WEM Westminster ECL82 anodes glowing

[qualityten]

I'll post some better quality pics.

Meanwhile, just for interest, here is a link to another repair job on the same amp by Champ Electronics, which has one clear pic of the board. I see that the 200R 3W resistor has equally scorched that board and that the repairer also complains about the physical layout. http://www.chambonino.com/work/wem/wem1.html

[qualityten]

Originally Posted by pwdrive

Another good check to do would be to check the primary winding of the sound output transformer.
This MUST be done with the unit switched off.
Measure the resistance from the center tap (the one that goes from the transformer to the HT resevior line) and the other ends which go to V2 anode pin 6 and V3 anode pin 6 respectively the two results might not be exactly the same (this is normal) but should be pretty close.

Sorry to have been quiet for a couple of days.

Resistance measurements to centre tap: v2 pin 6 is 112 ohms, V3 pin 6 124 ohms.

[GrimJosef]

That sounds close enough, unless anyone knows better.

Cheers,

GJ

[ms660]

Hi G J in a previous post I mentioned a possible extra resistor shown on the O/P trans. connected to the tone correction cap. The gentleman posted 2 pics early on in this thread,,,see what you think.
Yes the O/P trans. readings do not look far out, maybe swap the anode connections to see if the other valve glows instead just to eliminate the transformer ?

Cheers.

[qualityten]

I just soldered new connections between the point where the .01uf capacitor that I lifted connects to the PCB. I noticed that the ECL82 glowing was now in V2 (this is a new valve). I moved it to V3 position and noticed that it glowed red there.

I then replaced it with an older ECL82 and the result in V3 was worse. Yet there was less hum. I attach a pic which looks a little worse than it is because of the camera working in low light.

Tried it again with another valve in V3, same thing. Yet, as the voltage was rising, these valves had that healthy blue glow for a while.

Puzzling.

[qualityten]

Originally Posted by pwdrive

Just been looking at the photo's you posted...to the left of the tone correction capacitor on the sound output transformer there appears to be what looks like a 1/2 watt resistor connected to one end of the tone correction capacitor but I cannot see where the other end goes (I need glasses!!) if it is a resistor then it could possibly be a low value one but I can not say for sure, the other resistor..4.7k..is part of the tone correction as per the circuit.
Not sure about the 1/2 watt resistor if that's what it is.
Might be worth looking into.

Screen feed is via the 1.5 k filter resistor.

Any other members see what I mean ?

Hi PW, I have posted another pic from above that should be clearer. I think you are talking about the mustard coloured 2n2 cap on the output transformer.

[valvekits]

Hi David
Ouch that's glowing!
Just in case you have some RF oscillation, short pin 3 of both output valves to ground.
If they still glow then you are only left with managing the dc conditions which are too high.
If this is the case first step, you can just double the cathode resistor value by putting the old one in series with the 220 you fitted. No harm to just see where that takes us.

Eddie

[ms660]

The "other resistor" on the the old pic clearly isn't one on the new pic (it's a transformer lug!)..so thats sorted.
Phew! that valve looks seriously hot to me, I am trying to figure this out, the voltage measurements you took seem not to bad, the HT was a bit high but nothing way out, there is something bad to cause the valve to glow like that, looks like there is umpteen +Ve volts on G1 but you have measured them, the value of the cathode resistor that you repaced is correct from what you say. It's a pity that no members of the forum are near you to check it out.
You could try shorting the G1 grid (pin 3) to ground and see if the valve still glows.

[ms660]

Hi Eddie ...I was typing my last post while you were posting. It always happens!

[qualityten]

Thanks to you both. Will short out the pins as requested. And then add the resistor to try dropping the DC.

[GrimJosef]

Originally Posted by pwdrive

... there is something bad to cause the valve to glow like that ...

We're still stuck with the problem that these valves are being run with much (60% ?) more internal power dissipation than the manufacturers recommend. In those circumstances it's easy for things to run away. Shorting the grid (pin 3) to ground with as short a lead as is practical would be a worthwhile test though.

Cheers,

GJ

[electrogram]

This amp is definately supposed to use ecl82 not ecl86 ? is the pin-out the same ?

[GrimJosef]

No, the pinouts are different. And the WEM schematic (post #1) explicitly says ECL82.

Cheers,

GJ

[Diabolical Artificer]

Hi David,I cant contribute much I,m afraid as other members are far more knowledgeable than I am. However John Chambers at Champ Electronics may be worth giving a call for general background on this amp. If you do decide to contact him,I think he prefers to talk on the phone after 12pm rather than Email. He,s a nice bloke. Best of luck,Andy.

[qualityten]

I have just shorted out pin 3 on both valves and V3 still glows, so I put in a 500 ohm cathode resistor. V3 still glows, but doesn't look as bad. Tried again with 1K, and the glow is dimmer still. How high can I go with that resistor?

[ms660]

Hi there...I wouldn't as you risk exceeding the working voltage rating of that resistors by pass capicitor if it is quite a low voltage.
An anode current measurement for the overheating valve would be the next step so everybody can see what current we are dealing with.
So....To do this...Isolate from the mains reinstate the 220 R resistor and disconect the wire on the O/P trans. that goes to v3's anode (pin 6 I think) if you disconnect it at the transformer you can easily connect the free end to one of your meter leads and connect the other meter lead to the transformer your meter needs to be set to DC amps at 1 amp say...switch the unit on 'till valve warms up and switch down range on the meter till you can take an accurate measurement. I do not know the max Ia off hand for that valve so post it up and someone should be able to clarify.
Cheers.

[glowinganode]

What voltages are you getting across the cathode resistors?
You have replaced the electrolytic by-pass caps haven't you?
Rob.

[GrimJosef]

Yes I wondered about the cathode capacitor too. But post #12 suggests it was replaced.

I'm afraid we're going to have to bite the bullet and measure anode currents as pwdrive suggests. With measured voltages and measured currents there will be almost no place left for this fault to hide (the only one I could think of would be some strange high-frequency oscillation and shorting the grid to ground should have nailed that). Incidentally, a reasonable indication of current can be had without breaking the anode circuit by measuring the voltages across each half of the output transformer primary. As far as DC is concerned these are just resistors (112 ohms and 124 ohms according to post #62). Perhaps 40 mA through each half should give 5V or so across them. High currents might lead to some heating which could change the resistance a bit but if the answer is very different from 5V then that would be very informative. Take care while doing this of course. Both the windings are at HT potential.

Cheers,

GJ

[qualityten]

V3 anode current measured 61mA, when warmed up. It might have still risen a bit more but I switched off for the valve's sake.

On V2, the anode current rises to about 38mA, and then starts gradually falling away.

[Sideband]

Could it be a problem with the output transformer?



SB