Tube Amps / Music Electronics
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|6/5/1999 12:30 PM|
||Silvertone Mdl 4707|
Hi, all. I picked up a Silvertone Organ Amplifier at the local thrift shop today, and I'm wondering if anyone has a schematic available. It's a large wooden box with the controls mounted at the top on the front panel. Inside is a sheet-metal chassis (like most Silvertones) and an alnico 12". Tube complement: 1x 12DW7, 1x 12AX7, 1x 6V6GT, 1x 5Y3GT. The circuit looks modified to me, and the vibrato is non-functional. If I can't find a proper schem, I may just turn it into a Vibro-Champ clone. Thoughts?
Thanks in advance
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|6/5/1999 7:32 PM|
I personally haven't seen too many Silver tone schematics. I belive Aspen's book (The Tube Amp Book) has some in there.
The 12DW7's are getting VERY hard to find and also expensive. These were mostly used in the older Ampeg SVT's. It sounds like the amp would be a nice sounding.
|6/6/1999 7:16 AM|
---~p~ Mark Knapp Sun Jun 6 13:12
I have a 4708, which is about the same. The trem is great if you can get it working. The oscillator caps have probably gone bad or the tube has gone. I have a hand drawn schematic, but it hasn't been scanned.
Somewhere out on the web it a society of organ techs that has the schematic listed for sale, but I can't find the link. The trem is very similar to the magnatone type. It uses varistors. That part of the circuit is similar
to the one on page 424 of "A Desktop reference of Hip Vintage Guitar Amps" by Gerald Weber. The third triode in the signal chain is what I am refering too. The rest of that schematic is totally different.
I have used a 12AX7 in mine instead of the 12DW7. The problem is that it seems to make the input have more gain, and the amp distorts. You might like this.
|6/6/1999 4:51 PM|
I'm not sure we're on the same page about the trem circuit. It seems to me that the 1st half of the 12DW7 (the low-mu half) is the first gain stage, then the second half (the hi-mu) is the "switch" for the trem, you know, the part that cuts the bias for the pream tube. However, there are a couple of odd looking bits in there that _could_ be varistors. They're sort of dogbone or barbell looking jobs, red-brown all over with a single yellow dot paited on. I thought they were caps but now that I think about it... Anyway the feedback caps for the oscillator were bad. I subbed in a .02 for one of the .01s to make the trem go slower and threw in a 10M pot and a 100K resistor to change the trem speed.
I left in the 12DW7 -- but I did bypass the cathode resistors on the 1st half of the 12DW7 and the last half of the 12AX7 to get a bit more grind. It's a killer little recording amp. It stands on those screw-in furniture legs and my girlfriend and I are going to upholster it in black naugahyde with flames on it. Should be pretty happening.
Thanks for the help,
|6/7/1999 11:26 AM|
Yeah, the dogbones are the varistors. The bias of the preamp is not changed in this circuit, though I can't explain how it does. I could never get the thing to work right with the 12DW7, so I changed to the 12AX7. I changed the first stage to be typical Fender (1.5K/25uF bypass) and raised the output feedback resistor value. I also put in an attenuation network so the first stage doesn't overdrive the second one too much. Lastly, I put in a switch to switch the first stage around the trem section. The signal path is almost "Champ" like then. It has less grind but about the same volume and more bass. One of these days I am going to paint it cream color.
|6/7/1999 7:43 PM|
I've been playing with it today--I think the varistors (used as voltage-variable resistors) make a low-pass filter with one or two of those .02 caps. Change the voltage on the varistors and presto! you've changed the time constant of the filter. Phase modulation is pitch modulation (or near enough!) and therefore vibrato. It makes much more sense now!
Anyway -- about the amp itself, the power amp is fine. The two main problems are with the use of the 12DW7 and the lack of cathode bypassing on either tube stage (makes the thing _real_ top-friendly...) So what did you do? Go 100K plate, 1.5K cathode, 25/25 bypass? Both stages? It looks like the feedback comes off of the 4ohm speaker tap from the OT? What resistor val. did you use? Higher? That would nuke the midrange and make the sound seem deeper, right?
And the switch -- SPDT to move the signal from the plate of the 1st stage to the grid of the 2nd gainstage or to the grid of the modulator tube? I was thinking of more of a Fender-style job where I disconnect the osc. cathode from ground...
|6/7/1999 9:54 PM|
That trem explanation makes sense. Real vibrato - that is why the Magnaton trem is so revered.
"(makes the thing _real_ top-friendly...) "
Is yours trebely? mine has always been too midrangey, though I don't have the original speaker. It was gone when I got the amp.
"... what did you do? Go 100K plate, 1.5K cathode, 25/25 bypass? Both stages?"
Yes on the values, but just on the first one. I didn't change the second one. On mine, the Vibrato does not seem to work right if the first stage gain is lower, but it tends to distort now...
"It looks like the feedback comes off of the 4ohm speaker tap from the OT?What resistor val. did you use? Higher? That would nuke the midrange and make the sound seem deeper, right?"
I think you are right about it being a four ohm tap on the OPT. I have not measured the voltages there. If I remember right, I made the feedback resistor about 1.5K. That makes it more trebely. In my case, that was a good thing. If you have lots of highs already, you probably don't want to change the feedback resistor.
"And the switch -- SPDT to move the signal from the plate of the 1st stage to the grid of the 2nd gainstage or to the grid of the modulator tube? I was thinking of more of a Fender-style job where I disconnect the osc. cathode from ground..."
I really would have to give you a schematic...What I did was to put the switch pole on the "top" connection of the volume pot. It selects the input from either the coupling cap after the first stage or from the output of the Vibrato circuit. The former selection has less distortion and more bass. The input to the vibrato circuit now has a 470K res from the cap to the grid of the vibrato stage (so I can use a 12AX7). The volume is about equal, but the trem stage is more raw sounding. I love it, but you have to tyurn your guitar volume way down to use the vibrato - it distorts real early.
I guess I would actually recommend leaving it as stock as possible, especially if you already like the tone. The Fender style oscillator killer is a good idea and easy to implement - much easier than what I did. Also, working in that rats nest is not easy...
What type of speaker is in yours? A Jensen? P12R maybe? I have been thinking about getting a Weber for mine, but I don't know which one.
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