PPIMV LOUD HUM

[rockstah]

hums when its on zero and is quiet on 10....

[Jana]

Are you certain that you have it wired correctly? Is it possible that the wires going to the pot are connected to the wrong lugs? With it on 10 it wouldn't matter but at anything less than 10 it would matter.

If the wiring checks out okay, what happens if you jumper each of the 2.2M with a 220K?

[rockstah]

Are you certain that you have it wired correctly? Is it possible that the wires going to the pot are connected to the wrong lugs? With it on 10 it wouldn't matter but at anything less than 10 it would matter.

If the wiring checks out okay, what happens if you jumper each of the 2.2M with a 220K?

positively wired correctly. 250k pot. why would i wire in two 220k's?

[Jana]

Why 220K's?

There are a couple of possibilities for the situation you are describing. One is that the balance of the differential signals is somehow being compromised which results in the increase in hum. There is hum present at this stage of the amp but it gets nulled out because of the opposite polarity hum on the other half. But, if one half is being fed a stronger signal of hum than the other (at a setting of less than 10) then the hum is no longer canceled out. That is what prompted my question about the wiring. But, the wiring is correct.

Another possibility is that one of the pots is losing contact with its trace--either through the wiper or one of the outside lugs. I have seen this happen with pots before and they only made solid contact when fully one way or the other. But, you changed the pot.

There could be an oscillation occurring when the pots are less than 10.

So, my suggestion to jumper with 220k is a stab at trying to figure it out. Personally, I think 2.2M is too large for the safety resistors. I would use 220K or so. But that is just my opinion. Having said that, if for some reason those safety resistors are in fact in this case active, meaning doing what they are supposed to be doing--provide an alternate path for bias because of some other error or failure, and the power tubes are not seeing a 250K resistor but are seeing the 2.2M, then maybe their grids are not being held to a stable reference point. 2.2M is much too high of a grid resistance for the power tubes. Hence my suggestion to jumper in 220K over the 2.2M to see what happens.

[Colossal]

So, my suggestion to jumper with 220k is a stab at trying to figure it out. Personally, I think 2.2M is too large for the safety resistors. I would use 220K or so. But that is just my opinion. Having said that, if for some reason those safety resistors are in fact in this case active, meaning doing what they are supposed to be doing--provide an alternate path for bias because of some other error or failure, and the power tubes are not seeing a 250K resistor but are seeing the 2.2M, then maybe their grids are not being held to a stable reference point. 2.2M is much too high of a grid resistance for the power tubes. Hence my suggestion to jumper in 220K over the 2.2M to see what happens.

Jana,

2M2 was chosen as the paralleled resistor so as to keep the power tubes from running away if one (or both sides) of the dual gang pot failed but also because 2M2 || 250k gives 224k which is very close to the typical grid leak value of 220k found in most Marshall and Fender amplifiers.

[rockstah]

Why 220K's?

There are a couple of possibilities for the situation you are describing. One is that the balance of the differential signals is somehow being compromised which results in the increase in hum. There is hum present at this stage of the amp but it gets nulled out because of the opposite polarity hum on the other half. But, if one half is being fed a stronger signal of hum than the other (at a setting of less than 10) then the hum is no longer canceled out. That is what prompted my question about the wiring. But, the wiring is correct.

Another possibility is that one of the pots is losing contact with its trace--either through the wiper or one of the outside lugs. I have seen this happen with pots before and they only made solid contact when fully one way or the other. But, you changed the pot.

There could be an oscillation occurring when the pots are less than 10.

So, my suggestion to jumper with 220k is a stab at trying to figure it out. Personally, I think 2.2M is too large for the safety resistors. I would use 220K or so. But that is just my opinion. Having said that, if for some reason those safety resistors are in fact in this case active, meaning doing what they are supposed to be doing--provide an alternate path for bias because of some other error or failure, and the power tubes are not seeing a 250K resistor but are seeing the 2.2M, then maybe their grids are not being held to a stable reference point. 2.2M is much too high of a grid resistance for the power tubes. Hence my suggestion to jumper in 220K over the 2.2M to see what happens.

i understand what you are saying and i have measured the pot and there doesn't seem to be anything wrong with its readings. tried two different pots as well.

i just wired two 220k's one in parallel for each 2.2m resistor and it still hums on zero and makes no noise on 10. when i say it makes no noise on 10 i mean no hum, the amp functions as it should otherwise.

[Jana]

Colossal,

Yes, I understand why they are there and the logic behind the chosen values. I still question the wisdom of relying on a 2.2M grid leak resistor, however. The data sheets for the power tubes tend to lend credence to my opinion. Again, this is my opinion and preference (I prefer 100K bias feed/grid leaks for greater stability with the power tubes).

Rockstah,
I've used up my brain cells on this one--I even brought cell number two online! The next thing I would do is hook it up to a scope and a signal generator and snoop around. Maybe it's the solar flares acting up.

[rockstah]

i just pulled the board out... driving me nuts over here...

[Colossal]

i just pulled the board out... driving me nuts over here...

Have you tried reversing the leads going from the PPIMV out to their respective halves of the power section?

Also, have you tested the bias voltage on each side the pot when the volume is at 0 and again at 10?

[rockstah]

i just pulled the board out... driving me nuts over here...

Have you tried reversing the leads going from the PPIMV out to their respective halves of the power section?

Also, have you tested the bias voltage on each side the pot when the volume is at 0 and again at 10?

didn't try the reversing because i can see that they are hooked up correctly.
if it were hooked up incorrectly the amp would be squealing like a pig if i turned up the PPIMV and they were hooked up wrong.

it was -47.89, center lugs = -47.89 input lugs =46dcv at any position of the pot.

[Colossal]

didn't see that they are hooked up correctly.
if it were hooked up incorrectly the amp would be squealing like a pig if i turned up the PPIMV and they were hooked up wrong.

I was referring to the leads going from the PPIMV to each side of the power section, not the output transformer leads (which would howl if reversed).

Good that the bias voltage is staying the same over the travel of the pot. So the buzzing is not related to changing bias condition.

Have you checked for leaky output coupling caps?

[rockstah]

didn't see that they are hooked up correctly.
if it were hooked up incorrectly the amp would be squealing like a pig if i turned up the PPIMV and they were hooked up wrong.

I was referring to the leads going from the PPIMV to each side of the power section, not the output transformer leads (which would howl if reversed).

Good that the bias voltage is staying the same over the travel of the pot. So the buzzing is not related to changing bias condition.

Have you checked for leaky output coupling caps?

wouldn't leaky output caps show up on the input lugs of the pot?

that doesn't make sense to switch the leads because they are in the correct places on the output tubes unless you're saying to try it as see if resolves and then that would lead to an answer as towhy they work when they shouldn't work hooked up that way?

[Colossal]

unless you're saying to try it as see if resolves and then that would lead to an answer as towhy they work when they shouldn't work hooked up that way?

Yes, that was I was saying; to try to see if there is a difference (although there should not be)...potentially a phase issue.

[rockstah]

i hear ya. let me get it back together and ill take it from there... it has be baffled.

[jjman]

Are the output heaters wired properly? If not, there might be hum in the output that might be masked by reversed phase hum coming from the PI outputs.

Wild guess....