Plexi Channel Bleed

[NoisyVia]

If you're plugged into a Bright input on a plexi without jumpering, is the bleeding between channels just about the fact that there's signal flowing through the Normal channel mixer resistor to the Volume II pot?

Is it correct that I think that when I turn up Volume II, i'm increasing the resistance to ground and therefore the level into the CF and also raising the impedance seen by the Bright channel plate, which creates the slightly warmer and gainer sound? Is there any contribution from the Normal channel V1A plate?

[pdf64]

It shouldn’t happen, that it does is probably just due to signal coupling via parastic capacitances.

[pompeiisneaks]

wouldn't it happen if the pot doesn't go to 0 resistance on the bottom leg? Therefore 'some' portion of the signal still bleeds through? I know I've heard of this phenomenon before. You can easily test it by testing the pot's resistance to ground from the wiper at 0 volume.

~Phil

[sluckey]

wouldn't it happen if the pot doesn't go to 0 resistance on the bottom leg? Therefore 'some' portion of the signal still bleeds through? I know I've heard of this phenomenon before. You can easily test it by testing the pot's resistance to ground from the wiper at 0 volume.

Yes. That's just the nature of using resistors for a passive mixer.

[pdf64]

wouldn't it happen if the pot doesn't go to 0 resistance on the bottom leg? Therefore 'some' portion of the signal still bleeds through? I know I've heard of this phenomenon before. You can easily test it by testing the pot's resistance to ground from the wiper at 0 volume.

Certainly that can cause it, or be a contributory factor.
But sometimes the wipers can be hardwired to 0V yet the signal still bleed through.

[NoisyVia]

Just so i'm clear, when we're talking about a non-jumpered situation any bleed through from the unused channel is sourcing from parasitic capacitances within the tube and it's getting through because the pot wiper may not go all the way to 0ohm?

[sluckey]

Just to be clear, describe what you are hearing that makes you think something in the unused channel is bleeding into the used channel. Or is this just a theory discussion?

[NoisyVia]

Warming of the tone, fills it out a bit. It's less "in your face", which sometimes is a good thing.

[Roe]

its not bleeding but you're changing the voltage divider (the mixer resistors) when you adjust the volume of the unused channel.

To simplify matters slightly, consider a bassman or jtm without the 500pf bright cap across the 270k mixer resistor. plug into one channel and watch the voltage gain increase as you turn the unused volume from 0 to 7, since the voltage divider of the channel mixer changes from 270k/270k to 270/(270k+500k)

[pdf64]

Sorry, I think I may have read more into post #1 than was actually there; what I understood 'bleeding' to mean was, from the scenario in post #1, the signal that may manifest and be controllable by vol 2 when vol 1 is turned down to minimum.

[wpaulvogel]

I’m just wondering how or why it’s a concern. We’re talking about a Plexi. We play Plexi’s with both volumes turned up. We don’t turn on a plexi, turn the volumes to zero, plug in our guitar and play air guitar.

[Roe]

Is it correct that I think that when I turn up Volume II, i'm increasing the resistance to ground and therefore the level into the CF and also raising the impedance seen by the Bright channel plate, which creates the slightly warmer and gainer sound? Is there any contribution from the Normal channel V1A plate?

yes, you're changing the voltage divider of the mixer resistor. You get max gain by cranking the brilliant channel and setting the normal channel at ca 7 with a audio taper pot (the latter will be about midpoint then, with 500k to ground and 500k to the AC signal of the normal channel. the latter works as a virtual ground basically. This is why you loose gain by turning the volume from 7 to 10 on the normal channel)

[NoisyVia]

Thanks Roe that's exactly what I was looking for. Sorry I've been busy and haven't checked back everyday. To answer the questions on why I asked, the practical use here is I'm putting together a 3 channel schematic with one of them being a Plexi channel; in this case I'm only going to use one triode of the first 12ax7 and therefore need to hardwire the normal channel volume to what I prefer during test. Just wanted to be sure I understood the change.

Cheers