DR201 Housekeeping

[theSeeker]

Everything is running clean and quiet, but here are a few questions about my '78 DR201 (6 x EL34).

1. I have a new set of winged C EL34's and after moving them around to balance side to side, I have measured the following:

Plate voltage: 675 Bias Voltage: -37

Left: 32.3, 35.1, 32.9 mA (delta V 1.774/20.2 ohms = 84.0 mA)
Right: 35.1, 35.3, 26.0 mA (delta V 1.679/20.0 ohms = 87.8 mA)

I think the sides are balanced the best I can, my question is should I bother to change the bias resistor or let 'em run hot?

2. My screen grid resistors are all discolored like this:

IMG_0952a.jpg (290.09 KiB) Viewed 2653 times

Should I replace them?

3. It's hard to photograph, but all the caps have a little bump as shown on the center one here:

IMG_0958a.jpg (342.31 KiB) Viewed 2653 times

It is in the exact same place on every one. Do they need replacement?

[mhuss]

Too hot. I would much prefer if these ran closer to 25mA.

As long as the screen resistors still measure 1k +/-5% they should be okay.

The electrolytic cans need to be replaced.

--mark

[theSeeker]

Thanks Mark, that leads me to the next question. The bias resistor is 22k right now. I have values of 24k, 27k, etc in increments of 3k. I would like to avoid solder, test, solder, test, so in your experience can you tell what a good increment would be to get close with the operating parameters above?

[mhuss]

I take it you have the separate bias winding and two parallel bias trimmers in series with the 22k? In this case it's more of a winding limitation than a resistor value. I'd try something way high (say 100k) and see where that puts you in terms of maximum negative voltage.

--mark

[theSeeker]

No this one has the alternate configuration, still a separate bias winding but fixed. I see your diagram shows -46V, I only measure -37V right now, so you think it will take a large change in resistance to bring it in?

[mhuss]

Probably. You could also try the 1M pot in series that people have used with success on the fixed bias network DR103s.

--mark

[theSeeker]

I need clarification in the values I posted above measured the cathode current, and by measuring plate current through voltage drop the average is ~3-5 milliamps lower, which I understand is screen current.

Plate voltage: 675 Bias Voltage: -37

Left: 32.3, 35.1, 32.9 mA (delta V 1.774/20.2 ohms = 84.0 mA)
Right: 35.1, 35.3, 26.0 mA (delta V 1.679/20.0 ohms = 87.8 mA)

So the calculation I am shooting for 0.7*25/675=26mA. This is plate current only right? And I should expect the individual cathode currents to be in the neighborhood of 30mA?

[mhuss]

Yes.

--mark

[theSeeker]

Got it, one 27k resistor and she's idling happily. It sounds awesome, and LOUD!

IMG_0992a.jpg (84.13 KiB) Viewed 2576 times

[mhuss]

Nice.

--mark