Marshall JTM/1962 , of 1998, amp clipping

v3p1 anode is 220V and v3p6 anode is 168V so leaky .1uF letting through some of the neg bias from the output bottles , I assume

Can't really follow that.

If you pull the phase splitter, are its grids getting an undistorted signal measured at the empty socket?

Similarly, if you pull the output valves and put back the phase splitter, are they getting a good grid signal?

Gareth.

"Nutcase Kook"

** ROTFL...

"Nutcase Kook"

** The output tranny may have shorted turns.

I test them by driving the speaker winding with an audio generator set at

400Hz and checking the AC at the anode of each EL34 / 5881 with a scope.

The amp is powered off for this, of course.

.... Phil

I forgot to say swapping output bottles did not swap the problem side. Trying a 600 ohm sig gen backwards into "8 ohm" so about 50mV pk-pk there,

0.36V on the (from low V3 anodes) problem side "white anode" and 0.5V on the red other . I assume they should be much closer than that. I suppose the conclusive test is swapping red and white wires and see of the low splitter V3 anode voltage swaps halves.

of

of

signal

I may as well try removing the output valves and trying on 80 percent mains or so ,and check their cathode signals, but looks more and more like a turn or few , shorted turns in the Tx

a corroded break rather than shorted turn/s? now taking Tx readings DVM White 61.8R , Red 75.3R was 61.3/66.4 RLC(L) 20.9mH, 34.0mH RLC(R) 215 imp, 338 imp Things certainly not right but what is wrong inside? tugging the tails makes no difference

"Nutcase Kook"

** Valve OTs do not get hot enough to melt the enamel insulation on the wires, as often happens with power trannys. What brings about their demise is the very high voltages that exist during operation into a loudspeaker, particularly when driven into clipping.

Voltage peaks can then occur on the anodes of the output tubes that range up to thousands of volts - resulting in insulation breakdown and arcing between adjacent layers of the primary winding.

Sometimes an OT will test OK with a multimeter or at low power only to arc internally as soon as full power operating conditions are applied. This is due to carbonised material in the area of the previous arcing reigniting in a new arc.

Marshall amps using EL43s are the prime example of this kind of OT failure.

Some amps, notably Music Man and some Boogie models, have high voltage diodes fitted from output valve anodes to ground to prevent the sort of high voltages that damage OTs.

.... Phil

You either got raised resistance of the cathode resistor(s) of the splitter , low voltage to an element of the outputs or shorted something on the outp ut.

Do you know how to use a scope in the DC mode ? Whatever the cathode voltag e is on the outputs, remember that and set the trace to a line somewhere, o ver it. If the grid drive gets up near that level then the problem is after . Has to be in the load or the bias of the output stage then.

If however you see that the output grid drive clips before getting near the cathodes' DC level (of the outputs of course) then look to the splitter st age. The way to tell if the cathode resistor value(s) has gone high is that the plate voltage will be high. Usually.

J

You either got raised resistance of the cathode resistor(s) of the splitter, low voltage to an element of the outputs or shorted something on the output.

Do you know how to use a scope in the DC mode ? Whatever the cathode voltage is on the outputs, remember that and set the trace to a line somewhere, over it. If the grid drive gets up near that level then the problem is after. Has to be in the load or the bias of the output stage then.

If however you see that the output grid drive clips before getting near the cathodes' DC level (of the outputs of course) then look to the splitter stage. The way to tell if the cathode resistor value(s) has gone high is that the plate voltage will be high. Usually.

** OTOH - the amp might just be hexed.

Don't you think ?

.... Phil

Do you know how to use a scope in the DC mode ? Whatever the cathode voltage is on the outputs, remember that and set the trace to a line somewhere, over it. If the grid drive gets up near that level then the problem is after. Has to be in the load or the bias of the output stage then.

If however you see that the output grid drive clips before getting near the cathodes' DC level (of the outputs of course) then look to the splitter stage. The way to tell if the cathode resistor value(s) has gone high is that the plate voltage will be high. Usually.

J

+++++

The owner reckons he knows someone across the country with a spare Tx and ability to fit it.

So what now, he ships the thing there ? I think it would be better to ship the Tx than the whole amp. Of course some people are nuts.

Really though, if this thing runs class AB, if the Tx is shorted (turns) ca using this, I would imagine the output tubes would redplate if you keep on pushing it no ?

Maybe you should plug the primary of that transformer into the mains and se e if it smokes.

HEY, that is not stupid as it may sound really, but I would definitely remo ve it from the cabinet first. Think of it, you put 220/110 at 50/60Hx to it at the primary ? That shouldn't bother it at all. If it does, it's bad any way. Difference is, this is a source with some balls (current) behind it. T he impedance SHOULD keep it from smoking.

It's called follaw the smoke, I've done it meself sometimes.

J