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Nasty oscillations in 2xEL84 amp

5/16/2000 11:39 AM
Dominik	Nasty oscillations in 2xEL84 amp Hi there, I worked on an old power amp (12AX7, 5U4G, 2xEL84 aka 6qb5) that came out of a Hammond organ A100 (AO-44). Since the PI was for symmetrical input I had to redo that and went for a typical PI like the one that can be found in the generic schematics: 820R to two 1M each going to grids, 22k to cap (I think 0.1) going into Presence pot (2kb) going to secondary of OT via 4k7. When I hooked it up, I got a nasty low frequency hum and feedback. Somewhere I read that reversing the leads on the OT might cure this, but i forgot how it really should be done. One more thing: This amp is cathode biased with 100uF caps going from each output cathode to a common resistor. How do I measure bias?? Dumb in Regensburg, Dominik

5/16/2000 11:39 AM

Dominik

Nasty oscillations in 2xEL84 amp

Hi there,
I worked on an old power amp (12AX7, 5U4G, 2xEL84 aka 6qb5) that came out of a Hammond organ A100 (AO-44). Since the PI was for symmetrical input I had to redo that and went for a typical PI like the one that can be found in the generic schematics: 820R to two 1M each going to grids, 22k to cap (I think 0.1) going into Presence pot (2kb) going to secondary of OT via 4k7. When I hooked it up, I got a nasty low frequency hum and feedback. Somewhere I read that reversing the leads on the OT might cure this, but i forgot how it really should be done. One more thing: This amp is cathode biased with 100uF caps going from each output cathode to a common resistor. How do I measure bias??

Dumb in Regensburg,
Dominik

Replies:

Carlo q{When I hooked it up, I got a nast... -- 5/16/2000 3:04 PM