Hot Rod Deville Bias Runaway

[Telebuckers]

Hello again-
I've got a Hot Rod Deville here that's acting up. The 470ohm1w resistor in R61 was blown so v5 wasn't getting any bias- I replaced that and re-biased and the amp seemed to be fine. However after playing for about 5 minutes on the yellow channel it makes a horrible noise and the bias jumps up from 60mV to over 300mV or 25mA to 190mA. Throwing it back into standby and back will correct the problem for a few minutes. If anyone could give me a pointer or two on why this might be happening I'd appreciate it. here's a link to the schematic:

http://www.schematicheaven.com/fende...od_deville.pdf

Thanks.

[Enzo]

OK. The 470 ohm resistor R61 is a screen grid resistor. It has nothing to do with bias, sorry. It is however very common for the screen resistor to burn up when a power tube shorts out internally.

I would say you need a new pair of power tubes. The existing V5 is failing.

While you are inside, I suggest also that you inspect CLOSELY the solder on the pins of the two power tube sockets. Those like to crack and that causes various bad things.

[Telebuckers]

Well I'll have to double-check everything when I get a chance- but v5 didn't have any bias on it- and I found that resistor to be bad. When I changed it the bias came back- and I've tried two sets of tubes with the same result.

[Albert Kreuzer]

What Enzo said.

When the screen resistor is open, the tube goes into cutoff. The bias voltage is still there, but there is no current flowing. So you measure zero at the bias test point.

[Enzo]

I think we are using the terms differently.


I am now thinking when you say you lost the bias, you mean the idle current through the tube, which is set with the bias voltage. WHen I heard loss of bias, I think of loss of the bias voltage.

[Telebuckers]

Yes this is what I was getting at was that there was no current running through v5. That situation has been taken care of- now I just need to figure out what is causing this other problem. I'll be getting back on this one in a couple hours.

[Telebuckers]

Ok so I just tried a third set of tubes and I'm still having the same problem- I can kick the standby switch off and back on and it goes back to normal for a short while.

[Telebuckers]

Also I checked out the solder connections on the power tube sockets and chop-sticked them and they seem to be ok. The amplifier seems to work ok on the clean channel, although my coworker said it did it on the clean channel after about an hour. On the yellow channel it starts screwing up after about a minute.

[booj]

Replace the bias supply caps

[robertamp]

Have you considered the possibility of the bias control pot being faulty?Ive had a few problems with creeping bias on the Hotrod caused by the track on the bias pot being damaged by the hard resin fender seal it with after moving it to rebias new tubes??

[Mars Amp Repair]

Good point. I also had one where the red threadlock paint had gotten into the slider in the trim pot & would intermittently cause it to disconnect & throw the bias way off...g

[booj]

The coupling caps with the neg. bias voltage on the one side and the plate voltage of the driver on the other could be leaky also

[Telebuckers]

Well I set this one aside for a while and now I've come back to it. It's still having the same issue. I checked for continuity between the socket pins and where they're soldered to the board and that checks out ok. I replaced C37 through C43- same issue. I checked all the diodes and they all check out ok. I checked the bias trimmer pot and it seems ok, but I put a new one in there just to be sure- still having the same issue.

Also there is now another bizarre thing happening when I switch the amp to the yellow channel. K1 is physically buzzing on the circuit board, it's a pretty loud buzz. K2 is also buzzing a bit. If I press up against it with the chopstick it quiets down. This amp is killing me.

[Enzo]

First off, regarding the power tube socket pins. Your ohm meter cannot tell you if the solder is cracked, thermal and vibrational movement can make them intermittant. That is why a visual inspection with a hand lens is recommended, and then resoldering them anyway regardless of how nice they look.

If your relays are chattering, you might be losing low voltage supplies, or the filtration thereto. Also, the FS decoding uses an AC voltage that is rectified and clipped into comparators. If a diode in the strings gets out of whack this can happen. COmpare the voltages in the swithcing circuit with the schematic - there are test voltages for each state printed on there. I recently had a Fender something or other with a buzzing relay, and one of the LEDs in the switching circuit had its junction drop way off, confusing the comparators. New LED fixed it.

[booj]

C26 and/or C27 could be leaky, causing B+ from the driver stage to affect the bias voltage.