Just fired up an early AC15 build and need to add a PPIMV

[mort]

I see commonly people use a dual gang pot but I've also read that it's not necessary. Anyone care to nudge me in the right direction here.. what's the best way to implement?

[mort]

Do I simply add a dual pot where indicated by the orange boxes on the drawing?


This is one example I found of the PPIMV implementation. In my case I don't think I should be removing any resistors though, only adding a pot and the 2.2m resistors on the pot and the shielded wire.

[pdf64]

The tracks of the dual pot replaced the 220k power tube grid leaks, R53 and R54; the 2M2 resistors ensure a path for grid leakage current should the regular circuit via the pot tracks fail, eg a pot terminal becomes loose.
But if you're concerned, then there's no harm really in leaving R53 and R54 in circuit somehow, one side of the pot or the other; they would just affect things such that it could no longer be said that the master was effectively 'out of circuit' when maxxed.

'illustration by rockstah' - I think he died a year or three back; so it's nice for some of his work at least to live on.

[mort]

ok I see.. thanks. So it should look like this then?

[pdf64]

No, as the control would have negligible effect.
The output from the pots to be taken from their wipers.

[mort]

oops please allow me to take a mulligan on that one for reason of temporary insanity.


So it should look like this then...

[pdf64]

That's it!
If you'll mainly be using the master to get domestic level volumes, then audio taper tracks may work better for you than linear.

[mort]

ah ok thank on the taper suggestion. I understand what the difference is between the two types of taper, but is there a rule of thumb or otherwise easy way to figure which would work better for various applications used with tube guitar amps? I've read that an audio taper can have too fast an effect at a certain point in the pot, but that's what's usually spec'ed for a volume control... so I'm not sure I understand.

[mhuss]

The change is resistance in a log taper pot (very) roughly corresponds to how our ears perceive volume: in a logarithmic way. This is why almost all volume controls use log taper pots. Linear taper pots are better for cases where you want a fairly consistent voltage divider ratio end-to-end, e.g., in some tone controls.

[olddawg]

In my experience with these types of amps..... An output attenuator like a Weber Mini mass works much better in application. Just a thought if you don't get what you want out of it.

[teemuk]

Ok, there are at least two PPIMV implementation schemes, one of them (along with its variations) has been discussed here already, the second one is right there in front of your eyes and labelled "Top Cut". It controls the degree in which inverted and non-inverted output signals are mixed together and thus signal amplitude at each output. Due to way its wired it has a nickname "cross-line". Don't let the point that the control is initially designed to affect upper frequencies only fool you, you can bypass that. It's done by by either shorting the coupling cap wired in series with the potentiometer or by increasing capacitance of the said capacitor so that it introduces low impedance for low frequencies as well as for highs. You can obviously wire different kind of switching variations to achieve either goal.

e.g.
http://www.trinityamps.com/Trinity15schematicrev10.gif


There are differences between these two schemes:

The dual potentiometer scheme (assuming potentiometers are high impedance) presents a rather constant and high load impedance for the phase inverter stage. Effectively at all settings of the control the PI stage's clipping thresholds for positive and negative half waves remain soundly in those defined by stage's B+ supply voltage and stage bias. Input signal's amplitude controls amount of overdrive and the potentiometer works as an attenuator only for the PI stage's output signal.

The "cross-line" phase inverter, however, presents a constantly decreasing load impedance for the phase inverter when you turn it towards the attenuating end of the dial. To retain correct output signal amplitude to decreased load impedance the phase inverter stage draws higher current but eventually it runs to its limits and starts to clip the output signal. The clipping thresholds are therefore effectively current-defined, as well as clipping characteristics. Clipping to stage's supply current limit may sound entirely different than clipping to stage's supply voltage limit, and usually does. In essence the control is therefore not a simple attenuator like in previously discussed scheme but has great effect to overall level and threshold of PI stage's overdrive. This characteristic can be circumvented by wiring high-ish impedance series resistors to each PI output. Those will introduce a limit for the load impedance of the PI at extreme settings of the dial and ease up the job of the phase inverter stage.

[Matt R]

Have you considered installing a VVR instead?

[mort]

quick question, in the drawing attached the first post, on the top cut pot, should the wiper be tied to the .005 cap or the .01 cap? Or does it not matter..