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Nasty oscillations in 2xEL84 amp

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5/16/2000 11:39 AM

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 3:04 PM

quote:
"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."

The two wires you need to swap are coming from the OT primary and connected to pin 7 of each EL84. So all you have to do is unsolder the wires from pin 7 and swap them with eachother. Make sure the amp is off, unplugged and that there's no voltage present on these pins before you start.

quote:
"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??"

With the amp off - measure the resistance from the cathode (pin 3) to ground. If it's one common resistor for both tubes, you'll get the same reading for each tube. If there are two separate resistors (one for each tube), measure, and write down, the resistance for both.
Now power up the amp and measure the voltage (DC) at pin three for each tube.
To find the bias current divide the voltage by the resistance. I=V/R
If it's one common cathode resistor then the answer to the equation above will be the current draw for both tubes. If there are two separate cathode resistors, you solve the equation for each tube individually.

Hope this makes sense,
Carlo

5/16/2000 4:47 PM

Just a clarification:

If the cathode bias resistor is bypassed with an electrolytic capacitor, it will be tough to obtain an accurate reading of the resistance. The cap will start charging by the battery current from the meter. The measured resistance value will drift. The parallel R-C circuit has to be temporarily broken to measure just the resistor. Unsolder one end of either component (whichever is easier). Measure the cathode resistance value accurately, then write it down for future reference. Resolder the connection, then follow Carlo's method for obtaining the bias current by measuring the DC voltage drop across the cathode resistor with the amp operating (idling, no signal), and divide by the resistance value you measured previously.

5/17/2000 9:34 AM
Dominik	Re: Nasty oscillations in 2xEL84 amp, THANK YOU Thanks for the help!

5/18/2000 9:52 AM

Re: Nasty oscillations in 2xEL84 amp

Hello,

If the cathode bias resistor is bypassed with an electrolytic capacitor, it will be tough to obtain an accurate reading of the resistance. The cap will start charging by the battery current from the meter. The measured resistance value will drift. The parallel R-C circuit has to be temporarily broken to measure just the resistor.

or you could use 4 point resistance measuring.

K.

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