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Re: Choke replaced; AC30 still screaming at me

10/13/2005 7:58 AM
R.G.	Re: Choke replaced; AC30 still screaming at me Given that it screams with the phase inverter pulled and that it was not in fact the choke, it seems from the description that the power stage all by itself is oscillating. In addition, pulling one of the four output tubes stops it dead. Oscillation needs a loop gain of greater than one, feedback, and a phase change of at least 180 degrees. The one sensitive output tube is somehow getting those conditions satisfied. It's obviously getting the gain, but where is it getting the feedback? Here are some tests to find that out. - First, make sure the tube contacts are tight and clean. - On an EL84 tube, there are three inputs - the grid, the cathode, and the screen. You can eliminate the grid by clipping a capacitor from grid to cathode temporarily, or by solid-grounding the grid. doing that, does it still squeal? If the squealing gets worse, you may have identified grounding-induced feedback. - In the same vein, you can stop feedback on the screen by clipping a BFC (really big capacitor)of sufficient voltage rating across the screen-to-ground right at the screen. Does that stop the squeal? - Is that cathode solidly connected to the cathode resistor/capacitor? Check with an ohmmeter while wiggling the tube. - If it's still squealing, that points to the power supply and/or grounding. Have you tried temporarily bridging the screen supply with an extra filter cap? Then there are the miscellaneous ones: - long lead under the board semi-shorting to chassis or conductive grundge on the chassis; stick a sheet of cardboard under the board and see if it stops. - cracked traces (if it's a PCB version) or bad solder joints; I would just spend a few minutes remelting all the solder joints in the power amps and power supply.

10/13/2005 7:58 AM

R.G.

Re: Choke replaced; AC30 still screaming at me

Given that it screams with the phase inverter pulled and that it was not in fact the choke, it seems from the description that the power stage all by itself is oscillating. In addition, pulling one of the four output tubes stops it dead.

Oscillation needs a loop gain of greater than one, feedback, and a phase change of at least 180 degrees.

The one sensitive output tube is somehow getting those conditions satisfied. It's obviously getting the gain, but where is it getting the feedback?

Here are some tests to find that out.
- First, make sure the tube contacts are tight and clean.
- On an EL84 tube, there are three inputs - the grid, the cathode, and the screen. You can eliminate the grid by clipping a capacitor from grid to cathode temporarily, or by solid-grounding the grid. doing that, does it still squeal? If the squealing gets worse, you may have identified grounding-induced feedback.
- In the same vein, you can stop feedback on the screen by clipping a BFC (really big capacitor)of sufficient voltage rating across the screen-to-ground right at the screen. Does that stop the squeal?
- Is that cathode solidly connected to the cathode resistor/capacitor? Check with an ohmmeter while wiggling the tube.
- If it's still squealing, that points to the power supply and/or grounding. Have you tried temporarily bridging the screen supply with an extra filter cap?

Then there are the miscellaneous ones:
- long lead under the board semi-shorting to chassis or conductive grundge on the chassis; stick a sheet of cardboard under the board and see if it stops.
- cracked traces (if it's a PCB version) or bad solder joints; I would just spend a few minutes remelting all the solder joints in the power amps and power supply.

Replies:

David While you're at it... -- 10/13/2005 11:15 AM