the bottom end. The speaker used wasn't in a cab either it was just lying on a box, so it's probably not the best advert for the sound.
I've since tried it through one of my Twin's speakers and it sounds a lot warmer and smoother. I'll get hold of a mic and do some better soundclips.
I don't know how it would stand up next to a proper low wattage tube amp but as an experiment I'm more than happy with the results.

Power stage: Is that class A in parallel?
Why not push-pull as in Firefly?

Yes it's single ended class A with 2 triodes in parallel to get a reasonable volume. Push pull would probably be better but I think I'd need a proper output transformer for that and I wanted to do this on the cheap. Now I know the idea works I might try a push pull version.

Caferacernoc

Re: Low voltage tube amp experiment
« Reply #11 on: September 17, 2008, 03:21:17 PM »
"The speaker used wasn't in a cab either it was just lying on a box,"

      In that case the clip sounds incredible.  :-)

slacker

Re: Low voltage tube amp experiment
« Reply #12 on: September 17, 2008, 04:14:27 PM »
cheers.

sean k

Re: Low voltage tube amp experiment
« Reply #13 on: September 17, 2008, 04:16:38 PM »
Slacker, have you wired the heaters on each tube reverse phase to each other? The other trick is to have a variable resistor between the pins to adjust the midpoint. I've picked them up in old valve tape machines but have yet to find a 200ohm wound pot. Another trick with heaters on AC is to do the centre tap with 100 ohm resistors then attach the mid point to a reasonable high DC point at high impedance. You could try about 24V.

Oh yeah, and the sound is noticably 12AT7! I used to be able to get old Tesa brand 12AT7's for a few bucks each and made loads of things with them.

sean k

Re: Low voltage tube amp experiment
« Reply #14 on: September 18, 2008, 04:38:32 PM »
Oops, I see now that you're running your heaters on DC so none of the AC tricks would apply. But from your photo it looks like your power in is those twisted pairs and the heater pair, are they the grey and orange?, aren't twisted anywhere near tight enough and they could be set alot closer to the chassis and away from the other wires. The best trick I got for grounding tube amps was that all the grounds should spider to the highest impedance point and that was always the input.

Given your power supply is coming in as DC then the grounding issues are whats bringing in AC 60Hz noise 'cause even ripple is at 120Hz so your grounds must be an antenna to bring in 60Hz... ??? unless it's coupling into the DC lines in. A wooden chopstick to push twisted pairs about is the best tool on p-p amps.

slacker

Re: Low voltage tube amp experiment
« Reply #15 on: September 18, 2008, 05:01:20 PM »
Cheers for the advice Sean, I've still got to sort out where I'm going to mount the rectifier and regulator so once I've done that the wiring should hopefully get a bit neater. I'll definitely try increasing the amount of twist in the power and heater wires and route them as out of the way as possible.
I think I just need some extra filtering on the power supply to get rid of the hum, but I'll have a better look at it at the weekend.

Boogdish

Re: Low voltage tube amp experiment
« Reply #16 on: September 18, 2008, 05:29:02 PM »
I'm the junior assembly guy at Kendrick Amplifiers, I don't have a proper college education and I'm constantly learning, so take this advice with a grain of salt.

One thing to keep out noise/hum is you could try mounting less on that board and more on the panel mounted components.  Instead of having signal go from a pot lug to a wire to a cap on the board to another wire to another pot lug, just mount the cap from lug of one pot to the lug of the next, you'd eliminate about a foot of wire (antenna wanting to pick up noise).  Also, make sure you're not daisy chaining your grounds, connect each ground point to something that is directly connected to your chassis which is nearby (sleeve of a jack, back of a pot, grounding lug on one of the bolts holding in your transformer or tube socket), this eliminates the chance of ground loops within your circuit. 

Anyways, congrats on the build and good luck with tweaking it out and getting it as proper as you want it to be.  From your photos, I couldn't tell exactly what was going on all the time, so if some of what I wrote above was something you were already doing, sorry.

Oh and also, I agree with twisting those wires into tighter pairs.

sean k

Re: Low voltage tube amp experiment
« Reply #17 on: September 18, 2008, 05:40:12 PM »
Yeah, tubes are such a completely different kettle of fish compared to transistors. High Voltage at high impedance sharing the exact same space as Low Voltage at a low impedance so the territory becomes rife for all sorts of magnetic coupling to occur.

I absolutely love point to point wiring on tube amps and have to admit that when an amp works quietly re; S/N, it's usually quite beautiful, aesthetically, as well. Tubes are mystical, and getting them to work right makes life alot easier to live as if being able to balance those two extreme variables, HighV and Imp and LowV and Imp, somehow affects the psyche and works on ones enviro and Inviroment...   

Sorry Chaps... Tubes are like that for me!

kurtlives

Re: Low voltage tube amp experiment
« Reply #18 on: September 18, 2008, 05:44:45 PM »
I'm the junior assembly guy at Kendrick Amplifiers, I don't have a proper college education and I'm constantly learning, so take this advice with a grain of salt.

One thing to keep out noise/hum is you could try mounting less on that board and more on the panel mounted components.  Instead of having signal go from a pot lug to a wire to a cap on the board to another wire to another pot lug, just mount the cap from lug of one pot to the lug of the next, you'd eliminate about a foot of wire (antenna wanting to pick up noise).  Also, make sure you're not daisy chaining your grounds, connect each ground point to something that is directly connected to your chassis which is nearby (sleeve of a jack, back of a pot, grounding lug on one of the bolts holding in your transformer or tube socket), this eliminates the chance of ground loops within your circuit. 

Anyways, congrats on the build and good luck with tweaking it out and getting it as proper as you want it to be.  From your photos, I couldn't tell exactly what was going on all the time, so if some of what I wrote above was something you were already doing, sorry.

Oh and also, I agree with twisting those wires into tighter pairs.
If there is one thing I have learned from Gerald it's this. ;D

bioroids

Re: Low voltage tube amp experiment
« Reply #19 on: September 18, 2008, 11:19:58 PM »
PS.: Change your name: nobody will remember what it is...  Even if it is not your "real" name... We don't care. ( Well, I don't, as long as I (and others) can remember you )

I think his name is Bradley, only some letters are transposed to numbers. I'll sure remember him now that I cracked the code!  :icon_cool:

Greetings

Miguel
Eramos tan pobres!