VM2466H Unstable Bias

[knowtice]

Hello, so I have an issue with my VM2466H head. Just a little background I've owned it since 2007 and it's been the best amp I've ever played. Early on, maybe 2 years in it needed a screen grid resistor replaced, V6. This was the only service the amp has ever needed, and I've changed tubes and biased ever since with no problems. Until recently, my tubes that i've honestly used for the last 6 years finally died, or so I thought. I know the sound of power tubes failing, it' s usually, complete loss of body and power, thin sounding, so I made the assumption that was the problem. Ordered new Gold Lion KT66's, put them in, biased at 78mv, and the amp sounded better than ever. After about a month, I was recording, and noticed the tell tale sign of tubes failing, lost all my tone, so I couldn't believe it, I checked the amp and V8 was flaming out! I immediately shut it down and assumed the tubes were bad so I got a replacement quartet. This time my suspicions were high, so I checked the bias continually instead of just when dialing it in. Before all this there's been noise that comes and goes and I thought it was the bad tubes. It's like a hum will come out of no where, then other interference type noise and I just thought it was like some sort of power interference. However I found that the noises correlate with the bias jumping around. When the hum came in, the bias had jumped to 85mv, the dropped to 65, the up to 78, so it's jumping around all over the place. Are these the tell tale signs of another screen grid resistor? If so not too big of a deal, I think I could swap that, but I'm wondering if anyone can tell me how to test for that. I'm concerned of it being a bigger problem. As such I've heard of Marshall DSL PCB's being faulty and causing unstable bias because the paths are so close, so I hope it's not that! Thanks in advance!

 

[Pete Farrington]

I checked the amp and V8 was flaming out!

I read 'flaming out' as flames are being emitted from the valve in V8. Is that what happened?

 

[RPR]

Could it be a coupling cap that is failing ?, that could explain the hum .Maybe also a cap in the power supply ?. There's not much else in the power amp section other than the output transformer. I had lower power on my 2466c and r55 and a tube went .With r55 open I wasn't getting any bias voltage on that connector.Im not sure if resistor going bad can do that, I guess it's possible but usually they're good or bad(open).I haven't heard of issues with the VM like the JVM 2000 has.Could be a cold solder to if something is intermittent.

 

[knowtice]

I read 'flaming out' as flames are being emitted from the valve in V8. Is that what happened?

Internally it was like it was on fire inside the tube is the best way I can describe it. It wasn't like fire outside of the tube

 

[knowtice]

Could it be a coupling cap that is failing ?, that could explain the hum .Maybe also a cap in the power supply ?. There's not much else in the power amp section other than the output transformer. I had lower power on my 2466c and r55 and a tube went .With r55 open I wasn't getting any bias voltage on that connector.Im not sure if resistor going bad can do that, I guess it's possible but usually they're good or bad(open).I haven't heard of issues with the VM like the JVM 2000 has.Could be a cold solder to if something is intermittent.

Interesting R55 is exactly what was replaced on my amp earlier on in it's life.

 

[knowtice]

I also wonder could preamp tubes have anything at all to do with this? Like if my phase inverter in V4 is bad would it affect my bias at all. I tested it again last night, and it seems like it's a drifting runaway bias. It's starts at 72mv, then, just keeps slowly climbing as the amp warms up, 72.5, 73, 74 and so on all the way up to 80 and I shut her off. My other question is could different brand power tubes not have this problem? I would think not, seems like an internal amp issue, however, I have no noise whatsoever and I will have to test again, but the bias seemed stable with my old Shuguang's. The new Gold Lions are where I'm seeing the problem, but this may just be coincidence. Thanks in advance!

 

[RPR]

I googled red plating and found an interesting video.It could be caused by a few things and probably not a bad tube.Could be a coupling cap, resistors that the value has changed and the output transformer.Have you ever measured the voltages on your preamps anodes ?, Mine is 164v and I think it's too low, it could be me but I think my amp sounds different.I wonder for you if you have a resistor that is changing value when it heats up, or something with a few windings on the transformer when it heats up .

 

[knowtice]

I googled red plating and found an interesting video.It could be caused by a few things and probably not a bad tube.Could be a coupling cap, resistors that the value has changed and the output transformer.Have you ever measured the voltages on your preamps anodes ?, Mine is 164v and I think it's too low, it could be me but I think my amp sounds different.I wonder for you if you have a resistor that is changing value when it heats up, or something with a few windings on the transformer when it heats up .

I sure hope it's not the output transformer! I am ignorant to what coupling caps are, or where they are located. Is there any way to get a hold of a service manual? I have not measured those voltages from the preamps anodes, tell me where to measure and I'll let you know what I've got.

 

[RPR]

The coupling caps connect the output of one tube to the next tube.The output of a tube has the high voltage on it and the guitar signal.The DC high voltage doesn't pass through the cap but the signal does.So if the cap is bad this DC voltage gets sent to the next tube.Which is bad.I have to pickup my wife, I'll get on my computer later. You can get schematics on the internet.Watch out for those voltages though.

 

[Pete Farrington]

Internally it was like it was on fire inside the tube is the best way I can describe it

I can't conceive of a fire inside a valve - there's no oxygen necessary for a flame, so I can't visualise what might have been happening.
Common output valve failure modes are arcing and redplating. I suggest you search for images of these and try and work out what matches what was happening in your amp.

It's starts at 72mv, then, just keeps slowly climbing as the amp warms up, 72.5, 73, 74 and so on all the way up to 80 and I shut her off.

It's normal for anode current to increase a little over the course of say 30 minutes, as valves warm up and settle in. Hence the above isn't necessarily indicative of a problem.

it needed a screen grid resistor replaced, V6

R55 is exactly what was replaced on my amp

Yes, from https://musiciansroadhouse.com/viewtopic.php?t=11594 R55 is a screen grid resistor. They tend to fail if its (output) valve shorts but the HT fuse fails to blow quickly.

When the hum came in, the bias had jumped to 85mv, the dropped to 65, the up to 78, so it's jumping around all over the place. Are these the tell tale signs of another screen grid resistor?

Any kinda bad solder joint of a screen grid or other resistor or wire related to an output valve might cause the above.
Solder joints that are suject to heat and mechanical stress over time will tend to fail, especially so with lead free solder.
If you're competent at soldering I suggest to remove the bulk of original lead free solder and reflow with good quality leaded solder.

 

[knowtice]

I can't conceive of a fire inside a valve - there's no oxygen necessary for a flame, so I can't visualise what might have been happening.
Common output valve failure modes are arcing and redplating. I suggest you search for images of these and try and work out what matches what was happening in your amp.

It's normal for anode current to increase a little over the course of say 30 minutes, as valves warm up and settle in. Hence the above isn't necessarily indicative of a problem.




Yes, from https://musiciansroadhouse.com/viewtopic.php?t=11594 R55 is a screen grid resistor. They tend to fail if its (output) valve shorts but the HT fuse fails to blow quickly.


Any kinda bad solder joint of a screen grid or other resistor or wire related to an output valve might cause the above.
Solder joints that are suject to heat and mechanical stress over time will tend to fail, especially so with lead free solder.
If you're competent at soldering I suggest to remove the bulk of original lead free solder and reflow with good quality leaded solder.

thanks for your time on this Pete!
the valve was glowing and burning a bluish orange flame, definitely didn't look right, it looked like a big pilot light for a furnace? It looked like a flame inside the tube.
Yes, 30 minutes, but I went from 72mv to 80 within 2 minutes...
So, my HT fuse should blow if i have a bad screen grid resistor right?

 

[knowtice]

So i just checked my bias again, let it warm up more, and it seemed to slow down a little on the rising bias....then it gets erratic, it was starting to settle around 77-78mv, then it'd drop to 24, 44, 58, 65, 14 , etc all over the place like that. I made sure i was holding the leads consistently so it wasn't me. Then a hum comes in out of no where... and goes away... then comes back louder... so that's what's going on. TIA!

 

[RPR]

I also didnt mention that when a coupling cap goes you can hear the dc voltage , that might be your issue , and like someone said above , could be cold solder joints as well , these problems occur when it heats up.
This is the link to the red plating video


You can ge tthe schematics by googling it.

A few examples of coupling caps are c2,c3 for V1 and c34,c35 for the inverter tube , the c34 and c35 go to the power tubes.
The voltages im looking for are at pins 1 and pin 6 for v1,v2 and v3. So it would be pin1 and the other lead to chassis same for each pin.

Mine reads 162v ,I think it should be higher.
And sometimes I find my amp doesnt sound the same , ealier today the ldr was horrible , not breaking up , then later its much better.
Does your ldr sound the same as it was ?

 

[Pete Farrington]

it was starting to settle around 77-78mv, then it'd drop to 24, 44, 58, 65, 14 , etc all over the place like that. I made sure i was holding the leads consistently so it wasn't me. Then a hum comes in out of no where... and goes away... then comes back louder...

The good news there is that it's not shooting up, so nothing to indicate the bias feed to, or V DC in general at, pin 5 is dodgy. Which can precipitate bad things happening.
Something dodgy in the screen grid supply (to pin 4) seems more likely.
So the solder reflow idea still seems your best option.

my HT fuse should blow if i have a bad screen grid resistor right?

Screen grid resistor failure is a symptom / collateral damage. The root cause is a bad output valve, shorting out and drawing too much current. Or a bias supply failure.
Ideally the HT fuse would blow before the resistor gets damaged.
It's more likely to work like that in a 50W than a 100W, because the HT fuse can be a lower value, eg 500mA.
Whereas the 1A HT fuse of a 100W won't blow, or will take a while to blow, if the current that the bad valve draws isn't greater than 1A.
Another complication hampering the fuse responding is that the design has it at the transformer winding, protecting the whole HT system, so it has to be a time delayed T type, in order to accommodate the start up surge current required to charge up the caps.
A fast acting F fuse after the reservoir cap might be an improvement.

the c34 and c35 go to the power tubes.

C36 & 37 are the only coupling caps whose failure could cause a serious problem for the output valves, ie by buggering up the bias voltage at pin 5.

C34&35 just couple the LTP phase splitter anodes to the type 1 master volume tracks. If they leaked a bit of DC, all that would probably happen is low V DC on the volume control track, causing staticy rustling noise when it was turned.

 

[RPR]

Does anyone know what voltages I should get on the anodes of the preamp tubes , Im pretty sure on ldr mode it used to break up more.I had a bad power tube and r55 was bad.The tubes and resistor have been replaced but I think something is still off.

 

[Kuga]

Try push on FX loop buton. Without cables connected on it. Do you hear a volume drop?

 

[Kuga]

If you get a volume drop when turn on FXloop it's wrong FET3 from FXloop.
This it's not related on your LDR issue. But a power section failure can destroy FET's on FXloop as other components. Did you been using FXloop when power valve fails?

 

[RPR]

Yes, always using the fx loop.Ill check the other stuff after supper, thanks.I was wondering about the voltage on the anodes of the preamps as mine are 162v and someone else measured 220v.

 

[Kuga]

Yes, always using the fx loop.Ill check the other stuff after supper, thanks.I was wondering about the voltage on the anodes of the preamps as mine are 162v and someone else measured 220v.

I bet FET3 from FXloop it's wrong. Mesure voltage on the anodes of preamp valves without FX loop on.

 

[RPR]

Ok, I disconnected the cables and tested the loop, it seems good, and today it seems to distort more than other days .I'll try measuring with the loop in bypass, do you think the voltage is low ?