Fender Pro Reverb - One Tube Flashing and Making Tremolo/Chopper Sound

[Yclepticon]

Hi There,

This is my first post. I'm having some troubles with my black faced Fender Pro Reverb. There is a louder chopper like sound when I'm not playing and when I play it produces a tremolo effect. There are 2 6L6 power tubes and the Left (when looking at the back of the amp) is flashing on/off blue. This is when I'm plugged into the regular speaker out, when I have it plugged into the ext speaker the amp is drastically under powered and distorted... But, the tremolo and blue flashing goes away.

When this issue first started I was able to get rid of it by tapping the right tube with a drum stick... This doesn't work at all anymore. I've since changed my power tubes and re-biased the amp hoping that would help... However, I can't get any reading from the right tube...

Let me know if there are any pictures of any part of the amp you'd like to see and I'd gladly post.

Thanks for looking/your time.

[Enzo]

You can't run the amp without using the main speaker jack. WHen that jack is empty, it shorts across the output. SO if you plug the speaker into the AUX speaker jack instead, there is still that shorted output. SO the power is greatly reduced and it sounds crappy. As you found out. The AUX jack is ONLY used for a second speaker, never for the only speaker.

If tapping on the tube changed things, the tube was shot.

Pull the power tubes, power up the amp, take voltage readings on the power tube sockets. You should get B+ voltage on pins 3 AND 4 on all power tube sockets. And there should be whatever the bias voltage is on the pins 5. If any voltages are missing, we need to solve that before the tubes can work right.

[Jazz P Bass]

I do not think that you can plug into the "Ext" jack without also having a speaker in the "normal" jack.
So that test is not valid.
Just because the tubes are new does not certify there condition.
Short of ripping the chassis out & testing the output section in a proper manner, new tubes are called for.
The flashing & lightning effect is a bad tube.

[Yclepticon]

You can't run the amp without using the main speaker jack. WHen that jack is empty, it shorts across the output. SO if you plug the speaker into the AUX speaker jack instead, there is still that shorted output. SO the power is greatly reduced and it sounds crappy. As you found out. The AUX jack is ONLY used for a second speaker, never for the only speaker.

If tapping on the tube changed things, the tube was shot.

Pull the power tubes, power up the amp, take voltage readings on the power tube sockets. You should get B+ voltage on pins 3 AND 4 on all power tube sockets. And there should be whatever the bias voltage is on the pins 5. If any voltages are missing, we need to solve that before the tubes can work right.

Thanks for the quick reply - it's much appreciated.

B+ is ~475 (as written on the inside of the amp by the person who black faced it)

I'm getting:
Right tube: Pin3 485. Pin4 113 Pin5 -52 Pin6 480
Left tube: Pin3 485. Pin4 485 -52 Pin6 480

(I edited my original post as I had the L and R tubes mixed up)

[52 Bill]

Check the value of the large resistor on the right tube socket. It should read 470 ohms. The voltages there show that it's probably open.

[Yclepticon]

Check the value of the large resistor on the right tube socket. It should read 470 ohms. The voltages there show that it's probably open.

Hey 52 Bill, thanks so much! I just checked and it looks like the resister is blown - the color of the bands is faded and one is missing... I'll replace it tomorrow and get back to the thread.

[Yclepticon]

I just swapped the 470 ohm resistor and the amp is working great. Thanks so much for your help. I'm going to go play through it for a few hours!