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RE: Q to John Swenson or others about Transconductance amp..

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In Reply to: RE: Q to John Swenson or others about Transconductance amp.. posted by C.Y on October 19, 2007 at 01:24:38

The CCS will run the cathode at whatever voltage is necessary to maintain the current through the tube. As long as the B+ is high enough so the output tube can maintain that current with a negative grid, it will do so.

If you have built one and the grid to cathode voltage i sclose to zero or positive, that probably means the screen voltage is too low. There is a voltage divider feeding the screen, you can change the screen voltage by changing the resistor values. This needs to be done so the grid has enough voltage range for the current swings you are trying to achieve.

John S.

Follow Ups:

A question about the role of tube? - ychungmd 17:54:06 10/24/07 (2)
Hello John

I have a plan to make a Transconductance Amp with SE transformer output. Battery PSU without Pentode.

I am thinking about the role of tube in TraC amp as below.
1) heat dissipating when using high voltage.
2) another pentode cascode above Q2 mosfet?
3) adding character of tube sound in TraC amp?

How about your opinion?
Thanks so much.

Regards

ychungmd
- RE: A question about the role of tube? - John Swenson 18:18:38 10/25/07 (1)
  The pentode is primarily there to dissipate the heat and handle the voltage.
  
  You can certainly do it with just the MOSFETS, just realize that the voltage rating of the MOSFET has to handle the total voltage excursion of the output, not just the idle voltage. You will probably wind up with fairly high current and low impedance of the OPT.
  
  Lets say you use Mike's 800 ohm OPT (EXO-800?) if you run at 300mA idle current, thats 600mA peak to peak, which is 480V swing on the OPT. Half that is 240V + 50 to make sure the CCS is running at all times, thats 290V B+. Plus the 240V = 530V peak. That comes out to 36watts output and 87watts dissipated by the CCS, which I think is a little bit pushing things. For this service you want a fairly low current, but high power dissipation device, say rated for 2-5 A with at least a 100 watt dissipation. And of course a humongous heatsink.
  
  Oops thats way past the power rating for the EXO-800, so lets say 200mA, thats 320V swing, 16watts out, 42 watts dissipation, 370V max voltage, much nicer. Still a pretty big heatsink though. Thats a 200mA 210V B+, so finding the right power transformer etc is going to be interesting. And then you need the right plate choke etc.
  
  It can be done.
  
  The "sound" with the tube is almost identical to one without the tube so don't don't worry about that. And you don't need filament supplies etc. But you don't get nice glowing things.
  
  John S.
  - Thanks, I know a fault in my idea. Need Help - ychungmd 22:44:01 10/25/07 (0)
    Thanks, I know a fault in my idea. Need Help
    
    Hello John
    
    I did not consider the voltage swing in transformer.
    
    I compared your CCS pre and Zen-like CCS pre which use one IRF 820. Actually this modi was done by C.Y on my request.
    
    Sound is different. Your pre has more impact especially in bass. Zen-like pre is more airy, but deficient in bass compared to yours. I prefer airy sound.
    
    I�d like to convert it to SE transformer output amplifier as posted. I do not have any idea for a grid reference in this design. Any simple solution? I need your help.
    
    A great appreciation.
    
    Regards
    
    ychungmd
RE: Q to John Swenson or others about Transconductance amp.. - C.Y 19:22:52 10/23/07 (1)
Thanks John,
Before reading your reply I just have finished transconductance EL84 amp.
What a sound, again I am happy for myself that I built this after your circuit.
Mine uses two EL84s per channel with total 80mA per channel. This is because I wanted to use Dynaco ST-120 cage which is not tall enough to house KT88. Also I used Gary's battery CCS that uses 12 volt batteries for bias. I went crazy and I used resistor loaded parafeed, using two 10K 50W WW resistors in parallel and 5K OPTs.
I made your CCS preamp too, and tried SLA battery power. This caused a little stir in my hobby club. take a look on the link.
I appreciate again the sharing of brilliant circuits, from you and Gary who recently put his site back up.
Cheers,
C.Y
- CCS pre amp picture (Open in New Window)
Edits: 10/24/07
- RE: Q to John Swenson or others about Transconductance amp.. - kenev 01:04:58 11/06/07 (0)
  Indeed, John's circuits are brilliant, innovative, excellent!
  
  I had originally tried the EL84 version, as a first start to see what is all about this current fed single diver speakers and it was absolutely amazing on my Fostexes, although a little underpowered even for 95 dB speakers.
  
  Next, I built the higher power KT88 version, which I am currently using, with CCS fed VR tubed shunt regulators (one per channel) instead of the original resistive voltage divider screen supply - it sounds a lot better now.
  
  I have plans, also, for building the "high-power" 813 version, later in the not so far future.
  
  I have not tried the transonductance pre-amp, but I would like to give it a try - after all it's not very difficult to build.
  
  Once again,thanks John for sharing your brilliand ideas.
  
  Regards,
  Evangelos

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