Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 06, 2018, 10:55:47 am

Title: Princeton Reverb B1270 extra capacitor on PI
Post by: mresistor on February 06, 2018, 10:55:47 am

Good morning all. I've got a '72 PR here that has an extra,  what looks to be 25/25, electrolytic cap going from pin 3 cathode of the PI to ground at the ground lug of the tremolo pedal. It has nothing to do with the tremolo that I can tell.  I guess it was just a convenient point for them to ground it  It's not on the schematic or layout so I was wondering why CBS Fender would have used this cap there? Extra filtering? Lowering the freq response? The PI cathode already has a 25/25 electrolytic cap in the bypass circuit. Guess I could take it out and see what happens when I get this amp running. The lead dress on this amp is horrible. It appears that there are no under-board jumpers and some wires are so tight between points.

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: mresistor on February 06, 2018, 11:04:14 am

Here's a picture

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: sluckey on February 06, 2018, 11:24:31 am

I think you will find that cap is connected to V3 pin 3 which is the reverb recovery. There is no cap on the cathode of V4A which is the PI. Earlier PRs had that cap on the board connected parallel to the cathode resistor.

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: mresistor on February 06, 2018, 11:41:19 am

It is on V4 pin 3.  Here's a better look and you can see it clearly.  There is no cap extra cap to gnd on V3 pin 3 and the B1270 and A1164 schematics/layouts don't show one there either. Did someone err at the factory?

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: sluckey on February 06, 2018, 01:44:45 pm

OK, I see what's going on now. The cap is still not on the PI. The PI would not work if it was. The cap is on the cathode of the tremolo oscillator. The schematic has the pin numbers right but calls pins 1,2,3 V4B and calls pins 6,7,8 V4A. That's backwards from my normal thinking. The cap is physically in the correct place. If you look at this schematic you will see the cap connected to V4 pin3, which is the cathode of the trem osc.
 

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: mresistor on February 06, 2018, 02:01:51 pm

Sorry   my bad   it is connected to the trem osc side of V4  and I'll be danged on the board there is only the 3.3k resistor, so they mounted that cap over on the tube.   For some reason I don't think I've ever seen that before..   Thank you.

Title: Re: Princeton Reverb B1270 extra capacitor on PI
Post by: mresistor on February 06, 2018, 02:37:42 pm

I see now why they did this. The eyelet where the negative lead of the cap would be soldered has 4 wires in it already.