|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
50.132.29.220
In Reply to: RE: SET and PSET with 6S33S posted by mstolpner@gmail.com on May 05, 2015 at 10:30:22
With transformers that are configured to be highly flexible, there come other sacrifices that you wouldn't otherwise have with a unit designed for one application only.In this instance, I would find this to be particularly true for the PSE 6C33 instance, as you have absolutely the wrong transformer for that. (Go custom - 300:8 @600mA)
Unfortunately, you will really want to pick whether or not you want to go PSE, then you can narrow down on experimenting with iron.
As for the Melquiades, I am a little disturbed at the parts count and complexity just to present grid bias voltage to the driver tube. There are plenty of decent ways to get 3.4V of bias under the cathode, and most of them don't involve their own power supply. If you have a preamplifier with a transformer coupled output, especially one with low impedance, you will lose bias adjustment on the driver, and the operating point will shift. (It needs a blocking cap at the input)
The 6E5P is a great choice for a driver tube, but a real opportunity here has been wasted. This is a tube with high Gm and low Rp, and you have an amplifier with 200V of B+ already. There are many decent plate chokes that are suitable here - a Magnequest EXO-001 will give you 100H at 20mA and drop under 20V doing so, you could just use this between the 6E5P plate and B+ supply for the 6C33, you've just eliminated a power transformer and a bunch of parts that you have to buy and shove into the amp, and heating in your chassis will be reduced. To bias the 6E5P, try a 270 Ohm resistor, a 270 Ohm resistor bypassed, and maybe a blue 3.5V LED.
Edits: 05/05/15Follow Ups:
Are what I use under the 6E5P driver in my 833C amps. Gives just about the right voltage for this application, too. Sounds great!
Interesting! Do you have a cap on the input? A schematic would be helpful.
No cap on the input. Not even a cap between the stages or in the cathode of the power tube.
Edits: 05/07/15
Speechless! :-) I bet it sounds wonderful! Thank you for sharing.
Hey, we are neighbors!
Regarding biasing. It's a good point. Romy claims that this biasing technique is beneficial for the sound quality and I never tried validating it. I kinda trust his claims. This technique also sets 0V DC on the input, so I do not think a preamp can impact the biasing without having a DC voltage on its output?
6S33S needs a large voltage swing on the gate, do you think 6E5P will be able to provide it with 200V B+? Again, Romy claims it's a sweet operating point for 6E5P.
I would totally go custom OPT route. However, I could not find a reputable manufacturer willing to do it. One told me right upfront that he did not want to compete with Landuhl. (FWIW) I would appreciate a recommendation.
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.
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.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: