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Add A Gain Control To An Old Tube Amp


stevenhoneywell

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I've got an old tube amp "Selmer Treble & Bass 50w reverb" which I would like to tinker with.

It hasn't got a input gain control so I can only get power amp distortion from the output valves at full whack

If I were to change the the 68K to variable pot of a same / lesser value would I have control of the gain or would i be killing me and / or the amp

here's a link to a copy of the circuit diagram

http://www.drtube.com/schematics/selmer/t-and-b50rvbsv.gif

Thanks in advance

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Glad to help, but some of us do not look here every hour!

An amp with a single volume control and designed in the "correct" engineering manner is set up so that that preamp does not overload (except may be with really high output pickups). This one has the volume control after the first stage on each channel as one would expect. Getting preamp distortion is not as simple as just adding another pot; you need more gain. For example, on one of the two channels, you could add another preamp tube stage, followed by another pot, which would become the "master volume". Then the current volume control for this channel would be the "gain". The interesting thing is that I see nine preamp tube sections on the schematic, and so one of the dual triodes should have an extra section. So it is possible without drastic surgery, I think, but this would have to be examined, designed , and executed by someone who knows what they are doing.

Another approach would be to throw out the reverb; then you could use the existing stages associated with it for a really high gain preamp channel.

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The schematic is a little fishy itself. The way it's drawn, the input to the phase inverter is coming off the B+ line? Chances are good that the 500pf to the right of the second tube is really the input cap to the phase inverter.

Some schematics have intentional errors, others may be accidential.

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...shame no-one here can help...
:D Silly boy, of course we can help! :D Here's my advice: first of all, switch to decaf, and stop acting like other people with real lives are obligated to provide for your instant gratification. Second, realize that just because an amp has tubes in it doesn't make it a prime candidate for modding into a homebrew ├╝ber-Marshall. What you have there is a great little classic semi-clean amp, so if you want distortion, you can try running both channels in parallel (a la the way Clapton, et al, used Marshall Bluesbreakers), you can add a gain stage or you can use either a clean/treble booster (as JohnH suggested) or a Tube Screamer/OD250/Dist+/Megablast Belchfire 2000/choose your favorite distortion stompbox (just like SRV did) to dirty it up. It's never going to get you into Diezl/Bogner country, but it will get you some nice old-school crunch and singing sustain. If you're into death/thrash, your best bet is to put it on Ebay - you just can't get there from here. HTH
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hi,

AFAIK, selmer amps increase greatly in value with time. i wouldn't make any changes to that if i were you. just baby it and have it checked out by a tube amp pro every now and then.

i could be wrong on this, but this is what i'Ve read on the net and in some magazines.

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How about just a claen solid-state booster before the input........John

The problem with a clean solid state booster is that it you cannot keep it clean and get a lot of gain; you can get a moderate amount with typical battery voltages, but the final SS stage will overload if you use a lot. You could put a tube followed by a pot in front; then the current volume control becomes the master gain.

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The problem with a clean solid state booster is that it you cannot keep it clean and get a lot of gain; you can get a moderate amount with typical battery voltages, but the final SS stage will overload if you use a lot. You could put a tube followed by a pot in front; then the current volume control becomes the master gain.

Thats what I mean, use an SS circuit to boost the signal but only in the clean range, and then let the tube amp go from there into overdrive. If theres not enough to get the result from a 9V supply, two batteries could be used to boost using a circuit designed for an 18V supply.

I use JfETs for this type of thing, so that if the booster does saturate a bit, the result is not bipolar-nasty.

John

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Thanks everybody. I'm switching to decaff as we speak :D

I'm gonna try the booster idea and see how it goes. Upon further reflection I don't want to be one of those people who tinkered with a future "classic" (imagine being one of those people who had an early strat refinished....I would never forgive myslef).

If it doesn't work out the way I want I'll sell it and put the money towards a ax84.com project or similar and have some "painful fun" tinkering with a designed kit. I'll probably learn loads from it aswell

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ok i have a harmony 535 tube amp with the same situation. no preamp gain control. i have been studing a circuit (a marshall design) and believe it will work if put in the right place. which i would break in the signal chain just behind the first gain stage on either chanlle of your amp. this is an untested design and really needs one of the guru's here to look over it and give me thier thoughts. if you pm me i will send a copy of the circuit to you.

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