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Old 16th May 2008, 05:51 AM   #1
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Default Need help rebuilding old PA amplifier!

Hi!

I found this old PA amplifier from the 40s, made by an old unknown company here in Quebec, Canada.

Click the image to open in full size.

Here is the schematics I made.
More pictures here.
A gut shot.

I want to build a 20/30W guitar amplifier with this while reusing as much components as possible. It has a nice Hammond 273X (350-0-350 @ 110mA) and 1649 (10K / 30W), 4x6EU7, 2x6L6 and a 5U4 rectifier.

Anyone has suggestions schematics for me to look at ?

Mat
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Old 16th May 2008, 08:42 AM   #2
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Hi,

That schematic looks messed up to me :-\

Pair of 6L6 into 10K load?? Is it tapped down on the transformer for 6.6K perhaps? If so, you can get 50W guitar without it working up a sweat

Cheers!
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Old 16th May 2008, 12:12 PM   #3
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You have a great start for your project. All the valves in your amp will work fine. The 6EU7s can be treated just like 7025/12AX7s. Your 6L6s look like 6L6GCs, which have large, heavy anodes for long life in your application. The 10k output load is a little high but really won't hurt your output much. If you are so inclined you can try an eight ohm speaker on the 16 ohm tap to see if you like it better. I would expect output power to be higher than 25 watts with fixed bias and a 350-0-350 power transformer. You should have 430 to 440 volts available at your first filter capacitor.

As for schematics, look at the Fender Bandmasters 5E7, 6G7 and AB763. They use basically the same valves you already have. Your present output stage is already suitable so the main thing you need to concentrate on is the input stages and tone stack stages.

Best,

Wade
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Old 16th May 2008, 06:39 PM   #4
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Just swap the in some 1/4 inch jacks for the input and speaker output and give it a ride! You can then look into having one input channel feeding into another, or putting a volume pot after the input stage to control tone some.

Replacing the the electrolytic should give good results as well.
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Old 17th May 2008, 11:47 AM   #5
GordonW is offline GordonW  United States
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Not to be the contrarian here, but if it were mine, I'd be tempted to re-wire the input tube sockets to use 12AX7 rather than 6EU7s. 6EU7s are really expensive, compared to 12AX7s. Also, there's less "tube-rolling" opportunities, and less availability of replacement tubes in an "emergency" situation (say, you're doing a guitar gig out "in the sticks" somewhere and a tube dies) due to the fact that 6EU7s aren't made anymore. OTOH, 12AX7s are at every guitar store, pretty much.

Don't get me wrong- IMHO, the 6EU7 may well be a better tube for hi-fi use... but in a guitar amp, you probably want a "tried-and-true" bottle in there, so that the circuits "sound right"...

And, if you're rewiring for tone stacks and such anyway, changing the tube socket pinout for a 12AX7 is not much work. And if they're in good shape, you should be able to sell your 6EU7s for enough to buy probably MORE 12AX7s than you have 6EU7s, too...

Regards,
Gordon.
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Old 17th May 2008, 12:59 PM   #6
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That is pretty cool. You could rewire V5 as a CF and change the tone stack to a more traditional guitar stack. Then with all of those input tubes you could do a lot of neat stuff. Trem channel, bright, normal, cascade. Or try the cascode input style as found on the AX84 site.

Rewiring for the 12A#7 family seems like a good idea and use the preamp valves from this amp for a hi-fi project. In playing testing out the amp that I am building for my grandson we found that using an EH 12AY7 as the input tube v.s. a Chinese 12AX7 reduced the microphonics tremendously with very little loss of gain. You have the potential for plenty of stages.

Another thing you could do given all of those sockets is set it up to work with two guitars at one time so you and your buddy can play together.

mike
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