12DW7 in Princeton Reverb PI...

[lookah]

MWJB, sorry for the mixup. Yes, it was tubeswell who said "it may work better with 22k plate and cathode resistors".

Ok, so if I lower the cathode load resistor (to ground) from 56k to 22k, What is the smaller value you suggest I replace the stock 1k cathode resistor with? 820 Ohm? 470 Ohm? And I can leave the plate resistor stock 56k? Also, are there a measurements I can take on the 12AU7 PI side that will help me find the best resistor values?

Many thanks!

[MWJB]

Plate load & cathode load resistors should be the same value & matched, e.g. both 56K or both 22K/'both whatever value you choose'. The 1K biases the tube, the 12AU7 side should dissipate 3.5W...but preamp tubes are typically not run at anything like max dissipation, use the value that sounds best, measure dissipation by voltage drop accross the plate resistor, multplied by plate to cathode voltage.

[lookah]

Great! That really clears things up...thanks again.

[lookah]

Hi again. I performed the 12DW7 "rewiring mod" on my Princeton Reverb and tried to optimize the resistor values for the 12AU7 side. I'm a bit confused with the results I'm getting when I plug the numbers in the dissipation formula above so hopefully someone can help. Here's some readings with 2 different cathode resistor settings. Which one is more on the right path of properly biasing the 12AU7 side?

JJ ECC82 (12AU7) Data Sheet says Max Plate Dissipation is 2.75W


1)

lowered stock 56k plate resistors to 22k
increased stock 1k cathode resistor to 3.9k


Voltage drop across 22k plate resistors = 38v
Voltage drop across 3.9k cathode resistor = 6.8v
Plate to Cathode Voltage = 146V

-------and------

2)

lowered stock 56k plate resistors to 22k
lowered stock 1k cathode resistor to 680ohm


Voltage drop across 22k plate resistors = 63v
Voltage drop across 680ohm cathode resistor = 1.9v
Plate to Cathode Voltage = 80V

[tubeswell]

Hi lookah

When you say 'voltage drop across 22k plate resistors = xyV", are you meaning voltage drop across each 22k resistor?

[lookah]

hi tubeswell

sorry for late reply and thanks for asking. i ended up studying up on this material after my last few posts and now have a much better understanding of the cathodyne PI. ultimately, learning how to create load lines off of the tube data chart was the real break thru. In fact I'd like to share my findings as well as the actually mod I've ended up with. I'm enjoying the sound of the mod which has provided more focused bass response and less harmonic madness when dimed. I would much appreciate any feedback on what I've done...

Here's what I did:

1) "stokes mod" to increase B+ voltage to PI (stock was 230v...now it's 270v)
2) Rotated V4 180 degrees and rewired it to accomodate 12DW7 (JJ ECC832)
3) changed the two 56k Ra+Rk resistors to 12k (each)
3) changed cathode bias resistor (Rb) to 1.5k
4) added large, 1M grid stopper resistor




The blue "X" below is where the 12AX7 was performing





The blue "X" below (a bit hard to see here) is where the 12AU7 side is currently performing (plate to cathode voltage is 162v)

[Rick Erickson]

This reminds me of swatting a fly with a sledgehammer. If you want to tighten up the bass, just replace the .1uF coupling caps between the PI and the power tubes with .022uF's. This will also increase the depth of the vibrato (tremolo).

[lookah]

fair enough but i was namely going after a better cathodyne/concertina PI by using a 12au7. and one of the results is a tighter bass and smoother distortion when cranked...

[lookah]

Hi MWJB and Tubeswell (and anyone else who wants to chip in). Still very interested if you have any feedback for me regarding the load line calculations and subsequent mods I performed on my Princeton (pics and data are above). Thanks!

[TheElectricMoron]

Fly/hammer thing seems true to me too. Way easier to do the Stokes mod and the PaulC mod and get a decent speaker.

hint hint a DeluxeReverb OT will make your day and with a 12" speaker in there you will be in another amp world.

JMO of course YMMV

Charlie

[lookah]

Thanks Charlie. No disrespect to you or any kind folks who have replied but...I did the mod because several folks starting to talk about it on forums once JJ has re-released the 12DW7. I then ran across Lord Valve's free article on Cathodyne PI's, enjoyed it, was curious enough to try it, and went ahead and plugged in some of his formulas in order to obtain the values for the resistors I was changing. So what I'd really love, is hearing if anyone can chime in on whether I applied the theory "correctly". For example, one thing I still haven't figured out, is how the value of Rg should be calculated. Thoughts?

Cheers,

Luca

P.S. I have changed speakers...going back and forth b/w a celestion g10 and a emi copperhead. both great. although I will say that the original oxford blows both of them away in "vintage mojo" up until 4 on the volume

[tubeswell]

Hi MWJB and Tubeswell (and anyone else who wants to chip in). Still very interested if you have any feedback for me regarding the load line calculations and subsequent mods I performed on my Princeton (pics and data are above). Thanks!

If you like it, it must be good

[dang!]

? reverse 12dw7 ECC823 ?

hi tubeswell,

thanks for sharing your extensive knowledge

i have a 65 bf pr that i'd like to use the reverse 12dw7 u mentioned as "ECC823" in V4

do u or anyone else know where can i get these?

i'd rather not mod a vintage amp...

thanks again for your help!

[dang!]

another question:
if i use the 12dw7 or even a 12au7 in V4 of a princeton reverb, do any other parts of the circuit need to be changed to accomodate the difference?
or is 12au7/12dw7 a safe substitue without any other mods?
btw
i was able by chance to track down a reverse 12dw7 on ebay!
thanks again for your help...

[lookah]

"do any other parts of the circuit need to be changed to accomodate the difference"

that's what this whole thread is about...just read the earlier posts.

fyi/ after going through all the mods i listed above for the 12dw7, i've reverted my 71 PR to stock with the exception of a bias pot and a new speaker. happy with it again. it is what it is