12DW7 in Princeton Reverb PI...

[lookah]

Hi folks. I'm going to perform the mod that reverses the wiring on V4 of my '71 PR in order for it to accomodate a 12DW7 (1/2 12ax7 + 1/2 12AU7). I stumbled upon the archived post below and tested my amp with a NOS 12AU7 first. Indeed it really does wonders to the bass...cleans it up and makes it nice and tight.

Here's my question: I've ordered the tube but won't have time to mod the amp until the weekend and I have a few gigs before then with my PR. Is it safe to run my amp for a full gig with the 12AU7 in V4? I don't care about the vibrato being temporarily disabled.

Here's the thread post by "Bill"

I would recommend a simple test to see if the 12DW7 mod will improve your amp- simply replace the stock 12AX7 with a 12AU7 and play. In the case of the PR, you will lose the Vibrato during the test (a 12AU7 can't get the oscillator oscillatin'), but your testing the tone, not the vibrato. That is why the 12DW7 is specified.
I was able to score my tubes off of ebay. 2 (one British, one Japanese) for under $20.00 including shipping. New 12DW7 tubes are sold under the EI label (and possibly others).

[tubeswell]

FWIW JJ sells a tube that is already a 12DW7 in reverse 'ECC832' (or 'ECC823'? can't remember which)- to save you doing the mod.

[lookah]

Hmm. Are you sure? I got the mod idea from the Torres website and they sell the JJ 12DW7 tube specifically for this mod and say to switch the wiring

DOWNLOAD The Newest Coolest PRINCETON mod

JJ new 12DW7 dis-similar dual triode

[tubeswell]

Yup - The info is on the JJ ECC832 datasheet

http://www.jj-electronic.com/pdf/ECC%20832.pdf

the reversed triode set is called the 'ECC823'

[lookah]

Ah, I see now. But what's the deal with the reverse pin wiring "on request" bit?

[lookah]

never mind. figured it out. looks like the JJ ECC 823 is what i want

btw, would you know the answer to my original question...if it's safe to temporarily run the 12AU7 in V4 until i receive the JJ tube?

thanks!

[tubeswell]

Probably not 12AU7 in V4. AFAICT the cathodyne phase splitter triode part of it would function fine, but IIRC, you need a minimum gain of 29 for the LFO tube in a 3-stage RC network (a 12AU7 has a gain of 17). A 12AY7 or a 12AT7 might work well tho'.

[lookah]

thanks very much for the info. and if i'm not concerned about using the vibrato at all, will it hurt the amp if i use the 12au7 simply for the PI function? or is the amp expecting to "see" a gain factor of 29 or more regardless whether i engage the vibrato or not?

[tubeswell]

If you're not worried about the LFO, then go for it and see what you think. running a 12AU7 as a cathodyne stage with 56k plate and cathode resistors will work, (although it may work better with 22k plate and cathode resistors, but you could just tack a 47k-56k in parallel with each 56k if you wanted to check out the difference)

[Enzo]

No, it won't hurt anything, go ahead and try it.

[lookah]

great. thanks for your replies

[lookah]

Hi. One more question...if I go with the lower 22k plate and cathode resistors when running the ECC 832, will 1/2 watt Carbon Comps be safe? I found this quote on an other forum regarding this mod that suggests otherwise:

"Be careful It's a very good idea but the bias will be way off on the
12AU7 section and it's going to run very HOT! You will need lower
value plate and cathode resistors on the PI. Those resistors need to
be bigger as well (5W) because they will take a lot more current than
on the 12AX7 PI. (around 10mA)"


Thanks!

[MWJB]

5W would appear to be overkill. A 12AU7 can kill a 1W resistor in a big, 6L6 Fender with a LTPI, but the cathodyne in a PR runs much lower voltage, in a 5E3 w/56K, for example dissipation is under 1/4W per triode, won't be that much more in a PR. It will draw less current with a 22K & larger cathode (you won't use 22K for the cathode) so half watt will probably do, 1W safe as houses.

[lookah]

That's great info...except now i'm confused by your saying "you won't use 22K for the cathode". In your previous reply you mentioned using 22k for both the plate and cathode. Did I misread?

BTW, thought you might like to know that I found out JJ only produces the ECC 823 (reversed 12DW7) for orders of 1000 or more. No wonder no one carries it!

[MWJB]

"That's great info...except now i'm confused by your saying "you won't use 22K for the cathode". In your previous reply you mentioned using 22k for both the plate and cathode. Did I misread?" You're confusing me with someone else, I never suggested changing any resistor values...personally I'd just stick the 12DW7 in there, reverse the wires to the 2 triodes if you want the low mu side on the PI. Cathode load resistor (to ground) can be 22K, but cathode resistor, where the 1Meg grid load?PI coupling cap connects, will be smaller (stock 1K?).