Kalamazoo Bass 30 Tone Stack

[bobloblaws]

STOP!!!!!!!!!!!

Your bias voltage dropped after the attempted filter cap substitution. There's a good chance that whatever happened to the power supply when you tried to connect that extra filter with the amp on did some kind of dirt to the bias circuit. I'm thinking a shorted (or partially shorted) capacitor. But, of course, it could be something else. It's certainly causing stress on the tubes. As Jason suggested, you should remove the tubes and start at square one. I haven't even looked at the drawings and schematics yet because I wanted to post this before something craps the bed.

Yes, I have removed the power tubes, once I noticed the red plating and killed the amp I only fired them back up again briefly to get the plate and cathode voltages. Since then I've disconnected the filter caps and alligator clipped in 4 brand new filter caps.

[bobloblaws]

It's possible they are damaged since they now draw more bias current from redplating. Tube testers don't always tell a lot about tubes more or less a pass/fail an amp circuit is the best test.
nosaj

Fair enough, they were probably red plating for awhile before I noticed. But, I don't believe this has any effect on the bias voltage. I see the same bias voltage with tubes in or out. Please correct me if I'm wrong.

[Helmholtz]

A few things looking at the schematic are you have a half wave rectifier which in my experience seem to have a little hum to begin with. The other is the amp is cathode bias which means the tubes adjust themselves. The only thing you can adjust in this bias circuit is the 100 ohm resistor.

From what I see in the schematic, the amp has a full wave HT rectifier and a combination of negative grid bias and additional cathode bias. The 100 Ohm cathode pot allows to adjust power tube symmetry, which should also minimize power stage hum.

[nosaj]

Fair enough, they were probably red plating for awhile before I noticed. But, I don't believe this has any effect on the bias voltage. I see the same bias voltage with tubes in or out. Please correct me if I'm wrong.

What about the bias caps?
nosaj

[nosaj]

From what I see in the schematic, the amp has a full wave HT rectifier and a combination of negative grid bias and additional cathode bias. The 100 Ohm cathode pot allows to adjust power tube symmetry, which should also minimize power stage hum.

Full wave uses 4 diodes while a half wave uses 2, no?
I stand corrected Full Wave Center-tapped Rectifier.

nosaj

[bobloblaws]

What about the bias caps?
nosaj

Yep, I tried swapping those out as well.

[nosaj]

Yep, I tried swapping those out as well.

Ok how about this the bias winding if lifted from power tubes gives what?
nosaj

[Chuck H]

The amp is full wave rectified. The reason for a greater diode count would be to increase electrical capacities on the components, but it's really the arrangement of the circuit that counts. Most full wave and full wave bridge rectifiers do use four rectifiers. Some full wave's use six (three in series from each branch), but one diode from each leg of the HV winding is fine if the diode chosen is up to the electrical demands of the circuit.

[g1]

Please check those diodes (HT rectifiers). And post what the B+ reads for DC and AC voltage with power tubes out.
Just in case something in the HT circuit is messing up the bias circuit, usually they are more isolated than this.

[bobloblaws]

Please check those diodes (HT rectifiers). And post what the B+ reads for DC and AC voltage with power tubes out.
Just in case something in the HT circuit is messing up the bias circuit, usually they are more isolated than this.

I removed the rectifier from the circuit and tested the two diodes with my Fluke diode setting. Typically when I test a generic diode with the red lead going to the anode and the black lead going to the cathode the DVM beeps and gives me a reading of somewhere in the neighborhood of 0.5V (forward voltage?). In the case of this rectifier the tester is not beeping and the readings are 2.1V on both sides. Expected? With the rectifier back in and the tubes out, on the PT secondary I'm seeing 730 VAC and B+ of 493. Between the 390K resistor and the bias diode I have 152 VDC and on the other side of that diode I have -12 VDC. At the power tube grid pin we have -10V (same with tube in or out), should be -22V based on what I saw yesterday.

[Chuck H]

Between the 390K resistor and the bias diode I have 152 VDC and on the other side of that diode I have -12 VDC. At the power tube grid pin we have -10V (same with tube in or out), should be -22V based on what I saw yesterday.

What do you mean by "I tried swapping those out as well" WRT the bias caps? Did you replace them? Did you parallel them? Did you put in new caps and then take them back out? I'm unclear. At any rate...

Test the bias supply resistors for value. Replace if necessary. Just put a new diode in there. The typical 1n4007 will do (and you likely have one on hand?). See to it there are new caps in the bias supply and be sure the + terminal is the grounded end of the bias caps. If, by chance, you have a cap in the bias supply installed with incorrect polarity you must replace it now rather than just flip it around.

[bobloblaws]

What do you mean by "I tried swapping those out as well" WRT the bias caps? Did you replace them? Did you parallel them? Did you put in new caps and then take them back out? I'm unclear.

The answer is "...put in new caps and then (took) them back out..".

See to it there are new caps in the bias supply and be sure the + terminal is the grounded end of the bias caps. If, by chance, you have a cap in the bias supply installed with incorrect polarity you must replace it now rather than just flip it around.

Whoah, this might be my issue. Does the cap that bypasses the 47K resistor need to be reverse polarity too? I have it in with negative to ground. I knew the other one needed to be reversed.

[Chuck H]

Does the cap that bypasses the 47K resistor need to be reverse polarity too? I have it in with negative to ground. I knew the other one needed to be reversed.

They should both be in "reverse polarity" (as it were?). That is, + to ground. You should just put in new caps. And by new caps I mean new. Not caps that have been languishing in a drawer for over five years because sitting unused is very bad for electrolytics. The bias supply is critical for safe tube operating parameters. Why take any chances.?.

[bobloblaws]

They should both be in "reverse polarity" (as it were?). That is, + to ground. You should just put in new caps. The bias supply is critical for safe tube operating parameters. Why take any chances.?.

Will do, thanks. (I) learn something new everyday. I'll come back and edit this reply with the result.

[bobloblaws]

They should both be in "reverse polarity" (as it were?). That is, + to ground. You should just put in new caps. And by new caps I mean new. Not caps that have been languishing in a drawer for over five years because sitting unused is very bad for electrolytics. The bias supply is critical for safe tube operating parameters. Why take any chances.?.

Can you comment on my diode test of the rectifier diodes? It looks to be labelled "SCXE" on the drawing. I couldn't find anything in that respect via Google.