Silvertone Organ Amp

[Enzo]

I hate to disagree, but those single triode phase shift LFOs have been running fine for decades. A zillion Fenders use it, to mention just one.

I agree there ought to be some cathode bias resistor, and that odd connection for the cathode of the other half of the tube sure looks wrong.

Are those feedback caps among the ones you did not change? How did you test the ones you left? A hand meter will tell you the value, but not the ESR nor more importantly the leakage. Your meter runs on a volt or so, and a cap that checks fine at a volt could leak like a sieve at say 40 volts, and thus not work here.

This oscillator is a basic circuit and should work as built if it has good parts. And get the schematic right. When I encounter a dead trem oscillator, I verify the tube is conducting, and if it looks like the tube is working, I just wholesale replace that whole string of caps. Once I get the oscillator swinging, I can worry if the interface part works.


I think you switched the center taps on the power transformer. Connecting the4 HV center tap to the power tube cathode would be hard to defend. All the ripple current through the B+ would be added to the power tube. That should run straight to ground. ON the other hand, connecting the cente tap of the heaters to that cathode makes a lot of sense. It elevates the heaters to some DC voltage positive to ground, thus reducing potential for hum in the preamp stages.

Also, on the output transformer, that green tap with the couple resistors is hard to explain. You sure that connects to the primary?

[smoreland]

Thanks guys for the responses. I'm working on some of the things you mentioned. I'll report back tonight when I get a chance to check some things!

-Steven

[smoreland]

Ok, I printed out the schematic and followed it to the circuit of my amp and there are some differences. I don't know why I didn't do this to start with. I'm attaching the updated schematic to see if this looks any better. The things that differ from the one I posted earlier are in RED. Thanks for your help! I'm going to go ahead and change out all the ceramic caps though.

-Steven

[smoreland]

I took the other side of the panel off and could see a mistake I made in the new schematic. On the output transformer, there is blue, red, green, white, and a bare wire. They all come out the same side, but I think the Blue is to plate, Red is B+, green is to resistors then ground, white is secondary +, and bare is secondary -.

Does the green go into the screen? Like the pic I attached?

[Enzo]

I believed the schematic you drew with the screen going to B+(A) in post #13.


The fact the wires all come out of the transformer on the same side of it physically says nothing about how they are wired internally. In the post #14 above you show the green wire as a center tap of the primary./ That means it will have continuity with the red and blue wires. (Standard American color code for output transformers has primary wires red to B+ and blue to tube plate for single ended transformers like this) But in the post before that you have it as part of the secondary. You must determine with an ohm meter which winding - primary or secondary - this wire and resistors is part of. It can't be both.

And having those two resistors in series is weird. Any chance something was connected to the point between the two resistors where they join? I am imagining something like using them as a voltage divider to sample the signal off the speaker winding for use as a output for a slave amp. A sort of poor man's line out.

[smoreland]

I got around to the voltages on the pins:

12DW7:
1-(217.2)
2-(28.2)
3-(62.5)
6-(183.7)
7-(0)
8-(1.75)

12AX7:
1-(284)
2-(0)
3-(2.4)
6-(-.049)
7-(-0.8)
8-(0)

6V6:
3-(334.5)
4-(287.5)
5-(.035)
8-(15.2)

5Y3
2-(364)
4-(.022)
6-(.022)
8-(364)

[smoreland]

Here is the NEW SCHEMATIC. This show what the green wire coming from the output tranny does.

-Steven

[smoreland]

Ok, I got the vibrato working. When I took the voltage readings on the pins, obviously the problem was the plate at pin 6 on V2. I went back from there and found that the 270K resistor had opened. I replaced it and viola...vibrato. I'm going to add a speed pot so it can be controlled. Are there any suggestions to make it sound any better for those of you who know more about schematics than me?

[smoreland]

I know I've been having a conversation with myself the last few posts here. Haha, oh well, here is the schematic as it is right now. Sounds really cool and everything works! Still wondering if there are some things I should do to make it more guitar friendly. Vibrato pitch shift is pretty cool and the speed pot help a lot. I've seen a 25/50V cap on the cathode of the 6V6. What purpose would that serve?

-Steven

[Enzo]

None of your cathodes are bypassed, but any of them could be. It tends to raise the gain some and add to the bottom end response.

But really, if you wonder what a 25uf (or 22uf as a more common value) cap on the cathode of the power tube will do, tack one on there and find out.


Try adding one across the 3k9 cathode resistor of the first stage for a gain boost. You may like it or it may be going in a direction you don't like, but it is simple to try it and it isn't like it would be permanent if you hate it. I don't think that 220k is necessary at the input, I;d be happier with maybe 22k instead.