I just put new tubes and biased my old 115-65 and now I have an issue with the reverb that didn't exist before. When the reverb is switched on it squeals. The more reverb, the more it squeals. It also has a pronounced hum when the reverb is switched on and turned up above zero. This problem does not affect the tremolo circuit. Anyone have any ideas of what could be causing this?
Sat, 07/11/2020 - 15:02
#1
Cable Check
Were any of the reverb tank cables disconnected when performing the bias adjust? If yes, check to make sure they didn't get reversed. Also, does the reverb problem happen with the foot switch disconnected? -mgriffin
The reverb is "on" when the
The reverb is "on" when the foot switch is not connected. When the reverb is on and turned to "0" there is no hum through the amp. Whenever the reverb is turned up there is a hum. If you turn the reverb off using the foot switch the hum disappears. So the hum is there regardless of the foot switch being connected. Could the new power tubes and subsequent bias adjustment have changed a gain structure or level in the reverb circuit? I noticed there are two adjustment pots on the circuit board. The bias at pin 8, by the 10 ohm resistor was at 1.42 and adjusted to 0.5 for the new tubes. The old tubes were original to the amp. The reverb had no issues before resetting the bias.
Keep Looking
Thanks for clearing that up. Changing tubes and resetting bias should have no effect on the reverb circuit. Even though it worked before, try troubleshooting the reverb circuit anyway because something has gone wrong, for whatever reason. Does the humm go away if the reverb tank is disconnected or if the IC is pulled? -mgriffin
Always check the obvious..
Always check the obvious...right! One of the rca jacks for the reverb tank must have come unseated. I never checked it because I never had to unplug it. It wasn't unhooked but needed tightening. Thank you so much mgriffin. You are the best!