Type 2/LarMar PPIMV in 5F6A build goes silent when maxed

[TitaniumValhalla]

Just finished up a 5F6A Bassman build today from a Hoffman board - sounds fantastic already by the way - just have a few minor kinks to work out. The major weirdness is I added a type 2 PPIMV (to the so-called Lar-Mar specs), the dual 250k/a pot and 2.2m resistors, replacing the 220k grid leak resistors, and it works nicely all the way through the pot until around 90%-100% of the range of the pot, when suddenly all volume cuts out. I was under the impression when it was cranked that it should more or less "disappear" so I assume something is not right here. Anyone have any thoughts on what might be going on or if this is expected when this type of PPIMV is cranked?

[kagliostro]

I hope I've understand you correctly

If you mean that decreasing power rotating the pot at 100% sound disappear ... that is right

What do you expect of different ?

If you don't want that this happen you can add a resistor in series with the ground connection of the pots

Franco

[tubenit]

Only thing I can think of causing that would be parasitic oscillation or something is wrong with the bias control when maxed? Have you checked bias voltages?


A schematic and photo would be useful.



With respect, Tubenit

[sluckey]

Definitely check the voltage on pin 5 of both output tubes while turning the MV pot. It should remain steady. Your pot is faulty or you have mis-wired. I would suspect wiring error first.

[kagliostro]

OK, I understood the opposite wise 


Franco

[Ed_Chambley]

Check your dual pot travel if you have 2 meters. I have had using cheap pots, one measure at the end like the wiper loses contact.
I have had them be very far apart. Way over 20%.


Precision dual PEC pots cost too much, but I can find clerostat and Allen Bradley quite often or under 10  shipped. Also, check with Steve at apexjr.com. he j

Keeps a good supply of Nos pots. The 100 and 250 dual Potts are more common as they were used in older stereo receivers and amps.


I have used dual concentric, but you must be carefull using them. These are the ones that have a dual knob to adjust them. Outer knob and inner and can be set separately. You can hear when you get them too far apart and one tube gets more signal. It is a good way to hear a slight tube imbalance which sometimes sounds really good.


If that amp sounds good in the middle of travel and does not when wide open it could be the pot.


I always use my best tracking pots for master volumes and I measure all of them and write the true value on them.

[TitaniumValhalla]

Thanks guys. My bias supply voltage was varying with the MV. I took the MV out completely to confirm the bias circuit was working properly, and it is now. Now that I have it out I can see where my wiring was incorrect for this board. There are a couple of jumpers I laced into the board from Hoffman's layout that shouldn't be there for this mod. I have the parts to try an easier type 3 PPIMV so I think I'm going to try that out first and see how it sounds before I tear up the board to go with the type two.

[robrob]

[TitaniumValhalla]

Thanks Rob! That's what I was working from. I just pieced yours together with the Hoffman board layout to produce this - does this look right?



As you can see you would need to remove the jumpers on the original layout between the .1 coupling caps and the lugs where the 220k resistors are on the original layout, move the grid wires inward one lug each and install the PPIMV between these two sets of lugs. The other end goes to the wiper on the bias pot and the underboard wire that would have gone from that to the other ends of the 220k's is removed.

I laced my board up according to the original layout so to make these changes I'd need to do some surgery on the board so I'm going to try the type 3 and see how it sounds before I go to that effort.

[robrob]

Yes, that looks good. I spec 1.8M pot resistors to get the max grid leak resistance down to the circuit standard 220k but that's no big deal.