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Thoughts about tube output stages...

Hi all, Never worked much on tube circuits, though lately my interest has increased for this old technology. I thought some questions could clarify a few things:

  1. The bias current through the tubes, this is pretty much as the idle current in a solidstate design, right?
  2. How is the idle current set?
  3. As far as I can see, the idle current flows through the output transformer, why doesn't this saturate the transformer?
  4. What function does the screen grid have? Some design apply full HT to these grids while others apply HT through a resistor. Do these have an influence of the gain in the tube and/or the idle current?

Thanks, I'm sure I'll come up with some more questions, hope it's ok to ask.

Regards Steve

Reply to
Steve
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Yep, analogous.

By the bias on the grids. Typically, there'll be a DC negative voltage set to the grids via resistors, or through the centre tap of the driver transformer. It's common to have a pot to adjust this. Normally there'll be a few specified conditions in the data for the tubes, typically they'll be 'class AB', with some standing current, and 'class B' with significantly less. As you get closer to class B linearity becomes more of an issue.

Not a problem for a pushpull stage, for single ended the tranny has to be designed for it. Single ended power amps are very inefficient, you'd rarely have one above a few watts IME.

Generally the screen supply will be lower than the anode voltage. Once again, the specs provide the correct value. A screen supply above what's specified can damage the tube, because the grids are relatively fragile. A correctly set up tetrode or pentode will have higher gain than an equivalent triode.

There are unofficial variants. I remember driving a pair of 807s in a modulator with the input on the screen grids and the control grids fixed to control the standing current. It really smoked.

HTH

Reply to
Bruce Varley

For a complete exposition on the function of the screen grid you need to find an old ARRL handbook, RCA tube data book, or other text from the day.

Basically, a screen grid screens the cathode from the plate, so the cathode current isn't affected by the plate voltage. For most of the tube's operating region most of the electrons are captured by the plate even if its voltage is lower than the screen's, so the effect of the screen grid is to significantly raise the plate resistance. This also increases the "power sensitivity", or the ratio of power out to voltage in, significantly over a triode. The downside is that you must pay attention to the care and feeding of the screen.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

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