Re: Question on dc-coupled pair using separate tubes...

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5/29/2000 7:30 PM
Bruce	Re: Question on dc-coupled pair using separate tubes... I run my amps with the center tap of the 6v line at +30vdc.   Very very nice compromise for the cost of two 1/2w resistors and a 63uF/100v cap.   Bruce

 

5/30/2000 4:10 PM

Andrew M

Bruce said:   "I run my amps with the center tap of the 6v line at +30vdc.   Very very nice compromise for the cost of two 1/2w resistors and a 63uF/100v cap."     Can this work with a non-center tapped filament transformer, maybe using the usual two 100 ohm resistors? Any comments on the Ampeg method of referencing the heaters to the top of the cathode resistor in a cathode biased amp.     Lastly, how do you get italics for quotations?     Andrew.

 

5/30/2000 11:32 PM

Bruce

quote: "Can this work with a non-center tapped filament transformer, maybe using the usual two 100 ohm resistors? Any comments on the Ampeg method of referencing the heaters to the top of the cathode resistor in a cathode biased amp."   Yes, no trouble at all to do that with the 100ohm resistors.   My little 5watt Soulkicker Jr has a pair of hand picked 5% 1w 100ohm resistors connected to the 6.3volt line and the center tap point of those two resistors are connected by a single wire run to the +20v generated across the bias resistor on the cathode of the 6V6GTA.   Same thing only getting there a different way.     quoting:   Use the key followed by a q then the { bracket followed by the text to quote and then closed with the } bracket.     Bruce

 

5/30/2000 2:36 PM

kg

quote: "...In a perfect world you want the plate voltage of the first half running at approximately half the rail voltage."     actually, carl, you want it at about 1/3 of the rail voltage.     if you put the plate at 1/2, then the grid of the DC stage is at 1/2 B+. that means the cathode is sitting at approximately the same voltage as well.     if the cathode is sitting halfway between ground and B+, then you've only got another HALF to distribute from rail to cathode. this drop in potential must occur across the tube (from plate to cathode) AND across the plate load (from plate to rail). obviously, that does not give you equal voltage drops across Rp, Rk, and Vak, which is what you want for max voltage out from a cathodyne.     set the directly coupled Vp and Vg at 1/3 of B+, use equal value Rp and Rk for the cathodyne, and you're all set.     ken

 

5/31/2000 8:13 AM

Steve Ahola

Ken:     actually, carl, you want it at about 1/3 of the rail voltage.         So if I was running 300vdc to the plate of the cathode follower, I would want 100vdc on the cathode? Sounds a bit low to me, although your point may be that the B+ should be 500vdc for an optimum cathode voltage of 166vdc... (anything lower than 166vdc on the plate of a 12AX7 sounds pretty "brown" to me).         Are we talking about hi-fi or guitar amps?     --Thanks!     Steve Ahola

 

5/31/2000 11:15 AM
Dave H.	Steve,     You and Ken are not talking about the same circuit. Ken was describing the cathodyne PI where the signals are taken from the cathode and plate of the tube so you would want 1/3 of B+ across each resistor. The 5F6A DC coupled pair tone stack driver output is a cathode follower which has no plate resistor so you would want to drop ½ B+ across the cathode resistor.     Dave

 

5/31/2000 1:15 PM
kg	exactly.     thanks dave.

 

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