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Old 5th November 2009, 12:07 AM   #1
2wo is offline 2wo  United States
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Default Replacing the cathode resistor with a current source in SE ?

I did a quick breadboard with a 6V6 triode wired, 320V on the plate. I used a IXYS 1045 set to about 30ma. This gave me about 24V at the cathode.

All I got was a little distorted sound, before I ran out of time to play. So Can this work in SE? Was the signal fighting the CS or maybe I don’t have enough voltage compliance cathode to ground…John
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Old 5th November 2009, 12:41 AM   #2
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A CCS on the cathode in a single-ended Common-Cathode
circuit will give essentially no gain. It would work OK in
a Common Plate (follower) circuit.

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Old 5th November 2009, 12:51 AM   #3
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If you bypass the CCS with a suitable cap,you should have sound.
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Old 5th November 2009, 12:51 AM   #4
rdf is offline rdf  Canada
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Did you bypass the CCS with a largish cap? A CCS in the cathode has the nice side benefit that variations in B+ from hydro fluctuations cause a linear instead of geometric rise in static plate dissipation. Plate voltage changes, plate current doesn't.
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Old 5th November 2009, 01:02 AM   #5
tomchr is offline tomchr  United States
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I had issues with the IXYS 10M45 in a similar setup. I was using it to bias the pentode section of a 6LU8 compactron. I wanted 55-60 mA but couldn't get more than 40 mA. I wonder if I was running out of headroom on the 10M45 part. The voltage across the 10M45 was supposed to be about 30 V.

You can verify the current of your CCS by measuring the voltage drop across the "programming resistor".

~Tom
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Old 5th November 2009, 05:17 AM   #6
2wo is offline 2wo  United States
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Thanks everyone, the point was to eliminate the bypass cap. I will hang one in there just to try it…John
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Old 5th November 2009, 07:37 AM   #7
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Quote:
the point was to eliminate the bypass cap
All problems solved if you go fixed bias instead with the cathode grounded.
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Old 5th November 2009, 08:13 AM   #8
mos57 is offline mos57  Italy
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Quote:
Originally Posted by revintage View Post
All problems solved if you go fixed bias instead with the cathode grounded.
No cap discount here.

If you use fixed bias, then the AC signal is closed on negative power supply cap.
So, you move this problem in another place, only.

Francesco.

Last edited by mos57; 5th November 2009 at 08:21 AM.
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Old 5th November 2009, 09:05 AM   #9
OneyedK is offline OneyedK  Belgium
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Quote:
Originally Posted by mos57 View Post
No cap discount here.

If you use fixed bias, then the AC signal is closed on negative power supply cap.
So, you move this problem in another place, only.

Francesco.
True, but I thought it gets another magnitude.
Instead of Ra+Rp, Rg1 (much bigger) comes into play...
Doesn't have the negative power supply cap less impact
than the cathode resistor bypass cap?
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Old 5th November 2009, 09:52 AM   #10
mos57 is offline mos57  Italy
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Quote:
Originally Posted by OneyedK View Post
True, but I thought it gets another magnitude.
Instead of Ra+Rp, Rg1 (much bigger) comes into play...
Doesn't have the negative power supply cap less impact
than the cathode resistor bypass cap?
Have you thinking about the other components that you must use for this?
  1. the cost and the limitation band of the input transformer?
  2. the cost, the complexity and the influence in sound, of another power supply with its diodes,smoothing caps, and peraphs the inductor?
  3. the added power supply noise injected directly in the input circuit?

Francesco.
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