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Old 5th September 2012, 02:57 AM   #1
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Default Silvertone Organ Amp

I know this already has a couple threads, but I can't get my answers from them. Here is a Silvertone Organ Amp Model 4721. I'm converting it over to a guitar amp. I have already changed over the pots and added a 1/4 input jack. Changed the on/off switch to an actual switch and going to add a standby as well. I removed the master volume and am going to add a tone knob to the middle pot. I changed all the electrolytic caps and I also changed out the paper caps. I left the ceramic caps as they all tested fine.

My problem is I can't get the tremolo to work. This layout is very strange to me. Any advice would be appreciated. Here are the pics. (The tube layout is: 12dw7, 12ax7, 6v6, 5y3)


Let me know if you need better pics. Thanks for any help!

-Steven
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Old 5th September 2012, 03:05 AM   #2
Pano is offline Pano  United States
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Hi Steven, Nice pix but I had to resize them, they were really big.
Please see here:
Pictures -- Why Not attach Them ??
Cheers!
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Old 5th September 2012, 03:21 AM   #3
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Oops! So sorry. This is my first post. Will follow the norm next time. Thanks!
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Old 5th September 2012, 06:58 AM   #4
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Circuit schematic??

They look like magnatone varistors (ie. vibrato) ?
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Old 5th September 2012, 11:55 AM   #5
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I don't have one and haven't been able to find one. I'm just learning how to read schematics, but don't really feel like drawing one up would be doable for me.

I read about the fact that they might be varistors. Should/can I replace them?
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Old 5th September 2012, 12:53 PM   #6
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To start faultfinding an amplifier it always best to have a schematic and have some experience in testing circuits with hazardous voltages. It is a good learning curve to make up a schematic - it tests if you know the parts and how basic circuits are connected.
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Old 5th September 2012, 01:49 PM   #7
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Yeah, that's why I'm trying to learn. I've just been looking at layouts which I know only shows where to connect things, not why. Here's a schematic I found a while ago, but it looks partially done. But I'll attach it anyways.

I think I going to just start by replacing the ceramic caps in that section to see if that helps.
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Old 5th September 2012, 03:53 PM   #8
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That double .01 uf cap arrangement with a .022 uf cap going back from the plate of the V2 to feed the pi network looks just like a phase shift oscillator from the GE transistor manual seventh edition that I have been unable to make work. It is a cheap design oscillator that works sometimes, but not in any of the 7 versions I have tried. Do you have about 300V at the plate of V2? Is the 3mohm resistor very clean? Dust on a 3mohm can make a large difference in the value seen by higher voltage signals. (Your DVM measures at about 2V, usually). Same problem with the 10 mohm resistor coupling the oscillator output into the grid of the second half ot V2.
Having fooled with the phase shift one active element oscillator for months without success, I would make a better design 2 transistor oscillator to replace that one gain stage tube phase shift oscillator. You're not getting the classis "tube sound" out of an audio frequency oscillator, anyway. You can drop down 10 ma from the b+ to drive it with a resistor ladder, or rectify the filament 12 VAC to make a low voltage supply. Transistors are good for some things, just not as good at final stage amplification of guitar signals.
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Old 5th September 2012, 11:41 PM   #9
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To add to indianajo's comments, and to firstly focus on the phase-shift low frequency oscillator - it would be good to check the following.
What is supply voltage at 'B'.
8K4 resistor is wrong - it should be something like an 820k.
The 270k//270k looks wrong - perhaps they are in series? - there would normally be something like 470k to 1Meg at that point.
Check the vibrato pot is 1 Meg (otherwise it will load the LFO too much).
Do you have a high-impedance probe to measure AC voltage to check if the LFO is oscillating (eg. by connecting to the vibrato pot wiper output) - eg. an oscilloscope probe with 1Meg input, or an AC meter with similar input impedance and measuring down to 100mV at 10Hz.
Some early LFO's appear to be marginal due to the 3M3 grid leak - increasing that value may help - or alternatively adding a cathode bias (eg. 2k2//25uF, or a red LED with on voltage about 1.5V to 2V).

Check the circuit around the cathode of the 12AX7 before the Volume pot - it's missing a cathode bias.
What are 'L?' ? (Are they the brown tube parts with yellow dot marking?)

Ciao, Tim

P.S. check the Magnatone Estey T-32 schematic - especially the vibrato modulator section with the varistors - see if that is similar to your circuit ?

Last edited by trobbins; 5th September 2012 at 11:48 PM.
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Old 6th September 2012, 12:24 AM   #10
kevinkr is offline kevinkr  United States
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This really belongs in the Instruments And Amplifier forum since it is a guitar amplifier. We try to spell that out in the sub header of both Tubes / Valves and I &A.. Moving it..
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