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Re: PP amp design

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KevinC

(M)

In Reply to: PP amp design posted by akltam on September 12, 2003 at 07:37:02:

Alan,
What you propose will work just fine, but....
As I mentioned in the reply to Daniel's post, I have found that step-up in the interstage only works sonically with a very low Rp driver tube. The real plate impedance of a 6C45 seems to be about 2.4-3K, which makes the driving impedance seen by the grid of each output tube something like 12K. I would prefer to see something less than 3000 ohms. I used a driver stage like you propose for a while in a P-P 300B amp and felt that the sound was just not pleasing enough. I changed it to the 3 stage design that I posted for a very large improvement.

Kevin Carter
K&K Audio
www.kandkaudio.com

Follow Ups:

Re: PP amp design - akltam 17:51:09 09/12/03 (5)
Kelvin,
I checked from the spec for 6C45, they said the internal plate is only 1200 OHM. But certainly the real implementation and listening experience does matter.
The reason I don't want to go for what you have done (6sn7 and 6BM8) is that I do not want to expand my tube collection choice.
What about two stage 6H30 but doing the other way, to step it down after the two stages - using ration 2.25+2.25:1+1 ? I am just exploring other possibilities.
Alan
- Thank you! - akltam 01:38:15 09/14/03 (0)
  Kelvin and others,
  I am thinking about 5687 and 7044 also. They also have low plate resistance and large current (less than 2K).
  Kelvin, it seems that what I am going to do will end up very similar to yours. Thank you
  Alan
- Re: PP amp design - KevinC 10:56:24 09/13/03 (2)
  I like the 6H30 idea better. You could choke or resistor load the first half of the 6H30, then direct couple to the grid of the second (interstage transformer loaded) half of the 6H30, using a bypassed (Black Gate) cathode resistor. This keeps small value coupling caps out of the path.
  
  Kevin Carter
  K&K Audio
  www.kandkaudio.com
  - Re: PP amp design - Tre' 22:03:46 09/18/03 (1)
    With the first half of the 6H30 direct coupled to the second half, won't the cathode resistor be a larger value allowing the use of a film bypass cap?
    - Re: PP amp design - KevinC 04:42:37 09/19/03 (0)
      Yes, it might at that. I haven't done the careful math to know what the exact value would be, but at 20mA and 100v a 5uF capacitor would get the roll-off below 10Hz.
      
      Kevin Carter
      K&K Audio
      www.kandkaudio.com
- Re: PP amp design - Horn-a-copia 10:32:54 09/13/03 (0)
  I think you may be on the right track with the 6H30P. This tube has a fabulous voice, a low Rp and loves to be loaded with a coil. I agree with Kevin about avoiding a step up between the driver and the output. Stay with 1:1 or even a step down and the sound will really open up. You could use both halves of this tube to get two gain stages in one bottle if you need.

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