Hi all,
TLDR; I've got a tube amp from an old hifi I'm hoping to convert into a guitar amp and am hoping for some circuit analysis help/advice.
A while back I picked up a pair of Symphonic 1260 stereo consoles which came with mono tube amps (schematic below). I gutted the consoles and refinished them and they are now lovely living room accessories with all new electronics. So NOW I've got a pair of at-least-partially-functional (one works but hums after a while, the other one seemed to be badly need all the pots cleaned and maybe other stuff).
(this isn't mine, but this is the same amp I'm working with if you want a pic: https://reverb.com/item/12970609-symphonic-model-1260-vintage-1957-6l6-tube-amp-works-great)
I'm hoping to recondition these and build them into a matched pair of guitar amps for a friend and I (and by "guitar" I mean "cigar box electric ukulele", it's a companion project to the amps). I'm not a pro musician, these will probably never be used to perform, I don't care if they sound amazing or not, but it'd be a delight if they didn't sound like garbage. I'm not currently an electric guitar player and have never owned a guitar amp, so a lot of the details I've read about are lost on me.
My general plan:
I realize I could probably just get these running and sell em off to buy some amps, but they have some sentimental value and I'm pretty interested in them just for the fun project value.
TLDR; I've got a tube amp from an old hifi I'm hoping to convert into a guitar amp and am hoping for some circuit analysis help/advice.
A while back I picked up a pair of Symphonic 1260 stereo consoles which came with mono tube amps (schematic below). I gutted the consoles and refinished them and they are now lovely living room accessories with all new electronics. So NOW I've got a pair of at-least-partially-functional (one works but hums after a while, the other one seemed to be badly need all the pots cleaned and maybe other stuff).
(this isn't mine, but this is the same amp I'm working with if you want a pic: https://reverb.com/item/12970609-symphonic-model-1260-vintage-1957-6l6-tube-amp-works-great)
I'm hoping to recondition these and build them into a matched pair of guitar amps for a friend and I (and by "guitar" I mean "cigar box electric ukulele", it's a companion project to the amps). I'm not a pro musician, these will probably never be used to perform, I don't care if they sound amazing or not, but it'd be a delight if they didn't sound like garbage. I'm not currently an electric guitar player and have never owned a guitar amp, so a lot of the details I've read about are lost on me.
My general plan:
- Switch in a solid state rectifier
- Replace all the filter caps. Probably add a bigger one right after the rectifier.
- Clean all the pots real well and attempt to verify that things are basically working. Replace tubes or other components if necessary.
- Excise/disconnect the existing tone controls (got that schematic from here: http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/index.html, will pick values later but probably just go with one of the two listed there)
- Does my proposed new schematic (i.e. removing and replacing the tone control) make sense? Is that generally where I want to be hooking things up?
- Do I need to fiddle with the preamp circuit at all (the first 1/2 of the 12ax7)? I also have no problem adding a different/additional preamp to this if it needs it (something like https://till.com/articles/GuitarPreamp/)
- Volume pots: should I add more?
- The linked article suggests one after tone control (circled in blue), and perhaps a dual pot on the inputs to the push/pull tubes - does it make sense to add both?
- If you were going to add in another volume control, where would you put it and why?
- What are reasonable values for these (500k? 1M? log taper I assume?)
- Am I completely off base and crazy?
I realize I could probably just get these running and sell em off to buy some amps, but they have some sentimental value and I'm pretty interested in them just for the fun project value.
You have drawn a Fender tone control circuit. I would rather have the Baxandal contol as it has a smoother response.
Just reduce the negative feedback to increase the THD, connect your guitar lead into the Tuner input and there you go; One guitar amp.
Just reduce the negative feedback to increase the THD, connect your guitar lead into the Tuner input and there you go; One guitar amp.
Is this just a matter of increasing the resistor between the output transformer and the 6au6? Any opinion on a reasonable replacement value?
Removing the tube rectifier and replacing it with solid state diodes will do several things:
Increase the B+ voltage, it may come close to the capacitor's 450V rating
Increase the transient rectifier current into the 40uF first filter cap
Heat up the transformer's B+ secondary
Run the 6L6 outputs hotter
And, the original spec was for 110VAC to 120VAC . . . and if your power mains are now 120VAC, with the solid state rectifiers, everything will get hotter.
Increase the B+ voltage, it may come close to the capacitor's 450V rating
Increase the transient rectifier current into the 40uF first filter cap
Heat up the transformer's B+ secondary
Run the 6L6 outputs hotter
And, the original spec was for 110VAC to 120VAC . . . and if your power mains are now 120VAC, with the solid state rectifiers, everything will get hotter.