Plexi or jtm 45 PPIMV: could it fry output tubes?

[dariez75]

Huy guys,
I've read somewhere (I don't remember where...) that PPIMV made using just dual 500K log alpha pot (rich mod as described in metroamp forum)could fry or damage output tubes. What about that? Is this correct or are there some problems with that kinda mod; the original Mashall use a PrePIMV just after the tonestack and by mean a single pot.

[pdf64]

A dual 500k pot doesn't seem correct.
As the pot track replaces the grid leak resistors (normally 220k), the pot track value should not exceed their value. Well, 250k is within tolerance.
The max grid leak value is often specified in the manufacturer's tube info.
Pete

[dariez75]

Yeah, this is true for LaMar mod via removing 220K resistor and installing on 250K dualpot two 2.2M resistors.

[pdf64]

So that we are clear, it would be best to post details of the mod, or a link to somewhere that documents it.
It may be helpful to refer to http://www.blueguitar.org/new/schem/tra ... _pages.pdf in which the various master volume methods have been described as types 1 to 4, starting page 40 on the page / page 29 of the pdf file.
The rich mod is likely to be a variation of one of these, just as the LarMar is an implementation of the Type-1.

[martin manning]

...just as the LarMar is an implementation of the Type-1.

Type 2. :^)

[dariez75]

If I correctly read, Type 2 using 100k linear and is traiwreck exclusive? Any schematics?

[pdf64]

Thanks Martin. I was in a rush and Type-1 looked close enough.

[martin manning]

Type 2 PPIMV with Lar-Mar mod goes like this:

[Structo]

Yep, and mind the shields that carry bias voltage.

[dariez75]

Great Martin, you are an artist! What is the power rating for 2.2M restistor? 1/2 W.? Could I have tonal change swaping the two bias resistors from 220k to 2.2M? And last but not the... What is the main improvement despite my PPIMV using only 500k log dual pot as suggested by George Metropulos?

[martin manning]

1/2 W is fine for the 2M2. Notice that the circuit without the MV has 220k from each grid to the bias voltage. When the MV (as shown) is turned all the way up there is 250k//2M2 or 224k from each grid to the bias voltage, and the circuit is essentially unchanged. Using 1M8 resistors would make it even closer. Those resistors also provide some safety margin in case the pot wipers lose contact.

When turned down, this type of MV does decrease the relative feedback to the PI, but most people are very happy with the result.

Using a 500k dual pot would alter the frequency response at the output of the PI, increase the time constant associated with bias excursions, and place too much resistance between the grids and the bias supply when the MV is turned up. If there is too much resistance between the grids and the bias voltage, the grid bias voltage can drift upward and cause a failure of the power tubes.

[dariez75]

N.1 Martin. Sonner I will buy the parts. Which kinda sheleded cable? Dual wire with sheld?

[martin manning]

Which kinda sheleded cable? Dual wire with sheld?

Yes, exactly.

[dariez75]

Thanks to pd64 for trainwreck pdf file. I don't known about this, very amazing Ken Fisher's document!

[dariez75]

Ok guy I've done. The new master volume is fine and even almost noiseless! Yeah!!!