SET Asylum

RE: Budget speakers to pair with SET amp

199.187.211.16

Whats-a-vacuum-toob?

In Reply to: RE: Budget speakers to pair with SET amp posted by Whats-a-vacuum-toob? on September 16, 2021 at 10:11:12

Thinking about this a little more...

I don't know the output impedance of the amp and I have no documentation. Are SETs amps limited by design to 4 Ohm, 8 Ohm, or something else?

Is this something that I could determine if I can remove the shield and get the transformer part number?

Thanks again!

Follow Ups:

RE: Budget speakers to pair with SET amp - Paul Joppa 08:24:30 09/17/21 (5)
There are two resistances to consider - the source impedance and the intended load (speaker) impedance. Most amps are designed around 4, 8, and/or 16 ohm loads, but the source impedance depends on the actual design. The ratio of load impedance to source impedance is the damping factor, and most modern speakers assume it to be very large but it's usually around 3 with SETs. That's why you want a speaker whose impedance does not drop much below the nominal value.

Your tube seems to be a beam tetrode, not a true triode. If the second grid is tied to the plate (usually with a resistor around 100 ohms) then it's a simulated triode circuit. But sometime you'll see a pentode/tetrode design with feedback, or even a screen-driven output stage, especially with high-perveance tetrodes such as you have.

You can measure the output impedance by feeding the amp a sine wave signal and measuring the output voltage unloaded and loaded with a speaker-size resistor. (If you don't have an oscillator, you can download a signal-generator app for a smartphone)
- RE: Budget speakers to pair with SET amp - Whats-a-vacuum-toob? 11:48:16 09/17/21 (4)
  Thank you, sir!
  
  Brilliant information. I noted the unusual choice of driver tube in my own research. Knowing the designer, it makes sense. His knowledge of analog video was encyclopedic. Of course he used a tv tube. He also had a tendency toward altruism. It would be just like him to try to improve a design by reducing the cost and barrier to entry. Hard to beat a $5 tube in an audio amp. The 6P36S must have some specific characteristic that he was after.
  
  I've been hesitant to crack the thing open for fear of messing up some subtle tuning, but it seems that some mysteries can't be solved any other way.
  
  If you're interested, I'll post what I find inside. It will probably be a few days. I have a huge collection of tubes to test for estate liquidation.
  - RE: Budget speakers to pair with SET amp - Whats-a-vacuum-toob? 08:08:50 09/25/21 (1)
    I got a lead from the guy that I bought speakers from. With help from the nice folks over at the Decware Forum, I found the original discussion about the amp that I inherited. The builder of the amp is the guy who designed the Catskin. The amp in my possession is the same unit discussed. It sounds like the design was experimental in many respects.
    
    For those who are curious: https://www.audiokarma.org/forums/index.php?threads/catskin-class-a2-set-with-secondary-flux-cancellation.780706/
    
    Thanks again for all of the help and recommendations!
    - Thanks! - Paul Joppa 08:38:56 09/29/21 (0)
      ...for the link and info. Very imaginative and creative design!
  - RE: Budget speakers to pair with SET amp - Whats-a-vacuum-toob? 16:21:31 09/20/21 (0)
    OK. Here it is. I've talked this amp up. Now it's time for the big reveal.
    
    I've posted a video about the amp and what's inside, here -> https://youtu.be/ikAjMVTd5b4
    
    Inside, the amp is about what I expected; complex but elegant. This is definitely not the 15-20 component-per-channel SET amp that I see in schematics online. This amp is more sophisticated.
    
    Two mysteries were solved. The output transformers are 8 Ohm and the driver tubes are pentodes that are configured as triodes.
    
    New mysteries have been discovered. Why were dual triode tubes used in the first gain stage? They aren't wired in parallel or push-pull configuration. What are the devices on the heat sinks? Maybe linear regs?
    
    More sleuthing is required, but at least I can be certain of the output impedance now.
  - RE: Budget speakers to pair with SET amp - Paul Joppa 17:44:29 09/17/21 (0)
    It would indeed be interesting to see what a video designer would come up with for an SET!

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