HELP!! 1966 Fender Princeton Reverb Low Bias & Pulsating Issue
My 1966 blackface Princeton reverb went out on me Friday night on the last song. I had no output so I looked in the back of the amp and noticed the 6v6gt’s were pulsating as if in time with the tremolo so I shut it down. First let me say one month ago I serviced the amp. All the coupling caps were replaced with new 50uf@50v and I replaced the electrolytic can with a new 20/20/20/20 @ 500v and the amp sounded absolutely killer for an entire month. So far I’ve checked everything I know to check. I checked all the original blue Mallory caps and I checked the little disc cap associated with the tremolo circuit thinking this has something to do with the tremolo considering it’s pulsating. Of course these PRs are fixed bias with the cathodes grounded and the grid runs to the tremolo intensity pot so I checked the intensity pot and it checks perfect. My plate current was running 33 and 34 milliamps when I finished servicing the amp a month ago and now it’s cut almost exactly in half with my analog milliamp meter needle fluctuating up to 18 and down to about 13 and speeds up/slows down as I adjust the intensity control?! All tubes test excellent and I even switched the power tubes just to see if it changed anything and it didn’t. If some of you have experienced this problem and can set me on the right track it would be much appreciated! Thanks
Re: HELP!! 1966 Fender Princeton Reverb Low Bias & Pulsating Issue
It sounds like the tremolo is "on". Do you have a footswitch connected- the tremolo is "on" when the tremolo pedal is not connected.
Re: HELP!! 1966 Fender Princeton Reverb Low Bias & Pulsating Issue
Quote:
Originally Posted by
Don
It sounds like the tremolo is "on". Do you have a footswitch connected- the tremolo is "on" when the tremolo pedal is not connected.
Correct. Backwards from most Fender amps.
Re: HELP!! 1966 Fender Princeton Reverb Low Bias & Pulsating Issue
Quote:
Originally Posted by
Don
It sounds like the tremolo is "on". Do you have a footswitch connected- the tremolo is "on" when the tremolo pedal is not connected.
No it does this with a footswitch connected and tremolo off. I just don’t understand why suddenly my bias dropped from 34 mA to 14 mA. Something is wrong but I haven’t found it yet. I assumed the grid resistor lost its value but it checks fine.
