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Old 4th July 2014, 10:31 PM   #1
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Default novice tube amp (re)build

Hello fellow DIYers and builders.
short version: i'm looking for help and advice building a tube guitar amp. Attached is a schematic of where I am now. It's a basic SE amp, with a pair of preamp stages on a ECC82 (12AU7) and an EL84 power tube. It works. What i'd like to do is to:
-connect the tremolo to the signal path
-add a master volume
-also, if anyone would be so kind, to point out problems with what i have already and tips where this can be improved (is the coupling sensible, the bias, etc.)

The long story:
So 2 years ago i built myself an electric ukulele, just to wrap my head around how a electric guitar works, what's the signal path and so on. While it's a crude instrument i'm quite fond of it. Mind you, this was my first venture into building something like that, and my first electronics experience.
After playing through a multieffect and headphones for a year i went out and found myself a couple of old amps. One of them was obviously a DIY effort from at least 30 years ago - i decided to make this one into a learning experience. It was a hissy and noisy rat's nest so i gutted it and went ahead rebuilding it more or less from the ground up. I kept as much of the original parts as i thought feasible. the primary and output transformers, the tubes, the speaker, controls configuration (and its what i'd like to keep in the future in this amp). I got new capacitors, resistors, pots...
For months i gleaned infromation from the AX84 site and the Valve Wizard, while soldering it together.
I'm happy that the amp works but i'm sure there ar problematic areas. I based a lot of it on the AX84, because it seemed closest to the original amp and was well documented.

What I feel are the problem areas: I was not sucessful in making the tremolo work. I'm not sure if the coupling is consistent and sensible. I tied various tone stacks for the 1 pot tone control and this low-loss configuration is what worked best. the other half of the tremolo tube is unused which feels like a waste, but i don't know what i could do with it. I'm also quite a novice in drawing a schematic, i hope it's okay.

So if anyone would like to dive into this mess (?) i'd be very happy for your comments.

Thanks,
Peter
Attached Images
File Type: png extra-solo-wiring-s.png (28.5 KB, 100 views)
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Old 5th July 2014, 03:40 AM   #2
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First off, 12AU7 is kind of a low gain tube, you happy with it?

And still on the 12AU7, doubt it has enough gain to get the tremolo oscillator started. A12AX7 would be a better choice. And speaking of the extra triode, you could use it as Fender had in their VibroChamp.

http://ampwares.com/schematics/champ_vibro_aa764.pdf

The second triode is shares the cathode resistor with the second stage, thereby introducing the tremolo. Other than that, the 10nF cap after your tone control is not nessisary, mind you it probably will not hurt. Might be better if you put a 470-1k resistor on your screen so that when the output tube is run into clipping you limit screen current. But then again, using a 12AU7 you probably are not getting much clipping. For a master volume, stick it where the 470k resistor is, use a 500k pot. The wiper goes to the output tube. Also for safety sake, put a 2,2M resistor on the wiper to ground, just in case the pot goes open.
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Old 12th July 2014, 12:20 AM   #3
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Hi and thanks for the reply.

I'm happy with the 12AU7 inasmuch as I'm not ready yet to try out other tubes. There are still some mechanical kinks I'd like to iron out before that - a scratchy volume pot and similar. The tubes I have probably need replacing, but they work and if something fails I won't feed bad about them.

Thank you for the tips about the tremolo. It seems that the 12AU7 hasn't been used in a tremolo so I guess that's one part of my problem. These last few days I put more research into how to couple the effect into the signal and I'd like to go for a preamp tube bias shifting trem like you suggested. But that is kind of above my current knowledge - calculating the voltage swing of the trem (or simply measuring it?) and pairing that with the cathode resistor, etc. I do like the circuit I found from a Supro s6616 (see attached), I'll try to work that into my amp.

I'll put off mounting a screen resistor for now since any resistors I put in the signal path attenuated it more than i liked (but that might have been a mistake in wiring on my part).

Attached is my updated work in progress. Please feel free to comment on it.

Quote:
Originally Posted by Printer2 View Post
First off, 12AU7 is kind of a low gain tube, you happy with it?

And still on the 12AU7, doubt it has enough gain to get the tremolo oscillator started. A12AX7 would be a better choice. And speaking of the extra triode, you could use it as Fender had in their VibroChamp.

http://ampwares.com/schematics/champ_vibro_aa764.pdf

The second triode is shares the cathode resistor with the second stage, thereby introducing the tremolo. Other than that, the 10nF cap after your tone control is not nessisary, mind you it probably will not hurt. Might be better if you put a 470-1k resistor on your screen so that when the output tube is run into clipping you limit screen current. But then again, using a 12AU7 you probably are not getting much clipping. For a master volume, stick it where the 470k resistor is, use a 500k pot. The wiper goes to the output tube. Also for safety sake, put a 2,2M resistor on the wiper to ground, just in case the pot goes open.
Attached Images
File Type: png extra-solo-wiring-s2.png (30.2 KB, 63 views)
Attached Files
File Type: pdf supro_s6616.pdf (528.0 KB, 5 views)
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Old 12th July 2014, 04:06 AM   #4
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You have the vibrato pot on the wrong side of the resistor, the wiper is attached to ground.
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Old 13th July 2014, 04:31 AM   #5
Poacher is offline Poacher  Canada
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Location: Uxbridge, Ontario
And my 2 cents, there is no coupling cap/blocking cap from the plate of the oscillator triode to the grid of the cathode follower. See the supro schem you posted.
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Old 13th July 2014, 09:40 PM   #6
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whoops, thanks for that!
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