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Voltage drop across c4s

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Hi there
How do you calculate the voltage drop across a c4s load ???
I have limited B+ headroom and am not sure what the drop would be .
I have the manual from the anticipation upgrades but it doesn't help with this.
Specifically I have 200v B+ and need 35 ma through the c4s - what voltage with be coming out ??
Obviously ohms law applies here somewhere but I am not sure what the R is?
Thanks in advance for your replies

Follow Ups:

Thanks all you're a great help !!! - SLM 15:13:35 04/26/02 (0)
Thanks guys
You've helped a lot - I will let you know how I go
Simon
Re: Voltage drop across c4s - VoltSecond 21:19:27 04/25/02 (0)
The C4S clips with 5V across it.
So with a 200V B+, the C4S clips at 195V.
The C4S behaves better with more like 20V instantaneous across it (estimate). So this means plan on 180V peak from your tube.
The static plate voltage on the tube will be closer to 120V (WAG) when you are all set and done. So you'll have 80V across the C4S at 35 mA for 2.8W loss, so buy a heatsink!
Re: Voltage drop across c4s - SpongeIt 20:18:43 04/25/02 (0)
This post by Paul Joppa may help at bit.
-matt
- http://www.audioasylum.com/forums/bottlehead/messages/24362.html
Re: Voltage drop across c4s - Jim Carlon 18:53:09 04/25/02 (3)
That depends upn what it is hooked up to. The tube and associated circuitry will all come into play. If you are using the C4S to replace a resistor in an existing circuit, you should be able to get very close to the same voltages.
Perhaps it would be best to describe the rest of the setup.
Jim
- Additional info - SLM 02:02:54 04/26/02 (2)
  I want to use it to replace a resistor on a monkey style amp between the cathode of output tube at around 200 or 220 volts and feed the plate of the driver at around 150-170 volts in the new circuit . It will need to lock in at 35ma and hopfeully drop obviously around 50 volts
  Driver tube is c645pi
  Thanks for all the help already - i have the c4s booklet but don't have this bit worked out
  - Re: Additional info - Jim Carlon 11:58:14 04/26/02 (0)
    If I understand correctly, in your existing design, the voltage for the plate of the driver tube is obtained from the voltage drop across the cathode resistor of the output tube.
    If this is the case, then the only thing you will need to be concerned about is how much AC drive voltage you need on the grid of the output tube to obtain the output you're after.
    Assuming 200 Volts available and 150 Volts on the plate of the driver, you have 50 volts to play with, less a minimum of 5 Volts compliance for the C4S, so your AC drive will be limited to a peak of 45 Volts. Paul and VS, please chip in here if I have gone astray!
    If this is adequate for your purposes, then the only other thing to be aware of is the 2mA additional current that you will need to bias the LEDs on the C4S. In other words, if you want 35mA for the tube, the C4S will require 37mA, 35mA for the tube and 2mA for the LED bias. I mention this so that you may figure out how that extra 2mA is going to affect the voltage drop across the cathode resistor of the driver tube.
    Hope this helps,
    Jim
  - Re: Additional info - AnandR 06:35:18 04/26/02 (0)
    You might want to also ask Jeremy Epstein about this. He has a monkey amp as well with this exact configuration except he used the LL interstage trannie as his current source.
    AnandR.

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