Mesa Rectifiter Solo 50 - one EL34 red plating

[disorder]

I have a Mesa Solo 50 here. Just one of the EL34's will red plate after playing at high gain for 5-10 minutes. I understand red plating due to too hot a bias but that usually occurs pretty soon after turning the amp off stand by. What could this be a sign of? The tubes are Electro Harmonix EL34's that are matched and I'm getting a bias of -39V at both tubes, 447V at the plates of both tubes, and 440V at the screens. Only one tube red plates.

[Pedro Vecino]

Tubes do not always fail fast. You can change tube in the other socket and see if makes the same effect for discard it but is necessary to know the bias adjust measuring the current flowing through each valve. -39V/447 not explain anything in that aspect,

[MWJB]

That tube is under biased, the 2 may not be closely matched, so the other might not be redplating. The harder you drive the amp, the more likely redplating will occur, it's possible that an underbiased amp played at low volume may never show direct symptoms.

-39v sounds to low to me.

[disorder]

I switched the tubes and it's the socket that is culprit. The tube that was not red plating before now red plates when in the bad socket. I'm going to first check the screen resistors even though they look ok and both measure 1K and make sure the sockets are up to spec.

[disorder]

Aside from a loose socket on pin 5... could there be another reason the tube doesn't start to red plate until the amp is cranked for a solid 5 minutes or more?

[MWJB]

If the screen grid resistor was blown then you would not get a clean tone.

We haven't ascertained for sure that the tubes are not underbiased, one socket may run at slightly different voltage to the other, OT/PI plate resistor imbalance could be pushing one tube harder than the other, if both are borderline then it might not take much to push the tube in that socket over the edge.

[disorder]

makes sense! I just tightened a few of the socket "clamps" and resoldered both sockets.

I'm either going to install some 1 ohm resistors off the cathode or finally build a separate tube socket wired up to read cathode current. I'll report back.

[Jazz P Bass]

Monitor the Volts ac on each bias pin when you are really cranking the amp.
If the reading is severely off on the redplater, you may have a phase inverter problem.

[disorder]

AC voltage on pin 5 of both tubes stayed pretty much the same when cranking the amp. I popped in some new Mesa branded EL34's that were matched and said to be "plug and play" for the Solo 50 but that same tube socket is causing the new tubes to red plate as well.

[Steve Conner]

I believe some amp designs drive one tube harder than the other.

[MWJB]

Who said that the Mesa EL34s were "plug and play"? Tubes red plate when they draw too much current, so how much current are they drawing?

[Rhodesplyr]

I've seen this problem in two different amps I've worked on over the past year. First, you need to determine that you don't have any sort of intermittent connection that's preventing the negative bias voltage from reaching its ultimate destination--the tube's control grid. It doesn't matter if you have the right bias voltage at a test point if it isn't getting where it needs to go. The problem can be bad solder joints, dirty tube sockets, poor socket-tube pin connection, or even a bad connection inside the tube.

Once you've eliminated faulty bias voltage or an intermittently shorting coupling capacitor from the driver tube, the issue becomes one of damage to the tube's control grid. The EL34's control grid should have some sort of coating (some are gold-plated) on it to prevent it from becoming a source of electron emission, known as "grid emission." Whether this coating is faulty from the factory or has become damaged over time, once it's damaged, the tube won't ever maintain a stable bias again. It will test perfectly fine in a tube tester, but put it in an amp, and in 5, 10, or 15 minutes, you'll start to see the bias current creep up, slowly at first, then accelerating.

I'm finding more and more that once an output tube has red-plated fairly severely, it's likely to be permanently damaged. Even if it originally red-plated for another reason, it can continue to do so because the coating on the control grid has been blown off.

In both cases I mentioned, another tech had fixed the original source of the loss of bias control, but had not insisted on replacing the output tubes.

[Steve Conner]

I agree with the observation, if not the mechanism. I always thought it was gas that came out of the tube's internal metalwork if it got overheated too badly. This is supposed to get eaten by the getter, but eventually the getter is used up and you start getting reverse grid current from the ionised gas.

The bakelite bases of some tubes also go conductive if they get too hot, and cathode material can get sputtered onto the mica insulators. I also heard that control grids can get contaminated, or simply get red-hot from excessive forward grid current, and start to emit electrons, but all I'm saying is, there are other causes of thermal runaway.

I once had an amp that would redplate whichever power tube was uppermost if you stood it on its end.

[Rhodesplyr]

Steve, You're good at keeping me honest :-) I'll freely admit that I don't know exactly what's happening inside the tubes from first-hand research. I'm going from what others have told me.

What I have observed in these cases is increasing voltage drop across the grid-stop resistors, indicating that current is flowing somewhere via the control grid tube socket terminal. It goes away when new tubes are installed. In one case, the tubes were EL84s--no bakelite base.

We can also note a consistent pattern to this mechanism, whatever it is. Something in the tube has to heat up for 5-15 minutes in order for runaway to begin. Cathode material stuck to the control grid sounds as plausible to me as damage to the control grid coating.

[jmaf]

I believe some amp designs drive one tube harder than the other.

Recently a MB Mark III kicked a friend of mine's butt for days and I tried to assist with the repair at his own shop so I got a feel for it as well.

The outter 2 tubes are driven off different taps from the OT like some variation on ultralinear and the 2 pairs seem to draw different current, have different resistors, etc. These amps also apparently can take mixed pairs, EL34 and 6L6 it seems, not sure about that. It was then I learned about Mesa's "simulclass" patent and that's what it's called. The constant warnings everywhere about patent infringement and lawyerspeak on the service manuals made me hate it before I had time to like it - IMHO this amp is typical 1980's marketing magic.

Also, it bothers me that they crammed 100 watts into a 20 watt sized amp, they made it heavier than lead, and then thought it was a nice idea to pack 200 functions on the front panel, every knob is push/pull and it takes a phd to use every setting on the thing. As a high gain tons of distortion amp, they had to give up on tone to make it distort to Napalm Death degree.

OK thanks for listening, I needed to vent that.