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Valve 45 amp questions

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I'm very excited about a 45 SET amp schematic I just came across some dsays ago. It was published in Valve and the schematic looks very simple. Simple enough to try it as a first scratch build project. The article can be found here: http://www.positive-feedback.com/Issue1/single.htm
Doc said that C4S could be used just as easily as the CCS. How can I calculate how to set up the C4s? I have the C4S manual and a couple of extra boards that came out of my Parabees when I did the JT upgrade but I could use some help here.
Another question would be how sensitive is this. Or put it differently: Would it need an active preamp?
Thanks for your help,
xavier

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Re: Valve 45 amp questions - Paul Joppa 11:31:29 03/27/06 (1)
The easy way would be to keep the ParaBee C4S boards unmodified. That gives about 6mA current, a bit over half the amount specified. But we found some years ago that it sounded better with a single driver triode, rather than using both sections paralleled. If you use a single section at 6mA and change R4, the driver cathode bias resistor, to 220 ohms the voltages should come out pretty close, about 120v at the plate of the driver.
Direct coupled circuits like this are notorious for voltages and currents interacting, so you may have to adjust something once it's running to get the voltages spot on. I think that cathode resistor is the easiest place to make such changes, so mount it in a way that you can get at it to replace it if necessary.
Note the original ParaBee power supply as well. There is a 15k resistor between B+ and the C4S, and a capacitor from that point to ground. This prevents excess voltage across the C4S. (In the original diagram, it's "R6,7" that serves this purpose.)
With a 5965, it will need a bit less than 1.0vRMS to produce full power. A passive control, or a unity-gain buffer, is adequate for use with a normal CD player which has 2vRMS peak output.
- Cool! Thanks Paul (nt) - xcortes 13:17:04 03/27/06 (0)
  nt
No help? - xcortes 11:23:44 03/26/06 (5)
Please
thanks
xavier
- Re: No help? - Seth Hensel 12:32:24 03/26/06 (4)
  That was the article that started me down the slippery slope.
  You have two options: One - build it as the schematic indicates. As I recall, there is a "primitive" CCCS specified. Two - should you wish to incorporate the current offering, my understanding is that the kit Doc offers has instructions for calculating the necessary values. If it does not, once you buy the kit, there is plenty of help here.
  Hope this helps,
  Seth
  - Re: No help? - sethhensel 12:38:11 03/26/06 (3)
    I just re-read your post. You said that you have the kit and the boards. Sorry.
    As for the active pre. My somewhat limited experience indicates that an active pre partners well with this kind of amp. Others, perhaps with different experience will suggest a passive pre or maybe a transformer volume control. If you are starting out, you really can't do better than a Foreplay.
    Understand that this is addicting and your tastes may change as your skills improve.
    Seth
    - Re: No help? - xcortes 15:58:49 03/26/06 (2)
      thanks seth. i will review the c4s manual when i have access to it next week. i hope i can get it from there. i`m traveling now. i have one foreplay iii but its on my main rig. i also have a foreplay 2 but i modified it and its a double buffer with no gain that i use to record from my seduction. i want to build the 45 for my bedroom and thats why i,d like to use it without a preamp.
      i will maybe build it and use just a passive and later try a foreplay or something different.
      xavier
      - Re: No help? - xty 01:19:33 03/27/06 (1)
        
        In Reply to: Re: No help? posted by xcortes on March 26, 2006 at 15:58:49:
        
        This might help a little: http://www.ecp.cc/c4s.html
        
        Thanks (nt) - xcortes 07:55:26 03/27/06 (0)
        
        In Reply to: Re: No help? posted by xty on March 27, 2006 at 01:19:33:
        
        nt

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