65 Blackface Fender Showman PPIMV

[tomcheck1212]

Hey Guys,

I'd like to implement a Post Phase Inverter Master Volume in a 65 Fender Showman.

I read quite a bit about PPIMVs and found in conclusion two designs which i'd like to put in a discussion with you.

First design is the I think the most common and known LAMAR PPIMV:



everything fine for me so far but I was wondering about the grid resitors (220k, see the attached aa763 schematic showman_aa763.pdf) over them the bias voltage (-52V) is delivered to the grids of the power tubes. If one would zero the master volume they seams to kind of disappear in the circuit. Does'nt that harm the amp in any way? Or the Tremolo? The bias voltage comes from the same supply the tremolo circuit seems to be refferenced to..

The second design from Chuck I found here in the forum (Thanks Chuck!!) flips the pots;



makes perfectly sence in my eyes because the pots in parallel with 2Meg2 resistors keeps up the 220k grid resistors all of the time. But what keeps me thinking is what happens to the phase inverter signals which seams to be shorted against each other when the master volume is set to zero. Wouldn't that be problematic to the phase inverter tube in terms of high currents or something?

Hope you can help me. And let me know if I forgot or beein completly wrong with something.

Thanks in advanced!

Thomas

[Chuck H]

Hello Thomas and welcome.

A couple of things to cover...

The original LARMAR circuit won't affect the trem on that amp. It's only an issue with amps that use bias tremolo on the power tubes by modulating the power tube grid voltage. Changing the arrangement as I did prevents changing the values for that sort of trem circuit when the master volume is used. I really only came up with this to help someone with that specific problem, BUT...

A greater advantage is that the different arrangement keeps the tube bias from relying on the pot wipers to stay on the power tube grids. Most pot failures involve the wiper and a loss of bias is a dangerous condition. The 2.2M parallel resistors help, but I still like keeping the bias off the wipers.

The lowest setting of the master volume in the circuit I proposed SHOULD meet the PI outputs at either ground (0V, so no shared anything) or at the output of a relatively low impedance bias circuit. Since there is (ideally) no DC on the PI outputs and the bias circuit (ideally) is filtered for AC below 60Hz there shouldn't be any interaction. Also, there is (ideally) no significant current on the PI outputs. The circuit acts as a voltage amplifier rather than a power amplifier. Even if there is a small amount of interaction it's no great shakes. In fact there is another PPIMV design that is simply a pot across the PI outputs to intentionally force interaction and therefor phase cancellation rather than referencing the signal to 0V.

There's no reason I know of not to try either of the schematics you posted. I think someone here mentioned they though my modified circuit might exhibit some less than desirable tonal effects, but I don't know why and no one that's used it has reported to that affect. The truth is that I've never tried either.

EDIT: And sorry for the late response. I've been much less frequent on the forum for a few weeks.

[Justin Thomas]

Dang, I remember seeing this a while back... I'll leave Chuck and others to the technical side, but I will say this:

Please, no drilling/filing/etc. on this classic amp! Unless it's already completely trashed, please don't drill any extra holes... I understand WHY you'd want to put an MV in this amp, but <please> give careful consideration to <how> you will do so.

I respect the fact that it is yours to do with as you please, though. So consider this my "getting on my knees and groveling..."

Justin

[Leo_Gnardo]

Dang, I remember seeing this a while back... I'll leave Chuck and others to the technical side, but I will say this:

Please, no drilling/filing/etc. on this classic amp! Unless it's already completely trashed, please don't drill any extra holes... I understand WHY you'd want to put an MV in this amp, but <please> give careful consideration to <how> you will do so.

I respect the fact that it is yours to do with as you please, though. So consider this my "getting on my knees and groveling..."

Justin

I second that emotion. Considering the popularity of master volume Twins, and how well (NOT) the MV works on them, why put a MV on an amp that's near-identical.

If you're after an overdrive tone, you're much better off pursuing it with a pedal.

If you're after an overall volume reduction, consider half power AND triode-mode.

What is it you're after, after all? Before you ventilate the chassis with a hole for your MV.

The Showman, for all its power, is a low gain amp. Works well for bass guitar, loud clean guitar, pedal steel, like that. An MV of any sort isn't going to send it to overdrive land.

[Jazz P Bass]

Not too sure what the OP is after with the MV.

Nothing to be gained with this amp.

[Tom Phillips]

I second that emotion. Considering the popularity of master volume Twins, and how well (NOT) the MV works on them, why put a MV on an amp that's near-identical.

If you're after an overdrive tone, you're much better off pursuing it with a pedal...The Showman, for all its power, is a low gain amp. Works well for bass guitar, loud clean guitar, pedal steel, like that. An MV of any sort isn't going to send it to overdrive land....

I'll put my vote with that. The showman just doesn't have enough front end gain to benefit from a MV.

Not too sure what the OP is after with the MV. Nothing to be gained with this amp.

Agreed again.

[Chuck H]

Well I think that's why the OP is going for the PPIMV rather than behind the PI. You can't get much preamp clipping, but you CAN get a lot of PI clipping from the circuit. IMHO it's not very good in high doses. A little crunch is ok, but once you get into heavy clipping the anomalous PI clipped wave form (that is usually pinched out by power tube clipping) becomes audible and isn't very musical. But different strokes and all.

As to cutting holes in that amp... Yeah, that's a bad idea unless there are already extra holes in it. Even if it's trashed it can be "restored" and it does qualify as a classic. They're never going to make more of those, yada, yada, yada...

Y'all know I'm a proponent of attenuators. And that's how I'd go for an amp like this.

From what I've read here it's not uncommon for vintage amps to become unstable with the addition of the LARMAR type master volume. This is almost certainly due to lead dress, grounding, etc. So it might be better to install it on the BACK panel of the amp anyway. There are a couple of ways to do this on a black face chassis without cutting extra holes.