Ashdown ABM500 EVO III Signal Lost In Preamp - No Schematic

[Relsom]

Well well well....thanks for the motivation. Clipping TR19 Pin 3 got some sound out of her. Fairly weak, but there. Then I clipped TR18 Pin 3 and it looks like she is at full volume. Still not all that clear as to how these JFETs behave. Normally, is it the + voltage through R114 that keeps the gates high and therefore not shunting the signal to ground on pin 1 of the JFETs?
Anyway, thanks for your help. Much, Much appreciated!

[Enzo]

yes.

The JFETs are a low resistance - check a new one with a meter just to see - unless turned off with a gate voltage. Normally that positive voltage turns them off unless that voltage is removed by the mute circuit. If the gate shorts on one of them, the other one cannot turn off. left alone in their ON state, they each put a low resistance from the signal path to ground at the volume control and the signal out to the power amp.

MAybe should have mentioned earlier, note that the MUTE LOW disables the JFET gates, and it is a ground shunt at the input jack. An empty input jack mutes the amp.

If the amp is for your use, and you don't plan to use the output mute switch, then snip out both transistors. If you don;t have undue noises from it , then leave them out.

[Relsom]

Thanks for the info. Interesting that Ashdown thought that it was muted though by the input jack not realizing that I was applying my test signal with a plug inserted. I had sold an EVO II version of the same amp a year or so ago thinking I was happy to move on to Class D lightweights. I missed the ABM so I took a chance on this one in need. She's not quite done yet. Looks like I got a hum that creeps in as the tube heats up. Goes away if the tube is deactivated. Good thing we're snowed in.

Edit: Bad tube. I'll take it down one more time to replace the JFETs before it goes back together with loktite and ty-raps.

To all that responded - Thanks again for your help.