Silvertone Organ amp, weird osc section.

[ThSpeakerDude88]

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.

 

[ThSpeakerDude88]

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

 

[Tom Bavis]

Sams index says it's in # 528 - Servicer. Not in my collection - sorry! Likely schematic only - looks like you have that worked out already.

 

[ThSpeakerDude88]

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

 

[tubewade]

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.

 

[m6tt]

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.

 

[ThSpeakerDude88]

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.

 

[ThSpeakerDude88]

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?

 

[tubewade]

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.

 

[ThSpeakerDude88]

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.

 

[m6tt]

Well, I opened up my organ amp, and low and behold, there was an oscillator! However, it seems to have been simply an onboard oscillator which provided a varying voltage to transistors offboard...it is grounded through some resistors to the cathode of the output tubes, but it seems this was a layout choice...a twisted pair goes from a polar electrolytic out of the amp (or to a plastic connecter which had been left off in my case). Connecting the two wires in the pair did nothing, nor did connecting them through a resistor (1meg). Grounding did nothing either. However, when I touched the wire to the output tube's grid for a moment, I distinctly heard an oscillator...I am wondering how I should use the output from the oscillator...I tried implementing something like a gibson reverb by putting the oscillators output between the P-P grid resistor network and isolating from ground with a 300k resistor. Nothing happened...I just touched the wire to the cathode of a preamp tube and it seemed to work, but I wonder if this is bad for the tube...the output on this wire seems to be around 5-10v.

 

[higbone]

Silvertone Organ Amp

This thread is a bit old, but thought I would try to rejuvinate it...

I just purchased an old Silvertone organ amp from E-bay...exactly like the one pictured on the links earlier in this thread. I was curious as to how the project of turning it into a guitar amp (Vibro Champ-type) worked out. What were the things you learned from the project?

Any body is free to comment...all thoughts are welcome!

higbone

 

[grecle72]

This thread is definitely a bit old now, but finding information on these Silvertone organ amps is not easy and this thread contains the only schematic that I've found for the silvertone 4707. I'm surprised that there's been no mention of the (Magnatone like) pitch shifting vibrato in this amplifier. I noticed that SpeakerDude88 made mention of bypassing the cathode of the 6v6 power tube with a 100uf cap. I have a 47uf 50v cap, would that work? Will this change the bias of the 6v6 at all? I'm sort of new to tube amp modification, though I have the essential capacitor draining measures down (not to worry). So, would it be safe to connect a bypass cap in parallel from the pin 8 cathode to ground? I've already bypassed the first preamp tube and it has more gain, now if I could only get more out of the 6v6... greg

 

[RWood]

Another "late to the party" owner of a 4710..... I got one today for $25.

Found Spkrdude's hand drawn schem on this site and another one here:
Silvertone 4710 organ amp

Is it true that this has a pitch-shifting vibrato?

And what are those two little transducer-looking things that were mounted in foam, behind the keyboard panel?

Thanks,

RWood

 

[RWood]

By the way, if anyone else is looking for info about this circuit it appears to be almost a direct copy of a Magnatone Model 410, right down to the 12DW7. Same power amp and rectifier, too.

One thing the Magnatone has that the Silvertone does not is a vibrato speed control, so this schematic shows where to add one.

These Silvertones were apparently made by the Thomas Organ Co., who had other ties with Estey Electronics (which merged with Magnatone in 1959). The date codes on my Silvertone are 1960 so this makes sense.

I also found that the two transducers are actually microphone capsules, mounted to pick up the electronically reproduced reed sounds. Truly an acoustic/electric instrument! As an organ it was probably woeful, but I am hoping that for guitar it will be a bit better.