Kalamazoo Bass 30 Tone Stack

[bobloblaws]

I have a Kalamazoo Bass 30 amp here, I read that this was a budget line for Gibson. I also read that Gibson's schematics couldn't always be relied upon. On this particular amp I'm seeing an anomaly in the tone stack circuit between the schematic that is attached to the cabinet and the actual circuit as it stands currently. I'm attaching the schematic and a rough diagram I drew up showing the current state. For the record, the reason I have it open is to try to track down a hum issue, but my question regarding the tone stack doesn't necessarily have anything to do with the hum issue at hand, I'm just curious if you guys think the schematic looks reasonable and someone modified the circuit (for better or worse) or if it's more likely that the existing circuit was the original design. Thanks!

[nosaj]

I have a Kalamazoo Bass 30 amp here, I read that this was a budget line for Gibson. I also read that Gibson's schematics couldn't always be relied upon. On this particular amp I'm seeing an anomaly in the tone stack circuit between the schematic that is attached to the cabinet and the actual circuit as it stands currently. I'm attaching the schematic and a rough diagram I drew up showing the current state. For the record, the reason I have it open is to try to track down a hum issue, but my question regarding the tone stack doesn't necessarily have anything to do with the hum issue at hand, I'm just curious if you guys think the schematic looks reasonable and someone modified the circuit (for better or worse) or if it's more likely that the existing circuit was the original design. Thanks!

[ATTACH=CONFIG]51791[/ATTACH] [ATTACH=CONFIG]51793[/ATTACH]

Well for one on the hand drawn circuit I don't see pot values which could change things as far as what was on hand at gibson.
Do tone knobs effect hum?
nosaj

[bobloblaws]

Well for one on the hand drawn circuit I don't see pot values which could change things as far as what was on hand at gibson.
Do tone knobs effect hum?
nosaj

Pot values are the same. Tone knobs affect the hum, but again, that isn't really what I'm asking, whether the tone stack is contributing to the hum. I only mentioned the hum because usually someone will ask why I have the amp on the bench to begin with. I'll ask it this way. Does the existing circuit make sense in terms of bass, treble and volume? I just want to make sure some hack that didn't know what they were doing didn't modify it in a way that didn't make sense.

[nosaj]

Pot values are the same. Tone knobs affect the hum, but again, that isn't really what I'm asking, whether the tone stack is contributing to the hum. I only mentioned the hum because usually someone will ask why I have the amp on the bench to begin with. I'll ask it this way. Does the existing circuit make sense in terms of bass, treble and volume? I just want to make sure some hack that didn't know what they were doing didn't modify it in a way that didn't make sense.

Well I'm just a hack myself....but if you remove V1 does the hum stop?

nosaj

[bobloblaws]

Well I'm just a hack myself....but if you remove V1 does the hum stop?

nosaj

As am I...anyway, I haven't been able to try that because I have another problem I need to solve. I seem to have buggered something up when I attempted to add some extra filtering as an experiment. I touched a wire connected to my additional filter cap to the positive terminal of one of the existing filter caps and, yeah, should have had the power off, duh, anyway there was a bit of a spark and the output level dropped from that point on. At first I thought it was just B+ because the voltages measured lower but that seems corrected now somehow, for the most part anyway. In any case, at one point I sensed something was getting really hot and sure enough the power tubes were red plating. When I measured voltages yesterday I had -22V on the power tube grids, 1V at the cathodes and about 470V on the plates. Right now the plates are sitting at around 435V, cathodes are 4.0V and grids are about -10V. I just spent most of the afternoon trying to get the bias voltage back to -22V but no matter what I tried I couldn't get it better than -10V. This included swapping out components in that bias circuit and trying different values in some cases. I even hooked up a pot in place of the resistor shown as 47K (actual is a 62K). Decreasing the resistance from roughly 62K pushes the bias toward 0V but going the other way did not yield better than -10V. I also tried lower values for the 390K resistor, I was surprised that the voltage drop remains the same with different resistance values. So if anybody can help me with this I would really appreciate it. I might start a different thread for the current problem. Thanks.

[J M Fahey]

I have a Kalamazoo Bass 30 amp here, I read that this was a budget line for Gibson. I also read that Gibson's schematics couldn't always be relied upon. On this particular amp I'm seeing an anomaly in the tone stack circuit between the schematic that is attached to the cabinet and the actual circuit as it stands currently. I'm attaching the schematic and a rough diagram I drew up showing the current state. For the record, the reason I have it open is to try to track down a hum issue, but my question regarding the tone stack doesn't necessarily have anything to do with the hum issue at hand, I'm just curious if you guys think the schematic looks reasonable and someone modified the circuit (for better or worse) or if it's more likely that the existing circuit was the original design. Thanks!

[ATTACH=CONFIG]51791[/ATTACH] [ATTACH=CONFIG]51793[/ATTACH]

The printed one "works", sort of, but is not very good.
The Treble control is almost right, but the Bass one is just a Bass cut , pot smoothly selects between a regular .022uF coupling cap when wiper is fully to the right and a Bass murdering 500pF one when wiper is fully left.

The hand drawn one is more normal, a James control similar to what Ampeg uses, maybe it was added by a later Tech.
Almost right, except the resistor from Bass pot to ground should be 22k instead of 220k.

Try to lose the .005 cap from right side V1 grid to ground, it´s a "mud" control.

[bobloblaws]

The printed one "works", sort of, but is not very good.
The Treble control is almost right, but the Bass one is just a Bass cut , pot smoothly selects between a regular .022uF coupling cap when wiper is fully to the right and a Bass murdering 500pF one when wiper is fully left.

The hand drawn one is more normal, a James control similar to what Ampeg uses, maybe it was added by a later Tech.
Almost right, except the resistor from Bass pot to ground should be 22k instead of 220k.

Try to lose the .005 cap from right side V1 grid to ground, it´s a "mud" control.

Perfect, thanks JM!

[nosaj]

As am I...anyway, I haven't been able to try that because I have another problem I need to solve. I seem to have buggered something up when I attempted to add some extra filtering as an experiment. I touched a wire connected to my additional filter cap to the positive terminal of one of the existing filter caps and, yeah, should have had the power off, duh, anyway there was a bit of a spark and the output level dropped from that point on. At first I thought it was just B+ because the voltages measured lower but that seems corrected now somehow, for the most part anyway. In any case, at one point I sensed something was getting really hot and sure enough the power tubes were red plating. When I measured voltages yesterday I had -22V on the power tube grids, 1V at the cathodes and about 470V on the plates. Right now the plates are sitting at around 435V, cathodes are 4.0V and grids are about -10V. I just spent most of the afternoon trying to get the bias voltage back to -22V but no matter what I tried I couldn't get it better than -10V. This included swapping out components in that bias circuit and trying different values in some cases. I even hooked up a pot in place of the resistor shown as 47K (actual is a 62K). Decreasing the resistance from roughly 62K pushes the bias toward 0V but going the other way did not yield better than -10V. I also tried lower values for the 390K resistor, I was surprised that the voltage drop remains the same with different resistance values. So if anybody can help me with this I would really appreciate it. I might start a different thread for the current problem. Thanks.

Well if it were me. I'd pull the tubes and start at square one at the power supply and check voltages across the amp, then one tube at a time checking voltages.

nosaj

[bobloblaws]

Well if it were me. I'd pull the tubes and start at square one at the power supply and check voltages across the amp, then one tube at a time checking voltages.

nosaj

Everything else seems to be in the ball park. B+ might be a bit lower than before the bias went south on me, but I'm guessing there small number of factors affecting the bias voltage. There is roughly 700VAC on the PT secondary and the components in the bias circuit all seem OK, the tubes are OK, what else could it be?

[nosaj]

As am I...anyway, I haven't been able to try that because I have another problem I need to solve. I seem to have buggered something up when I attempted to add some extra filtering as an experiment. I touched a wire connected to my additional filter cap to the positive terminal of one of the existing filter caps and, yeah, should have had the power off, duh, anyway there was a bit of a spark and the output level dropped from that point on. At first I thought it was just B+ because the voltages measured lower but that seems corrected now somehow, for the most part anyway. In any case, at one point I sensed something was getting really hot and sure enough the power tubes were red plating. When I measured voltages yesterday I had -22V on the power tube grids, 1V at the cathodes and about 470V on the plates. Right now the plates are sitting at around 435V, cathodes are 4.0V and grids are about -10V. I just spent most of the afternoon trying to get the bias voltage back to -22V but no matter what I tried I couldn't get it better than -10V. This included swapping out components in that bias circuit and trying different values in some cases. I even hooked up a pot in place of the resistor shown as 47K (actual is a 62K). Decreasing the resistance from roughly 62K pushes the bias toward 0V but going the other way did not yield better than -10V. I also tried lower values for the 390K resistor, I was surprised that the voltage drop remains the same with different resistance values. So if anybody can help me with this I would really appreciate it. I might start a different thread for the current problem. Thanks.

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. I hate to say it but you might want to read some books on repair. I can suggest some stuff if your interested.
nosaj

[bobloblaws]

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. I hate to say it but you might want to read some books on repair. I can suggest some stuff if your interested.
nosaj

Hmmm, so you're back to the hum again, eh?

Actually, I don't believe it is cathode biased. I think the 100 ohm pot is there for hum balance. There is a fixed bias circuit coming off one leg of the PT secondary. Maybe I should suggest some books to you?

[nosaj]

Hmmm, so you're back to the hum again, eh?

Actually, I don't believe it is cathode biased. I think the 100 ohm pot is there for hum balance. There is a fixed bias circuit coming off one leg of the PT secondary. Maybe I should suggest some books to you? ;-)

On your pic the arrow is hard to see I can see the pot now. Suggest all the books you want, I read whatever I can get my hands on.
But it is evident something is not right and everything can't test ok. So your missing something. Got another set of tubes?

[bobloblaws]

On your pic the arrow is hard to see I can see the pot now. Suggest all the books you want, I read whatever I can get my hands on.
But it is evident something is not right and everything can't test ok. So your missing something. Got another set of tubes?

No, but I tested the tubes and they are OK.

[nosaj]

No, but I tested the tubes and they are OK.

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

[Chuck H]

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.