Amp trouble - Blackstar (warning-non bass-content)
Tonight at practice, my guitarist's Blackstar HT-20 (20 watt, 2 el34's) head goes out, just the clean channel, drive channel is ok. So we figure its in the pre and swap the two 12ax7's, one ata time, with a known good one. Works for a while, 5-10 min and then fails again.
What it does is, after warm up playing 30-60 min, clean channel begins to fade like you put it on standby, then nothing. Drive channel still works. The amp actually does not have standby. All the tube filaments still glow. Tapping on or rocking tubes in their sockets produces no noise. Twice we shut it down for a few minutes to change a tube, it worked for a short while then clean channel fades out again.
Previously, the head fell from about 2 feet while loading out of a gig, and broke a power tube. He got a new pair of TAD's (OEM) and we re-biased, and straitened the trans mounting tabs which were bent from the impact. It's been fine for at least 5-6 practices, but now seems to have heat related issues with one channel.
I'm thinking circuit board traces/solder joints. Is there a component that might cause this?
Thanks for any help
I think it may be a case of opening the amp up and seeing if anything looks out of place. Then recreating the scenario and seeing whats going wrong with it
Sounds like it's heat related, but who knows?. Could have broken a trace on the board when dropped and as the amp heats up and things start shifting around (and they do, even if on a nano level) the trace or joint moves enough to stop through-put. Or maybe some other component affected by heat. It could be a hundred things and not even related to the drop, though.
I've been running tube amps for 30+ years and I can half-assed read a schematic and I can solder a little. But unless it's a bad tube (easily diagnosed), dirty connection (ribbon cable), or dirty effects loop I take my amps to a good tech. I leave it to the pros. And it sounds like you're kinda at that point, too.
Good luck with it. Hope it's something simple/cheap.
Thanks, I'm in agreement with your advice. We will go inside and give it good looking over. I'll plan on volt testing the tube sockets and see where that leads. It seems that letting it cool for a few minutes allows it to work again, briefly. If any one thinks of some thing that would not appear to be obvious please chime in.
Yes, I realize the dangers of very high voltages that will be present inside. Thanks in advance.
You can get some "cold" spray or compressed air and cool down parts of the amp to see if reducing the heat in specific locations gets it to work thus isolating the cause.
Let us know what happened once you get it fixed. :)
If you need a tech, maybe the Wisconsin Bassists Club can help you out.
If you do find a good tech, please post it here for future TBrs.
There is a chance a resistor or blocking cap fractured on the drop. Resistors can be found with DC voltage checks, blocking caps with signal applied and AC voltage or best a scope. The cold spray use could be very helpful (non-inflammable and non-conductive) here as mentioned before.
Thanks for the help :) I'll be sure to let you know what I find.
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