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FDP Forum / Amp Mods, Repairs, and Projects / Amp HONKS after Warm-Up

mbruffey

USA

Nov 3rd, 2018 02:35 PM   Edit   Profile  

On my little modded Carvin V16 I'm getting a honk after warmup. I suspected something was up a few weeks ago when about 5 minutes into warmup I heard "space signals" for about two minutes, then they would go away. I suspected a coupling cap. BIAS stayed in range and steady through the space signals. But a couple of days ago I got the telltale HOOOOOOOOOOOOOOONK / BWAAAAAAAAAAAAAAAAAAA about 20 minutes in. (My Dad's old Rickenbacker amp used to do this; it turned out to be a coupling cap in the preamp.) But here, tonite, I had the amp on for about 30-45 minutes with nothing plugged in, suddenly HONK. Turn off; connect multimeters to check bias. Turn on. Coupla minutes: HONK. EACH output tube jumped from 20mA to 75mA. Where would you start your diagnostics? Thx, m -- P.S. the last change I made was the addition of a Bourns bias pot (off the board) and the addition of bias test ports on the chassis. It's worked fine for dozens of hours since then.

Leftee
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VA

Nov 3rd, 2018 05:36 PM   Edit   Profile  

I’d chase that bias current issue down.

pdf64

UK

Nov 13th, 2018 06:50 AM   Edit   Profile  

Bias is checked at idle, ie no signal; if the amp is oscillating (honking), then it's not at idle.
You may just be seeing the normal increase in class AB power tube cathode current, ie in response to signal, whatever that signal happens to be.

Regarding the honking, intermittent faults are difficult to track down. A bad HT cap or its connections may be the most likely suspects.

A chop stick, freezer spray and warm air gun may help to identify the problem. An appreciation of how to keep safe when working in high voltage equipment would be very beneficial.

What controls affect the honk?
Accurate schematic link?

mbruffey

USA

Nov 16th, 2018 08:48 PM   Edit   Profile  

Sorry for the belated reply. I don't recv email notifictions on this thread. I was about ready to open up the chassis, but decided to troubleshoot the old-fashioned way. Removed all preamp tubes and turned on for a while. No HONK. Added in PI tube. After a while HOOOOOOOOONNNNNNKKKKKK. Put tube in tester (Sencore). FLASH. Heater blew. I'm guessing I was getting DC in some way related to a the failing heater. It was a cheap Chinese tube. I put a Sovtek 12ax7LPS in its place. Okay now, and even sounds like I have a bit more bass (who knows?) Thanks again, esp. PDF64, you've helped me many times on various forums. m

FDP Forum / Amp Mods, Repairs, and Projects / Amp HONKS after Warm-Up




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