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In Reply to: RE: Did some further testing posted by DAK on July 07, 2015 at 16:09:50
"First, I switched the primary leads of the OPT from one side to the other side of the circuit and that made no difference. "Did you move one of the primary wires from one of the two tubes in one channel to the other tube's plate in that same channel and then the same with the other end of that primary?
If so did you switch the screen grid wires as well?
With positive feedback present the amp should be very unstable so I don't understand how it could have "made no difference".
If it was right to start with, then after you switched them it would have been wrong and positive feedback is obvious.
Edit, I can't seem to get this right. Sorry.
It's push pull parallel so each end of the primary goes to two tubes.
What I am asking still applies.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/07/15Follow Ups:
Triode strap EL84's and ditch golbal feedback from secondary of OPT. If there is still a problem many of the usual suspects have been eliminated.
In my limited experience mistakes on the OPT wiring resulting in positive feedback have been painfully obvious.....as in one wouldn't dare play the amp like that for more than a second or two.
"Did you move one of the primary wires from one of the two tubes in one channel to the other tube's plate in that same channel and then the same with the other end of that primary?"
Tre, the amp is stereo so there are 8 power tubes in 4 pairs. Each pair's anode is connected and goes to a solder lug. Channel 1 solder lugs A and B. In my case OPT primaries are blue and brown . Originally blue on A and brown on B was oscillating. I switched them around and still oscillating. In both situations I left the FB tap the same which is on the Yellow lead to the speaker tap. regards, Dak
Did the gain change?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I can't really tell since I did not bring the amp up to full power.
That just makes me more confused.
Let's say there's 10db of negative global voltage feedback applied.
If that was reversed there would be 10db of positive feedback for a gain difference of 20db.
I would think you could tell, even if the amp is not working right, a 20db sensitivity difference.
What happens if you just disconnect the feedback?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Hi tre, I should have mentioned that in my amp I am operating in pentode mode so there was no ultralinear or screen taps. As far as the level of oscillation it is more of a high frequency buzzing but not squealing. I have had the situation where the OPT primaries needed to be switched from one tube to the other and the difference is immediately noticeable but not in this case. Thank you, Dak
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