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Cathodyne PI

5/28/2000 6:54 PM
Cathodyne PI
Can you direct couple the grid of a triode used as a cathodyne PI to the plate of the preceding stage? Is there some advantage to AC coupling in this configuration ala the Fender Tweed Deluxe. Might it work better to use a 12ax7 with the first triode as a standard gain stage direct coupled to the second triode with matching plate and cathode resistors? All input is appreciated.
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5/28/2000 8:53 PM
Ray Ivers

Yes, you can do this - it's how I would do it, for sure. You have to make certain that the phase inverter is properly biased, but this method has the advantage of presenting very little loading to the output of the preceding gain stage, allowing you to run a higher value of plate load resistor for more gain if necessary. If you know the B+ voltage and tube type you'll be using, I'll be glad to run the numbers thru TubeCAD for you. AC coupling would require another two resistors (and the coupling cap, of course) but it may be the more flexible approach since the DC conditions of the previous stage no longer affect the PI stage's design.  
Ray Ivers
5/29/2000 2:22 AM

What would be the best candidate from the following tube types: 6SL7, 6SN7, 12AX7, 12AT7? An EF86 is the front end of the amp which will drive the first triode of the tube I use (no tone stack). The available B+ for the tube will be about 260V. The PI will then drive a pair of cathode-biased EL84's in ultra-linear.  
EF86-->volume control-->triode-->triode PI-->EL84's  
What would be the proper values for the plate and cathode resistors on the PI triode? Thanks so much for your help in this matter.  
5/29/2000 4:22 AM

>You have to make certain that the phase  
>inverter is properly biased, but this  
>method has the advantage of presenting  
>very little loading to the output of the  
>preceding gain stage, allowing you to run  
>a higher value of plate load resistor for  
>more gain if necessary.  
Actually, with a typical common cathode stage found in most guitar amps, the DC voltage is a little bit high, and will bias the cathodyne so that one side clips sooner than the other. Switching to a higher plate load resistor in the common cathode stage (assuming its cathode resistor is left alone) helps line the DC up better for the cathodyne, in addition to giving a bit more gain.... Sounds like a "win-win" to me.... :^)  
BTW, I've seen an old circuit with a cathode follower DC-coupled to the plate side of a cathodyne phase inverter, so that both phases have a low impedance cathode-driven output. I guess this makes the amp work a bit more symmetrically when at the onset of clipping, since both sides have a fairly low impedance source driving them, rather than just one side.... I haven't tried it yet, but it does look interesting.  
C ya,  
5/29/2000 4:40 AM

Thanks for the response! What is an acceptable level of DC voltage for the PI's grid? My B+ line is only around 260V. Any advice is appreciated.
5/31/2000 3:13 AM

Just wanted to move it back up a bit. thanks!
5/31/2000 1:47 PM
Dave H.

With a 260v B+ 50v on the grid will give you about 50v across the cathode resistor, 50v across the plate resistor and 160v across the tube which would be OK. If you get the 50v by directly coupling the grid to the plate of the previous stage you will have to bias the plate to 60v which may not give the best tone.  

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