I have a Blackstar HT club on the bench. Its owner was noticing some intermittent volume fluctuation, tried a new set of power tubes, problem persisted, put the old set back in and sent it to me. I played the amp for over an hour, no issues. Turn it off for a while, when I turn it on again, no sound. Figure out one of the EL34 filaments was open. This amp runs 12v series filaments on a pair of EL34s with pin 7 grounded. There is no center tap on this 12v winding. Still get 12vac on the winding, 6v for each tube. No continuity between windings that shouldn’t be there.
New set of EL34s and the amp works fine. Haven’t found anything intermittent.
I would have assumed it was just a failing heater on the one tube if the guy hadn’t noticed the same problem on another pair. Haven’t had time to play the new set of power tubes for very long. Schematic recommends 50mv across their shared bias check resistor. So, 25ma per tube with 387v on the plates. Can’t remember the plate voltage on the original set, but it wasn’t anything out of the ordinary and the bias was at the factory setting. Base of each original tube was a little dark, but they were old. Definitely were not red plating while I had them. No signs of weird oscillations, amp seems pretty normal... Any ideas, aside from not working on Blackstars?
Blackstar with open heater
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Blackstar with open heater
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Blackstar with open heater
Sounds like just a case of time and fatigue, or maybe the other way around .
Combo amps played loud will do this eventually.
Combo amps played loud will do this eventually.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Blackstar with open heater
yeah, guessing the most logical thing is the amp is actually fine. Just curious if there is anything I'm not thinking of. I, of course, do not have the other set of power tubes he claimed had the same issue. Probably need to try it with those to be sure.
This amp has some odd features. Like, unplug your guitar and it cuts the power tube bias supply. Even with a schematic I feel more suspicious of things than I normally would. Microcontrollers are scary...
This amp has some odd features. Like, unplug your guitar and it cuts the power tube bias supply. Even with a schematic I feel more suspicious of things than I normally would. Microcontrollers are scary...
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Blackstar with open heater
Yes, and I have fixed atleast 5 of those that have had that bias control transistor go bad.
It's a great idea, but not with out some form of protection from receiving static shocks.
It's a great idea, but not with out some form of protection from receiving static shocks.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Blackstar with open heater
Thanks for the tip, will keep that in mind if I get one that isn't biasing properly. The added features seem nice on paper, but I definitely prefer well built and simple. It's not too bad compared to some of the overlycomplicated monstrosities I have looked at recently. At lest it seems more serviceable than average.
Re: Blackstar with open heater
He opted to buy a new pair of tubes from me instead of dropping off his other set, so I will never know. New pair of JJs worked flawlessly, with a couple hours in this amp. I replaced the cheap molded speaker plug/cable with a nicer neutrik one and some fresh quick connects just in case. It didn't fade when I wiggled the original, but that was the only other thing I could think of...