Dynaco ST70 bias issue - solved !

Hi all,

I have the tubes4hifi Classic board installed. Excellent sound. Mishap as follows
Installed quad of 6CA7's with the #6 pin intact.
The V3 tube flared and died minutes after biasing to 40ma. Blew the fuse also.
Reinstalled quad of EL34( no 6 pin). V3 tube lights up, but reads 0.00 on meter. All other tubes are able to be biased.
As a bonus, there is an audible transformer buzz, not present before.
Any ideas on what has happened ? I'm not technically inclined and will need a tech to interpret, LOL
Thanks ! Mike

wrayman wrote:Hi all,

I have the tubes4hifi Classic board installed. Excellent sound. Mishap as follows
Installed quad of 6CA7's with the #6 pin intact.
The V3 tube flared and died minutes after biasing to 40ma. Blew the fuse also.
Reinstalled quad of EL34( no 6 pin). V3 tube lights up, but reads 0.00 on meter. All other tubes are able to be biased.
As a bonus, there is an audible transformer buzz, not present before.
Any ideas on what has happened ? I'm not technically inclined and will need a tech to interpret, LOL
Thanks ! Mike

The cathode resistor ( assuming you have individual bias resistors) is broken, happens easily when a tube flashes. And yes, it should break, it acts as a fast
fuse to protect output transformer. Replace with similar.

And Yes, the 6CA7 that "flared" is dead ! Don't try to use it again !!

Thank you peterh and Bob. The resistor in question has very wonky measurements, so I'll replace it.
Thanks for the resistor link Bob, I'll go shopping tonight.

Cheers, Mike

Hello all,

Updated re the transformer "buzz" which started after the bias resistor blew. Repaired the resistor issue, all was sonically well since, except for tranny buzz.
The other evening this occurred:

-Loud crackle heard through the speakers after system on for a couple minutes.
-no sound within a couple seconds
-transformer went silent, no buzz, but amp still powered up.


All other components checked with alternate amp, no issues.

Does the above sound like a dead transformer or something else ?
Thanks for any thoughts !
Mike

wrayman wrote:Hello all,

Updated re the transformer "buzz" which started after the bias resistor blew. Repaired the resistor issue, all was sonically well since, except for tranny buzz.
The other evening this occurred:

-Loud crackle heard through the speakers after system on for a couple minutes.
-no sound within a couple seconds
-transformer went silent, no buzz, but amp still powered up.


All other components checked with alternate amp, no issues.

Does the above sound like a dead transformer or something else ?
Thanks for any thoughts !
Mike

check voltages and follow the fault-diagnostic procedure in the manual.

thank you peterh.

I'm sadly lacking in tech skills and cannot do much to check possible problems. Biasing was a big step for me, after years of owning a self biasing amp, LOL

I may have found a local person who can help, it'll be a few weeks before he can check it out. Until then, a trusty NAD power amp is pressed into service.

Cheers ! M

Update for this problem. Turned out to be a faulty rectifier tube. It was a Gold Lion GZ34, new production. Second new production tube to go in a year in my amp. I'll stick to vintage rectifiers from now on.

I would try a Weber Copper Cap rectifier. I use a WS-1 with thermistor. A WZ-68 work work well also. I use the time delay relay, recommended for when you use diode rectifiers.

Thanks Hops, I'll do some surfing on that. Cheers, M