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Op points for 750R cathode resistor?

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In Reply to: RE: Wright Mono 8 Schematic & MQ Iron posted by mqracing on September 17, 2017 at 16:39:56

Thanks for the info, Mike.

Now what's bugging me is the 750 ohm cathode resistor on the Mono 8's 300B tube. In order for that to work with the 60mA DC rating for the FS-030, it would need to be run at very low voltage for a 300B -- more like 2A3 voltage, actually.

If I run the 300B at 250Vak, the plate current is 63.7mA. Would that be too high for the FS-030?

If not, then either the voltage is a little lower than that (resulting in much lower output power than he claimed -- ca. 4W instead of 7W) or perhaps George had specially airgapped FS-030s for higher current?

Is it possible that he ran it in the 70+ mA range? That would allow for 300Vak and closer to 7W output power.

Follow Ups:

RE: Op points for 750R cathode resistor? - Palustris 11:08:51 10/18/17 (3)
750r with 60mA current gives us a bias voltage of -45V. That places the plate to cathode voltage at about 255V on the plate curves. This is a legitimate operating point for the 300B that ensures long life and low 2h and 3h distortion and about 4W output. So we have low distortion and long life for the tube.

Now, if you wish more power then you will need to raise the B+. That's entirely up to you. You can still have low distortion even as you maintain the 60mA constriction. For instance you can use a P-K voltage of 350V with 60mA plate current and -74V bias for 8.3W, at about the same distortion figures as above.

In the end it seems counter productive to slavishly copy a very dated circuit design someone else produced. There are many better alternatives that you can use; for instance a two stage direct coupled triode wired pentode as driver for the 300B will be much more transparent than a three stage RC coupled amp.
- RE: Op points for 750R cathode resistor? - anystereo 05:45:06 06/01/18 (0)
  I like this thread, low operating point for a 300b circuit, great. I've a pair of we300b's i'd like to keep for a real longtime. but more specifically a' triode wired pentode', I've also got a pair of d3a's I am itching to use, and am looking for a schematic to for them as direct coupled drivers for the 300b's. any thoughts suggestions would be great. -the output to go thru a TFA-2004/rgc-6 parafeed.
  
  thanks david,
- RE: Op points for 750R cathode resistor? - jdrouin 12:16:59 10/18/17 (1)
  Thanks Palustris. I found out that the HV winding on the PT is 620VCT, so yes at 310V per side it runs the 300B at 2A3 voltages.
  I realize that the circuit is dated and that there are much better ways to make a 300B SET. This project is more a labor of love and also a self-conscious experiment to go through the process of recreating something that is now Lost to Time.
  It's also not the only project I have going right now.
  Now that I know the formulae for calculating load lines, voltages, interstage capacitor values, and PS design, etc, I'm actually designing two 300B SETS, one for my 5K Electra-Print OPTs and another for a pair of Italian 3.6K OPTs that a friend keeps teasing me with. So I'll be breadboarding a WE91A clone with a 6SJ7 and am looking at various other input tubes.
  The previous amp on the breadboard was a WPA 3.5 front end with a 300B output stage run at 350Vak and 50mA into a 5K load. That sounded fantastic.
  Incidentally, the Mono 8 clone sounds really good on the breadboard even though my OPT specs are suboptimal. I have a complete schematic now and will post when I can get back to my computer.
  Really need to find a pair of TFA-204.
  
  Edits: 10/18/17
  - RE: Op points for 750R cathode resistor? - Caucasian Blackplate 20:35:40 10/21/17 (0)
    The WE91A is also a bit dated.

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