V4 Red Plating while playing

[VelvetGeorge]

If only V4 red plates, just replace the 82k plate resistor in the phase invertor with a 68k. This will make the output of the PI driving V4 closer to the output driving V5. And should alleviate red plating altogether.

The stock PI drives V4 harder than V5. This is common to all Marshalls. You can adjust that plate resistor to compensate, or try using a tube with lower gain on triode A vs triode B.

george

[planetjimi]

I'm wondering if you are confusing normal filament glow with redplating?

The big structure visible inside the tube is the plate.
When a tube is red plating this plate structure will glow cherry red.
When I say this I mean the whole plate will glow cherry red.

It's common in some power tubes for the filament to glow pretty brightly and it may appear that more than that is glowing.

What do you have the tubes biased at.

If you have the bias current too high, it will redplate.



Remember when you adjust the bias, you are setting the idle current.
It will go from that point to much higher when you are actually playing through the amp.

Also remember when setting the bias that all volume controls should be set to zero and no guitar should be plugged in.

Hi Structo,

It's the big grey plate in the tube. It's starts to glow in the center then expand outwards as I keep playing, but much less so than before. It doesn't ever get to be anywhere near the whole plate. As soon as I stop, it goes back to solid grey. This only seems to happen on V4 and only apparent in the very dark. These are the mesa tubes and they were biased at idle per the instructions you state. I kept them on the low side, they are at 31 and 25 I believe, so maybe the red plate would be worse if I brought them up a little more I don't know but they sound decent at this bias.

It does seem suspicious though that I can only get it to happen at full volume.. and of course I'm forced to use the weber mass, and that it goes away if I dial back the master volume a tad, so perhaps it is related to the load box. I had reproduced it some time back without the mass at full volume, but it doesn't seem to be as drastic now and I haven't had the opportunity to unleash the beast without the mass unfortunately. Having redone the whole amp and every solder point, I find it hard to believe that I've made a wiring error in the same place twice ( although it could happen). Unless there is some component attributing to it on the board who knows. But it's always that tube socket, and i"ve swapped out the .022 caps already. I'm also wondering if it's only on that tube because it gets hit harder than V5 ( per George).

Cheers

I've seen this happen in ALOT of el34 plexis that have been running for years with no problem. Your fine, leave it.

[redlir]

If only V4 red plates, just replace the 82k plate resistor in the phase invertor with a 68k. This will make the output of the PI driving V4 closer to the output driving V5. And should alleviate red plating altogether.

The stock PI drives V4 harder than V5. This is common to all Marshalls. You can adjust that plate resistor to compensate, or try using a tube with lower gain on triode A vs triode B.

george

Cool I'll try that and see if it helps.. I hope I have a 68k in the bag haha.

Cheers

[redlir]

I also just picked up a DR Z Air Brake at lunch so at least I have an attenuator I know can handle the punch of this Marshall.

There was an interesting note in the manual though. I wonder if anyone can make sense of it or would recommend this with a plexi?

"Helpful Hints:
When using the Airbrake with older vintage amps, mismatching the load will be a safer way to operate. This will eliminate any overheating of the output transformer. For example, a Vox AC30 or vintage 100 watt Marshall should be run at 8 ohms through the Z Airbrake with a 16 ohm speaker load."

[redlir]

If only V4 red plates, just replace the 82k plate resistor in the phase invertor with a 68k. This will make the output of the PI driving V4 closer to the output driving V5. And should alleviate red plating altogether.

The stock PI drives V4 harder than V5. This is common to all Marshalls. You can adjust that plate resistor to compensate, or try using a tube with lower gain on triode A vs triode B.

george

Hi George,

Well I installed the 68k in place of the 82k but it didn't seem to help unfortunately. What's intersting is both tubes start to red plate now if that makes sense. V4 still more than V5. I put the Winged C's I got from Metro amp in ( in place of the mesa's) and those tubes red plate a lot more. V4 almost has the whole plate glowing when I play. They were biased at 34 and 30.

I tried to use the DR Z airbrake instead but it has no where near the attenuation the weber has unfortunately so I could get it juiced up enough to tell. I'm at a loss I guess. If it's because of the weber mass that's fine, I just wish I knew for sure.

[VelvetGeorge]

That's strange. Something must be off then. Have you tested for red plating without an attenuator? That needs to be eliminated as a possible culprit.

You can also try setting the amp impedance to half of your cabinet load. i.e. set the amp to 8 ohms and run into a 16 ohm cab.

Regardless, the amp should not red plate when set correctly. Here's my check list:

verify that your screen grid resistors are in spec at 1k ohms.

verify that your OT secondaries are wired correctly.

verify that your OT primaries and secondaries are NOT bundled together. They can run parallel to one another, but shouldn't be cable tied together.

Measure the amp B+ voltage and bias voltage at pin 5 both at idle and at full output, when the tubes are red plating. These readings may help point us to the issue.

I know some of this is redundant for you, but sometimes a checklist can help.

george

[redlir]

That's strange. Something must be off then. Have you tested for red plating without an attenuator? That needs to be eliminated as a possible culprit.

You can also try setting the amp impedance to half of your cabinet load. i.e. set the amp to 8 ohms and run into a 16 ohm cab.

Regardless, the amp should not red plate when set correctly. Here's my check list:

verify that your screen grid resistors are in spec at 1k ohms.

verify that your OT secondaries are wired correctly.

verify that your OT primaries and secondaries are NOT bundled together. They can run parallel to one another, but shouldn't be cable tied together.

Measure the amp B+ voltage and bias voltage at pin 5 both at idle and at full output, when the tubes are red plating. These readings may help point us to the issue.

I know some of this is redundant for you, but sometimes a checklist can help.

george

I'm sure I tested without an attenuator very briefly when I first noticed the issue. I'll try to find 5 minutes again to make sure, but it's VERY hard to do as I'm in an apartment, and the time of day this can even remotely be considered does not coincide with dark enough times to see the tubes red plate. I may be able to put the amp in the bathroom LOL and maybe surround the speaker with a mattress or something to do a quick test.

The screen grid resistors did measure 1K when I checked on the second rebuild.

looking at the inside back of the chassis from the front, the output secondaries are wired from LEFT to RIGHT Yellow, Green, Grey

The red/white primaries are cable tied together where they finally meet up on the 2nd build.. when I compare to your instructions I see where you have them parallel and there is a bit of a gap between them. Even when I look back at my first build attempt, they were parallel but still touching. I change this tonight so they look more like the instructions. Do you think it could be something as small as this?

Oye the B+, I'll measure again while playing since I haven't done so since the rewire. I won't confuse this post with the previous results, but at idle it's 447.

[redlir]

That's strange. Something must be off then. Have you tested for red plating without an attenuator? That needs to be eliminated as a possible culprit.

....
Measure the amp B+ voltage and bias voltage at pin 5 both at idle and at full output, when the tubes are red plating. These readings may help point us to the issue.


george

Ok just some more info. I had to work late so I wasn't able to do the no attenuator test. I did return the air brake today and picked up an 8 ohm HotPlate. I used two 16 ohm speakers in parallel into the back of the hotplate and used the 8ohm tap on the amp into the hotplate.

Ok so, when I used bedroom mode to really juice out the amp without the cops coming to my house, there was indeed red plating. ( man does the weber kick this things butt tone wise at bedroom levels though). The cool thing about the HotPlate though, is the load button which I assume is just an 8ohm load to the amp so theoritically this should be the same as a speaker?

Anyway, when I kicked in the LOAD feature, the red plating was still there while playing, but definitely went down in level. It was just a faint red glow on V4 only.


Now B+ was about 440 on each without playing anything and both channels cranked. Bias with both channels cranked was 38 on V4 and 33 on V5.

While playing/red plating, with either the attenuator on the load setting or max bedroom level, V4's B+ would drop to about 370 and so would V5's. The bias mV on V4 would jump to about 210 and V5's would be around 135ish.

so if these readings are fine, it may be something to do with a load box on the amp. THe fact that the red plating reduces when I go from quasi attenuation to a full load box seems suspicious. Although I suspect I may find this minimal red plating at full volume without a load box, because I'm certain I did this before if I go back and read my posts.


Oh and I did separate the primary and secondary OT wires so they weren't together. They are a good inch apart in parallel now.

[Roe]

did you get rid of the redplating?

[redlir]

did you get rid of the redplating?

Nope but it only happens with everything on 11 so I can live with it. I'm using a variac now too.

[Structo]

Is there a master volume on this amp?

When you adjust the bias, do you have the volume set at zero and if you have a master that should be set fully clockwise.

Tone controls at noon.

[redlir]

Is there a master volume on this amp?

When you adjust the bias, do you have the volume set at zero and if you have a master that should be set fully clockwise.

Tone controls at noon.

There is, but I had the issue with or without the lar/mar installed.

Cheers.

[redlir]

I just wanted to follow up on this thread for anyone who reads through it in the future.

I did rebuild everything from the floor up with the original parts, and the issue was still there.

I've since replaced V1 with a new tube socket as I noticed there was some damage to the original one caused by too much heat my first time around. The V1 tube wasn't seating all the way into the socket, and now it is.

I've also added a HO's Attenuator in place of Weber Mass or Hot plate ( the ho's just slaughters them). I may try a phantom some day, but I'm pretty satisfied with the HO for the time being.

I have the original SED Winged C's in and there no longer appears to be red plating. I'm thinking it was probably the socket over the attenuator, but regardless it's finally gone. I've also since replaced the Xicon 10uf caps in the bias circuit with spragues.

The squeal when approaching the amp was rectified by the usual method of a grounding shield covering the opening of the top side of the chassis.
In fact, with the shield, and the Larry grounding scheme, I can pretty much get right on top of this amp and there is so little noise it's astounding.

The amp is still a bit fizzy, but I've barely had a chance to play it. Reading some posts, I've decided to do a full burn in on the amp for 100 hours. I have all the dials at 10, feeding some pink noise into the bridged inputs. I'm interested to see how much of a difference there is to the fizzyness once the SOZO's and OT have been broken in a bit. Once it's done, I'll probably order some NOS preamp tubes and RFT EL34's to round her out.

Cheers to you all, and thanks for the great suggestions and help.