1959 100w Marshall Super Lead with PPIMV

[TIMBO]

Hi guys, a mate has asked me to give this beast the once over.
It's suffering intermittent volume loss. 
I have not turned it on just yet as I would first investigate some mods added.
The ppimv is the first job.
As per this schem  http://el34world.com/charts/Schematics/files/marshall/Marshall_marshall1959m2.gif using 82k grid leak resistors.
These resistors have been replaced with a dual gang 100ka pot to make the PPIMV.
I am have done many PPIMV mods but used 250ka pots and these had 2.2m resistors added to the wiper.
This mod DOES NOT have these 2.2m resistors????
I am questioning this mod as there is a crusty 5.6k grid resistor on one of the power tubes.
Can any one supply a schem for the PPIMV used on this amp. Thanks

[TIMBO]

A bit digging and a 470k resistor is used to create the 82k leak resistor.

[sluckey]

A 100K pot is a logical choice to replace 82K grid leak resistors.

A bit digging and a 470k resistor is used to create the 82k leak resistor.

Tell me more. Does that mean there is a 470K resistor across the wiper and one end of the pot (actually two 470Ks for two pots)? If so, then no need to add 2.2M safety resistors. The 470Ks are doing that job.

[TIMBO]

Hey sluckey, The 470k is used the same as the 2.2m.
Did a test run and the bias supply only fluctuated buy about 1v and the pot measured about 90k

[Ritchie200]

Definitely check out the crusty grid resistor!   But maybe go through and clean sockets, retube as needed, retension jacks as needed.  That way you have a good baseline to start with. These are the easiest anyway and usually the root of most evil on an amp that has otherwise been taken care of. Then bill him for 30 hours of repairs!

Jim

[TIMBO]

Thanks Jim, Test date is 78, makes the ecaps 38yo, are these ready for a rest. They seem the measure under their marked value.

[TIMBO]

Hi guys, CRUSTY resistor removed and a few problems have presented.
The bias supply has at some time had a resistor failure.
The schematic shows a 47k in series with the trim pot, but on the board is a 56k (original) I'm thinking this is because the trimpot is 22k and it looks like the schem shows a 27k trim.
The 56k resistor is "open" and a 100k was tacked across it  this then set the bias range between -60 and -64 and this equated to about 1ma across a 1ohm sense resistor.
Replacing the 100k with a 56k gave a much better range.
One input jack was removed to accommodate the PPIMV and the single jack wired to the 68k (200uf on the cathode), making that channel muddy and blerty, so paralleled another 68k makes it a bit better.
The second channel is very brite and tinny, and a .0047 brite cap was put across the volume pot wiper as per the schem, with increased volume the hash and fizz is huge. I have not had much to do with these amps so I don't know if it is common, replaced the .0047 with a 500p and still much the same.
I added a 1ohm sense resistor to the el34 cathode to get measurements.
The added 470k resistors work well and the PPIMV also working well.

[sluckey]

... that channel muddy and blerty... The second channel is very brite and tinny

That is the nature of the Plexi style preamp. One channel too dark and muddy, the other too bright. IMO, neither channel is useful when used alone. But when you jump the two channels together you will be able to use the volume controls to blend the dark and bright together for a nice full sound that has good bottom and good top end too. Try it. You'll like it.

I have two plexi preamps and they are both permanently jumped internally, with only one input jack.