Bias to high

[pnut5150]

I have a Fender Hot Rod Deville here, the tubes that were in it was biased cold, around 20mv, the highest I could get them was around 45mv. It has been pretty common for the same power tube on this amp to die out before the other. I installed two new Tung-Sol tubes in it today, with the bias wheel set to lowest point, it is around 103mv, highest bias setting is 150mv. Of course at the lowest level you can see tubes trying to red plate. Physically the plate and grid resistors look fine. I have been in these amps before and resistors show signs of heat. Where should I start here? Why the high bias readings?

[The Dude]

Physically the plate and grid resistors look fine.

Have you actually checked them with a meter? My bet is a screen resistor gone towards open. If you have a bias probe, check current for each tube. Maybe one of them is not conducting or one is conducting much more than the other.

Also resolder the output tube sockets if you haven't. A notorious problem with these.

[pdf64]

30 watt tubes shouldn't red plate under static conditions of ~25 watt plate dissipation (0.103/2 x 480), so as per previous, are you sure that both tubes are redplating?
I think that a blown screen grid resistor would put a tube into cut off, but perhaps that's the point.
In addition to previous, consider retensioning the power tube sockets
Check the 1 ohm current sensing resistor R66.
With no power tubes in place, check the voltage at terminals 3,4 and 5.
If all good and problems persist, check the voltage across C43 and if ok, try some different, known good tubes.
http://support.fender.com/schematics..._schematic.pdf

[Mick Bailey]

Quite a few tubes, but Tung Sols in particular, are difficult to bias in these amps due to high current draw. I usually snip R83 and in some cases increase R77 to 120k or 150k. Usually just removing R83 does it.

The bias reading is of course the sum of both tubes. If the screen resistors are OK measure each tube individually for idle dissipation (transformer method). They may be matched, but that doesn't mean to say they will pull the same when installed. Check the socket solder pads for cracks too - they very often fail.

Don't forget that the cathode current includes the screen as well as the plate current, so the plate dissipation is slightly lower if you subtract the screen current.

[pnut5150]

Thanks for the info guys, Ill check it out and let you know what I find.

[Enzo]

Are yu taking this reading at the test point? That reading is for two tubes.

If you are getting a way high reading, try pulling one tube and reading and then try just the other. You may find that most o fthe high current is caused by just one of the tubes or by just one socket.

Also, on this series of amps, always check the solder on the power tube sockets VERY closely. REsolder any cracks.