- Posted by chaslieb on May 05, 2001 at 15:24:06
I have a preamplifier using 12au7 tubes and was thinking of using a parafeed output transformer coming off the plates where a c4s is used instead of an output choke. If anyone has done this, please tell me your experiences. I was reading the latest Valve, re: C4S with directly heated tubes, and wondered if the C4S could be substituted for the choke of an output tube like a 2A3 in a parafeed arrangement? Any ideas?Charles Liebman
Follow Ups:
- Re: C4S instead of parafeed choke? Paul Joppa 13:55:47 05/06/01
(0)
- In Reply to: C4S instead of parafeed choke? posted by chaslieb on May 05, 2001 at 15:24:06:
A choke as a plate load has the advantage that it drops a very small DC voltage, usually less than 20 volts. The signal can swing positive and negative from the quiescent point.
A current source as a plate load needs to see a voltage drop, usually at least 5 volts, even at the greatest output voltage swing. Therefore at the quiescent operating point it will need to drop the peak output signal voltage plus the margin of at least 5 volts. That means the power supply voltage must be higher than for a choke (or transformer) load, and the current source must dissipate some heat.
An example: typically a 2A3 will have the cathode (filament) at 45 volts and the plate at 295 volts, so with 20 volts for the choke or output transformer the B+ will be 315 volts. Peak output is about +/-150 volts, so a current source will need to drop at least 155 volts, and the power supply needs to be 450 volts minimum. The current source drops 155 volts at 60 mA (the plate current), so it dissipates 9.3 watts as heat.
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