Marshall JTM 1982 of 2007,PbF

** The model number is wrong or incomplete.

There ain't no " JTM 1982 "

The number 1982 refers to a Quad box.

Maybe it's a JTM45 re-issue.

.... Phil

With 8V 400Hz over 8R dummy load, no drop off seen after 10 mins. Preheated rect bottle with low setting hot air and 10V over 8R, and short bursts of extra directed heating and a see-sawing of output, then over 10 seconds or so extreme swinging down to 1V or so and over 10V or so repetitively, tremolo type speed. At least I've shown there is a problem unlike the other 2 "repair" people who've looked at this amp, returned without fixing it . Next step to repeat with hot air, monitoring the DC and then try SS rect

Could not repeat while monitoring the HT. So as I can't believe an electrode or 2 failure,or whatever, in the GZ34 had cured itself by my external and internal heating, just a coincidence and some other problem? With original failure returning immediately to full volume, without cooling down, might suggest a "cured" internal problem. I tried heating the pcb with hot air and no return to failure mode. So valve test the GZ and the other valves, put a saggy SS dual rect in there after checking the GZ valve base. Some heavy prolonged testing and return to owner with fingers crossed?

As though a thermal protect measure is cutting in falsely, but all valve.

10 minutes at gig level, longer at practise level, initially looses bass and then over a couple of minutes vol drops to uselessly low. Switch off ,cool down, and fine again. I'll check with a variac, to simulate gradual loss of DC, but is that the symptom sequence of loss of DC to a valve amp, lose bass then lose vol. Valves have been swapped out , but not the rectifier bottle, likely suspect?, a saggy 2x4007 lump will go in there initially to check that. A hot-air gun looks a promising tool. No hum reported, unlikely a major cap problem,a minor cap problem, surely not a Tx problem

You could check it is not a dry jointed heater connection to one or more valves.

Gareth.

** I have seen any number of Russian and Chinese octal valves with badly soldered pins - the steel wires inside the hollow pins are dry jointed and can disconnect at whim. This is visible under magnification.

If this happened on heater pins ( 2 or 8 in a GZ34 ) the output would fade down to nothing over about 10 seconds. A high resistance, intermittent connection could produce the symptoms seen by the OP.

Variations in the two spots of red light at the top of the valve would give you a big clue as well.

.... Phil

I never saw this, but then maybe I never looked. One of the things to know is what you don't know. I know I have had tubes with intermittent elements, but it is too late to think back to all of them that were octal.

Becoming inoperative could be caused by any pin and will be a malfunction of course, I mean the screen grid, plate, whatever.

If a tube/valve has good emission and all that but is intermittent, I wonder if that base would withstand a quick dip in a solder bath.

These are newer ones that are lead free ?

I'd forgotten about the bad solder problem , internal to the hollow pins of such bases, combined with PbF solder presumably as this is the original valve in a 2007 amp with green RoHS sticker on the back. I was only thinking of failed spotwelds internally. BTW mistyping of heading, model 1962 reissue with FET1 tremolo gizmo

I tested this JJ GZ34S (no other info on it like date) and came up 2x

75% so good. I'll retry while applying hot air and vibrating it with my modded engraver tool (nylon bolt instead of active steel engraver pin) How to explore the internal solder of the hollow pins without disturbing the join between bakelite and glass at the base?

Been doing this repair since 1978...

Touch a very hot iron to the pin, about midway between the point and the base. Hold for a few seconds. That should remelt the solder inside and reset the connection.

One of the few legitimate uses for a 200W soldering gun - be sure to pre-heat first. The idea is to get the pin hot enough to reflow the solder before the base knows what hit it. It is too little heat for too long a time that does damage.

Peter Wieck Melrose Park, PA

I was thinking grind a small hole in each pin, introduce flux and ordinary soldering iron. Made up an adaptor (2 back to back QM connector pins , 1 male,1 female for each extender) to set the valve on Avo CT160 tester, raised up, so I can selectively hot-air heat the pins to see if failure recurs, then try resoldering the internal tail wires to the tube pins

** Which is long before Pb free solder or Chinese and Russian made tubes became the norm - so completely irrelevant.

IME, the solder is cracked near the end of the pin and that is the only place solder is ever applied.

A little flux, some 60/40 solder and a hot iron tip is all that it takes to effect repair - plus a pair of working eyeballs.

.... Phil

Heated up the pins and engraver vibrated , while on the tester, and no wavering or failure. So resoldered the pins and will try back in the amp instead of SS rect. Another thing I've never seen , one of the ECC83 valve bases is only held on by one pop-rivet, the other rivet bulb is held in the valve base ring, pulled thru the chassis hole, not been knocked, just normal valve estraction force was enough

became the norm - so completely irrelevant.

Phil:

SINCE - that means: Frequently since I started seriously in the hobby. And this also includes Chinese, Eastern Euro and Russian tubes. Some of the che aper Chinese octals have solid-point pins, so there is no way to introduce flux or solder within them without removing the entire base. In that case, heating the pin is the lowest risk expedient.

Peter Wieck Melrose Park, PA