Hi All - I'm looking for some suggestions for modifications to make to the amplifier from a Voice of Music record player (model 556a) to use it as a guitar amp - see attached schematic for details. It is a 6V6 push-pull amp with a 12AX7 being used as a preamp and a phase inverter if I'm understanding the schematic correctly.
I know enough about tube amps to keep myself alive while working on them, and the basics of how they work. I don't know enough to understand what changing values of resistors and capacitors is likely to do. I was thinking it would be nice to add gain to the preamp section to get some distortion with out cranking up the amp, and perhaps add another pot to vary the input signal level to control how much distortion is being produced. I think I could probably figure out how to adjust the input signal level, but I don't know what to do to bump up the preamp gain.
Other suggestions would be welcome!
Thanks,
Clark
I know enough about tube amps to keep myself alive while working on them, and the basics of how they work. I don't know enough to understand what changing values of resistors and capacitors is likely to do. I was thinking it would be nice to add gain to the preamp section to get some distortion with out cranking up the amp, and perhaps add another pot to vary the input signal level to control how much distortion is being produced. I think I could probably figure out how to adjust the input signal level, but I don't know what to do to bump up the preamp gain.
Other suggestions would be welcome!
Thanks,
Clark
Attachments
Try a 47uF 100V capacitor across the 200 ohm 5w resistor.
If you then still need more gain even with the pots all the way up,
also try a 10uF 16V capacitor across the 3900 ohm resistor.
If you then still need more gain even with the pots all the way up,
also try a 10uF 16V capacitor across the 3900 ohm resistor.
May do about nothing until overload- push-pull each side bypasses the other.
Bypassing first cathode gets double gain which is good but may not be ample.
A rule-o-thumb which is hard to beat: you need two gain-stages between guitar and power tubes. Yes there were pentode-power line-ups but mostly as cheap student amps of limited performance (unwanted overload). Triode-power tube only in midget amps for small bedrooms.
Here it depends on religion but a practical approach is to wedge a JFET first stage in front, then the Volume/tone, then the gain stage, then the cathodyne. The other way is use these two triodes as gain stages and add a MOSFET 'cathodyne'. (Once upon a time we would use a transformer but today that is the more expensive less toneful path.)
Since I'm going for more crunch instead of more loud, would it make sense to start with bypassing the 3900 resistor?
Would it be feasible/sensible to replace the tube rectifier with solid state, and use the rectifier's spot for a triode for another gain stage?
Yes to both. If you used the rectifier spot, you could get two more gain stages. Enough to add bass and treble, or a reverb tank.
You may have to add a dropping resistor to the B+ on the rectifier side, approximately equal to the ON resistance of the rectifier. Or just deal with the slightly higher B+ if it doesnt cause problems.
You may have to add a dropping resistor to the B+ on the rectifier side, approximately equal to the ON resistance of the rectifier. Or just deal with the slightly higher B+ if it doesnt cause problems.
A solid state device would use less power, I think, and more durable.
A valve system sounds different, but for something that can be made for about $50, I think modifying an existing amp is not worth the effort.
A valve system sounds different, but for something that can be made for about $50, I think modifying an existing amp is not worth the effort.
I like the idea of getting two more gain stages... adding a reverb tank would necessitate another output transform (I think?) which would be more hassle than I want to take on.
Unless its fun to see what happens, what you can get. I've nothing to add, save perhaps introducing some neg feedback from the output secondary to the cathode of the 1st stage. Did I get that right? Will reduce the gain however.
One would think there's plenty of guitar "boost" boxes that run from a 9V that would drive the s--t out of this.
Who knows where the tone control frequencies are set to and if they are appropriate for guitar. You could play around with the values in the circuit that's already there, or replace it with a "vintage" guitar tone "stack", er, control.
Nice idea! That will essentially get you a Fender Princeton (without reverb, and with some different value resistors and caps). The Princeton is a well beloved vintage tube amp - not at all a bad place to start!
If I were in your shoes, I would be very tempted to plug my guitar into a suitable "clean gain" guitar FX pedal, then plug the output of that straight into the tuner or phono input of the amplifier, and find out what it sounds like. 😀
IMO a graphic EQ pedal (like my old Danelectro Fish-n-Chips) may be the ideal one to use in this case. This will let you tinker with both EQ and gain to get a sound you like, and that in turn will guide you in the design and "voicing" of the two new gain stages you're planning to add.
As for reverb, these days a cheap reverb pedal from Amazon is a perfectly logical solution. Many include "spring reverb" models, without the headaches of an actual spring reverb tank, driver, and recovery gain stages.
Some years ago I used a Donner "Verb Square" pedal for just this purpose (adding reverb to a tube amp I'd built), and found it quite adequate for my tastes. It includes a spring reverb model, as well as hall, plate, studio, and so on. A lot of bang for only 50 bucks USD: https://www.amazon.com/Donner-Digit...t/dp/B0719CBYXJ/ref=sr_1_2?crid=11NFOAIBLD4I4
I'm assuming basic safety is already taken care of - the chassis is securely earthed to mains earth via a suitable 3-wire cord and mains plug, yes?
EDIT - don't forget to add screen grid resistors for the two 6V6 output tubes! They will keep your 6V6 grids from melting when you feed the amp overdriven guitar.
-Gnobuddy
You could make a cheap and easy high voltage gain stage using the LND150 mosfet.
Thanks for the suggestions! There is not yet a 3 wire cord and plug, but there will be before I plug it in. Right now the power cord is a 70 year old terrifying frayed mess.