Lar/Mar PPIMV

[dorrisant]

Martin is right and it's worth the wait!
You'll be disappointed with a dual 1M pot and probably the PO will be driving you nuts.

I hear you and believe both of you. Ordered another few of them from another vendor. Just a few days of waiting.

Tony

[novosibir]

Is there some reason that the shielding is connected to the bias voltage? Would it be as effective or even more so if it were attached to ground?

I ask because after installing it on an express build I'm getting what is obviously feedback or low frequency (~300Hz) parasitic oscillation when the Master is turned up around half way.

Any DC voltage inside an amp is as good as ground like ground itself.

You only have to watch for, that there's not too much AC riding on top of the DC voltage, what in an bias supply sometimes might happen, if the electrolytics already are worn out - or if there's ONLY ONE electrolytic in the bias voltage supply, like in some Fender amps.

Just check for GOOD electrolytics in the bias supply, but DON'T increase their values, otherwise the power amp of the amp might react somewhat stiffer than with the correct values.

Larry

[martin manning]

Thanks for popping in, Larry!

[Reeltarded]

Larry! Happy New Year, buddy!!!

[novosibir]

Larry! Happy New Year, buddy!!!

Thank you!

Happy New Year 2013 to all of you out there!
Wherever you are on this planet!

Larry

[sepulchre]

You only have to watch for, that there's not too much AC riding on top of the DC voltage, what in an bias supply sometimes might happen, if the electrolytics already are worn out - or if there's ONLY ONE electrolytic in the bias voltage supply, like in some Fender amps.

Just check for GOOD electrolytics in the bias supply, but DON'T increase their values, otherwise the power amp of the amp might react somewhat stiffer than with the correct values.

Larry

Thanks! They're brand new F&Ts.

And Happy New Year to everyone!

[sepulchre]

Since posting this I discovered that my "feedback" problem was an NFB phase issue. So problem solved.

I don't have experience you guys have (obviously) but I have been studying the LarMar PPIMV a bit more and discovered that the value of the pots is indeed important. With 250K pots and their 1M5 to 2M2 parallel resistors is an aproximation of the normal 220K grid leaks (aka Bias feed). Using values that would parallel out to much more than 220K would subject the power tubes to more input than they're meant to have, possibly more than they can handle with catastrophic results.

So 250K pots and 1.5M - 2.2M parallels work out fine, if you have the normal 220K grid leaks. I've got a 250K ganged pot. Unfortunately, the Jana 2204 mods I'm trying use 100K grid leaks. I would think these are for tone issues rather than to safeguard the tubes. But wanting to preserve the tone, I will use 180K parallels instead of the usual higher range. That works out to around 100K.

However, this brings up something else. Jana's mods start with a recommended HT of 375V. My PT is a bit hotter than that so to get the preamp voltages down I added 8K to the dropping resistor between the screen voltage and the PI node. Works great, but now I'm wondering. Are Jana's 100K grid leaks related to the 375V HT? If so, how does my ~425V HT effect that? Should I be using the higher grid leaks? And if I use the LarMar, adjust the values there too?

So my question is: are the grid leak resistor values set according to the HT?

tia,
Ken

[Reeltarded]

No. Tube type. 6550 would take 150k vs the 220k for 34s.

[sepulchre]

Okay, I'll go with the usual 2M parallels on the LarMar. But why would anybody use 100K?

[jjman]

Okay, I'll go with the usual 2M parallels on the LarMar. But why would anybody use 100K?

Some tubes/schemes call for 100k grid leaks. I use 100k larmar on my 6v6 cathode biased outputs since the LTPI drives them too hard (for me) with 220k.

[BarryW]

I was just about to do a 6v6 plexi and was trying to figure out what to do if I used the 100k grid leaks with a LarMar. Would you agree with 180K parallel resistors with the 250k pot? I'm finding out that I don't quite grok the math/theory behind calculating the proper value. I need to read/study more . . . as usual!

Okay, I'll go with the usual 2M parallels on the LarMar. But why would anybody use 100K?

Some tubes/schemes call for 100k grid leaks. I use 100k larmar on my 6v6 cathode biased outputs since the LTPI drives them too hard (for me) with 220k.

[sepulchre]

Yep, 180K parallel with 250K yields 104K according to my figgers.

[martin manning]

I was just about to do a 6v6 plexi and was trying to figure out what to do if I used the 100k grid leaks with a LarMar. Would you agree with 180K parallel resistors with the 250k pot? I'm finding out that I don't quite grok the math/theory behind calculating the proper value. I need to read/study more . . . as usual!

No, I don't agree.

Yes, 180//250 is 104k, and that's fine when the MV is all the way up, but what about when it's at 50% of it's maximum rotation ("noon")? For a typical 10%-taper pot, the resistance from the top to ground would be 225 + 180//25 = 247k. This changes the AC load on the PI and the frequency break point of the coupling cap/grid leak RC, and the MV is no longer "transparent."

I would use a 100k dual pot and keep the safety resistors at 2M2, which gives 96k at max and 99.95k at 50%.

[BarryW]

Thanks Martin, I'll give that a shot - I happen to have a dual 100k in my drawer. Very much appreciate the advice!

Barry

[sepulchre]

Sorry Barry. Guess I hadn't thought it through.