Strange? PPIMV

[DG45]

Installed a LarMar PPIMV when I built a an EL34 Marshall kit a while back. I was checking bias today and initially forgot to turn the MV all the way up. The mistake made me take notice of something that I thought was strange. Bias in one power tube jumps up with the MV all the way up and down as the MV is turned down. The other tube does the opposite, higher bias when MV is down and lower bias when MV is turned up. It's an alpha 250K dual linear pot that measured 238K and 237K. Checked wiring, and it appears to be right. So, is this strange behavior, or normal (and just showing my lack of technical savvy)?

Thanks.

[Carbia]

Isn't normal.

The bias does not oscillate too much really using a PPIMV.

Maybe you have a leaky cap on the phase inverter...

[DG45]

Isn't normal.

The bias does not oscillate too much really using a PPIMV.

Maybe you have a leaky cap on the phase inverter...

Leaky pi coupling cap? Or the filter cap feeding the pi?

One tube only varies maybe 1mv between ppimv being all the way up or down, the other varies 4 mv or so. So maybe a leaky coupler on that side?

Thanks

[DG45]

Think I found it. I swapped EL34 positions and it follows the tube. I've been doing some Googling and reading and found some information that tube leakage can contribute to varying bias readings with position of the PPIMV. Not that I understand it all but seeing how there is less than 1mv change with position of PPIMV with the one tube and over 4mv with the other tube, even when swapping sockets, I'm seems like there must be merit to it being related to the tube.

Feel free to let me know if I'm missing anything. Otherwise, I'm open to a good explanation from one of you more tech savvy members.

Thanks.

[Carbia]

I meant PI coupling cap.

but if the oscillation follows the tube... is the tube.

[DG45]

Thanks for the help Carbia. And everyone else.

[romberg]

Think I found it. I swapped EL34 positions and it follows the tube. I've been doing some Googling and reading and found some information that tube leakage can contribute to varying bias readings with position of the PPIMV. Not that I understand it all but seeing how there is less than 1mv change with position of PPIMV with the one tube and over 4mv with the other tube, even when swapping sockets, I'm seems like there must be merit to it being related to the tube.

I've never thought about this but it does make a lot of sense. In the past I have wondered what consequences there would be to inserting a LarMar type MV in front of the power tubes. The PI will continue to see a load equal to more or less the pots value (which is chosen to be close to the grid leak resistors it replaces). But what about the power tubes?

I've seen discussions about grid leak resistors that involve a tube having a maximum grid leak resistance that may exist. If the grid leak resistance is too large then the tube can loose bias and run away (red plate). A larmar MV varies this resistance between say 220k and zero. So, it never increases it and risks exceeding the maximum grid leak resistance. This is all I ever thought about the issue.

But there IS some small amount of grid current that does always exist. That is what the grid leak resistor is there for in the first place. And it does make sense that the grid leak current would vary from tube to tube. Adjusting the MV is of course changing this resistance (that is what it does after all). So, the small amount of grid current is going to generate a small voltage across the MV resistance and shift the bias as the pot is rotated.

Guess this may be one more thing a person could have tubes matched for if you are really, really hard type A.

Mike

[DG45]

Hi Mike.

In doing some reading, there seemed to be some debate about any changes in bias with the position of the PPIMV pot. There wasn't really a general consensus but what I took away from it was that, if everything was working right, any change should be very small. In the case of one of my tubes, it varies less than 1mv, with it being "hotter" with the pot wide open. As I mentioned, the "problem" tube varies by close to 5mv and is "colder" when the pot is wide open. That ain't right although I don't know if it's always been that way or even if it's indicative of the tube going south.

At any rate, I'm going to assume that the tube that varies by <1mV is more representative of the impact that LarMar should have on influencing the bias. The ironic thing is, that without the PPIMV in the circuit, I'd never know that the one tube is potentially problematic (leaky?) ... assuming that it is.

[romberg]

I'm sure that changing the MV setting will change the voltage. It is kinda basic electronics 101. There is a current flowing and you are changing the resistance it flows through. But I bet you are right in that normally this current is so small that the resultant voltage change of the bias is not even worth considering.

Unless you have a tube where the grid captures more electrons than normal. Looks like you've discovered a new on the fly tube tester .

I love this aspect of the hobby. It is fun learning new things. As simple as these circuits are there is always something else to be discovered. Thanks for sharing your story!

Mike