Metro JTM45 tube intermittently redplating in V1 socket...

[Heliotricity]

Hello all, first post here. I need some advice on troubleshooting a metro kit JTM45 amplifier which has one tube (V1) which intermittently redplates. The amp sounds great when it is working. Sometimes I can play it for 3 weeks with no problem, then it redplates, then maybe it will do it 2 days later then maybe again 2 weeks later. I know it isn't the tube because I've used 3 different tubes (KT66) and it only happens in the V4 socket. Had a tech check out the amp twice already and he is stumped. The tubes are biased correctly, the current and voltages in the amp are all correct and to make it worse it won't redplate ever when it is in the techs shop! The tech on the first visit added "swamp" resistors in series with pin 5 on each power tube and shortened some of the wire lengths, this removed a parasitic oscillation that he detected. The second time he moved the wires which go from pins 5 of the power tubes out from under the circuit board and sautered them on top because he thought maybe there had been a bad sauter joint underneath (see photo). Of course the amp worked and sounded great in his shop but weeks later I find it is still intermittently redplating.



I went through and checked all the capacitors and resistors with a multimeter and everything looked to be in spec. I also double checked all the solder joints and none look suspicious.

What could cause this to be happening? Could it be one of the coupling caps is leaky? Could it have something to do with the bias diode? Or should I suspect either the grid leak resistor or the grid stopper resistor on that valve?

Just looking for suggestions on how to proceed from here, any help appreciated. Attached is a metro JTM45 schematic for reference.

[big_teee]

Have you checked the negative bias voltage on pin 5 with the tube out.
Check it from the top with the tube removed in play mode.

[Heliotricity]

With the comment above in mind and as per the metro amp instructions I decided to check a few voltages. I pulled all the tubes except the GZ34 rectifier, turned the bias pot all the way clockwise and checked the negative bias voltage on pin 5 of V4 and V5, they both read -50VDC. According to the metro kit instructions these should be -45VDC which seems close enough. Next I plugged in the 3 preamp tubes and checked pins 1,3,6 and 8 on all three and the voltages were right on. Then I plugged in the power tubes and rebiased.

One thing I noticed was that measuring the B+ on the standby mine read 436VDC where it should be 400VDC. I factored that using the metro formula 19w / 436 = 43.5mV. 43.5mV being the maximum to bias to. Rebiasing I noticed that my tubes are quite mismatched, I ended up leaving V4 at 38mV and V5 at 31mV. I'm quite certain they were closer matched when the tech installed and checked them. So this may very well be why V4 has been overheating. Although it is possible that the couple times that tube has redplated has also damaged it (even though since the problem started I've been keeping a general eye on it while I played and shut the amp right off as soon as it starts overheating). That still wouldn't quite explain why it takes 3 weeks sometimes to overheat, then only 2 days the next time though.

Let me know if there is anything I may be overlooking here. In the meantime, I'm going to play through it as it is and keep an eye on the bias to see if it is stable or not then look into getting some matched tubes. I'd really hate to put the money into new tubes only to have the problem resurface and take out another pricey KT66, between the cost of tubes and the tech I'm going to be wiped out if I can't get this taken care of.

[Heliotricity]

I just realized that in the title of the thread I put tube overheating in V1 which is a mistake, it should read V4...if the moderator sees this and can change the title so it is not misleading that would be great.

[loudthud]

Before you do this, it is important to check that the amp is unplugged from the Mains and that the capacitors are discharged. The capacitors can retain a charge even when the amp is off that can cause serious injury or death. If you don't know how to discharge the caps, do a search here or on the web and do not proceed until you are sure they are discharged.

Check the tube socket. Look at from the top where the tube plugs in. Does the contact for pin 5 look bent or dirty? Is it flooded with solder? Try to find something that is the same diameter as a tube's pin. Do you feed equal tension on all the pins when you insert and remove the pin? Many times you can gently bend the contacts with a sharp tool like a scribe or dental tool to restore the tension so it makes good contact.

[Heliotricity]

Played through it last night for a couple hours and no redplating. Checked the bias again and it was the same as where I had left it. Out of curiousity I reversed the V4 and V5 tubes and the tubes still had the same mV readings. The lower biasing tube starts out and stays pretty much at 31mV but the other one starts at 31 and creeps up to 38 over about a 5 minute period, there were even a few moments when it read 45 and 50 for a second then went back to 38. I'm going to see if I have a closer matched tube in my backups.

Check the tube socket. Look at from the top where the tube plugs in. Does the contact for pin 5 look bent or dirty?

I took a look at the socket in V4 and it looks very clean, I'll drain the caps later and check the contacts closer.

[Jazz P Bass]

I think that 'climbing' tube is bad.

[Jess13]

Just so you understand I do not know enough to be giving any advice. But something I though of when you said it may be three weeks or two days before the problem would show up, this would be your mains voltage (120v house voltage). And then if you have some voltages in the amp that are on the edge and your mains jump 5v on you it might get you in trouble.


Here where I live I have seen the mains up and down by a surprising amount this time of year. I am only starting to even begin to learn about tube amps, but I wander how much such a thing could effect them.


Jess

[Heliotricity]

But something I though of when you said it may be three weeks or two days before the problem would show up, this would be your mains voltage (120v house voltage)

That could be a factor. When I measured the B+ at the standby a couple days ago it was 436VDC and when I checked yesterday it was 412VDC so there is some fluctuation. On the other hand, I do also use a Marshall 50 watt 1986 bass head and I haven't had any problems with it...

I think that 'climbing' tube is bad.

I think your right about that, whether it was bad to start or the fact that it has overheated a few times I think has sent it south. It redplated a couple days after I posted last. I did rustle up another tube which is much closer biased to the good tube, I have that in there now. Been playing through it for the last couple days and so far so good. It sure would be nice to think that the whole issue is just a bad tube. However, this is the fourth tube that is going in there. The original JJ tube that was in the amp when I got it burned out, a replacement Gold Lion burned out and now this other "climbing" tube which I've just taken out. I'm keeping my fingers crossed but 3 bad tubes? KT66 tubes I've heard are more prone to this type of problem than others. Regardless, I'll see how this new one holds up and keep you all posted.

I still have to drain the caps and check on the tension of the pins inside the socket, been procrastinating on that, will do that too.

[big_teee]

You might try working out the problem with cheaper 6L6 tubes.
They don't break the bank like higher dollar KT66s.
You may have to tweak the bias voltage with resistor change, but probably not.
I have a set of Sovtek 5881WXT+ 30 watt tubes I always put in new amps.
They don't sound that great, but are battleship tough.
Then once I get everything working properly I go to the better tubes.
I would also try running with less bias current, maybe between 35-40ma.
I now run most of my amps with colder bias current.
They sound fine to me!
T

[Heliotricity]

You might try working out the problem with cheaper 6L6 tubes.

Thats exactly what I did after the Gold Lion blew out. I left in the good GL and the tech sold me a 6L6 which matched it (well it matched it at the time, that is the "climbing" tube I just took out). And the tube I replaced it with in there now is also a 6L6.

I would also try running with less bias current, maybe between 35-40ma.

Right there with you, I've got it biased so the GL is 37 and the 6L6 is 34.5. I had these biased colder and it was a little mushy but where it is now sounds just fine.

[Heliotricity]

Am I losing my grip or did posts disappear from this thread? I'm pretty sure I had made a few comments and that a proper schematic (rather than component layout) had been posted or linked to?
Anyone else notice this?

I got a notification in my email with this post by "g-one" but don't see it here in the thread. Thought I'd post it since it didn't show up and also to point out that there may be a thing or two awry in the general vicinity of JTM redplating tube threads...

[g1]

I deleted that post out of embarrassment .
As far as the disappearing posts I mentioned, it was probably a memory lapse, I think I had typed out a reply and tried to post it, but the forum was in "glitch" mode, so I just gave up and it didn't get posted (and I forgot it was no go).
Or the forum is eating posts . Probably wouldn't be the first time. Anyone notice anything missing from this thread, maybe a proper schematic?

[big_teee]

I deleted that post out of embarrassment .
As far as the disappearing posts I mentioned, it was probably a memory lapse, I think I had typed out a reply and tried to post it, but the forum was in "glitch" mode, so I just gave up and it didn't get posted (and I forgot it was no go).
Or the forum is eating posts . Probably wouldn't be the first time. Anyone notice anything missing from this thread, maybe a proper schematic?

In your Defense, there has been several threads similar, and you have posted in them.
T

[Heliotricity]

Update: No issues so far with the new tube, although I liked the last tubes sound better, this one is a cheapo Shuguang and it makes for a more shrill sound than the last tubes. I'll be happy to get it out once the problem is resolved!

On another note, I was checking out the photos of another metro JTM45 just like mine a guy has for sale, it uses same transformers and caps and is nearly identical but I noticed his has a 47nf 630V cap coming off pin 3 of V5 going over to one of the electrolytic can cap terminals. Mine doesn't have this and I checked out the schematic and don't see it on there. Can anyone tell me why that cap would be on his amp?

Other Metro JTM45 with 47nf cap:


Close up of cap:


JTM45 schematic:


And here is the underside of my amp again for reference: