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Tomatito 29th December 2002 08:06 AM

Several reports of Black Gates failing
Hi all,

I don't know if anyone else has heard of these rumours, but I've heard of 4 instances now where the 100uF / 100V Black Gate cathode bias decoupling cap popular in 300B circuits has failed. It blew up in mine too but I saw it happening and pulled the plug immediately.
I'm going to replace it with a 160V type.


jean-paul 29th December 2002 08:12 AM

Could it be that it took a higher voltage than 100 V ? Every electrolytic will fail if used over it's ratings.
The 4 instances you mention were all the same kind of circuit ?

Tomatito 29th December 2002 08:45 AM

Yes, they were all the same circuits and I think the caps failed at a lower voltage than rated, hence the warning.


jean-paul 29th December 2002 09:26 AM

Did you measure the voltage over it at start-up ?
What type or series of BG are you using ? It could be a too high voltage in that application or a series of BG that has this problem.
( just to prevent everybody soldering out their BG's ;) )

Tomatito 29th December 2002 09:39 AM

I don't know about the others who had this problem, but mine was a VK series. I replaced it with a FK. Yes, I did measure and it was about 74 V.


jean-paul 29th December 2002 09:46 AM

If you use a digital ( who doesn't nowadays ? ) multimeter a start-up peak voltage is virtually impossible to measure right. A good old analog meter will show the peak voltage. Probably it is higher than 74 V.

jean-paul 29th December 2002 12:10 PM

Is it the Ceasar amp in which the BG's fail ? I have heard reports of blown 300B tubes as well in this amp.

fdegrove 29th December 2002 01:23 PM


Dick measured 85V on the cathode of the Ceasar amp.

In theory the 100V insulation should be enough but I can well imagine that you surpass it when playing loud.


dhaen 29th December 2002 02:48 PM


but I can well imagine that you surpass it when playing loud.
You'd be distorting a hell of a lot.:xeye:
Even us single enders don't like that much distortion;)

A more credible explanation for me is that if the anode voltage rises (due to mains voltage rise), the cathode will rise much more, as the operating point stabilizes.

Could be a mixture of both of course.

fdegrove 29th December 2002 02:56 PM


Another good reason to regulate the B+.

After all what's the point in carefully designing operating points if you're still going to depend on the vagaries of the mains supply?

Can't help but wonder how a well regulated SE amp would would probably have a far better bass response,much more stable stereo image....

Just throwing a pebble in the big pond.:D


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