Early pre-amp tube failure in Twin Reverb '65 Reissue FSR Blonde

[judgeamps]

Hey all, I have another weird one I thought I would throw out there. I had a Fender Twin Reverb Reissue that was purchased new around 2016, it's a special edition with blonde tolex. The problem is this particular amp is burning through pre-amp tubes. The owner says he went through 3 sets before bringing it to me a few months ago. He had tried Mullard and EH's. I put in new JJ's, checked voltages at every test point and all seems good. All voltages at every pre-amp tube pin are pretty spot on. There was one screen resistor on a 6l6 I replaced, but the other 6l6 tubes test and sound fine. A few months later, all of the pre-amp tubes are bad again. I did notice that this particular amp appears to have a different brand of transformers than all of the other Twin Reissues I've worked on. The owner has reached out to Fender, but they provided no help.

I'm realizing I don't have a clear idea of what to look for that would cause early pre-amp tube failure if voltages are all in range. Any ideas?

[sluckey]

Are you saying that all 6 little tubes are failing at the same time? What kind of failure? This is very rare. It's also very hard to believe.

[Latole]

Are you saying that all 6 little tubes are failing at the same time? What kind of failure? This is very rare. It's also very hard to believe.

100% right; Hard to believe

[pdf64]

‘burned up’ - what exactly is meant by this, ie what is the failure mode/s of the affected valves?
What heater VAC does the amp put across the filament line?
What’s the owner’s wall voltage, how stable is it (eg does their lighting ever go bright or dim?), same for the places it gets used, if different?

[judgeamps]

Yes, it's very weird. All of pre-amp tubes are failing. A previous tech replaced them all twice. He also replaced the power tubes the second time even thought they tested fine. The owner brought it to me complaining of very low volume. I tested all of the tubes in my tube tester and they were all weak and barely passing a signal. I replaced all tubes and the volume and tone was good for 2 months or so and now they all failed again. I can't wrap my head around this one.

[judgeamps]

pdf64, the owner is using a power conditioner since the second time it happened and even moved to a new location since then. Based on my notes, there was 6.2 VAC across the filaments. I had the same thought that maybe it was very high and checked that first. It has a hum balance pot as well.

[Latole]

All tubes were all weak and barely passing a signal after 2 months and heater voltage is the good one 

I don't know what the owner do with is amp, it is incredible 

I'll check all DC voltages.
Any mods on amp ?

[SoundCity85]

Maybe he has a tech who makes money on pulling his tubes for spares, and selling him fresh ones.

[pdf64]

As I understand it, power conditioners just absorb spikes, transients, they do nothing for high / low mains wall voltage, other perhaps than provide an indication of it.

[Williamblake]

Maybe he rolled it on the tubes for transportation?

[66Strat]

Yes, it's very weird. All of pre-amp tubes are failing. A previous tech replaced them all twice. He also replaced the power tubes the second time even thought they tested fine. The owner brought it to me complaining of very low volume. I tested all of the tubes in my tube tester and they were all weak and barely passing a signal. I replaced all tubes and the volume and tone was good for 2 months or so and now they all failed again. I can't wrap my head around this one.

How well do you know this customer? Is the customer wanting you to replace the tubes on your dime? Are you sure that the faulty tubes currently in the amp are the ones that you put in the amp? Assuming a 50% fail rate, there is a 1.5% chance of all 6 tubes failing at the same time. Either there is an underlying fault in the circuit, or the customer is a scam artist.

[Latole]

This story that the amp destroys the tubes without outside intervention does not seem credible to me .

[judgeamps]

I don't know the customer well at all, but he paid me once quickly and wants to pay me again. He also paid a previous tech multiple times it seems. He's very frustrated and his plan is to call it a lost cause if I don't have any ideas.  My 2 thoughts are bad power at the source (having the issue at 2 different buildings sort of negates that) or a power transformer that intermittently performs out of spec, but I've never seen or heard of a new power transformer failing in that way. I'm the type who goes for the most simple explanation first, but this one makes no sense to me.

Thanks to everyone for their insights. I'm very grateful.

[Tom_Hull]

Does any of his appliances need replacing early..could be imbalanced voltage supply.
I had problemsfor many years appliances not lasting . and  then .


My fans started burning out ,one at a time .
.then the washer burned .
Then I turned on my tube stereo. It started smoking ,
It s going to be rebuilt.


Then I phone .the ellectric company.
They fixed the house cables .the connections did not even look like metal .

[acheld]

I'm curious about your comment about the PT being a different brand.  Do you have a photo you could share?  Part #?

Doubt that it will make a difference, but . . .

[judgeamps]

I'm resurrecting this post. The customer brought the amp to the nearest Fender Certified Technician who also called him crazy after all of the pre-amp tubes were bad again. He's bringing it back to me for one last look. I'll take a photo of the transformer when I get it.

[PRR]

Does it happen gradually or all of a sudden?

Does it happen just after gigging a certain venue?

[judgeamps]

The customer says the amp never leaves his rehearsal space and seems to gradually happen over a period of time.

[jordan86]

Do you test the “burned up” preamp tubes every time he brings it back? Are they the same tubes you installed? It sounds like you’re certain the tubes are being abused to the point of not functioning. That just seems so far fetched.

As other have noted, the filament winding could be the culprit. Maybe it only affects the preamp because they share a third connection in parallel?

Not sure if it would be the issue but have you checked the heater secondary center tap and related components? Or maybe something at the phase inverter heater connection (the “first” preamp socket in the heater line) that is blowing everything else down the line?

[acheld]

Well this is the kind of project that will drive a working amp tech to the ground.  On the other hand, for those of us trying to ward off dementia, it would keep us busy in our workshops for months.

My personal plan would be to test the PT to an inch of its demise, recheck all voltages to establish a baseline, and then turn the amp on until it begins to fail.   The problem will be to recreate the heat and vibration levels the amp gets in its day job.  Think of it like a stress test.    Like I said, this may take some time. 

The folks on this site will undoubtably have a better (eg, faster) plan.   

The story actually sounds like it is straight out of Car Talk, with Tom and Ray . . . 

Your customer will likely sell this thing.   I would if I was in his shoes(as a working musician), though it sounds like he does like the amp enough to keep at it. 

[Latole]

Jordan86 ask ;

"Do you test the “burned up” preamp tubes every time he brings it back?"

[judgeamps]

Yes, I tested the failed tubes with my tester and was told the other techs also tested them. They seem to be the same tubes I installed. I carry JJ, the other tech seemed to have EH. The poor guy pays all of is each time for new tubes and the labor, so can't claim a scam.

As a refresher and update to an old post, I am now 1 of 3 techs to look at this. The customer brought it to one tech, then to me, then to a Fender Certified tech, and then back to me, probably because I'm the most polite.

Most of you will probably think this is a waste of money, but we're trying a replacement power transformer. I ordered a Hammond. Worse case scenario, the customer has an extra power transformer. If it the failure continues, I will likely buy one back to use for a custom build.

[Latole]

Yes, I tested the failed tubes with my tester and was told the other techs also tested them.

What is the tester said ? What part of the tubes failed ?

[judgeamps]

I just wanted to follow up with this. Customer had me install a Hammond Power Transformer and reports everything is still working well 4 months later.