Lar/Mar Bias Question

[Chevelle]

I've had a lar/mar in my 100w 69 metro build for a year or so without any problems. Recently, when I cranked up the MV, I got red plating on v4 and v5. After checking it out, I noticed that the lar/mar was controlling the bias voltage to v4 and v5, but had no effect on v6 or v7. So when I cranked up the volume with the mv, I was seeing the bias go way up at v4 and v5 as well.

I am thinking the outer or back pot on the dual gang pot is the problem since this is the pot connected to v4 and v5. The pot tests ok in terms of resistance.

I'm going to try an new pot, but wanted to see if anyone had any other thoughts.

Thanks

[toner]

You may notice a slight drop in negative voltage on pin 5 of each power tube as you turn the MV up (about 4 volts or so). If you have a large change (more than 10V) on one pair of tubes, that side's output coupling cap may be leaking and needs to be replaced.

I would temporarily remove the MV and wire it up like normal and check the bias voltages and current to verify if the dual pot is the problem. You could just replace the MV but there still may be a problem in the PI area.

[Chevelle]

Thanks for the help Toner. On pin 5 of v4 and v5, I am seeing the negative voltage from from 36.7 to 32 as I turn the MV from min to max. On pin 5 of v6 and v7, I see less than a volt change. Give these results, do you think the coupling cap on the v4 and v5 side is leaking and causing the bias problem?

[toner]

The one cap is probably leaking a little DC. Both bias feeds in one of my amps with the Lar/Mar MV varied about that much at one time. I've changed coupling caps since then (from Sozo regulars to real mustards). I haven't checked them since but never had any problems (red-plating, etc.). In your case, since only one side varies, it may be more important.

As long as your bias current doesn't get too high on V4 and V5 with the MV dimed, you'll be okay. EDIT: You were getting red-plating so you'll either have to set the bias colder or replace the cap.

You can do a basic test of the cap by unsoldering one end of it and checking the neg voltage at pin 5 again with the MV dimed. Compare it with what you got before (-32).

[Chevelle]

With the cap disconnected on one end, my voltage on v4 and v5 now matches v6 and v7 with very little change if I move the MV from min to max. So does that mean I need a new cap there?

[toner]

With the cap disconnected on one end, my voltage on v4 and v5 now matches v6 and v7 with very little change if I move the MV from min to max. So does that mean I need a new cap there?

Yes.

If your tubes happen to be mismatched you can put the two with lowest bias current in V4 and V5 as a temporary fix. I would replace the cap when you can though.

[Chevelle]

Interesting thing happened, I resoldered the cap back in and now the problem seems to be gone. Moving the mv doesn't seem to have the effect it did on the bias. Do you think I should still replace the cap or was something in the solder connection causing it?

Thanks again Toner, I really appreciate the help.

[toner]

Oh sure, throw another wrench in the mix! I'm not sure but you may want to leave the amp on for a while and re-check it. Maybe power off/on and check it again.

You're welcome for the help. I'm just paraphrasing Larry (novosibir) anyway.

[Chevelle]

OK, so I think I do need that cap. After the amp warmed up, the problem came back. Definitely getting a new cap. Changing the cap is easier than putting a new mv pot in there.

Again, thanks.

[arledgsc]

Great tip toner. There should be a new forum section (wiki perhaps) for troubleshooting and repair where issues like this are condensed and categorized for easy referral. Scott

[Chevelle]

So I got a hold of new .022uf cap to put in there and thought it was fixed, but after about 15-20 minutes of the amp being on, the problem came back. After the amp cools off and I restart it, the pattern starts over where the larmar has no effect on the bias until after the amp has been on for a while. I do notice that when the larmar is affecting the bias, the neg voltage at pin 5 of v4/5 will drop 2 or volts. The negative voltage at pin 5 of v6/7 doesn't fluctuate at all.

I am thinking the next step is to take the larmar out of the amp to see if the problem persists. Any suggestions on what may be causing this would be greatly appreciated

Thanks

[toner]

That sucks.

You won't like this but... As far as I can tell, it's still a leaking cap since lifting it before stopped the bias voltage change. I don't see any other possible cause but maybe someone with more experience will have an idea.

If it only varies by a couple of volts that shouldn't drastically change your bias current. Hopefully it will be usable without red-plating.

[toner]

Your issue got me curious so I opened my amp and found the following.

- adjusting my lar/mar PPIMV from 0 to 10 changes my negative voltage on all pin 5's from -41.4 VDC to -34.0 VDC so I've got a 7.4V change.
- this would indicate both my output coupling caps are leaking but I did not test them
- the above bias voltage change HAS NO EFFECT on my bias current via 1 ohm resistors. I can't explain this because adjusting the bias trimmer does affect both bias voltage and current.

Maybe there is a difference between tubes and how they are affected by the negative bias voltage change via the MV. My tubes are Winged C's.

[toner]

Regarding my last post, it seems my cheap meter is causing the negative bias voltage readings I got. The voltage only dropped because of the voltage divider formed by my meter's low internal resistance and the PPIMV pot. The actual voltage on pin 5 wasn't really changing as the PPIMV was adjusted. It only appeared to be while the meter was "in the circuit". That's why the bias current wasn't affected by the PPIMV in my case.

Chevelle,
Since your bias current IS affected by the PPIMV on V4/V5, I believe that side's coupling cap is leaking (even the new one). There may be another explanation beyond my knowledge but that seems to be the obvious cause, given the results you have had.

Thanks to SDM for the above info. Cheap meters suck.

[neikeel]

I think there might be a duff batch of dual 250k pots?

I had the exactly the same problem (I have one cheap meter and two expensive ones ( a Fluk and AVO). I thought I might have a leaky PI output cap but no it was the pot with part internal short. Fortunately I had spare pot and now working normally