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Cathodyne PI vs. Long Tail PI?


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5/19/2003 8:32 PM
Bob Predaina Cathodyne PI vs. Long Tail PI?
I'm building an amp into an existing chassis that only allows for two miniature preamp tubes. The objective is to build a single channel amp with "organic" natural sounding overdrive using two 12AX7 (four triodes).  
 
I've been thinking about the following topologies, and I'd appreciate your opinions about the strengths and weaknesses of each:  
 
1) two gain stages into a long-tail PI, like the BF Fender Deluxe: input, triode, tonestack, triode, long tail PI.  
 
2) three gain stages into a cathodyne PI, like a Push-Pull Princeton: input, triode, tonestack, triode, triode, cathodyne PI.  
 
in either case, the amp would have a BF style tonestack, not a treble rolloff pot/cap.  
 
I had originally thought about the BF deluxe topology, with the two gain stages going into a cathodyne PI, but I have wondered how the overall gain of the amp would compare to the later Princeton type layout with three gain stages going into the cathodyne.  
 
I understand that with a cathodyne PI you're faced with no more than unity gain in the PI, which may negate any "advantages" of having 3 preceeding gain stages. In this situation, which topology would offer more net gain? Would it be a wash?  
 
tia!  
 
bob
 
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5/19/2003 9:32 PM
Man o'Blues
IME, the BF Princeton circuit with an LTPI drives the outputs into a nice distortion, unlike the wimpy stock concertina circuit.  
 
If you go with a concertina, you'll need to pad down the gain of preceding stage(s) to prevent signal overloads using split plate loads or unbypassed cathode resistors etc.  
 
I really do like the sound of the concertina PI if it is in fixed bias. I use a 12DW7 with the AU7 half as the PI in fixed bias in a couple of my amps and they rock! One of them has the PI direct coupled to the preceding driver stage and this is my favorite.
 
5/20/2003 10:04 PM
Bob Predaina
quote:
"IME, the BF Princeton circuit with an LTPI drives the outputs into a nice distortion, unlike the wimpy stock concertina circuit."
thanks for the response.  
 
are you talking about taking the AA964 circuit and replacing the cathodyne with a long tail PI, and leaving the rest the same? replacing the AA964 cathodyne with a long tail PI would be the topology that i had mentioned as #1 above - just like a BF Deluxe without tremolo.  
 
the other option (#2) of three gain stages plus a cathodyne would be like a BFPR without the reverb and trem circuits.  
 
going to three triode gain stages plus a LTPI isn't an option for me -- I don't have 5 triodes to work with, as there is only room for 2 dual triode tubes.  
 
i wasn't sure which topology you were talking about.  
 
thx.  
 
bob
 
5/21/2003 12:04 AM
Man o'Blues
quote:
"are you talking about taking the AA964 circuit and replacing the cathodyne with a long tail PI, and leaving the rest the same? replacing the AA964 cathodyne with a long tail PI would be the topology that i had mentioned as #1 above - just like a BF Deluxe without tremolo."
 
 
Yes, this would be option #1 that would sound nice.  
 
quote:
"the other option (#2) of three gain stages plus a cathodyne would be like a BFPR without the reverb and trem circuits."
 
 
Yes, this is the topology I was refering to. If you want to get close in tone to a Princeton Reverb sans reverb, I would add the 3M3/10pf series network between the 2nd and 3rd triodes, 500K grid return and -ve feedback to the bottom of the bypassed cathode resistor as per the PR schematic.  
 
Option 2A would be converting the PI to fixed bias as per the "PaulC" mod.  
 
Option 2B would be using a 12DW7 for the PI tube, with the AU7 half as the PI. The PI is still in fixed bias. This is basically how my PR is set up, and it is SO MUCH BETTER than the stock sound, it's scary!  
 
Option 2C would be direct coupling the 3rd stage to the PI. This is the one I'm having fun with in a homebrew using a 6SN7 for the output tube.
 
5/20/2003 12:12 AM
Chill

IIRC, the long tail gives you a gain of around 10, while the cathodyne plus the triode in front gives you a gain closer to 30-60 depending on whether you bypass the driver's cathode. Cascade 'em all and stick in an MV either right in front of or right behind the PI. But then I like preamp gain. :)  
 
Check out Steve Ahola's blues express schematics in the trainwreck stuff at blueguitar.org.
 
5/20/2003 10:20 PM
Bob Predaina
quote:
"Check out Steve Ahola's blues express schematics in the trainwreck stuff at blueguitar.org."
thanks for the point to Steve's amp. I didn't realize that Steve A had built the Blues Express using 4 triodes at the front end.  
 
Unfortunatly, I can't use a Trainwreck Express type of topology, as I don't have 5 triodes to work with -- I've only got room in the chassis for four.  
 
I see that Steve A's "Blues Express" variant used a cathodyne instead of a LTPI, and reduced the triode count from 5 to 4.  
 
I'm wondering how the Blues Express A5h amp would compare gain-wise if the 4 triodes were changed to 2 gain stages and a long tail PI instead of 3 gain stages plus a cathodyne.  
 
Has anyone tried comparing the two topologies? Steve A???
 

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