PPIMV in SF Twin 135

[Zouto]

What is the best dual pot value for a PPI Master Volume in a SF 135 Twin? It replaces the 47K resistors, so it should be 2x 50K, but I'm not quite sure. Can I also use a 2x 250K pot with 2M2 to gnd? The amp is otherwise standard, no blackfacing, except an additional bias pot connected to the "output balance" pot.
Thanks for help
Zouto

[Chuck H]

I think you may be mistaken bro. The 47k resistors are plate loads. The standard PPIMV should replace the bias feed resistors. On that amp there is a bias balance control and the feed resistors are 47k and 68k and the balance control adjusts the leeway. Unless you've modded the bias circuit to bypass the balance control? But if you did the better choice would be 68k for the bias feed resistors. Aside from that I'm puzzled why you would want to isolate the preamp and PI distortion of a 135W UL Twin?!? I wouldn't think the preamp and PI distortion of that amp is worth isolating and it negates the point of that amp. Sort of like lowering a pickup truck. Which people do, I just don't know why.

[Zouto]

I think you may be mistaken bro. The 47k resistors are plate loads. The standard PPIMV should replace the bias feed resistors. On that amp there is a bias balance control and the feed resistors are 47k and 68k and the balance control adjusts the leeway. Unless you've modded the bias circuit to bypass the balance control? But if you did the better choice would be 68k for the bias feed resistors. Aside from that I'm puzzled why you would want to isolate the preamp and PI distortion of a 135W UL Twin?!? I wouldn't think the preamp and PI distortion of that amp is worth isolating and it negates the point of that amp. Sort of like lowering a pickup truck. Which people do, I just don't know why.

Thanks Chuck, you're absolutely right, I'll leave the original MV circuit well enough alone and keep on restoring that Twin. I'll remove the pull pot scratch distortion circuit and use the switching function for some other stuff.

[pdf64]

On the other hand, the stock master volume doesn't give a particularly pleasant tone (though it may help if its bright cap is removed).
The Type 2 master that Zouto was proposing doesn't integrate very well with the bias balance system.
However the Type 3 may be more suitable, allowing more gain then the stock (Type 4) master and possibly providing a better tone.
See http://www.blueguitar.org/new/schem/...reck_pages.pdf page 43
Pete

EDIT - A 250k audio taper pot may work better than the 1M suggested in the Trainwreck Pages, due to the overall lower impedances with the SF phase splitter.

[Chuck H]

On the other hand, the stock master volume doesn't give a particularly pleasant tone (though it may help if its bright cap is removed).
The Type 2 master that Zouto was proposing doesn't integrate very well with the bias balance system.
However the Type 3 may be more suitable, allowing more gain then the stock (Type 4) master and possibly providing a better tone.

Good point. There IS already a master volume. Why not?

[Zouto]

Thanks Pete, I used Type 3 with 500K A pot for a VOX AC30 and a SF Bassman 50 successfully, so it also might be a good idea for the Twin135. The stock master volumes gives a rather thin tone, even with the cap removed. As soon as the amp is running again, I'll give it a try. Another mod will be to make the NFB switchable, but it will add some hiss when removed.

[jimboyogi]

I don't know what MV 'type' numbers refer to.

But WRT making the NFB switchable, if you install a post PI MV, then the MV is inside the NFB loop.
So when you turn the MV down, you are also reducing the NFB. At the same time you are also reducing the signal voltage that is transferred from the PI to the output tubes. So as hiss increases in the PI due to reduced NFB, the signal from the PI is attenuated, which reduces the hiss transferred to the output!

My point is that using a post PI MV may give you both elements that you are seeking at the same time - possibly better sounding pre-amp/PI distortion (may need to play with PI component values to fine tune), and variable NFB.

[Chuck H]

Also... The actual hiss frequency isn't that useful for guitar. A small cap value (like .0022uf) could be added in series with the FB loop instead of disconnecting it completely. Notice that the NFB loop currently has a .01 cap bypassing the loop series resistor. Maybe the top end is down right unpleasant or unstable without it. Keeping some high top end NFB might be the best way to go for this amp.

[Zouto]

I don't know what MV 'type' numbers refer to.

The numbers refer to http://www.blueguitar.org/new/schem/...reck_pages.pdf. It's the so-called cross line MV.

[jimboyogi]

Thanks Zouto, I just looked up the Trainwreck pages and found the numbered MV systems. #2 is the one I was thinking of when I posted above, and #1 would behave similarly.

[pdf64]

Type 2 (and 1) has the potential for the wiper to lose contact with the track, resulting in the loss of bias for the power tube.
To mitigate for this, a coupe of amendments have been suggested; the 'Lar-Mar' has a couple of additional fixed resistors, eg 2M2, between the bias voltage and the wiper. The 'Chuck H' has the pots turned around, 5E3 style, so that the phase splitter plates connect to wipers, the power tube grids are fed from the track ends.
Pete

[Chuck H]

The 'Chuck H' has the pots turned around, 5E3 style, so that the phase splitter plates connect to wipers, the power tube grids are fed from the track ends.

I just offered it as a solution for someone that was using a bias wiggle trem. They reported that it worked well. So I suppose it's an alternative to having the bias depend on the wiper. I've never actually wired one up this way.

[isaac]

When I worked on one of these 135W twins, I made the preamp blackface COMPLETELY. I then proceeded to remove all pull boost and master volume circuitry, I guess that goes with the preamp but just for clarification... But I didn't touch anything after that. So PREAMP and MV +pull boost, black face that. It involves removing a bunch of parts and some wires and I think adding one jumper wire where the MV would have been in the circuit. Don't forget the Reverb circuit too, it has some changes. BUT IT SOUNDED FANTASTIC.

Although this post doesn't really have quite the same answers to your problems, I say these things to everyone that has these amps. It works well, and sounds great. Don't let the UL drive you off, of course you already have one... But they sound great, when They have blackface parts. Now you could also add a normal bias supply, and change the 4 or 5 resistors in the PI to blackface spec, but I loved my twin, it sounded so good. The reverb was lush and beautiful, and the tremolo was great. the clean tone.... everything.

[isaac]

I should still have the schematic of what I did, if you'd like it.