rooster wrote:KGW - OK, do me a favor? If we've done this before tell me...
Pull V1 and V2. Is the hum still there? Are your tubes biased to 40 mas? Do you have a good set of EL34s? Sometimes, especially if you have hum with V1 and V2 pulled, you could have a single bad pwr tube making the hum.
OK, if all of this is hum-free, put V2 back in. Does the hum come back?
Yes it goes away with V1 and V2 pulled. There is about 4.5mV difference in the bias between the output tubes. I have tried adjusting the bias and the hum level barely changes.
Also, I tried switching the 12AX7s and have them in the quietest combination. I also tried some other 12AX7s (JJs). The Tungsols are way better than the JJs.
However, there is a difference between the tubes as far as hum is concerned.
rooster wrote:OK, here's a problem. If it does, this would be very unusual. Most hum in these circuits come from V1. Now, if the hum was still there with V1 - AND V2 - pulled, you are dealing with an very strange beast....
........which leaves us with that 220K bias supply thing you have installed. Here I am blind. But it does occur to me that you might have one pwr tube taking a bias - and the other not. Is there a way - using an external bias type device - that you can check the bias of EACH tube?
Get over the 220k placement
![Twisted Evil :twisted:](./images/smilies/icon_twisted.gif)
- I've done it both ways. Also, I've had this amp for 2 years now.
rooster wrote:For that matter, as a last ditch effort here, pull all the tubes. Now put 1 pwr tube in. Does the amp hum? Pull that tube and put the other pwr tube in. Does the amp hum? If good, then plug them both in. Hum? Switch sockets now and start over with 1 pwr tube, etc.. Hum?
Of course no hum with both pulled. Hum with either or both.
rooster wrote:I have to think, and only because what you have done here is different than what I have done - and the fact that you say the hum is quite obvious. It could be a pwr tube issue, related to biasing.
"quite obvious?" well ... it is there. Much better than most amps. Still very playable. Maybe a recording would help. Sorry, can't do that right now though. Maybe measuring it on the scope would be a better metric?
rooster wrote:OK, get back to me on this? Oh, one last thing? Are you running with the Tonesluts trannys? When you turn you amp on, but not switching the 'standby' function, does the pwr tranny hum?
Heyboer trannies (I think I got them from toneslut - it was a while back).
There if hum (not though the speakers) in stadby mode.