UL Vibrosonic (Twin Reverb) Phase Inverter question

[cejay825]

I been rebuilding a very worn out Vibrosonic (Twin Reverb)
So Far:
New preamp and output tubes
New filter caps
Replaced poly (blue drops) caps and ceramic caps
Rebuild B+ rail components and power supply components
Replaced preamp and power tube plate resistors
Replaced any resistors that were out of spec
30ma bias
Ok, so the amp sounds great ! all measurements appear to be normal, very pleased with the amp BUT the 22k tail resistor has .485 volts dc on the ground end going to the 100 ohm resistor to ground. I don't know if this is normal or not...the problem is its puts said voltage on the master volume pot and makes it scratchy sounding. All the resistors in the PI have been replaced; 470 cathode, the 2 330k, the 100 to ground, the two 220k split resistors and the 22k tail resistor.
Measurements:

330k 87.1 vdc pre/ 107 vdc post
330k 86.8 vdc pre/ 107 vdc post
470 109 vdc pre/107 vdc post
22k 107 vdc pre/ .485 vdc post

Is this normal ?

[pdf64]

It's just a reference point, in just the same way that grounding it would be; there shouldn't be any Vdc across the pot.
Check for that, and if there is, investigate the source.
This may be due to coupling caps leaking a small degree of dc, or that the circuit board isn't behaving as a perfect insulator.

[cejay825]

Yes, there is .485 vdc on the pot.........coming from where the ground lead of the pot ties into the end of the 22k reference to ground point. I've gone as far as to isolate the phase inverter and ground the tail resistor through a seperate 100 ohm resistor at a different location...same thing still has the .485 vdc on it, all caps are new. The source of the .485 vdc is the end of the 22k tail resistor before it hits the 100 ohm resistor.

[cejay825]

Also for testing purposes the entire Phase inverter section is lifted from the board....to discount board conductivity. I'm stumped....unless it's by design ??

[Leo_Gnardo]

the 22k tail resistor has .485 volts dc on the ground end going to the 100 ohm resistor to ground. I don't know if this is normal or not...the problem is its puts said voltage on the master volume pot and makes it scratchy sounding. All the resistors in the PI have been replaced; 470 cathode, the 2 330k, the 100 to ground, the two 220k split resistors and the 22k tail resistor.

Is this normal ?

For the life of me, can't understand why Fender did this. Chalk it up as a misteak by those vaunted CBS ingenieurs who couldn't leave well enough alone. (Decaf boys, DECAF! I think they did a lot of designing/modification without listening.) When I run across these I move the 'bottom' of the pot to ground. Try it, takes a minute, solves DC skritchy scratch on the MV.

A shot of Caig D5 in the pot and you're doing as well as you can. Never was terribly satisfied with pot quality on these, or their replacements with switches. Of course if you don't need any switch function, wire as you please for the switch and use a pot that isn't a problem. CTS, Alpha, like that. (I've heard too many complaints about PEC, not going there but those who like 'em, suit yourself for $10+ each)

[52 Bill]

I'm stumped....unless it's by design ??

The voltage is there because the 100 ohm resistor is there. It's similar to the presence controls in that same position that are always scratchy.

Unless you redesign the MV circuit, I guess that it will always be there.

[cejay825]

You're right Leo- Last night I removed the pot ground from the 100 ohm resistor and moved it to a different ground location....the voltage is still present but not on the MV pot.....I guess I'll install a new pot and ground it in a different location.....the amp seems to work just fine this way. Only thing I don't know is that if I ground the pot lead thats getting the vdc, does it have to go through a 100 ohm resistor or just straight to ground ??

52 Bill; correct, if I take the 100 ohm resistor out all vdc disappears. Thanks for the Presences pot reminder !

[Enzo]

Look at the schematic, it says 110v on the PI cathodes. From there to ground is a series resistance of 470+22k+100 ohms, or 22,570 ohms. So Ohm's law tells us that there will be current of... 110v/22,570ohms... Whatever that is, I just forgot, now take that current through the 100 ohm, and I calculate 0.478v. APparently I am off 2 little millivolts from your measurement. And i am sure if i measured the 110v and all three resistors, the numbers would then match.


So yes, it is expected we will have the half a volt there.


A cardinal rule of internet ranting is that when you criticize someone for doing something wrong, you will inevitably make an error yourself.

So just so you know, the work is "mistake". Or was that purposeful?


The 100 ohm resistor is there to form a voltage divider with the NFB resistor.

The MV is a 1meg pot, do we really think it would notice any difference between ground and 100 little ohms to ground? (No, it will not care.)

[pdf64]

I don't see why referencing the pot to the 100 ohm feedback tail resistor, rather than ground, would make the pot scratchy?

Presence pots are scratchy because there's Vdc across them, whereas there's no reason that would be the case here.

Granted, I accept Leo's point that there's no readily apparent benefit from referencing at this unusual point.

My hunch is that the pot will still be scratchy when ground referenced, due to dc leaking from somewhere.
Pete

[cejay825]

Great thanks everybody !!, I'm going to ground the pot lug separately and let it ride...seems to work just fine that way anyhow !

[Leo_Gnardo]

A cardinal rule of internet ranting is that when you criticize someone for doing something wrong, you will inevitably make an error yourself.

So just so you know, the work is "mistake". Or was that purposeful?

Uh-oes, you found me out. LG's self deprecating humor. I do it a lot. Read what I write carefully, sometimes you find a hidden "easter egg." Sometimes I don't even plan it. Good on you for finding this one. With respect, as always Enzo! You da man! All the best to Wenzo too. LG.

[pontiacpete]

cejay, I had a similiar issue with one these. http://music-electronics-forum.com/t36637/

[Leo_Gnardo]

My hunch is that the pot will still be scratchy when ground referenced, due to dc leaking from somewhere.
Pete

There's one cap on the way in and another on the way out. If scratch persists replace these then all that can be done, has been done.