Non-Marshall PPIMV

[EFK]

Can someone tell me exactly where in this circuit one would install a Ken Fischer-type, or similar, master volume? Also, I have seen recommendations for pot values ranging from 250K (Fischer) to 500K or 1M (a Collins diagram I saw somewhere but now cannot find). Do certain values favor different circuits? This is a Gibson amp which I think leans towards a Fender circuit. Any help much appreciated!

[Chuck H]

The pot for the KF PPMV mod replaces the bias feed (grid load) resistors so you pick a pot value close to that resistor value in your amp. Other PPMVs effect the circuit differently. That's why there seems to be such a wide variance in pot values for the different types of volume controls.

Do the KF PPMV mod to your amp like this:

[EFK]

Thanks! That makes sense. Do I understand correctly that the 220K resistors are removed completely? Also, the 4.7 grid resistors on pin 5 of power tubes downstream stay in place, right? I appreciate the help.

Any thoughts on how changing the value of the pot - say, using a 500K vs. a 250K - would alter the sound? Can it be inferred in some way, or is it just a crap shoot and best to stay close to original resistor value?

[hasserl]

The pot for the KF PPMV mod replaces the bias feed (grid load) resistors so you pick a pot value close to that resistor value in your amp. Other PPMVs effect the circuit differently. That's why there seems to be such a wide variance in pot values for the different types of volume controls.

Do the KF PPMV mod to your amp like this:

That's a modification of the KF style MV, in which the wipers of the pot would be going to the power tube grids, not attaching to the feed from the PI. That mod has the advantage that the bias voltage to the tube grids is not dependent on the wiper maintaining continuity with the bias feed terminal, but it will effect the function of the volume control. I haven't tried it, but it's bound to be different from the standard KF style MV.

Note that you can avoid the potential issue with loss of bias voltage to the tube grids using the standard KF style MV by adding some high value, say 2M, resistors between the wiper and trhe bias feed terminals. The high value will not cause any functional volume control issues and will maintain the bias feed to the grids in the evcent the wiper looses continuity.

Personally, I've installed dozens of the KF MV and have never had a loss of bias voltage due to a failure of the pot wiper, but I admit the potential exists, just doesn't seem to be much of a problem in dozens of amps with the MV installed. I think the worry is a bit over stated, but that's just my opinion.

[Chuck H]

Oh, hey, your right. I whipped that out a little fast then. I don't know what those "extra" leads are on the power tube grid leads between the bias feed and grid resistor. They could be bleeder caps, but maybe not. At any rate, my design won't compromise any relationship in this circuit so I'd probably try it this way first. thanks for being vigilant.

Chuck

[EFK]

Those extra leads tie in from the reverb/tremolo circuit (which could be cut right out of there as far as I'm concerned). Thanks guys!

[Chuck H]

Those extra leads tie in from the reverb/tremolo circuit (which could be cut right out of there as far as I'm concerned). Thanks guys!

I thought it could be something like that. Are you sure they are part of the reverb circuit? I figured for the trem. But that would be a wierd place to inject the reverb. And if that is the case the reverb would not be effected by the master volume the way I have arranged it. Making it possible to get 100% wet signal, for better or worse. I don't know if the standard KF type master would work with the trem. I don't think I would want bias modulation voltage on the grids with no resistances in between the grids, trem oscillator voltage and ground. But I dunno. You could try it both ways and see. I'll draw up a proper KF version of the master and re-post.

Chuck

[EFK]

Yep you're right, it is the trem only. I've attached a layout of what I have been planning to install. I think this is a modified version of the 2nd KF type - maybe a 'type 3?' I'm a little unclear as to how the trem usage could be affected - if the 220K resistors are being replaced with a pot which itself has 250K resistance, isn't the resistance therefore still in place albeit variable?

[Chuck H]

That would be the standard KF PPMV (+ shielded lead)

Three points to make:

1) If you wire it this way you will be grounding not only the power tube grids, but also the trem oscillator voltage. At the very least you will limit the trems function as the master is decreased. Even if you don't use the trem I don't see the point in adding a circuit that undermines another of the amps circuits during operation. At worst it may increase oscillator current when you ground that voltage supply and burn up some componants. In this case I would wire the PPMV the way I have shown.

2) I would skip the shielded cable. It's a source of unwanted capacitance and makes for a crowded, messy mod. Also, grounds in this part of the circuit can do funny things when they interact because there is more current in the ground leads. So you could actually INCREASE hum by introducing ground loops. Shields are more important for early preamp stages. Just make sure to keep your new leads away from the preamp but as short as is convenient.

3) Be sure that point where the two resistors use to be grounded is an actual ground point. That is, if you lift those resistors, was there a lead to ground at that node? If so then you don't need to ground at that node. You can ground the new circuit where that node was grounded. If it makes for a neater build and a better layout.

Chuck

[EFK]

Thanks for the continuing info Chuck. I am planning to ditch the shielded cable. Have heard nothing but complaints about it! However, as I am not a tech but rather a diy'er (and a very careful one at that) I'm now confused. To my untrained eye, what you posted looks the same as what I posted. Are you saying to use the dual pot to replace the 220K resistors only, w/ the wipers essentially replacing the resistors, and no added wiring? I do agree with you in that, while I never use the trem, at the same time the amp maker included it and it's already there so it would be silly to do away with it.

Is this the same thing?

[Chuck H]

The way I have it on my first post is like this. Notice that the PI outputs are on the wipers instead of the power tube grids being on the wipers as in the original circuit. This arrangement will allow the trem and cathode bias to keep the same relationship (this is a bias modulation trem circuit) no matter where the master is set. The standard way the KF PPMV is wired won't do that for your amp.

Chuck

[EFK]

Got it! That is perfectly clear. Thank you VERY MUCH for taking the time to help me understand this!

[EFK]

OK. Now I have another question. Chuck, if I wire it this way, what happens if the pot fails? I suppose one could ask the same question of the original resistors. I'm not sure what hasserl is alluding to above (i.e. 2M resistors added... where exactly? ...as backup) but I do like the idea of a backup. Any thoughts or advice? Where would such a thing fit into your above diagram?

[Chuck H]

You wouldn't use those resistors with this arrangement. With the standard wiring the power tube grids are on the pot's wiper. The wiper is more likely to fail than any other part of the pot. So with the standard wiring, if the wiper fails it leaves the power tube grid floating with no bias. The tube goes into "runnaway" and melts. Adding the 2.2M resistors in this circuit prevents that by routing the bias voltage for the power tube grid across two paths instead of one unreliable one. With this modified wiring niether the bias voltage or the power tube grid is on the pots wiper. The bias voltage gets to the power tube grid across the pots two outside lugs so it's just like having fixed resistors in there.

OK, that's the explaination for educational purposes. And to be honest I personally wouldn't bother with the 2.2M resistors if it wer my amp. But... Pots do fail (even on the end lugs) more often than fixed resistors. They regularly go "open" on me after a couple of experimental component changes. If you re-melt the solder on a lug more than a couple of times there is always a chance that it's going to short. Even if it seems to be working now. So... To be absolutely safe you could use the 2.2M resistors with this arrangement by soldering one across the "outside" lugs of each pot gang.

Chuck

[EFK]

Thanks again Chuck , so much! Very well-explained. From a theoretical perspective, I don't understand how putting a big value (2 or 2.2 M) resistor across the pot can have no effect on the sound. Doesn't that resistor then figure into the equation all the time? Regardless I've seen quite a bit of reference to it so I'm sure it works well. Not sure whether or not I'll use one, just thinking about a MV as opposed to an attenuator, which initially I liked but after using it for 6 months or so I can definitely hear a very decided effect on the un-attenuated tone of the amp and character of the overdriven sound. An effect which I don't care for all that much. Thin and fizzy.