|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
141.239.172.248
In Reply to: RE: Anything wrong with this schematic? posted by DAK on July 06, 2015 at 23:31:59
First, I switched the primary leads of the OPT from one side to the other side of the circuit and that made no difference.
I removed all the 12ax7 and found no noise at all. So, problem is definitely in the small tube circuit. I installed the splitter tubes and had some buzzing sound at about 70% of full voltage which increased as the voltage increased.
At the 70% setting I also read 11vdc on the output of the .068uf coupling caps. The other .068 also had a similar voltage. I don't know if this is normal for a circuit like this? I did test my VitaminQ caps for leakage before installing and there was none. And the fact that both sides of the circuit show about the same voltage makes me think it is not the capacitor that is a problem. I will try to get a photo posted soon. regards, Dak
Follow Ups:
If you're seeing +11 volts on the grid of the phase splitter then something is wrong with those coupling caps. If you have another cap around of any value, swap those out and re-measure. That's very strange although I have occasionally encountered multiple bad caps from the same production run.
You have a pair of ECC83 drawing current through a series connection of 1K and 22K resistors. Figure each half does 0.001 amps. So you have 0.002 amps going through a 22K resistor. The grids are referenced to that point through 2M2 resistors. 2 millamps across 22K should develop 44 volts. So I would expect the grids to be 44 volts above ground. The cathode has an addtional 1K resistor would would make the cathode 2 volts above the grids...as in 46 volts above ground.
Or maybe I see it all wrong?
Hi Russ, thanks for explaining that. I was sure that the circuit was supposed to have DCV at that point but since I was tracking down the problem and did not have voltage charts I was not certain of what the voltage should be at that junction. unfortunately, I did change out the 2 caps just to be on the safe side, with a pair of 800vdc ERO film caps. But I think I will put the VitaminQ ones back. BTW, what type of phase splitter is this called? It is not that common as I only ran across it a few times and long ago. thanx again, cheers, Dak
Edits: 07/11/15
"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: 07/07/15
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
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: