issues with a GZ34 and open circuit anode resistors.

[bikerhifinut]

Any ideas gents?
I was running up my EL34 monoblocs prior to running a few tests before I take on the task of the off board PSU. One of them on switch on had an instant fault. The GZ34 rectifier flashed quite alarmingly, fireworks in side the bottle and then although the valve stayed lit up the amp was very obviously dead.
I was fearful that the output transformer had gone S/C to cause this but on examination and measurement all I could ascertain was that the 100R anode resistors on the GZ34 had both gone O/C but no distress marks on them at all. These are fairly cheapo white cement coffin jobs from maplin rated at 7W which should be more than ample. So is it just **** resistors or something else? All I can think is the PSU electrolytics could be to blame as they run very close to their 500V rating and have on the odd occasion gone a bit over when testing and setting the amp up (High mains volts and transformer with only 230V taps). I can find no problems elsewhere in either amplifier. So my instinct is to junk the "Ruby" USA made (I think) PSU caps from Hi Fi collective and source some better branded jobs, maybe wire em as series pairs to handle any voltage surges.
The valve is definitely sick now too so was it the valve failing that caused the resistors to go open circuit or a fault elsewhere that took the valve out too? The thing is that the flashover occured pretty much at switch on so that is a mystery too.
for the record I have gone through a few anode resistors on the rects so that may point to a fault in the PSU? All circuit voltages check out as bang on design values by the way and I am sure cathode bypass caps are good as are coupling caps.

Andy

[bob orbell]

I would say that it is only the valve that was at fault Andy, do you have a variac ?, they are very handy with this game. BOB

[bikerhifinut]

Its an item I am sadly lacking Bob, I can probably lay my hands on one.
however these monos have a history of causing rectifier resistors too go OC, in the past they have usually shown signs of overheating first, not every time but this time there was a fast and obviously catastrophic failure of them. I think its a combination of lousy quality resistors and something else PSU related that's causing the stress, hence my suspicion of the main reservoir and smoothing capacitor. by the time the volts get to the phase splitter and first stage supply the stress on the smoother/reservoirs is bugger all.

I was minded to try a polyprop first cap rated at 630V I don't need 50uF before the choke. 16uF would be fine and 32uF ample. The original design i adapted these from used only 16uF after the rectifier.

A.

[pre65]

What make is the GZ34 ?

Does the "flashing" start immediately the amp is switched on ?

Check the GZ34 data sheet, because there is often a maximum size for the first capacitor.What value were you using ?

Was the value of the anode limiting resistors calculated by yourself ?

Do you use a thermistor on the amp mains input ?

[bikerhifinut]

Originally Posted by pre65

What make is the GZ34 ?

Does the "flashing" start immediately the amp is switched on ?

Check the GZ34 data sheet, because there is often a maximum size for the first capacitor.What value were you using ?

Was the value of the anode limiting resistors calculated by yourself ?

Do you use a thermistor on the amp mains input ?

It's a genuine Blackburn mullard as is its mate in the other amp but even a Chinese one would be adequate at the voltage and current I am using. It's venerable but as far as i could see still perfectly healthy.

It flashed just the once at switch on, after that I assume the anode resistors had failed fairly instantly thus leaving it O/C and therefore no load on it.

Trust me Philip I have been around long enough to know about the maximum capacitance straight off the cathode, I was using 50uF easily inside the 60uF maximum.

The anode resistors are fine at 100R, at that voltage that's the Mullard recommended value off their data book, and with the transformer internal resistance of 47R thats a good 150R so a bit in reserve to limit current inrush. The Leak amps never used any sort of limiting resistor relying on the transformer resistance to provide the load and mine have never been a problem.

I see no obvious reason to use a thermistor on the mains input, on an EI transformer you don't see one on commercial or even the WD designs. it's got a fuse nicely rated at just above the load current which seems to me to be enough. Now if it was a big toroid then no doubt at all use a GEC MOVistor or similar device (varistor) to counteract any voltage spikes on switch on or switch off. This is something i will need to watch when I build the offboard PSU for these amps as its a pretty massive toroid that I have had custom wound by a friend along with dire warnings about correctly protecting it!

The circuit looks and measures fine, no obvious shorts or component failures anywhere. I am starting to wonder if the type of anode resistor is the issue as they were cheapo (except they aint that cheap) maplin sourced white cement coffin jobs. I know JC has a low opinion of them and I now think i can see why. They seem to go open circuit arbitrarily.
The other issue is possibly the type of electrolytic i am using. They are "Ruby" 50 + 50uF at 500V sourced from hi fi collective connected across a Hammond 5H choke which runs very cool so not concerned there. I suspect they are barely capable of withstanding their 500 rated dc volts and a fair bit of ripple not withstanding they were advertised as suitable for PSU use.
My gut feeling right now is to swap the first 50uF out and shoehorn in a 10 to 16uF polywotsit 630V job. If the Rubies are ok still I can safely parallel them to give 100uF south of the choke. I can only assume its either faulty electrolytics or just too much inrush current (the faulty elcos having a similar effect) which either takes out the resistors and/or stresses the rectifier valve. I am minded to try silicon but frankly as I am modding the amps anyway its a waste of time as they will be fed from a good silicon rectified supply once thats built.
No i think the main issue is that I may have used underspecced PSU capacitors and have possibly trashed them when I should have used series connected pairs at the front of the PSU to get the working volts up above 700 or so to give a safety cushion. Not an expensive solution in the scheme of things.

As an aside i checked the other mono as it seemed to be humming a bit too much and the already very noisy mains transformer seemed to be throbbing more than its usual racket and lo and behold one of the rectifier anode limiters/droppers had also gone O/C, no sign of overheating or distress of any sort.
I think its simply crappy wirewound resistors not fit for purpose and possibly the elcos straight off the cathode.
Back in the day Harold Leak used 16 uF here and in the higher powered TL50 8 uF as first capacitors. I always assumed because in those days at those voltages bigger electrolytics just weren't available, at least at any sort of reasonable price.
But now I'm wondering if there were other reasons. I'll dig the data out again and see if I missed something in the small print.
I could always use a 5U4 and take the voltage drop on the chin bearing in mind I'd have to keep the capacitor below 10uF in that case. the transformers can handle the extra amp of heater current.

Phew.

Andy.

[Richard]

Did you check the voltage at cold switch on Andy? Thinking the caps may be going short. My Matamp 5-20s had valve rects straight onto 2 seried 300uF caps (=150uF), no choke or second filter before going to the output, 100Rs on the anodes and no problem or hum. I tried several types without problem inc Mullard GZ34s and preferred the old coke bottle style 5U4 winged C (5U3 Russian).