trouble checking bias

[xtian]

I have this amp from a client. It burned up one of his NOS EL84s. I'm trying to check the bias. First I checked the cathode voltage (10v) and divided by the cathode resistor (120ohm) to get 84.75ma. Next I need to measure the voltage drop across the screen resistor. Looks to me like that's the 1.5Kohm resistors connected to pins 8 (unused) and 2 (screen).

When the tubes are removed, I can measure the screen resistor at 1.5Kohm. But with the tubes in and the amp running, not on standby, guitar playing works OK, I cannot read a voltage drop across these screen resistors! Just says 0.000v. What am I doing wrong?!?

[martin manning]

Next I need to measure the voltage drop across the screen resistor. Looks to me like that's the 1.5Kohm resistors connected to pins 8 (unused) and 2 (screen).

Those are grid stoppers, since pin 2 is the control grid. The screen is pin 9, and I can see the jumper from one socket to the other, and a wire over to one of the cap cans... looks like there are no screen resistors. Maybe adding some would prolong the life of the tubes?

[xtian]

Ah, yes. Teach me to work late at night.

I added a 4k7 5w shared screen resistor and was able to calculate plate dissipation at 19 watts. This confirms my suspicion that the tubes were burning way too hot.

The current cathode resistor is 120ohm. On hand, I have 220r, 360r, 430r, and 620r. What's my best first try?

If this was a linear equation, I'd say 19watts x 0.63 = 12, therefore 120r / 0.63 = 190r. So I'd try the 220r resistor.

But I'm guessing it's probably not linear…

[martin manning]

I agree of those you have the 220R is probably the best bet. I think your 4k7 shared screen resistor is too big, though. I'd use 470R to 1k. Try that and see what plate voltage you get, then you can tweak it if necessary.

[xtian]

Thanks, Martin!

I ended up using a 360r for the cathode resistor. Leaves me with 12watts on the money, using biasing calculations on these two pages:

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

http://www.webervst.com/tubes1/calcbias.htm