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HD410 (2275-130 w12ax7) noise at turn on


I have a 410hd that was having problems in the bias circuit, the pot ended up being intermittent. However, I have one lingering issue that may or may not be a problem. If power the amp off and quickly re-power it, the amp makes a terrible humming noise. If I use the Standby or let the amp discharge, it doesn't occur.

I am assuming it is more an issue of "don't do that" but wanted to see if anyone can verify if that is normal or if I need to continue to look for a problem.

The amp seems very stable now except for that issue. FYI, the amp has had the power supply recapped not to long ago and all the 105uf/50v caps were just replaced. Bias is set at .48 volts across the 100 ohm cathode resistor,


mm210's picture

Yes, that IS a case of just

Yes, that IS a case of just don't do it. Use the standbye switch, THAT'S what it's for. As for the bias, I will harp here again. Don't do that measurement like that. I would suggest getting 1 ohm resistors and going from the pin to the 100 ohm resistor on every tubes and then measure EACH tube separately for the voltage across the 1 ohm resistor. With the crappy tubes we have these days and the wide variance, you are measuring the total for all the tubes so one could be very cold, one very hot. It's just one way to be safe with these older amps. Also, I'd make sure that each tubes is biased with less than 23mV per tube across the 1 ohm. 21 mV won't hurt you and the tubes will last longer. Mike.


It ended up being an issue

It ended up being an issue with the JJ 6L6 tubes I installed. Switching the amp back on, if it hadn't fully discharged, would cause the amp to start to oscillate. An older set of 6L6 didn't exhibit the problem so since I had to re-order tubes, I installed a set of 6CA7, like the originals. My choice for the 6L6 was based on the info from other posts I have read that JJ's can handle higher plate voltages better. However, in this case, they didn't work out well.

RD112's picture

6CA7 is not the same as 6L6

Hi Vintasonic,
You need to verify the circuit for your amp first. Check the "Downloads and Resources" section of this website for circuit diagrams etc.
The 6CA7 is not equivalent to a 6L6 tube. 6CA7 and 6L6 are very different and it is no wonder the amp went crazy with 6L6 tubes if the originals are 6CA7.

EL34, 6CA7, and KT77 are all drop-in replacements for each other. You can use 6CA7 or KT77 in place of EL34. Electrically, they are the same.

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