problems with original wad 300b

[julianr]

Hi
I made one of these for a friend and it has worked great for 10 or years, only things done have been to change the 300bs a couple of times and change the 5687. Anyway it looks as if one of the 300bs have gone - the cathode resistors on one side are measuring 67 Ohms instead of 1.2K rest are ok, the amp got very hot and the gz37 glowed cherry red - he put it off immediately.
With all the valves in except the 300bs voltages are good, and the amp does not draw too much current. When I put in the 300bs and start up with a variac as the heaters kick in the current shoots up and some of the 300bs get very hot - presumably the one that doesn't is the blown one.
Is it possible that anything else is wrong. I have got it home now so can test it more fully.

thanks Julian

[Clive]

Hi Julian,

There will be those that know the amp much better than I do. All I can suggest is that this sounds the same as my KaT34. One EL34 went cherry red, a cathode bypass cap had failed and shorted out. I suggest you remove the dubious cathode bypass cap and re-measure the cathode resistor.

Clive

[julianr]

Hi Clive
I have removed the resistor to take the measurements 67 Ohms instead of 1.2K, not sure if the cathode caps have gone, they look ok??

Julian

[Clive]

You're saying the resistor measures 67R when removed? If so and if a 300B has gone too then either the valve went and took out the resisistor or it happened the other way around. Are the cathode caps an expensive type?

Is just one resistor down or is it both on that channel?

[julianr]

Clive
It is only one set of resistors that went the others on that channel are fine, cannot remember the price of the caps (but they weren't cheap), I think I will maybe just order the cathode resistors and start it up.
Thanks for your help.
Julian

Hi Julian,

I would follow Clive's advice, and replace the 100uF Cathode Capacitors (if you can't directly measure them) just in case - and replace all the Cathode resistors with 1.5k values. The latter is commonly done to stop the circuit 'caning' the triodes quite so much.
http://www.world-designs.co.uk/forum...ead.php?t=1343

As an aside, my version of this amplifier recently killed two 300Bs but that was due to a dry(ish) joint that took ten years to finally give way!

[Clive]

What worries me is why both resistors died. Did only one 300B die? You should check the OPT as well....there are others on the forum who can advise you better as to the likely causes.