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Old 23rd February 2012, 04:56 PM   #1
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Default silvertone organ amp conversion. 4704

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
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Old 23rd February 2012, 07:27 PM   #2
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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.
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Old 24th February 2012, 10:30 PM   #3
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Default thanks

How your particular guitar pickups match to the resistor network at the grid of the first 12AX7 can also be critical
not sure what that means . thanks for all the helpfull advice too.
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Old 25th February 2012, 02:51 AM   #4
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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


Quote:
I picked up one for 80.00. Rewired first stage and input like a typical blackface, 100k plate load and 1.5k cathode resistor bypassed with a 25/50v sprague, 1meg to ground with a 68k input resistor. I just used an rca to 1/4 adaptor. Tried bypassing the second stage but had waaaay too much gain. Left second stage stock. Replaced all bypass caps with .02 spragues, which took away the "boxy" sound it had, really openned up the sound. Replaced another 25/50v electrolytic with a sprague of the same value. Replaced the stock Oxford with a Weber alnico sig 12 speaker (45.00 wanted to go realatively cheap since it was a project).

Didn't have enough grind for me so I lowered the input resistor to 33k and raised the plate load resistor on the first stage to 200k, used 2 100k's in series. I would have rather lowered the last stage resistor on the filter cap to increase plate voltage but I didn't have the resistor I needed so I just raised plate load resistance. Plate load was around 165-170v before I raised the resistance (forgot to measure it afterwards).

I may experiment more with the second stage but, since and organ amp is pretty much a bass amp, I didn't want to loose too much of the low end. Gotta get some more components to play more with it.

I was trying to get what I wanted and still use the 12dw7, because the trem doesn't work on mine either and I wasn't sure if the extra oomph of a 12ax7 would affect the trem when I get to that. Stuck a 12ax7 there for more gain for now and I'm pretty much in Tweed territory, but I think with more minor tweeking (when I get the components I need) I may be able to do better.

Looks like something from the old "Dick Van Dyke" show from the early 60's. Very retro looking with the wooden legs. It's kinda like an end table looking tweed champ (vibrochamp when I get the trem working).
Silvertone organ amp 4707 anyone seen one? - Telecaster Guitar Forum

Silvertone Mdl 4707 - AMPAGE Archive

Silvertone Organ amp, weird osc section.
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Old 25th February 2012, 12:55 PM   #5
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thnak you soo much guys. I will post my progress and results. thanks foir the info. I will read everything you linked for me. i really did try to find that stuff myself but was not too lucky. great happy here. this forum rocks. and so do you guys.
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Old 25th February 2012, 07:07 PM   #6
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Quote:
Originally Posted by insanegrafix View Post
thnak you soo much guys. I will post my progress and results. thanks foir the info. I will read everything you linked for me. i really did try to find that stuff myself but was not too lucky. great happy here. this forum rocks. and so do you guys.
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
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Old 25th February 2012, 07:34 PM   #7
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wow this is exactly what I need. nice thanks again/
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Old 28th February 2012, 02:07 AM   #8
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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.


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