JJ 6CA7 a better option for 6L6 amps w/ excessive B+?

[tubeswell]

Edit - Ulysses, that's a weird problem that I'm nowhere near equipped to figure out... But bias creep always wigs me out.

Its related to tube current (which changes as the tube gets hotter). I.e. when you dime the amp, tube current goes up and everything heats up, and when you stop it takes a while to cool down. Bias is affected by tube current, which is affected by screen voltage. If you take a Marshall Plexi and increase the screen voltage, but keep the bias voltage the same, the tube current will increase. If you decrease the screen voltage below a certain point e.g. below 250V, it takes a heck of a lot less bias voltage to get the tube to conduct.

When biasing up a new quad of EL34s in a 1959 plexi, a practical way to see if tubes are going to match is to temporarily decrease Vg2 to a workable voltage e.g. 350ish, and set the bias voltage to about -45, and measure the tube current in each tube. These approximate real life conditions (Plates at 515ish, but with screens at a safe operating voltage). You will get low tube current (10-12mA) in good tubes but an increase of at least 10mA or higher (e.g. 30-35mA) in the tubes that are going to redplate in that amp.

(Edit: FWIW, all it takes to lower Vg2 to 350ish in a plexi is to disconnect the screens from the screen supply node, and hook them up to the middle of the screen supply filter cap stack (which is normally at 1/2 of the screen HT voltage) but you also need to swap out the lower 56k balancing resistor with something like 270k to 330k, which sets this voltage at about 350-380*. If you leave this node at 250ish, you'll find the tubes won't conduct with the bias voltage set at -45. So its one resistor swap and moving the screen supply wire for this test. When you flick the standby on, if you have everything metered, you'll see the Vg2 gradually rise up to the peak point, and you'll see the tube current rise at the same time.

* also make sure your filter caps are rated to take this voltage change)

[Old Tele man]

Hey, tubeswell, I see that you too are well acquainted with "working the B+ ladder points" of an amp (ha,ha)!

[tubeswell]

Every day is a ladder best climbed one-step at a time.