High Power Princeton?

Fender Amp Discussion

Moderators: pompeiisneaks, Colossal

Post Reply
fusionbear
Posts: 478
Joined: Thu Nov 01, 2007 4:42 am
Location: Southern California

High Power Princeton?

Post by fusionbear »

I have a client who is inquiring about a higher powered Princeton build. He wants to retain the cathodyne phase inverter. Any ideas???

Thinking about 30-45 watts, used as a clean amp only...

Thanks!
Learning to learn...
User avatar
cbass
Posts: 4347
Joined: Sat Jul 23, 2011 6:17 pm
Location: Between Pomona & Bakersfield

Re: High Power Princeton?

Post by cbass »

DLR is a Captain beefheart wannabe
User avatar
rdjones
Posts: 818
Joined: Tue Aug 31, 2010 2:20 am
Location: Music City, TN

Re: High Power Princeton?

Post by rdjones »

fusionbear wrote:I have a client who is inquiring about a higher powered Princeton build. He wants to retain the cathodyne phase inverter. Any ideas???

Thinking about 30-45 watts, used as a clean amp only...

Thanks!
That would be a sorta Princeton Boogie, although my "Prune Boogie" has an LTP.
There's no reason a cathodyne couldn't be used on a higher power amp.
The Sunn 2x6550 (80W) circuit has cathodydne.
The difference is it's a higher current triode than a 12AX7, either 7199 or 6AN8.

If you use the 12AX7 with 2x6L6 just follow Merlin's grid stopper suggestions for both the output grids AND the PI grid.

rd
printer2
Posts: 142
Joined: Sat Jun 26, 2010 5:14 am
Location: Canada

Re: High Power Princeton?

Post by printer2 »

You might want to raise the resistor values from 56k to 100k as the larger output tube needs more voltage swing on the grid.
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: High Power Princeton?

Post by brewdude »

Would a 12AU7 provide more voltage swing at the grid than a 12AX7?… Isn't there a tube that is half 12AU7 & half 12AX7?
printer2
Posts: 142
Joined: Sat Jun 26, 2010 5:14 am
Location: Canada

Re: High Power Princeton?

Post by printer2 »

The cathodyne phase inverter stage runs at a gain of just under one, so whatever is at the input you have the same signal size at the output. So if you had a 12AX7 or a 12AU7 in that position because of the 100% negative feedback both tubes will give the same output, both produce a gain of one.

The problem with using a 12AU7 is that the other triode in that tube is used in the preceding stage. Rather than having a 100X triode there you only have a 20X triode (theoretical gain, in the actual circuit there is less gain). So you do not gain anything in the cathodyne stage by using the 12AU7 but loose in the preceding stage.

With increasing the plate and cathode resistor it develops more voltage across the resistors and less across the tube. When the cathodyne follows the input voltage it have more room to swing the signal across these resistors. The voltage swing across the resistors is what gets translated through the coupling capacitors, effectively giving you more signal capability. Ampeg used the cathodyne in some of their high power amps and they were running 100k resistors.
beasleybodyshop
Posts: 1069
Joined: Tue Jul 02, 2013 12:51 am
Location: East Texas (Yee Yee!)

Re: High Power Princeton?

Post by beasleybodyshop »

brewdude wrote:Would a 12AU7 provide more voltage swing at the grid than a 12AX7?… Isn't there a tube that is half 12AU7 & half 12AX7?
isnt this a 12DW7 tube?
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: High Power Princeton?

Post by brewdude »

12DW7... Thanks.

When I read Merlin's pre-amp book (1st ed.) I thought he implied that a 12AU7 would provide enough voltage swing to drive a pair KT88's. Did I misunderstand.
printer2
Posts: 142
Joined: Sat Jun 26, 2010 5:14 am
Location: Canada

Re: High Power Princeton?

Post by printer2 »

brewdude wrote:12DW7... Thanks.

When I read Merlin's pre-amp book (1st ed.) I thought he implied that a 12AU7 would provide enough voltage swing to drive a pair KT88's. Did I misunderstand.
Sure it would. But you still have to have enough gain to give the 12AU7 something to swing.
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: High Power Princeton?

Post by brewdude »

would a 12DW7 work with AX7 side driving the AU7 side?
User avatar
jaysg
Posts: 1211
Joined: Tue Feb 07, 2006 11:16 pm
Location: Sandy Eggo

Re: High Power Princeton?

Post by jaysg »

Look at the bigger Fender 5e circuits, like the low power tweed twin and bandmaster.
printer2
Posts: 142
Joined: Sat Jun 26, 2010 5:14 am
Location: Canada

Re: High Power Princeton?

Post by printer2 »

brewdude wrote:would a 12DW7 work with AX7 side driving the AU7 side?
Sure.
Post Reply