One Tube Red Plates

[s2amps]

I'm working on a 100W Plexi. Both power tubes on one side red plate within a few seconds of powering up. The tubes on the right side are fine. So far, I have:

1. Checked all the connections a million times
2. Tried several sets of tubes
3. Reheated all the solder joints
4. Replaced the OT
5. Run through the amp with a scope
6. Found nothing out of the ordinary so far

Anything I might be missing? Any suggestions from experience, oh wise ones?

[Enzo]

Pull the tubes out of the sockets. Now what voltage appears on pin 5 of the bad sockets? It should be something like -50. If it is something else, then either there is an open resistor or a leaky cap from the PI.

REplaced the OT???!!!???

[R ski]

My best educated guess on the output tranny plate windings should read about
40 ohms to each leg from center tap (B+) Usually the readings vary a little.

IE: 35 R & 39 R

But if one is 38 R and the other is 10 R........then the tranny is causing the red plate....the bias is very important....as well.....as Enzo....mention'd

[Mars Amp Repair]

HMM,
Can't say I ever heard of a OT causing red plating, unless while being driven hard with a signal & overtaxed the tube. Sounds like this one is red plating at idle.

If a shorted OT could cause redplating at idle, then the OT shunting bias method would also cause red plating.

A shorted windings in an OT at idle with no signal would probably not cause any noticable difference, unless they were shorted to the core.

At least that makes sense to me...glen

[Don Symes]

If the negative voltage on the grids checks OK, you may need to retension the socket pins on that side so they make good contact (a loose grid socket pin that heats up and loses contact replates a power tube just like any other loss of bias).

Hope this helps!

[mykey]

I'm working on a 100W Plexi. Both power tubes on one side red plate within a few seconds of powering up. The tubes on the right side are fine. So far, I have:

1. Checked all the connections a million times
2. Tried several sets of tubes
3. Reheated all the solder joints
4. Replaced the OT
5. Run through the amp with a scope
6. Found nothing out of the ordinary so far

Anything I might be missing? Any suggestions from experience, oh wise ones?

the bias voltage is missing on pin 5 of both red tubes.
bad resistor usually, you replaced the OT? boy, do
you need a technician. you are guessing and that's
going to be expensive but not successful. get a
marshall tech, fast.

[Enzo]

Actually a bad tube is more likely that a bad resistor, and since the grids are common on each side, one shorted tube will bring both grids down.

And I believe the man is trying to learn about amp service on his own. That is why we are here after all.

[Amp Kat]

Come on Mykey give it up dude. If your trolling this ain't the place and you will be kicked off. Give these guys some help in a inspiring way would you. If you have the experience you say you do then give em a hand not a boot.

[mykey]

Come on Mykey give it up dude. If your trolling this ain't the place and you will be kicked off. Give these guys some help in a inspiring way would you. If you have the experience you say you do then give em a hand not a boot.

well you could switch the right tubes with the left tubes and this will indicate if its a bad tube or a bias failure, if the red stays on the same side its a bias failure. and i have seen bias failure many times in these amps. more often if one tube is bad only that one will turn red, not both. but you are right its possible if shorted internally one tube can bring down both. sounds more like a bias failure.
are you really going to kick me off? I'm waiting. this is an experienced person giving good advice, which was asked for. pardon my experience.

[Jag]

Based on your posts, I'd actually consider you more arrogant than experienced. Based on the evidence, you don't seem very thorough either. Read some of the other responses before you do the loose canon thing.

20-some posts, many not very informative or helpful.

[Amp Kat]

well you could switch the right tubes with the left tubes and this will indicate if its a bad tube or a bias failure, if the red stays on the same side its a bias failure. .

Not nescesarily mykey. He allready posted he swapped all of the tubes and I guess it is possible that an out of box failure is possible but not likely on the same side again. A bad coupling cap on one side would red plate that side and the other could remain constant but it would drift a hair but not enough to redplate it. Monitoring the negative grid voltage on the bad side would really tell the story which is not very hard to do. Take your meter and ground the black lead to chassis and pull one of the red plating tubes and put the red lead in the sockets pin #5 reading VDC. If it starts to drop less negative as the tube heats up it's the coupling cap. It could also be a bad connection on the tube socket as Don posted.

I'm not threatning to kick you off Mykey, your going to do that yourself. Were just trying to help people fix their amps in a friendly way. Your not doing a very good job of that. Sure theres several instances when a newbie should take their amp to an experienced tech but hell that takes all the fun out of it dude

[R.G.]

are you really going to kick me off? I'm waiting. this is an experienced person giving good advice, which was asked for. pardon my experience.

I think you should take a few deep breaths and calm down. It is possible to give good advice in a way that ensures that it will never be taken. The idea is to help the guy, not run him off.

We every one started with no knowledge of amps or anything else. Some have come further up the curve than others. New ones will be starting long after we're gone, and they may even learn more than we know.

I just thought of the perfect way to gauge your replies. Put yourself in the position of the person who asked - with their level of knowledge, mystification and frustration, then tell them what you would like to hear and in the manner you'd like to hear it.

This guy may not have all the experience in the world, but he might get you backstage passes one of these days... 8-)