JMP 50 with factory wrong grid resistors

[timbraun]

I think I finally realized why this JMP 50 I have had a history of problems (burned two PT's). There are 82K grid resistors in (as if it were a 100 W with 6550's) instead of the 220K that the schematic shows. It's a Canadian build, so would have come with EL34's.


Anybody out there see anything like this? It appears to be a factory error. The resistors sure look original, like the others.


I found a topic / thread on http://music-electronics-forum.com/t36971/ where somebody else had a 50 W JMP (same year, 1975) with 82K grid resistors. With a blown PT.

[Leevi]

I don't see that the grid leak resistors could be a reason for the burned PT.
But let's see what the other members say.


/Leevi

[tubeswell]

I gather that Marshall made changes to grid resistors in some models. EL34s have always been finicky tubes, even in the glory days of tube manufacture. The 'problem' with EL34s is their being prone to grid thermal issues because the grid is in very close spatial proximity to the cathode (to get more gain out of the tube), and the electrode cage is longer than your typical 6L6 or 5881 cage. As a result, the grid is prone to heating up and emitting electrons, esp when the tubes are running full blast, as they tend to do in Marshall amps where the screens and plates are run in the high 400s and the load resistance is low (3k4 for a pair, 1k7 for a quad). These conditions are quite severe on the poor old EL34s, especially when they are biased at the 'optimal' 70% setting recommended for PP amps in Class AB1. If the lost electrons can't be replenished fast enough to the control grid, the grid loses bias voltage and the heating/charging cycle spirals putting the tubes into melt mode.

The experimentation with changing grid leak resistors also needs to be understood in the context of general decline in the quality of EL34 tubes produced towards the end of the 20th century. I remember hearing somewhere that Lord Valve recommended that modern production EL34s should only really be regarded as having a nominal plate dissipation rating of 20W (instead of the 25W stated on datasheets). Even on the 1970 'unicord' schematic for the 1959 Superlead 100, it looks as if the grid leak resistance is written as '120k' (although its a bit hard to tell). (However, the RI schematic shows 220k). On another old 1959 schematic (I've seen somewhere), there is a handwritten note about the grid leak resistance needing to be 150k - I'll have a hunt for it. I've certainly come across a bit of debate on various forums about this. There was a good old debate about EL34s in Marshall amps over at Ampage a couple of months ago. If I can find the thread, I'll link it.

The trouble is that lowering the grid leak resistance too much really destroys the impedance bridging from the PI.

[Ritchie200]

Tubeswell - The 1959 MKII 6550 schematic shows the 82K, but that is the only one.  All the other 100W 6550 models show 150K (of that era). Debate?  I would think the only debate would be how to try and keep the new manufacture tubes alive in that torture chamber!

Tim - I would say factory error, but like Leevi said, I'm not so sure that has anything to do with a PT failure.  Maybe tube failure for sure.  Something to take a look at would be the line tap you are using.  If you got it on the 110v leg, you might be beating the crap out of everything with today's line voltage.  Also, if those are the original caps, they got to be taking a poop about now.

Jim

[Leevi]

Maybe tube failure for sure.

... which can lead to big current and burn the PT.


/Leevi

[Ritchie200]

Naw, that will just pop a fuse - unless it is of the foil wrapped variety!

Jim

[PRR]

> 82K grid resistors in (as if it were a 100 W with 6550's) instead of the 220K

I can't see how smaller grid resistors is any danger to the amplifier.

> a history of problems (burned two PT's). .... 1975

So two PTs in 42 years? 20 years each? And I believe old Marshall transformers have a reputation for being marginal life?

I don't think this is A Problem. I'd expect more from Fender or Sunn/Dyna, but I have seen many transformers give up the ghost after a couple decades. Transformers that might last a century sitting open on a bench, but instead jammed with hot tubes, life decreases rapidly with temperature rise. None of the amp makers designed for "lifetime" operation.

You can put another (perhaps clone) PT in there and odds are it won't be back before you close your shop for full-time fishing. Or you can shop Hammond's catalog for an oversize, or ask MM or other upgrade PT maker for a long-life model. Better shellac, better paper, maybe larger for better cooling. (You may be thinking "fan" but very few small fans run 20 years.)

[DummyLoad]

shorted diodes and/or dead/dying power rail caps likely blew the PT.


--pete

[timbraun]

Tubeswell - The 1959 MKII 6550 schematic shows the 82K, but that is the only one.  All the other 100W 6550 models show 150K (of that era). Debate?  I would think the only debate would be how to try and keep the new manufacture tubes alive in that torture chamber!

Tim - I would say factory error, but like Leevi said, I'm not so sure that has anything to do with a PT failure.  Maybe tube failure for sure.  Something to take a look at would be the line tap you are using.  If you got it on the 110v leg, you might be beating the crap out of everything with today's line voltage.  Also, if those are the original caps, they got to be taking a poop about now.

The PT that was in it when I got it didn't have voltage taps, and the output voltages were decent when I set the biasing. Like 431 V on the power tubes, that neighbourhood.


I do suspect the B+ caps are leaky and may have been the cause of the smoked PT. I've got replacement caps, but I'll test the old caps with a series resistor before I swap them so I have a chance to find "root cause of failure".

shorted diodes and/or dead/dying power rail caps likely blew the PT.

Yeah, that's my current theory (caps leaky, diodes were okay). I think the EL34's survived the latest PT smoke.

[Leevi]

I do suspect the B+ caps are leaky and may have been the cause of the smoked PT. I've got replacement caps, but I'll test the old caps with a series resistor before I swap them so I have a chance to find "root cause of failure".

Yes, the filter cap(s) can be the root cause. If you have possibility to add a fuse (500mA) before the first filter cap it would protect the PT.


/Leevi