Is this tube redplating?

[ajsmcs]

I'm in the process of retubing a Fender Prosonic combo that started acting up a few months back. (Huge loss in output volume) It belongs to my brother who stores it at my parents, as a christmas gift to him I'm getting it working again so he can play it when he's in town for christmas. It had the factory tube set still installed, and it was rarely ever played at stage volumes. But it was nearly always played with the preamp maxed. I was hoping the fact we never beat on the power tubes that hard would mean the stock ones were OK, and it would only need a rectifier tube, while the preamp tubes wouldnt hurt.

My background is in aerospace engineering with 10+ years of experience, and while I'm mainly propulsion and mechanical design-oriented I'm reasonably competent with the electrical side of things. However I've honestly never tinkered too deeply with tubes. I buy my amps, I play them, I am happy. So admittedly Im a novice with this.

So I replaced all of the preamp tubes and the rectifier, and it seemed to fix the problem initially. Then the problem recurred. I took this pic to send to my brother to show it was working, and I noticed the inconsistent glow in the power tubes. "Ok, maybe they're bad too." Ordered those. "Maybe one was on its way out and the new rectifier did it in.

But I was still a little skeptical that its only the power tubes, so I kept digging and learned of this whole "redplating" phenomena. I went back to the photo, and I saw this red glow on the rectifier.

Does this look like redplating?

The amp isn't with me at the moment so I can't take more photos, but if I need another GZ34 I want to get it ordered in time to be here before Christmas.

If this IS redplating, could a bad power tube have killed it? Note that this was in the Fixed Bias setting/Position 1/Class A setting, so I can't imagine it was do to incorrect biasing. Could it just be a bad tube?

If its not, is it just two dead power tubes that killed it?

 

[schmee]

That tube seems to be putting out a lot of light. But is that tiny spot redplating? I don't think so, normally redplating would be the two grey plates around that spot glowing red, not that center dot. Rectifiers often do have a red glowing small spot.

Not sure what rectifier that amp had in it to start with, but many of the new import rectifiers fail. I don't use them much, but of the ones I have had, I'd say 2 of 4 failed early on. That may just be a coincidence though. The old production Rectifiers last a long time, the one you took out may be just fine. The old ones I'm still using are probably 50 years in service!

I can't say it's totally unusual for one tube to glow blue and one not. I'm not even sure why some sets get that great glow and others don't but work fine. But it could be an indication that there is still something wrong with the amp. Have you looked inside for burned resistors? Especially on those power tube sockets?

 

[ajsmcs]

Thats a bit of a relief to hear about the rectifier. Although it is a bit weird that it would work fine for a few minutes then go back to what it was doing before. I have new power tubes on the way, so I'll know on Thursday whether or not they were the culprit for sure, but I want to make sure I have what I need to get it running during the brief window he's in town.

The new tubes are all JJ, while the amp came with all Fender-branded Sovtek tubes which I presume to be the originals. I held onto them just in case they were actually fine.

The amp itself is about 25 years old and has a lot of play time on it. My brother had a bad habit of leaving the amp on with the guitar's volume rolled all the way down when he was taking breaks from playing to do other stuff. That amp would be on for like 8 hours straight on the average saturday or sunday when we still lived at home. I wouldn't be surprised if some of those tubes were well-worn.
But those days are way in the past, and the amp only started giving us trouble about 6 months ago.

I haven't pulled the chassis out to look for burnt components yet, but my gut feels like it would be strange for a wear item like a tube to survive that abuse but a non-wear item like a resistor to be torched except due to a major accident, like a fluid spill into the amp or something. Doesn't mean it didn't happen, and I'll look next chance i have to go over there.

 

[ajsmcs]

UPDATE: Curiosity got the best of me, and I went and picked up the amp.

1) The glow I'm seeing is just a reflection off some copper tape directly behind the tube thru a gap in the plates. So unlikely to be an issue.

2) The actual issue seems to only be on channel 2, which works normally right after its initial warm up, then once its set for a few minutes channel 2 will play at a very quiet volume regardless of the position of the master volume nob. The clean channel continues to function as normal. That makes it seem like its an issue with the Preamp tubes as I initially suspected, but only on the hi gain side...despite having new tubes.

All in all very weird. Im gonna keep playing with it and I'll post what i find.

 

[Wally]

Fixed Bias setting/Position 1/Class A setting

Note that this statement cannot be accurate. They ‘Class A‘ option has cathode Biasing with tube rectification.…30 watts. The other two positions have fixed Biasing…one has tube rectification—-50 watts—-while the other is solid state rectified—-60 watts output.…claimed.
Know that the two modes share the first two 12AX7s. V1B is the input gainstage for both modes, and V2A is the last gain stage prior to the tone stack for both modes. The other two triodes of V1 and V2 are the two gain stages that are dropped in to creat the gain for the Lead mode.
The Gain 2 circuit in the Lead mode is a midrange boost circuit that is not in a tube.
The schematic will exhibit the signal paths for those two modes. Either of those two tubes could have a problem, or another aspect of the Lead mode circuitry could have a problem. If the Clean mode exhibited the same problem, one would want to clean the effects loop jacks.
This amp is getting to an age when one starts to be concerned about the electrolytics. Great amps, imho.

 

[ajsmcs]

EDIT: I realized I'm using the wrong terminology here. What I meant is that Class A doesn't rely on the trim bias setting, so the adjustment of that trim should be irrelevant to the malfunction.

I'd be a little surprised to see electrolytics die at that young of an age provided they're good quality. I have a 1984 Juno 106 with all the original caps thats never had issues, as well as some old Pioneer components from the 1970s.

But on the flipside, I had to recap an old Sega Game Gear whose caps were so bad it basically didn't function at all. That problem is infamously due to a bad quality batch of caps, but still I can't totally rule it out.

 

[Wally]

EDIT: I realized I'm using the wrong terminology here. What I meant is that Class A doesn't rely on the trim bias setting, so the adjustment of that trim should be irrelevant to the malfunction.

I'd be a little surprised to see electrolytics die at that young of an age provided they're good quality. I have a 1984 Juno 106 with all the original caps thats never had issues, as well as some old Pioneer components from the 1970s.

But on the flipside, I had to recap an old Sega Game Gear whose caps were so bad it basically didn't function at all. That problem is infamously due to a bad quality batch of caps, but still I can't totally rule it out.

It is correct that most Class A amps, which are very few in number despite manufacturer’s claims, almost always have cathode biased outputs. They are ‘self Biasing’. The other two modes of operation in this amp are fixed biased, and the bias should be set with both modes observed since they each draw a different current and operate wih the same bias voltage. I have read of a way to bias a fixed biased amp into Class A in Dave Funk’s book.…but that is another subject.
Re: e-cap lifespan. I agree that one might wish that electrolytics last longer, but there comes a time when the sonics are affected by weakened e-caps long before those caps start yielding gross effects like hum or outright failure. Many people hold that 10 years with modern
e-caps is old enough. I am not in that camp, but I have replaced a 9 year old electrolytic in a Fender amp just a few years ago due to total failure. With a 25 year old amp, I have some small misgivings about spending a lot of time chasing certain types of problems while there are old caps in the amp. ommv..and I too have older amps that I have not recapped…..my Super Champs from 1983 and 1984. They have cans, which seem to hold up better over time. Someday soon I will recap them and take note of the before and after sonics, which still sound as they did 30 years ago….no out of tune subharmonics, good attack, good high end sparkle, and good dimension to the sound stage.

 

[Wally]

I also like to understand how certain tubes are working in a cathode biased circuit. I want to know the current draw and the plate dissipation numbers.

 

[dan40]

I was going to suggest that the switching contacts on the effects loop jacks may be dirty but Wally beat me to it. The contacts in the jacks will oxidize over time and lose their ability to pass the signal easily. The effects loop handles both channels though so it looks like they are not the issue.

 

[ajsmcs]

Now that I have it at home and have had time to play around with it, here is what happens:

Turn it on, warm it up. Plays fine for a little while. Then it will suddenly drop in volume, but not statically. It will fluctuate in a very random manner, both in the speed of fluctuation and intensity. It almost feels like the power supply can't keep up with the amp demands. It doesn't sound like theres a loss in preamp gain while it happens. Its just...quieter. And as before the clean channel is as loud as its supposed to be, but i do notice odd artifacts every so often. Hard to describe. I'll play with it again tonight and see if I can do a better job of describing it.

Im still open to it being a non tube issue - like caps or what have you - but I want to try the new power tubes (which should be here tomorrow) before I pull out the DMM and start hunting thru the boards.

And by the way thank you all for your help.

 

[dan40]

Starting with fresh tubes is always a smart idea because most problems with tube amps will boil down to faulty tubes. Be sure to set the bias properly after installing the new power tubes.

 

[ajsmcs]

Well...the old power tubes were on their way out, but the problem still happens. I'll be playing for 5 minutes, everything works the way its supposed to, then POOF sudden volume drop to almost nothing - you can still hear it, but its very quiet - and the Master knob does nothing. This was only in Mode 1: Cathode Bias. I didn't have the time to pull the chassis out and set the bias, but that will be the next thing I do.

And while its out I'll look for visibly bad components, fader-lube the pots and switches, dust, etc, and hopefully its something obvious.

I have this theory it might be a broken PCB pad somewhere, and that as the amp heats up and metals expand this part lifts far enough away that it reduces the current flow to almost nothing. We'll see.

Stay tuned!

 

[2L man]

Grid leak resistor circuit (tube pin-socket-resistor-ground) might fail? Coupling capacitor between PI and power tube grid might leak PI tube anode voltage to control grid? Both of these allow grid voltage rise and it increase cathode current. Voltmeter should show this possible grid positive voltage but voltmeter internal resistance might make voltage drop but there should be definite difference between tubes?

 

[Wally]

This was only in Mode 1: Cathode Bias.

If this means that the problem has not arisen in either of the other two modes, that would be an indicator of a problem in that cathode biasing circuit…or so it seems to me.

 

[ajsmcs]

It happens in the other two as well - when the volume went out, i briefly flipped over to see if it was still there - but I didn't want to spend too much time in those settings since I hadn't checked the biasing yet.

 

[Wally]

Thanks for that clarification, @ajsmcs