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I plugged in my 240 this afternoon and one of the power tubes flashed blue and a *crackle* came out of one channel. I unplugged it and switched the power tubes (thinking one of the Svetlanas went bad) and now no sound comes out of either speaker. The tubes all light up okay. I just got this amp back after having it worked on (I've played it a few dozen hours, so I don't think it has anything to do with that) and I really don't want to ship it off again. can anyone tell me what this might be? anything I can check on my own? I know a bit of electronics, but I've generally had other people work on my mac. Thanks.
Follow Ups:
Michael Samra, I read that you had worked on some Dukane amps a while back, is that true? Looking for some info on them...
I did work on dukanes..Which ones do you have and what do you need to know?
Michael, can I run the 25v line into a 8 ohm speaker, or is that too low an impedance for the 1A475 pa amp? thanks, Randy (btw feel free to contact me...)
Hi, Michael, I have a pair of 1A475, 1A395, and 1A435, I would appreciate any help of ideas that you have as well as your experiences.. Thanks, Randy
Hi, Michael, I have a pair of 1A475, 1A395, and 1A435, I would appreciate any help of ideas that you have as well as your experiences.. Thanks, Randy
do a visual, check the voltages, and if you can't figure it out from that, take it to a reliable tech that works on that kind of stuff, like it was said above, fixing a portion of the amp can have stress older parts to failure, or replacement in certain areas with metal films can cause them to fail, especially cheap metal films. Of course, if I had need of whatever your job is, I would probably send that work to you, same with a E tech...
Will
Everyone of these guys gave you excellent advice and it boils down to a basic componet failure..I would say steve is correct in that it being the fuse if you have to narrow it down to a componet that is the probably cause..If you have any soldering skills and you live in the usa or canada,I can call you up and walk you thru the process as I have free unlimited long distance.
mike
Thanks for your very generous offer, Mike! I was having trouble with my Dynaco a couple of years ago and I think you also very generously offered to help me with that one too (I managed to figure out that one on my own). The Mac is a bit more intimidating to me than the Dynaco. I've tweaked and rewired my Dynaco, the whole nine yards, but I've always had professionals work on the Mac. I know the fuse Steve refers to and I checked it and am 99% certain it hasn't blown. I also didn't see any burn marks, etc., but it was rather dim and not long after the amp stopped working - before I posted here. I should put it under a good strong light and be a little more methodical about it. I'm kind of tied up today, but hopefully tomorrow I'll be able to give it some attention. Thanks again for your help.
You should check the high voltage supply. For both channels to be out, something interrupted a common source. The most likely break would be in the high voltage supply.
I would suspect that there has been a failure of a resistor somewhere in the power supply. Corerosin is on the right track as the high voltage doubler is part of that circuitry.I've seen this with older tube gear after its been worked on - the failure of some associated components after the amp has been run in a bit. I tend to want to keep someone's amp for about a month for use in regular operation to ensure that there's no drift in spec from the original resistors that are left in the amp.
I've seen these resistors 1) develop cracks which cause noise or arcing, 2) drift out of spec and/or 3) flame out from heating. Lastly, I've also had new, replacement metal-film resistors of the proper wattage fail due to high voltage arcing across the surface of the resistor when used in high voltage circuits. In this case, the fix is to use a larger wattage resistor of the same value in order to increase the spacing between the two ends of the resistor.
I'd suggest you pull the cover and look for part that looks 'heated' in some way. Post some pics, if you'd like, for others to see, too.
As always, know that there's a lot of high voltage in that amp so I do not recommend you attempt to work on it unless you're comfortable you know how to get around in there.
Hope that helps.
Cheers,
Thanks to both of you. I'll open it up and see if anything looks burnt, etc.Isn't it odd that the tubes still light up? Does that narrow it down at all?
but it does indicate that the main fuse if ok and there's current being passed through the power transformer to the filament windings.It doesn't rule out the power supply.
Cheers,
...It's located on the small rectifier/bias supply terminal board. This fuse may have blown. It rarely blows so one needs to find the root cause of failure before replacing it and repowering.
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