Princeton Reverb screen/grid resistors

[POWDOG]

Hey guys. Just finishing up a Princeton Reverb build. It's cathode biased, and I pretty much followed the AA1164 schematic, but added a VVR circuit. Down to the last solder joints, and notice that some PR schematics and layouts show the 470/1500 screen and grid resistors and some solder straight to the pins. Are these resistors just for tube longevity? What's the difference with or without as far as sound? Thanks!

[EL34]

It's best practice to add screen grid resistors and the grid stoppers.

The screens should have slightly less voltage compared to the plate when there is current flowing.

Other wise the electrons are attracted to the closest highest voltage and that is the screen

You can exceed the screen current ratings without the screen grid resistors

[xtian]

These safety measures should have very little effect on the sound.

[Blackburn]

I'd just go with 470R on each if you're wanting to get classic Fender.

[POWDOG]

Hey thanks for the input. Finished the build, and found that I was getting an oscillation above 4 on the Volume, first motor boating then high pitch squeal. First d/c the NFB lead with no change. Checked all voltages, and they were pretty close. With a 330 ohm cath bias resistor for the 6V6's, amp was dissipating about 15 watts per tube. Changed to 400 ohm cath resistor, now dissipating about 12.4 watts per tube, with nice deep tremolo. Still had oscillation, so swapped out new EH tubes for NOS pre's. No change until I swapped 12AT7 reverb driver for 12AU7. Oscillation is completely gone, amp sounds great. Only measurable difference is 18 volts on 12AU7 cathode instead of 8 as per 12AT7. I know this hasn't fixed the problem, only masked it. Any ideas where I should be looking? Thanks in advance!

[xtian]

Similar problem with the reverb driver in my PR build--reverb was distorting. Had to put in a voltage divider to tame it.

If your reverb is good and strong, and the oscillation is gone, I'd say you solved it!

[EL34]

Reverse the OT wires on pins 3 and see if that does it.

What you are describing sounds like positive feedback to me

I just fired up my PR build and had the same issue

I left the OT wires long on purpose until I could fire it up and determine the correct hookup

[martin manning]

...found that I was getting an oscillation above 4 on the Volume, first motor boating then high pitch squeal. First d/c the NFB lead with no change...

No change until I swapped 12AT7 reverb driver for 12AU7. Oscillation is completely gone...

There is positive feedback somewhere; maybe a lead dress issue around the reverb driver? Is the pin 1 to pin 6 jumper routed like the Fender layout? Likely the lower gain of the 12AU7 (17 vs. 60) got it out of trouble.

[EL34]

Reverse the OT wires first and see
It's an easy fix and a common one on new builds

[martin manning]

Doug, note the OP disconnected the NFB lead and the squeal was still there.

[EL34]

Ok, sorry, I missed that
Thanks

[POWDOG]

Hey guys, thanks for the continued support. Whenever I've wired the plate leads backwards with a NFB amp, the oscillation squealing starts at about 70 volts when ramping up the Variac. The oscillation on this build doesn't start till about 4 on the Volume, so I really didn't suspect that to be the problem. Went ahead and d/c the NFB lead anyway, just to rule that out. I DID wire the reverb driver just like the Fender layout, so I'll try mucking around there. I do however like the 12AU7 in the reverb driver position. Fender reverb is always "way over the top" for my taste. This gives me more latitude on the control. Even thinking about adding the 2000pF cap across the 220K to ground resistor as found on the B1270 version, to darken it up a little. Thanks again. I'll post the final result.

[billyz]

I actually like the 2000pf across the 220k . Especially with modern tanks. They seem to sound different than the old Gibbs and accutronic tanks.

[POWDOG]

With the amp "as is", the 12AU7 is getting pretty hot. The socket is still biased for the 12AU7. Is there enough of a difference that I should re-bias the socket for the 12AU7, or leave as is? Resistor value? Thanks!

[POWDOG]

Sorry guys. I meant that the socket is still biased for the 12AT7, with a 2.2K across both cathodes.