PPIMV after a split-load PI would be pointless right?

[Iplayloud]

Having installed PPIMVs in a few amps lately with cool results I was eyeballing my AA1164 Princeton Reverb clone. But it has a split-load PI and I hear they have a vely low gain and don't really distort but only pass on the signal?

[BosAmps]

Even if they have unity gain or less, it might be worth considering, because a well designed split-load PI does have a nice sound of its own when it is working on decent signal-levels.. Give it a shot and find out if it works for you, otherwise you can always use one half of the stereo pot to make an ordinairy pre-PI mastervolume..

[Iplayloud]

Ok great info thx, one last thing.. if I put a regular 1Meg MV instead, would it replace the 1M resistor just before the PI? I never really understood if it was connected to the 56k for convenience being an almost-ground or if it really had any business in the PI (between the 1k and 56k).

[BosAmps]

-You should not replace the 1MΩ resistor, connected between the 56K and 1K resistors in the cathode of the PI. The 1MΩ resistor is your grid-leak resistor for the PI, which is a part of the tube's bias-circuit. Kind of essential part to make the tube work. In an ordinairy preampstage you can replace the gridleak-resistor with a potentiometer, because there is always a high resistance-path (1MΩ regardless of the volume being 'on or off') between the cathode and grid, through the groundpath, so the tube's grid can get it's bias-voltage (≈1,3V more or less depending on the circuit). In case of the PI's 1MΩ resistor, if you replace it with a volume-pot (which can be wired in several ways) you mess up the returnpath of the biasvoltage (coming of the tap between the 1K and 56K resistors) back to the grid of the PI's grid and so you mess up the bias-point on which the tube operates. And you will be routing your tube's inputsignal (normally on the grid) to both the grid and the 1K/56K point on the cathode.. Don't know what that will sound like.. And there are other ways of messing it up.. So better don't..
-If you want to use a regular mastervolume I would put it in front of the 1MΩ resistor and wire it like an ordinaiy gain/volume-pot.
-Or perhaps to the previous 12AX7, right after the returnpoint of the reverb-circuit. Be sure to connect the ground-wire of your volumepot to an actual groundpoint close to where the 47Ω resistor is connected to ground OR connect it to the grounding point of the 100K-reverb-level pot, so you make an easy/short returnpath for the biasvoltage from ground back to the grid. In the original circuit there is no gridleak-resistor and the tube's grid probably gets is biasvoltage from the cathode through ground through the reverb-circuit back up to the grid..

I hope it's of any help!! Good luck!

[BosAmps]

Small rectification:
In the way I wrote my former reply, you can make up of it, that the biasvoltage on your cathode is somehow also 'appearing' in the grids of the tube.. This of course is not true.. The 1MΩ resistors are only for ground reference of the grid, so the grid is '0 volts' and the cathode-bias-voltage is 'X volts' with respect to the same '0 volts' as your grid... I 'think' it is wise to fysically connect your 1MΩ-ground-reference resistor close to your cathode-bias-resistor (and bypass-capacitor) to minimalise the amount of interference caused by other (heavier) currents in your grounding-circuit.

sorry for the mix-up and unintended confusion..

[potatofarmer]

I haven't tried a PPIMV after a cathodyne, but I'm not sure I see the point. Which master volume were you considering?

I have replaced the cathodyne's grid leak with a pot; that works fairly well.

The cathodyne does have very low gain (typically a hair or two under unity) but it does distort when overdriven. It's typically characterized as a nasty sounding buzzy farty mess, though with tweaking supposedly you can dial in a little bit of 'swirl.' I have never managed to pull that off myself, so I went with Merlin's suggestions to minimize the bad overdrive sounds: The Valve Wizard

Actually that page should answer all your questions better than I could, so make with the clicky.

[BosAmps]

Hai Potatofarmer!

When replacing the gridleak-resistor with a pot, how did you wire it? Wiper to grid, just like a normal gain/volume pot?
Not to critize you, but I just didn't see it at that time!! LOL

[potatofarmer]

Yep; wiper (to grid stopper) to grid. Top of the pot to the coupling cap, bottom of the pot to the intersection of Rb and Rk.

Though this pot will typically be inside the global NFB loop (if there is one). So you still get the disadvantages of a PPIMV.

I have the cathodyne pushing a pair of cathode-biased 7591s which are biased around 20-ish volts so it doesn't take a whole lot of voltage gain to get them to full output. The "master volume" turned out to be more of a "clipping balance" control, where above 12:00 the power amp would break up first and below it the preamp would break up first. Or something like that, it probably wasn't exactly at half of the pot's rotation. It was novel to have a sort of "headroom control" but not terribly useful.

[Iplayloud]

Thanks guys, invaluable info for me in tger
When the pot would be at zero, the 50v between the 56k and 1k would be direct on the grid, not a problem?
How about if I put a pot after the reverb return, should I replace the 3.3M/10pf and the resistor to ground with a pot? (mine has no reverb but has the 3 stages before the PI)

[Chuck H]

Maybe, since a master volume type circuit is less than ideal for this amp, you could break with your PPIMV trend and try something different. I have a simple attenuator circuit that I've installed in combos as an onboard unit by building it on a bent aluminum plate to recess it between the rear baffles. Works great and you'll get the WHOLE circuit going without any PI or NFB issues to solve for.

The resistors are those aluminum housed jobbies mounted to an aluminum surface with thermal paste.