Post PI Master Volume ideas??

[markmalin]

Hi guys.

I wanted to install a MV on my Vibrolux clone.  Orginally I thought I'd just use a 1M Audio taper pot at the input of the PI, but was poking around the internet and saw a lot of discussion about Post PI master volumes.  I found the 2 schematics below.  I actually implemented this one using a 1M Audio dual ganged pot, 33K resistors and since my amp had 0.047 uf caps rather than the stock 0.1 uf's shown in the picture, I used another pair of 0.05 uf's. 

The sound is "OK", but maybe a tad harsh.  It seems if I turn the input up too far (above 50%) you start to hear overtones.  I guess my questions have to do with whether I should try exchanging parts or go with a different desing. 

In any case, let me ask this - can someone help me understand the effect of additional 33k resistor/0.1 uf cap network on the tone in the the first schematic?  Then, regarding the second schematic...is this a "better" way to do it, or just different but the same effect?

Thanks guys!
Humbly,
Mark.

[phsyconoodler]

The second way is the Lar-Mar master and it's definitely a better sounding master.
Here is the one I use with great success on Fendery builds.Sounds killer good.

[markmalin]

Thanks, P.N. That looks like a much easier circuit.  It would only take a few minutes to give it a try ;)

Mark.

[Geezer]

The second way is the Lar-Mar master and it's definitely a better sounding master.

That's the one (LarMar) I use & am very happy with it in every amp I've put it into.

G

[markmalin]

The second way is the Lar-Mar master and it's definitely a better sounding master.
Here is the one I use with great success on Fendery builds.Sounds killer good.

That's the one I use & am very happy with it in every amp I've put it into.

G

Did you mean the Lar-Mar one, or the one Psychonoodler posted??
Mark.

[Geezer]

Oops.....sorry, LarMar is the one (I edited my original post above to reflect the correct answer). 

[markmalin]

Oops.....sorry, LarMar is the one (I edited my original post above to reflect the correct answer). 

Thanks.  So .. is there a way to use that style circuit with a 1M ganged pot?  The diagram shows 250K and what I have are 1M pots.  Hate to make another $2 order from Mouser if you know what I mean ;)

Mark.

[Geezer]

You could put 330k resistors across outside lugs of the pot(s) to get them to ~~250k.....even 470k would get you close (effective 320k)

[markmalin]

You could put 330k resistors across outside lugs of the pot(s) to get them to ~~250k.....even 470k would get you close (effective 320k)

Thanks.  That makes sense.  In retrospect it was probably a silly question, but what can I say...
Mark

[phsyconoodler]

I actually use the trainwreck one as it seems to work just as well as the Lar-Mar.I've tried the Lar-Mar in Marshall amps and it was fine,but I can't really hear any difference between the two types to tell you the truth.
 

[bluesbear]

The LarMar version seems very close to the type 2 Trainwreck MV, except his uses a 100K dual ganged pot. As long as we're comparing, has anyone tried this one?
Dave

[Geezer]

If I recall correctly, the LarMar is directly derived from the type-2 TW.

The thinking is this: Replace the grid leak resistors (100k, 220k, 470k....whatever your amp "normally" has) with variable resistors so you can bleed off signal & thus control the output of the PI (input to the power tube grids).
The LarMar has the 250k pot(s) with a 2.2meg across the outside lugs (parallel to the pot) which results in an approximation of the desired 220k grid leak resistor(s). The 2.2M's also serve the purpose of acting as a "safety" in case the wiper lifts from the pot trace, allowing bias to remain intact, thus protecting the output tubes.

I think I have that right.....

[Willabe]

I've read that on the TR type 1 the 2'nd set of coupling caps need to be 10x the value of the stock coupling caps value to end up with 90% of the original bass content. 2'nd set can be rated as low as a 100v.


          Brad

[kagliostro]

Sorry Geezer I'm not able to understand well

The LarMar has the 250k pot(s) with a 2.2meg across the outside lugs (parallel to the pot)

the 2.2M resistors are between the wiper and one side of the pot not parallel to the complete pot ???

what do you mean with "outside lugs" ?

here it seems to me that you confirm that 2.2M resistors are between wiper and one side of the pot

The 2.2M's also serve the purpose of acting as a "safety" in case the wiper lifts from the pot trace

someone has a schematic for the Lar-Mar ? to me is easier to understand than reading layouts

thanks

Kagliostro

[Geezer]

Sorry Geezer I'm not able to understand well

The LarMar has the 250k pot(s) with a 2.2meg across the outside lugs (parallel to the pot)

the 2.2M resistors are between the wiper and one side of the pot not parallel to the complete pot ???

what do you mean with "outside lugs" ?

here it seems to me that you confirm that 2.2M resistors are between wiper and one side of the pot

Sorry......my mistake. You are correct, the 2.2m goes from the wiper to ground on a cathode biased amp (or the bias supply on a fixed bias amp)

[Fresh_Start]

One attached schematic is TW type 2 if I'm not mistaken.  It avoids having two coupling caps in series (.1 + .1 = .05 sort of) at the cost of having DC voltage on the MV pots (bias voltage).

The second schematic repeats the first one in this thread but is fully connected.

Hope this helps someone.

Chip

[Willabe]

Why is there an R 33K- 100K from the wiper to the 2'nd coupling cap?


        Thanks,    Brad     

[kagliostro]

Why is there an R 33K- 100K from the wiper to the 2'nd coupling cap?

Just now I remembered that the two .1 (before and after the pots) are to prevent DC on the pots with fixed bias circuits and the 33k-:-100K resistors purpose is to avoid change in tone when turning more or less on the pot

if you have a cathode bias circuit you can use only the .1 cap between PI and the PPIMV pot, without the second .1 cap (that is between PPIMV and the final tubes)

Kagliostro

[Willabe]

Thanks Kagliostro.


     Brad