Mark IV red plating

[dudu]

Hi all,

I have a Mark IV rev B where one of the outer tubes red plates. After swapping tubes around and eventually replacing all of them with a fresh set it always happens in the same socket, so it seems there is something wrong with the amp rather than the particular tube. This happens as low as 1.5-2 on the lead channel master and the master volume when running in class A mode (a bit louder than apartment volume but not loud enough to annoy the neighbors) and roughly at 3.5-4 on either when running in simul-class mode (roughly rehearsal level volume). The problem manifests in either mode fairly quickly after turning the amp up and playing for 5-10 minutes.

The local tech I took the amp to (who is endorsed by Mesa) has made some interesting claims that I would like to run by the forum:
1. Master volume on anything higher than 2 is too high to safely operate this amp in class A mode. Anything higher than 4 is unsafe for operating in either mode.
2. Using humbuckers is prone to throwing the amp out of control because they are so high output.
3. The amp is absolutely fine and I should just play it at a lower volume.

I am not an electric engineer, but I am an engineer nevertheless and can't quite see the sense in these statements. In any case, they do not corroborate my previous experience with other Mesa amps or other amps in general.

Thanks in advance for your two cents!

 

[woodbutcher65]

You've got an open screen resistor on the socket that red plates. Get that resistor replaced.

The other comments made by your tech are a load of nonsense. Find a better technician!

 

[stephen sawall]

Agree....get rid of that tech. I would send those statements directly to Mesa. They should know this tech is incompetent.

 

[dudu]

You've got an open screen resistor on the socket that red plates. Get that resistor replaced. The other comments made by your tech are a load of nonsense. Find a better technician!

Agree....get rid of that tech. I would send those statements directly to Mesa. They should know this tech is incompetent.

Thank you both for your replies. I thought I'd provide more context around the tech's statements to perhaps gather more wisdom from the forum.

I noticed the problem while recording using a Suhr Reactive Load. The tech was quick to blame it on that and the fact that I was probably cranking the amp high enough to take the power amp into distortion (I was not). He claims to not have been able to reproduce the symptom in his shop, where he uses a signal generator in front of the amp and a resistive load connected to the output. I found it rather odd that an amp tech does not have a guitar cab, but I am no electric engineer. After the first session in his shop I took the amp back home and played it a bit louder than usual (still not loud enough to annoy the neighbors) and, surely enough, the tube started red plating again. My thinking at this point is that either the resistive setup in his shop masks the issue (I can reliably reproduce it with two reactive loads - the Suhr and the actual cabinet) or he has not even tried to reproduce it. He took offense when I suggested that he gave the amp a spin in a real world setup instead of the controlled bench environment. As an engineer, I cannot understand why he would refuse to do that, unless he cannot be bothered to do the actual work.

I took the amp back to him for now as I have paid for a diagnostic and a fix that he has not yet provided so the onus is still on him (he was quick to charge for me a couple of hours of bench time). I'll give it a couple of days before I ask for a status report. If he is still stalling I will take the amp back and try another tech. Either way, I will forward Mesa a transcript of my conversations with him.

He did do some unrelated, low value, prophylactic maintenance work on the amp but has declined to share the details of that, invoking a Mesa NDA prohibiting him from doing so. I found this very odd, I thought I'd ask for a second opinion. Does this sound like a baseline experience with Mesa?

He has a CRM system we have been communicating through, here are some verbatim quotes from that conversation (the rest of his statements have been verbal):

After reading back on the previous exchanges of how you're operating I wonder if the amp is being over driven way too hard too early and by using the power soak disguises the fact of the considerable actual output being generated. Valve amplifiers will continue to give all they have until something gives and they generate additional harmonics resulting in a far greater output power than their stated value. An overdriven signal starts to turn to a square wave as a result of the even harmonics and seriously loads everything, the valves and transformer. If the soak is turned down to record with you don't fully gauge the power the amp is dumping into the load.
I base this on the fact of a humbucker pick-up because of the extra windings gives a far greater output than any other pick-up. Mesa's are notoriously excessively high gained amplifiers, we get many brought in because they hiss more than other amps because of the gain and people think it is a problem.

Running the amp in Class A means that the valves are working much harder due to the typical biasing and arrangement of how Class A operates, it just irons out the issue with bias crossover distortion. I've outlined this below in a simple demonstration of the amp running in tweed with same input signal level, no change at all.
Class A output power 72w outside valves current draw 87mA
Simu output power 130w outside valves current draw 63mA
The HT voltage measured inside the amp is around 450V ish for a 6L6 the standard limit of maximum operating is around 30W per tube, this equates to roughly 65mA, anything more and the tube is being overloaded which is where the output tube starts to distort and overdrive, occasionally it's not a problem but continued operation in this region will cause the red plating by simply too much current passing through the valve.

I have put my findings with noted observations and your situation to Mesa to answer and confirm any irregularities.
Under test conditions I have run the amp at 50W into a 8 ohm load with a constant 100hz signal for 4 hours, measuring both output and operating temperatures using a thermal imaging camera, running in Class A Pentode mode
1)Amp was run far exceeding maximum stated wattages in both Class A and Simul for 30 minutes each with no issue.
2)The existing valves fitted show an moderate imbalance across the positions when run in sumil mode, and the imbalance travels with valves. Added with the bottoms missing on 2 valves I would highly recommend replacements, nor knowing the history of them.
3)Upon testing no red plating observed nor measured under normal rated values according to manual.
4) I've replaced the valves with some workshop test ones and confirm that they all run even current with sitting idle, and then even when loaded with a test signal, therefore ruling out any circuit issue.
5)running with the original valves for the 4 hour test Class A, the mains transformer temperature reached 54deg C which is well within a transformer temperature operating range, both v1 v4 measure same temperature and pulling the same currents.
6) Output dropped by 2-3 watts over this time.
7)periodically switched using foot pedal between all channels, setting the outputs and gains to match that of R1 no loss in brightness of LED's observed.

It is important to note that in your amp despite having 4 output valves, they're not all working exactly the same in a push pull fashion evenly and depending on the running mode. The 2 outer valves get worked harder especially in Class A mode than the middle ones. As such the outer ones are always going to wear more in Class A mode only.

If you run in Class A lots, I would recommend rotating V1 for V2, and V3 for V4 every now and then to ensure even wear.
This amp is not like any other vintage Marshall in terms of it's output stage and should not be compared as such.
Having run the amp for extended periods at hugely increased output powers than the manufacturing limits I cannot seem to find an issue with the amp. Maybe when cleaning one of the sockets dislodged something, however I am unable to get the amp to redplate and everything is working as expected with a correct level in vs output.

 

[woodbutcher65]

If an amp is running in class A, then the amount of plate current the tube draws is at its maximum set value whether there's a guitar plugged in or not, turned up or not, volume and master controls on the amp at zero or at max, it makes no difference. Idle current in Class A is the same as full load current.

Your tech REALLY does not know what he's talking about. Sounds like he has lots of knowledge but is short on UNDERSTANDING, and that's never a good combination.

Engineers don't always make good technicians. Technicians are all about the real world of electronics, while engineers are all about what it's supposed to do IN THEORY. The best engineers are technicians FIRST. That way they gain a functional understanding of what they will be doing later.

 

[stephen sawall]

I would send the amp to MESA. Get any updates done at the same time. No one is going to be more familiar with the amp than them.

 

[dudu]

I have considered that, but as I live in the UK I thought I'd first try some of their local techs. I would not be surprised if it ends up costing the same or more than a two way trip to Petaluma.

 

[stephen sawall]

UK might be too far. I'm in Seattle so not that far. My Tremoverb, Heartbreaker and Mark IV all need to go in for maintenance. I'm not having issues.
I hope it works out for you.

 

[dudu]

Thanks Stephen.

Meanwhile, I had dropped the amp back with the tech (time will tell if that was the right decision), since I had already paid for a fix. The plot thickened some more today with the latest update he sent:

Hi, there has been some discovery on a fault. Although it seems to have been an accidental discovery of the issue.
It seems that some of the internal loom wiring is laying in such a position on the circuit that it is causing the same position valve to redplate without anything playing through it. When checking the signals around the area I noticed a very high frequency oscillation occuring with the one valve only. Moving the cable loom removed the oscillation, and can alter the frequency and amplitude by moving the cables around. There is no insulation damage, no wire breaks, it appears to be induced RF coupling from the currents of the cables running in parallel. I have only ever seen this on a handful of amps over the entire 15 years of doing this so is incredibly rare. It seems that the amp is more prone to it when put in the wooden chassis hence why I couldn't get it to fault on the bench in prolonged test, a wooden box isn't the kind of material you would think can cause a proximity issue but it seems to be coupling at the frequency is causing all sorts of issues.
I have sent my findings to Mesa and our repair representative is chatting to their most senior technician in the factory (he's so senior that he only works a couple of days a week now apparently).
From previous experience by re routing the cables should resolve these oscillation issues fully, I will wait to see what the tech says in response, but suspect they will recommend the same solution.
I'll be in touch shortly

I can't help think that an amp this old should not have survived if it has always had this problem. Unless this is not a problem in its own right, but a symptom for something else...