Silvertone Organ Amp

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Hello...I'm a newbie and have lots of questions. I tried reviving an old thread that discussed someone's project of turning an old Silvertone organ amp into a guitar amp - to no avail. I was wondering what ever became of the project, or if anybody else has had experience with this type of mod.

I am just looking for suggestions, unique ideas, problems to watch out for, how effective the tremelo mod could be...etc. By the way, I am not knowledgable on electronics, but have a "tubie" friend that will help along the way.

Thanks in advance for the insights!

If the amp works, it may be possible to convert it to a guitar amp with a few simple changes. Sometimes, however, that is a lot more headache than its worth. I've done several of these and know first hand.

Perhaps the best way is to find a well known amp (i.e. Fender) that uses the same tubes and controls, and then rebuild the amp with a circuit board from the new amp. You'll end up with a really cool amp.

What tubes are in your organ amp?
Hi...thanks for your reply.

The tube set-up is 12DW7, 12AX7A, 6V6GT, 5Y3GT. From what I am is similar to a Fender Vibrochamp. I have grown fond of low wattage tube amps, but I am cheap and don't want to pay top dollar for vintage (Silvertone/Fender, et al) amps. In the long run, I may end up spending a little money on tube service and materials to build a cool, "retro" cabinet - but a married man with little children needs an honest, late-night hobby!

The thread I previously referenced is at the following link, and my organ amp is exactly like the one described in the thread.

All thoughts are welcome!

The two basic options are:
- get the amp working as it is
- use the parts to build a VibroChamp

The first step is to assess the quality of the amp to see how much effort it will take to get it working and sounding good for guitar.

The chassis is pretty small, and it looks like it has a fixed vibrato. You may want to consider making it into a Champ, which is an extremely simple circuit. You'd be removing parts, which will make it a lot easier to work on.

Have you plugged it in to see if it works?
I am in Tallahassee. I checked out your Web page...really cool stuff! I am very interested in learning more about amp building, but for now rely on a couple guys at a shop called "Class A Electronics". They build custom amps under the name "Diabolic" amps and do repair work for the Tallahassee area. I'll have them check the amp out and give me an estimate.

How would you recommend getting my feet wet in learning about electrical wiring and tube amp building?

You can check out some of my copies of old blues songs at the myspace page if you like...

hey I have one of those! I turned it into a champ-like clone.

For a test bed amp its kind of an ugly duckling, but works great. I would sugguest gutting it except for the heater wiring and rewire it as an AX84 P1 (using a 6v6 instead of an EL84, just leave the cathode resistor there and add a 47uf cap across it.)

I messed a little with the tremelo, which was fixed by replacing the bad caps in that section, but as there was no way to adjust the speed I just removed it. I don't use trem anyways.

The power transformer is nice and puts out just the right voltage for a variety of 5 watt SE amps, and the output tranny is larger than your average 5 watt amp too. I will take pics of the amp tomarrow for ya if you want :D

I think that the chassis you have would be cool to build into a head because of its size. I used a new hammond 8x4x2 chassis for my amp because it was going into a combo. The organ came with an oxford 8" speaker in it which sounds great!

Are your tubes marked silvertone? If so, that 6v6 is either made by sylvania or by RCA, I don't think westinghouse actually made a 6v6. The other tubes are made by westinghouse and sound very good. 5y3 is a sylvania. This was all obtained by looking at the EIA codes on the tubes, as silvertone obviously never made tubes.

To get it up and running, make sure your wires are all there. Someone may have cut off the tremelo pot that was mounted on the front of the organ , that had the power switch on it. There is no fuse on this amp btw, and tubes generally are hard to beat up if their packed well. If they are not lighting up, I can almost garuntee someone cut the power line running to that pot on the front panel, as it takes a lot of work to tear the organ apart to get to the switch. See where the twisted white and black wires connect on the far right side of the chassis near the power line? If those are cut anywhere it won't turn on. Short those together and it should turn on .
If you could take a high res photo of the underside of the chassis so we can see what the current state of it is.
I would note that most organ amps do not light up when simply plugged in. Somewhere near where the powercord comes in, and near the fuse but before the transformer you will generally find a break in the wiring connected to an amphenol or plastic jack which would have run to the main power switch on the organ. You'll probably need to jumper this or add your own switch. I wouldn't necessarily gut it unless the circuit is uninteresting. Some old non-guitar amps had different design philosophies, and they can have a very unique tone. If it's going to be HIFI, you'd probably be best gutting it, but for a guitar amp you might wanna keep it as is. The tremolo can be modified for adjustability by replacing on of the resistors (generally 1-3Meg) in the anode-grid oscillator cap loop with a variable resistor. Make sure it's one between two caps, so there's no DC. It helps to look at other tremolo oscillators. That 12dw7 will make a better drive stage & cathodyne PI than any AX7, use the high gain half to drive the other half (the low gain side will be the split load/ cathodyne pi). Much beefier and reliable in this service than 12ax7s.
this would be a SE amp, not PP so no need for a PI.

I traced out the circuit, much of it is un-needed and uninteresting sounding. They cut corners a lot and the tremelo section is just weird. Also, this amp definatly benefits from a 50-100uf bypass cap on the cathode of the 6v6 as it adds stability, bass responce, and overall volume. I drew up a schematic of it here:
looking at the schematic, some of it is a bit weird. I don't know your voltages, but it seems like there are a couple of places where the cathode resistors could be smaller (6v6, first stage). I have literally no idea why the first stage is a 12dw7, and the tremolo does come in strangely there. I'd swap the ax7 for the dw7, adjust cathode resistors, add bypass caps, increase the resistor which is in parallel with the grid of the first stage (you didn't list a value but you can safely put a meg or so here without changing the biasing scheme). The 'heavy' half of the dw7 would make a good driver for the 6v6. I'd replace the 47k x 2 resistor with a pot with a similar value, or replace on of the 47k resistors in the tremolo circuit with a 50k pot. This will allow controlling the speed. Increasing the cap sizes will give a slower tremolo. You could just run the output of this to the cathode of the final driver stage (12dw7). Measure the output voltage so that you're not causing too much change to the bias of the preamp tube though, you probably only need -2 to +2 sine wave. A voltage divider + a pot for a depth control will work great. Alternately, wipe out the entire tremolo and use the extra gain stage for more drive, although you may not need it. You could use it to have a two channel input though, with different coupling caps and bias, gain, etc. (A hot channel and a mellow channel or something).
Thats what I did when I was messing around with it, I wired it as a high octane from ax84 with some mods and used another 12ax7 instead of the 12dw7. As far as the cathode resistor, it is actually pretty nominal ,if not needing to be a little higher. The transformer puts out around 325-0-325 on the 5y3 and after rectification its around 375v, dead on with what the original champs used to run at. Thats why I used a 470r cathode resistor.

Sorry to threadjack Highbone, just giving you some ideas here :D

By the way, I found its easiest to work with the wiring when you unbolt the sides of the chassis ( how many times have you wished you could do that on cramped fender chassis??)
Hey guys...thanks for the replies and suggestions. I just dropped it off at the "tube-guy" shop and he'll be looking at it next week. I will be sure to share the information with him. As mentioned before...I'm just a dreamer that likes to work with wood, and I have to pay for the electrical expertise from the local shop. Who knows...maybe when the kids grow up and I retire?!?!?

By the way, mine came with the 8" speaker, too. I most likely will build the amp into a head, then build separate speaker cabinet for the 8" speaker.
Sounds Good! Don't let your tube guy take any of the tubes!! this tends to happen to people a LOT who don't know anything about tubes. Their tech claims the amp needs new ones and "replaces" them with new russian or chinese junk he has laying around, just so he can have your nice RCA, GE or Sylvanias. Whatever.

Sounds good on the head idea, I would start looking into tube theory a little so you can do the mods yourself. if you can solder and have a little patience, it should be very easy. Your amp is already set up for two preamp tubes, so its possible to go with a high octane style high gain amp. I would reccomend giong to radio shack and getting the package of 1/2 watt resistors that they have for $12. Not nessicarily the cheapest around, but it has a lot of the common values you'd need for modding your amp. Also, they have some .01uf 400v capacitors for a dollar something each.

BTW- That oxford is similar to the OEM version for the tweed champ- but BETTER! Larger magnet and slightly better voice coil with the same size cone. Handles the power better than the original did without crapping out on you and has nice compression when cranked. I'd use it! Just don't make the cabinet too large, nothing too much larger than a blackface or silverface champ size cab.
There's something to be said about experimenting too after you've read that stuff (and the HV safety thread!)–I was born around the time the transistor became almost entirely predominant, and so I really only knew basic electric circuits with batteries and how to solder stuff back onto a PCB when the boards broke. I read what was available online and messed around with my only amp (salvaged from the dump) and broke stuff or got electrocuted a lot less than I was expecting :)! Just be careful and look at schematics for other stuff ( and see where it takes you. Tubes can handle some stupid wiring mistakes for surprisingly long, so long as you catch them in a reasonable period of time. I'd also recommend looking at geofex's tube amp debugging page (, it's a good resource–when your experiment doesn't work, the order of suspicion and click-through debug guide is often a good start. I'd also recommend getting a good multimeter than can test caps (I debugged and fixed my first amp without a meter, and that is *hard*) and a *good* soldering iron (I have crappy irons right now and I assure you they do not make it easier to learn!)
You both have been fantastic...thanks so much for the information! And once I get this amp done...I can always go back and play with it once I educate myself a bit further.

I will let you know what the guys at Class A Electronics (Tallahassee, FL) do with the organ amp.

I also have an old Hoffman "hi fi" amp (15 watts with two EL84s and two 12AX7s) that I plan to turn into a guitar amp head. An E-bay special that I happened to win, just like the silvertone.

Curiously, the Hoffman had an old 8" woofer that looks exactly like the one from the silvertone organ (except for a slightly different paper cone (or whatever you call it.) The magnets/wrap around metal piece on the outside are exact - and even the numbers stamped on them are similar. My question, might it be a neat experiment to do a 2 x 8" speaker cabinet? I know a few 10" or 12" is bigger and beefier, but I want to make use of the other 8". Any thoughts?

If you are curious about the stamped numbers on the speakers...


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