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Actually, it's probably more because your B+ & screen supply aren't regulated. Change the bias current, the B+ changes, which also causes a change in screen supply voltage, which makes the other channel move...

It'll be a little harder with 4 pots, but I think it's worth the extra effort. You'll never find tubes that are matched that closely, and they'll age differently even if they're closely matched when new...

Roscoe

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I can explain it to you, but I can’t understand it for you.

this makes life easy but at a cost
http://www.ebay.com/itm/Quad-Tube-Amp-Bias-match-Meter-6L6-El34-KT88-6BQ5-Probe-/291765592401?hash=item43ee95a151:g:AWkAAOSwZd1VcVnP

I am building one using a kit for the socket part from tube depot

That makes a lot of sense. I actually have another set of pots that sit between the grids of the output tube pairs and have the wipers connected to ground. These allow me to adjust the bias difference between the tube pairs. So I guess I really do have a 4 pot system. I just stupidly made these pots really hard/dangerous to access. I'll have to think of a way to address this.

That's really cool!

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Walt

Justin,

To expand a bit on Roscoe's wise post, the change in B+ per tube, when another tube is being adjusted is a reflection of your power supply's impedance. The lower the impedance, the less movement of B+ and screen voltage when load on the supply (current) changes. The better the power supply transformer, the better your results here.

As a potential fix, you can regulate the screen grid supply, one regulator will do. A simple Maida regulator works just fine. Or the "regulated screen" supply on your schematic.

The elephant in the room is the supply voltage to the sensitive input stages. A strong suggestion for a better sounding amp, is to regulate both the screens and input stages. A nice, low impedance regulator can supply all of those loads, just include a bit of decoupling (RC networks) to minimize interaction.

Once the screen regulator is in place, output tube idle current will not change very much, even with a plate voltage change. Regulating the screens won't change the fact that B+ drops with an increase in plate current, it just makes the tube less sensitive to that variation.

Now, think about how much B+ varies when you are playing music! You don't want that B+ variation to modulate the input stages' signal. Three ways to fix it come to mind: 1) separate power supply, including transformer, for the input stages; a little too late for that one; 2) regulating the input stage power supply as previously discussed, or; 3) providing high impedance plate loads for the voltage gain stages. The high impedance plate loads can be chokes (simple, but space consuming and expensive), or solid state (a little more complex but small and if the current x voltage dropped is low enough, not requiring a heat sink). Poor SS regulators won't sound good. Good ones won't affect the sound in a bad way.

If you are interested, look up Guy Riffle's PCB for Cascoded Constant Current Sources. Simple and compact, they work great. The design was Pete Millet's IIRC.

Personally, the choice suggested is regulate the voltage to the input and screen stages.

Others may reasonably disagree.

Stuart

Thank you, Stuart. Thanks for going so far as to look up my schematic! I am actually using the "regulated screens option." However it's set by a resistor divider off the B+, so it fluctuates a good bit with B+.

The changes you suggest sound good but seem like a lot of work just to set the bias precisely. I'm generally happy with the way the amp sounds. I'll probably sour on it one day but right now I'm not really in the mood for doing a major overhaul.

These test sets are at all price points and there are a bunch at eBay
Some just measure bias and some do plate voltage also

Justin,

As I recall, two of your pots were for ac gain balance, and two were for bias (dual-channel amp). As Roscoe mentioned, the screen voltage changing with bias current is the strongest effect with one bias pot setting affecting the other. Short of regulating the screen, you simply have to keep going back and forth between the bias pots and eventually they will all be OK.
David

That's correct! Thanks, David. I was just trying to figure out how close they need to be. Sounds like I should shoot for as close as possible!

I'm noticing that the bias seems to be a little different each time I power up the amp. Maybe this is because I am not waiting for the amp to warm up sufficiently before setting the bias?