i have a WORKING tube amp from a silvertone organ model 4707. the tubes are TWO 12AX7-7025 tubes ONE 5Y3 and ONE 6AQ5A and ONE C-7 A--B Not removable. anyone have any ideas how I could turn this into a nice guitar amp. I played through my guitar on it and it was suprisingly louder than I though tit would be. this amp has two transformers and the 4 tubes and a fith tube? thats not removable. the c-7 tube. one voulome knob. attached to it still is the small panel with the on off knob vibrato that works. and some kind of tone control that has two selections strings (trebly) and flute (bassy) I have made boost pedals before and have added my own tone circuit, volume sag, feedback swith with trim pot, so I am not a total noob, but have not modified any tube amps yet. any ideas guys? am I biting off more than I can chew? I think can do it with a little advice, like where to put tone circuit and controls. between the preamp tubes or after>?? any HElp is apreciated. thanks guys
One transformer is the power supply transformer, the other is the otput transformer to drive the speaker(s).
Most of the tone comes from the preamp section(s).
Each of the 12AX7 is a dual-triode so you might have 4 preamp stages right there, plenty of gain for a guitar...you can put in a few more volume pots to act as 'master volume' in guitar-amp terms, and control the distortion, compression, sustain versus clean sound. Compare the first stages of the circuit to one channel of any typical Fender, and you'll see how to replace those tone controls with something more, well, Fenderish, and also put a bright switch with capacitor across the volume. Good luck, have fun. Trace the circuit and post it up! How your particular guitar pickups match to the resistor network at the grid of the first 12AX7 can also be critical. Look at one channel of a fender, or more ideally one channel of a Fender preamps section driven into another channel of a Fender preamp section. It's really nice to have lots of treble in early stages, then when later stage compression makes some distortion you can roll off some excess treble for a nice 'creamy' tone, or alternately carry the treble clean all the way thru. So two sets of tone controls and two preamp channels (four tube stages) really makes for a nice guitar amp IMHO. I'd much rather have two guitar preamp channels in series than two in parallel LOL.
Most of the tone comes from the preamp section(s).
Each of the 12AX7 is a dual-triode so you might have 4 preamp stages right there, plenty of gain for a guitar...you can put in a few more volume pots to act as 'master volume' in guitar-amp terms, and control the distortion, compression, sustain versus clean sound. Compare the first stages of the circuit to one channel of any typical Fender, and you'll see how to replace those tone controls with something more, well, Fenderish, and also put a bright switch with capacitor across the volume. Good luck, have fun. Trace the circuit and post it up! How your particular guitar pickups match to the resistor network at the grid of the first 12AX7 can also be critical. Look at one channel of a fender, or more ideally one channel of a Fender preamps section driven into another channel of a Fender preamp section. It's really nice to have lots of treble in early stages, then when later stage compression makes some distortion you can roll off some excess treble for a nice 'creamy' tone, or alternately carry the treble clean all the way thru. So two sets of tone controls and two preamp channels (four tube stages) really makes for a nice guitar amp IMHO. I'd much rather have two guitar preamp channels in series than two in parallel LOL.
The second 12AX7 is for the vibrato, the tube that you can not remove is probably the filter capacitor (C-7)
'60s Sears Silvertone 4707 organ amp conversion - Telecaster Guitar Forum
Silvertone Mdl 4707 - AMPAGE Archive
http://www.diyaudio.com/forums/tubes-valves/114405-silvertone-organ-amp-weird-osc-section.html
'60s Sears Silvertone 4707 organ amp conversion - Telecaster Guitar Forum
Silvertone organ amp 4707 anyone seen one? - Telecaster Guitar Forum
Silvertone Mdl 4707 - AMPAGE Archive
http://www.diyaudio.com/forums/tubes-valves/114405-silvertone-organ-amp-weird-osc-section.html
I am usually good at finding needles in haystacks, would like to help further but I am a little busy right now.
Best to learn a little of a basic amplifier. A really good document.
http://ax84.com/p1/P1_Theory_Document.zip
OK I thInk i have a handle on how tubes work now. kinda haha.. well I installed an input jack. I seperated the on off and tremolo swithes. (they were on same double stacked pot.) now I have a swith for on off and a pot for tremolo. I installed the light and red glow indicator thingy. so I am making progress.
Now if I understand right I can get rid of the tremolo and use that half of the tube for something else like a tone circiuit or a gain stage. AM I on the right track with my thinking? and I need to install a 3 pronged cord I am reading about a death cap or a grounding cap? I thought I would ground the third to the chassis BUt I guess theres more to it? and last question how can I make this louder? when I use a clean boost pedal in front of it its nice and loud but by itself its pretty weak volume. The tubes start to break up nice on about 3 and it just gets snottier as you go up. not louder but more drive. very nice. I know I need to install a fuse too. thought I would do that when I install 3 prong cord. so to recap my long winded post: how to make louder? death cap? can I replace tremolo with more gain or something in place of tremolo.
Now if I understand right I can get rid of the tremolo and use that half of the tube for something else like a tone circiuit or a gain stage. AM I on the right track with my thinking? and I need to install a 3 pronged cord I am reading about a death cap or a grounding cap? I thought I would ground the third to the chassis BUt I guess theres more to it? and last question how can I make this louder? when I use a clean boost pedal in front of it its nice and loud but by itself its pretty weak volume. The tubes start to break up nice on about 3 and it just gets snottier as you go up. not louder but more drive. very nice. I know I need to install a fuse too. thought I would do that when I install 3 prong cord. so to recap my long winded post: how to make louder? death cap? can I replace tremolo with more gain or something in place of tremolo.
Insanegraphix, could you please put up a schematic of the modifications you made? I'm a nubie when it comes to tubes, but interestingly enough, I stumbled upon the exact same project amp. I installed a power switch and input jack already.Your pictures helped greatly, but some mods are still unclear. Thanks.
I'm in the same boat @jezelman was (and I see his post was from five years ago). Any movement on this? I have one of these amps too that I'd like to get up and running. Would love to know, for starters, how you worked the input? I have hunch simply connecting the the existing mic line to a 1/4 inch mono jack, isn't going to work. Any info or pointers to other threads will be much appreciated.
Stephane, what I would suggest is that you look at the organ you have as a pre-built power supply (assuming caps have been replaced), and you will probably find you can also use the existing power amp section more or less unchanged, or with rather slight changes.
The rest of it, I suggest looking at as a collection of useful valves (tubes.) Maybe some usable pots and knobs and switches too. In other words, be prepared to throw away most of the existing wiring and small components, and rebuild the whole thing to guitar amp specs instead.
In particular, the preamp section - the 12AX7s - should be treated as raw material, nothing more. Ignore the existing preamp circuitry that's already in between those valves - rip it out. Rewire to taste. Schematics for most classic valve guitar amps are easy to find on the 'Web.
The thing is, it takes a fair bit of both theoretical and practical electronics knowledge to do this successfully, and to stay safe while doing it. (Remember those potentially lethal voltages!)
If you already have some tube guitar amp building under your belt, a build like this is quite feasible. If this is the first time you've ever tried to build a tube amp, you may find that converting an existing organ amp is actually harder, in some ways, than starting from scratch and building your own.
-Gnobuddy
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