Screen resistor voltage drop

[winterm]

Repairing a Fender Tremolux:

Rec'd with blown fuse. I replaced the fuse and found one of the power tubes was not glowing so I replaced it and also found that the 470 ohm screen resistor was burnt and replaced it also.

When I check the voltages on the tube, I see 440V on one side of the 470 ohm resistor and 340V on the other side which connects to pin 4 (screen). The other tube has 440V on both sides of the screen resistor which is what I would expect.

I swapped tubes but get the same results. I can't find any problems with the socket either.

[winterm]

recheck voltages

I'll recheck my voltages tonight because I should have some voltage drop on the good tube like about 5V.

[guitician]

Check the resistor values with meter. One may be mis-labeled.

[pdf64]

Voltage drop across 470 ohms will be minimal, < 1V, as the current will only be 1 to 2 mA, when there's no signal going through it.
You've either got a bad/wrong resistor on that screen, or the bias to that slot is screwy - maybe the 220k bias feed, or 1k5 grid stopper is bad. Check and compare voltages. Peter.

[JHow]

You don't have a 470K resistor in there by any chance?

[loudthud]

Make sure there is plate voltage on that tube. If the OT is open on one side, that tube will have crazy big screen current.

[winterm]

whoops

well, I'm embarassed because I did put a 470k in there instead of a 470 ohm.
That would explain why it didn't sound as loud as it should, I guess it would be operating in triode.

[JHow]

No need to be embarrassed. I made the same mistake last week - put a 560 ohm on an ef86 screen grid instead of 560k. To my eyes, orange, brown, red all look very similar on resistor color codes, especially if the resistor body is brown or tan. I try to measure before I install, but even that doesn't always work - if the meter gives you a reading like ".560k" for 560 ohms and you don't notice the decimel point.

What gave it away to me is that dropping 100 volts over 470 ohms would be .21 AMPs, not the .2 ma mentioned by the other poster. It suggested you were off by a factor of 1,000. Dropping 100 volts over 470,000, however, would be around .2ma, and since you didn't mention anything being on fire or smoking, I guessed your current draw was normal, but resistor might be off on the value.