Silvertone Organ amp, weird osc section. - diyAudio
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Old 28th December 2007, 09:02 PM   #1
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Default Silvertone Organ amp, weird osc section.

Anyone know where I can get a photofact for a Silvertone organ amp chassis number 157.47101 or 157.47210?

I pulled one out of a no-model cheapie reed organ. I got the whole shebang for $35, which included a nice 8" oxford 8HB. All the tubes ( silvertone branded) test new. Lineup is 5Y3, 6v6GT, 12ax7a, and 12DW7 (vibrato channel). Very nice sized output iron, and the power transformer is nice and small (perfect match for the size of the amp, not undersized or oversized.Runs cool @ 357vdc.)

Its going to make a nice cheap champ clone. Vibro champ anyone?

Anyways, I can't get the vibrato to work, I'm wondering if the caps in the oscilator section might be dried up?Theres an old dubiler cap and some "astron corp" paper caps in it. The rest are ceramic.

I can post a pic of the chassis if it helps, I can't really dig up much information on the net about any silvertone organs, let alone the amps. Of cource I could just gut it and build a champ circuit around it, but I want to try getting the vibrato working first. I have tried to schem it out but I'm finding some weird circuitry which I don't understand. The pre and post are typical gain stages, but the one in the middle that mixes the vibrato section is WEIRD. It almost reminds me of the entire vibrato section of a vibrochamp, and the oscilator section of a deluxe reverb. I'll take some pics and do a scan of my hand drawn schematic if it will help.
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Old 28th December 2007, 09:44 PM   #2
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Here are some pics. They are 8mp so they might take a while if you have a slow connection. I left them large so you can zoom in.


Silvertone chassis underside
Silvertone on workbench
Schematic
Closeup of preamp and vibrato section
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Old 28th December 2007, 10:34 PM   #3
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Sams index says it's in # 528 - Servicer. Not in my collection - sorry! Likely schematic only - looks like you have that worked out already.
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Old 29th December 2007, 02:32 AM   #4
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hey Thanks! I don't know if I have it worked out completly just yet... there are a few mystery parts that I wanna get the values for. I'll see if I can find the sams at a library or something in the mean time.


While I'm searching for the sams, does anyone have any insight on the second pre/vibrato mixer stage? I really don't understand whats going on there
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Old 29th December 2007, 02:34 AM   #5
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My suspicion is that the wax paper capacitors have dried out. I would replace these and see what happens. If you have a way of testing them that would verify the problem.
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Old 29th December 2007, 02:36 AM   #6
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I had a similar strange amp come out of an organ...I still haven't managed to figure out how the vibrato channel worked...the organ it came out of was a transistor generator coupled to a tube amp...in my case (I believe) the actual oscillator may have been offboard, and modified the bias on the outputs...I'm still not sure. One thing I have noticed with little spinet and smaller organ amps (especially mine, from a hammond model 2000 "everett" organ) that the internal circuitry is far, far more complex than guitar amplifiers. If the vibrato is actually a tremolo using an LDR, I have heard of the oscillator caps being to blame for problems. I would try reading values off everything connected to the oscillator stage and replacing with new parts. Before doing this, however, I would try dropping a new tube in there anyway. Just because it tests new doesn't mean it works (it probably isn't new ). Good luck!
ps. check voltages too...I assume mine was supposed to run the el84s pretty reasonably, as hammond tended toward over-engineering...I have about 425v (high!) going to the el84s and a maximum of about -13 (maybe not enough for this plate voltage) volts bias.
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Old 29th December 2007, 02:49 AM   #7
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I tested the tubes and they all tested very strong, but for good measure I did try swapping out tubes and it didn't make a diff. Some of those carbon comp's have drifted a little bit, but I dont think any are outside of their 10%. They probably were that way when new for all we know.

There is a similar unit made by silvertone that uses a 6aq5 output tube, and its circuit is even closer to a vibrochamp, or a smaller silvertone with a fender vibrato ( the smaller silvertones liked to use 6au6's for the vibrato channel.) I don't have a way of testing the caps but every single ceramic cased dubiler cap I have come across is almost always dead, and those paper ones arent much better. I'm going to replace them now with some I have in the parts bin. I'll report back soon.
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Old 29th December 2007, 03:22 AM   #8
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changed the caps, that did it- vibratos working now! Its a little slow though, and as I suspected the "vibrato" controll is only an intensity controll and I cant change the speed. Any idea where I'd add a speed pot in this circuit?
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Old 29th December 2007, 04:00 AM   #9
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The speed should be controlled by the RC time constant of the capacitors you just changed and their corresponding resistors. Try fiddling with the two parallel 47k resistors and see if you can get enough change by altering that alone.
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Old 29th December 2007, 08:23 PM   #10
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thanks, I tried changing it but it didn't change too much. I think I'm just gonna rewire it and use it as a testbed ( my original intention) before I gut it and put it in a nicer hammond chassis.
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