Volume control problem with Premier Twin 8

[bluzmn]

I still haven't had time to re-solder the old connections of components to inputs and pots (last 2 repair sessions turned into all-nighters), and now I'm having a new problem. When the amp gets hot, you get little or no volume until you turn the knob to maximum. But, if you turn up the tone knob, you can get volume at lower settings. It wasn't like this at first; I could leave the tone all the way to the left (as I usually do with a harp mic), and turn the volume knob progressively louder until the mic started feeding back at about the 3 o'clock position. What could be the problem? (This amp is going to bring back my ulcers!)

[52 Bill]

Leaky caps in the tone circuit?

[bluzmn]

This should probably be the subject of a whole different thread, but hopefully some of the other forum members will see it and comment on it. I've heard that you should replace all of those wax-covered paper caps in old amps when you're refurbishing them, but I'm afraid that all the differences in the individual tolerances and drifts in values are what give the amp it's unique sound; of course, never actually having heard what this amp originally sounded like, I guess it doesn't matter.

[MWJB]

Blzmn can you post a pic of pot wiring & changes to the circuit?

It's usually the caps in the can that get changed as matter of course.

[bluzmn]

If by "caps in the can" you mean the filter caps, I've already replaced those; those are the only changes to the circuit I've made. Do you want a picture of that? As for a picture of the pot wiring, I don't know if my camera has the resolution for that; it's like a bramble patch in there. But I'll give it a try. Oh yeah, I did make another change; I put the cathode bias cap for the 6L6 directly across the cathode resistor rather than have wires coming from the can (which I disconnected).

[Enzo]

The can caps are electrolytics, and those dry out over time. There may be other electolytics that are not in the can. ANY electrolytics that are 50 years old really need to be replaced.

The waxy caps are paper caps. These days the same job would be done by film caps. Old sealed caps sometimes are OK, but the wax ones are just about never OK. Not at this age.

Wiring a cathode bypass cap across th resistor instead of off in a can makes no difference - electrically it is the same. However, if the cap is right up against that resistor, it could be subjected to the heat of the resistor. If that resistor gets warm, it would be preferqable to move the cap away from it some. Heat kills caps.

If the amp doesn;t work, I am not sure what sound we are trying to save.

[bluzmn]

@MJWB-I haven't had time to take those pics yet, but I'll get to it ASAP
@Enzo-every time I make a repair, the amp sounds beautiful, then some other problem pops up. I suspected those other (non-electrolytic) caps might be leaky or something , so I guess I should replace them all. And yes, whenever I bypass a cathode resistor like that, I arch the capacitor as far away from the resistor as space permits (I also do that with filter caps if they are too close to a dropping resistor).

[Enzo]

When you have an old amp with suspect caps, each time you replace one, it allows the amp to work a little better, but it also then reveals the deficit in other caps. SOmetimes the replacement of a leaky cap in one spot allows a voltage to climb elsewhere enough that a cap there now shows as leaky.

If your amp has coupling caps all of similar nature - like waxy or white plastic or whatever - if one is real leaky, chances are they all are.

[Rhodesplyr]

I've heard that you should replace all of those wax-covered paper caps in old amps when you're refurbishing them.

I recently refurbished a 1950s Premier 120 and tested it with no tubes (with Variac--I know how to do this safely) just out of curiosity. Every single paper capacitor in it was unacceptably leaky.

You may feel better about replacing them if you understand why they fail. The fundamental problem is that wax doesn't create a very good seal. It's too soft when warm and too brittle when cold. Airborne moisture gets inside and does several things. It can significantly increase capacitance because water has a dielectric constant of ~80. It can allow DC current flow in and of itself, and it can cause electrochemical changes in the paper in combination with residual acids left in the paper and the DC bias voltage. In some cases, it dissolves molecules of the foil and creates conductive pathways through the paper dielectric. Combo amps, in particular, are terrible places for wax-dipped paper capacitors. Heating cycles cause them to "breathe," and vibrations from the speaker and from carting the amp around damage the seals around the leads.

A 1950s TV repair manual I ran across cited paper capacitors as the #1 reliability problem in early televisions. In humid climates, some repairmen resorted to to putting continuously burning light bulbs inside the sets to try to reduce ambient humidity around the caps. A lot of R&D money was spent in the 1950s in the quest to develop more reliable capacitors.

[bluzmn]

Pictures for MWJB

Well, I finally got around to re-soldering all those "crunchy" connections, and now the amp sounds like it's old sweet self. The volume control now increases gradually as you turn it from left to right. Since it is sounding good to me now, should I just replace all those paper capacitors as a matter of course, or should I just leave well enough alone?
There is one thing that concerns me,though; as I was "chopsticking" all the connections I had just re-soldered, I nudged the cathode tab of the 6l6 and there was a small "pop". I nudged a few more times to see if would happen again; occasionally it would, but most of the time it wouldn't. It also makes that sound when I wiggle the 6l6 in it's socket. Is this the harbinger of something worse to come, or do I just need to re-tension the socket?
And now here are the pictures MWJB requested; I'm no Ansel Adams, and I hope that whatever he's looking for is in there. As I suspected, the closeups of the pot wiring didn't turn out too well.

[bluzmn]

More pics for MWJB

@MWJB - Here are a few more pictures of the pot wiring; unfortunately, none of the close-ups of the front of the volume pot were clear enough to use. There's not much wiring to see there, anyway.

[MWJB]

Hi Buzman,

Looks like you have the pot issue sorted anyway now?

Yes, the power tube socket sounds like it could do with a retension (unplug from wall, drain caps, I use a small watchmakers screwdriver) make sure any parts connected to pin 8 aren't flexing & shorting against each other when you are probing.

[Enzo]

I agree, Ansel Adams would have shot these in black and white...

[bluzmn]

Thanks...

Thanks to all you guys for your help.