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In Reply to: RE: SET and PSET with 6S33S posted by mstolpner@gmail.com on May 05, 2015 at 15:06:40
Sowter will wind what you want, but it will be expensive.
With a plate choke, the driver can swing above B+, so 200V is plenty adequate.
If you look at the biasing scheme, there is a trim pot in the negative rail used to trim the voltage at the driver tube. If you assume that the input of the amplifier is shorted (as is the case at DC with a preamp output transformer feeding it), then this adjustment pot will be useless, as the output of the negative supply on the end of that 390K resistor shorts to ground.
Follow Ups:
Thank you for recommendation. I will check with Sowter, Lundahl is not cheap either.Good point, adjustment does become useless in this case. However, it does not impact the bias.
BTW, I wonder how would the driver with the choke sound vs resistive load? You are absolutely correct regarding the amp sensitivity - it could have been better. :-)
Edits: 05/05/15 05/05/15
Actually, the bias will pop up by about a volt, which is not insignificant.
Can you please explain how the bias can pop up?
Run the numbers for bias voltage assuming the input is shorted.
If you set up the amp properly, there is 0V on the input. What can happen if you short the input?
The trim pot adjusts the current across that 12K resistor to set the bias. If you ground the input, the current through that resistor is now fixed, and no longer dependent on the trim pot.
This trim pot is used to set 0V on the input. You can change the 12.1K resistor to adjust the bias if needed.
Yes, and if you have an output transformer with a low impedance secondary feeding the input of the amplifier, the trim pot will no longer function. You will indeed have to change the 12K resistor to get the correct bias.
I am in the process of building PSET 6S33S mono blocs using a circuit from Kevin Carter. Driver is a 6J49P. Metal work should be going to the paint shop to be powder coated soon. Looking forward to their completion and hearing them in my system.
big muck, I am curious to hear your opinion. Kevin was super nice to share the schematics with me too. It's very intriguing. Did you do it point to point or with a pcb? Can you share your impression on the sound?
Most of my projects I do point to point but on this project I am using circuit boards supplied by Kevin. I am building the chassis from scratch using .125" aluminum plate and 4" aluminum channel with Kevin's chassis work as a model. Wood will be attached to the front and side plates to enhance the cosmetics. Power transformer is potted and the Lundahl output transformer will be covered by a custom wood box made in my shop. I've yet to hear the amps but will share my impressions when I have completed the project. The chassis work has been a challenge at times but is nearing the end of fabrication.
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