PPIMV Issue

[philster]

HI Guys
I fitted a Lar/Mar PPIMV to my Bassman build. Pretty pleased with it but have found a issue - when I dial the MV to minimum attenuation there is a pop through the power tubes. You can actually see the valves "burp" for a millisecond. Is this normal? I'm thinking not.
I have removed it and checked every connection and can find no issues with the soldering. The pot also tested out alright. For the record I am using a 220k dual pot instead of the recommended (but harder to get) 250k. I have also increased the safety resistors to 18meg (the highest I had) to minimise the impact on the combined resistance as I'm not using the 250k pot.

So any ideas? Try another pot?

[img:768:1024]http://www.greatbritishtoolcompany.co.uk/ppimva.jpg[/img]

Cheers
Phil

[Colossal]

Can you reproduce exactly the point in the pot rotation that produces the burp? or is it only at the very lowest setting? Can you see which tube is burping? Have you swapped power tubes and does the burp follow? Have you isolated the pot and verified that the resistance is pretty well balanced on both sides of the pot, from 0 to 220k? Almost sounds like there is a part of the track missing and when the wiper goes over it as you sweep down, it temporarily has no resistance.

Something isn't right

[philster]

It happens just as you hit full volume (which means the pot is essentially out of the circuit?) Both tubes "burp" - with the lights off I saw a small spark on the pot as it happens, but since checking my wiring that has now stopped. It still burps though and after playing for about ten minutes the starts to fart out, slowly building from nothing to the point where you have to stop playing. No burning smell or smoke anywhere so I'm sort of stuck.
Phil

[Colossal]

It still burps though and after playing for about ten minutes the starts to fart out, slowly building from nothing to the point where you have to stop playing. No burning smell or smoke anywhere so I'm sort of stuck.
Phil

I'd replace that pot and if you have any doubt, the entire PPMIV circuit. Don't want to risk thermal runaway with a faulty installation. Something is definitely wrong there so if you can't or don't want to tease it out, replace it.

Have you also swapped the power tubes to ensure that that they are in no way contributing to this issue? So in other words, if you put a known working good set of power tubes in and the burp symptom followed it, then the problem absolutely lies with your PPIMV. Just can't assume without testing though.

[philster]

I'll swap my other pair in and try it. Have ordered another pot in the meantime!
Cheers
Phil

[Structo]

Sounds like a faulty pot to me.

Is there some reason you did not use the same spec parts and the Lar/ Mar MV?


Did you wire it exactly like the Lar/ Mar?

[philster]

Tom
220k pots are easy to find in the UK, 250k not so! Yes, I've wired exactly like the WIKI - I originally fitted 2.2meg resistors like the original but reading up I found a thread where someone had the same problem finding a 250k dual pot. They suggested a higher rated safety resistors (50meg I think) so that it doesn't alter the 200k the amp normally would see when the pot is fully open.
Anyone know where I can get a 250k pot in the UK?
Cheers
Phil

[Structo]

OK, sorry Phil.

Some guys modify pots by transplanting other wafers from other pots to create the needed value.

But I agree, a 30K difference isn't huge.

My thought on the MV pot in your amp is that it is somehow lifting from the trace when you peg it.

The bias is connected to the left lug of the pots so if the wiper lifts there is the problem.

[philster]

Alright, spent a little quality time with it. Measured the bias voltage from each half of the pots - the one side gave me from -46.1 to -39.9, the other -46.4 to -37. But the first side would randomly jump to from -39.9 to 0 if you turned the shaft too quickly or tapped it. So definitely a dodgy pot!
As I said, a new one has been ordered so I'll report back once it arrives and I can swap them out. This amp has been a real learning experience and it has taught me patience - you just can't rush things and troubleshooting is something not to fear. Much...
Phil

[Tillydog]

Anyone know where I can get a 250k pot in the UK?

Bluebell Audio

(Pots--> Alpha --> Dual 250k log)

....But I wouldn't sweat the difference, the tolerances on the pots are bigger than that difference in value, anyway.

(I would be uneasy about the effectiveness of such high value 'safety resistors' though - YMMV)

Bluebell Audio also sell Hammond Tx, btw. (No connections other than as a satisfied customer).

[rogb]

If you need a dual 250k I'm going to put a Mouser order in sometime in the next month and I am going to order one for myself for my Marshall build.

I can order another for you if it helps, they are like 2 quid or so

I have the LarMar PPIMV with dual 250k on my Deluxe and it works perfectly and I use it all the time FWIW.

Wouldn't be without one or a Marshall/Fender amp TBH. Don't need it on Dumbles as I have a Dlator to control MV.

EDIT: Or you can get one from Phil at Bluebell for £1.30 lol

[philster]

Cheers, Gents!
Probably going to build a Marshall next, so would the Hammond Tx's be a good choice?
Cheers
Phil

[Colossal]

Good to hear that you got it sorted out.

[philster]

Final update....
New pot arrived today (a 200k but upon testing both sides came out just over 250k!) so have fitted it (and gone back to 2.2meg resistors).
All tested, works great and amp sounds fine after extended playing.

Thanks again to everyone for their help, much appreciated! Now to play with the voicing of the amp.......

Phil

[Structo]

Great!

Enjoy your amp!