resistor instead of potontemeter

[andrew]

Hi all, I would like to replace a 250k MV pot with a resistor since I will be using a PPIMV. Any help please? Thanks

[Bob S]

Wire in a 250k pot temporary - set it where it works best for you & measure each half. Then replace with a 2 resistor divider.
Or leave both in.
The PPIMV will make it sound different, so your goto settings will change.
Also if you have regular feedback/presence it will be MIA with a PPIMV.
Good Luck.

[tele_player]

Also if you have regular feedback/presence it will be MIA with a PPIMV.

Why?

[Colossal]

Also if you have regular feedback/presence it will be MIA with a PPIMV.

Why?

The PPIMV functions to reduce volume by reducing the amplitude of the signal reaching the power tubes from the preamp and phase inverter (if present). It replaces the fixed resistors (commonly referred to as grid leaks) which keep the grid from succumbing to thermal runaway by providing a path to ground. The RC time constant provided by the phase inverter coupling cap (C) and grid leak (R) also determine the rate of recovery from bias excursion.

As you turn the PPIMV down, then the power amp sees less signal input and as a consequence, produces less power. If the magnitude of the signal coming out of the secondary side of the output transformer gets smaller, then there is less signal going back into the preamp in the form of feedback.

Therefore the properties imparted by the negative feedback loop are reduced as the PPIMV is turned down.

[Reeltarded]

Defeated NFB is wild and wooly.

Raw.

[tele_player]

Also if you have regular feedback/presence it will be MIA with a PPIMV.

Why?

The PPIMV functions to reduce volume by reducing the amplitude of the signal reaching the power tubes from the preamp and phase inverter (if present). It replaces the fixed resistors (commonly referred to as grid leaks) which keep the grid from succumbing to thermal runaway by providing a path to ground. The RC time constant provided by the phase inverter coupling cap (C) and grid leak (R) also determine the rate of recovery from bias excursion.

As you turn the PPIMV down, then the power amp sees less signal input and as a consequence, produces less power. If the magnitude of the signal coming out of the secondary side of the output transformer gets smaller, then there is less signal going back into the preamp in the form of feedback.

Therefore the properties imparted by the negative feedback loop are reduced as the PPIMV is turned down.

I respectfully disagree.

The main purpose of NFB running from the OPT secondary to the PI input is to reduce distortion. That isn't changed by placing a MV between the PI and the power tubes, since this is still 'enclosed' within the feedback loop.

Yes, the feedback voltage is reduced by the MV, but so is the signal, by the same proportion. The NFB continues to operate to reduce distortion. In fact, if the intention is to get distortion from the PI itself by driving it with a hotter preamp signal, the NFB will work against that.

Robert

[martin manning]

Yes, the feedback voltage is reduced by the MV, but so is the signal, by the same proportion.

The preamp output/power amp input signal is not affected by the PPIMV, but the output signal at the speaker is. As the PPIMV is turned down (reducing the open loop gain of the power amp), the gain reduction from open loop to closed loop will be reduced, or in other words, the feedback is less effective. To compensate, a proportional increase in the feedback factor (an adjustment to the FB resistor/PI tail resistor voltage divider) would be required.

[tele_player]

Fair enough, though I think it might matter more on a presence control than a straight NFB. Guitar players LIKE distortion, that's a big reason they like to crank the pre and lower the master.

Robert

[martin manning]

Feedback has multiple effects including linearization, increased bandwidth, increased damping, and frequency response shaping via presence, all of which are diminished as the PPIMV is turned down.

[tubeswell]

Hi all, I would like to replace a 250k MV pot with a resistor since I will be using a PPIMV. Any help please? Thanks

Do you mean that you want to take an existing PPIMV pot out and put a resistor in its place?

Or do you want to replace an existing grid leak resistor with a 250k pot for PPIMV?

[Structo]

Defeated NFB is wild and wooly.

Raw.

Yep, on my D clone amp, if I have to play at low volumes due to anybody objecting (wife), I have a switched pot for my presence, if I rotate it fully counterclockwise, it switches the NFB off, for a real raw tone I like.

Wild and woolly is a good description.

[Smokebreak]

Hi all, I would like to replace a 250k MV pot with a resistor since I will be using a PPIMV. Any help please? Thanks

Do you mean that you want to take an existing PPIMV pot out and put a resistor in its place?

Or do you want to replace an existing grid leak resistor with a 250k pot for PPIMV?

I assumed he meant he had a "normal" MV, 250K, and then wanted to use the panel hole/space for a PPIMV. If that's the case, a 250K to ground in place of the old MV would work, or just put the PPIMV on the back.

[andrew]

I am building an early 70's Hiwatt. The schematic has a MV but I thought I would not be needing it since I put a PPIMV in. I'm going to take Bob S's advice and wire up the MV pot and I find the setting I like then measure and sub in the resistors. Now the MV is a mini sized pot on a terminal near a preamp tube. I may just leave it there. Thanks all