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Old 3rd October 2005, 04:30 AM   #1
pho_boi is offline pho_boi  Australia
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Default question about 12au7

Hey everyone,

I have this intergrated tube amplifier that uses a pair of 12au7, a pair of 12ax7 and 4 el34s.

Just recently I been noticeing that one of the 12au7 flashes brighter then the other just as I turn the amp on. It then slowly dims out to a 'normal' brightness but still brighter then the other 12au7. I tried swapping the 12au7 around to see if it was a faulty tube or not, but the problem is still there.

Is this a sympton of a bigger problem inside my amp? Should I be worried? How would I go about fixing this?

I don't have the schematic for the amp, as its a commercial amplifier, its a Cayin A-50T.


Thanks
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Old 3rd October 2005, 05:57 AM   #2
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This has been covered before, but mainly for a 12AT7 IIRC.

Some types have a nasty heater turn on flash, others (particularily if they were made for a series string) do not.
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Old 3rd October 2005, 06:07 AM   #3
pho_boi is offline pho_boi  Australia
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Quote:
Originally posted by Geek
This has been covered before, but mainly for a 12AT7 IIRC.

Some types have a nasty heater turn on flash, others (particularily if they were made for a series string) do not.

I have read the previous posts, but the problems people had was with individual tubes. However, with my situation, it still occurs even when i switch tubes from one side to the other. As in, the tube that previously flashed is no longer flashing in its new position and the other tube now flashes when it wasn't previously???

Also, the tube that flashes stays brighter than the other one even after it dims down?
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Old 3rd October 2005, 07:00 AM   #4
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pho_boi

It could be possible that the suspect "socket" filament connections have been wired improperly. The filament connections should be to pins 4 and 5. I suspect tho I have no experience with the Cayin A-50T, that the filaments are wired series/parallel and used as the cathode load for the EL34's, based on your description.
Before you go poking around under your amp make sure it's "UN-plugged-un-powered" and discharge all power supply capacitiors! And not with a screw driver or some other tool! Test leads with alligator clips on each end and a 10k 1-2W resistor are the way to go.

Wayne
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