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SET and PSET with 6S33S

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Posted on May 5, 2015 at 10:30:22
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
Hi,

A year ago I was greatly intrigued by the Melquiades 6S33S amp http://www.goodsoundclub.com/PDF/Melquiades_SET.pdf . I could not find much information outside of the Romy's website, so I decided to build and try it. It took me a lot of time, but finally I built it with LL1693 OPT and I want to share my experience. I am not very good at describing sound. However, I like the sound a lot and I consider myself demanding. I much prefer it to my RM-10 or really any amp I heard to date. Melquiades produces a very well defined sound, good bass, reproduces various genres very well, brings up texture and highlighting sound of acoustic instruments. It's easy to listen to and not fatiguing.

I experimented a lot. I built SET as per Romy's design and PSET with 2x6S33S. I slightly prefer SET version to PSET, however, PSET gives some extra power which might come handy with my Meadowlark Hot Rod speakers. So I decided to build the final version in a way to easily switch from SET to PSET. It's actually quite simple with this amp.

I tried various operating points for 6S33S and I was surprised how much sound changed with changing B+ from 210V to 180V. To my ears it's a bit harsh and 'transistory' at 210V and mellow at 180V. I think my preference is somewhere around 190V. Moving it from 200mA to 300mA also changes the sound (not dramatically though) making the amp sound a bit more confident at 300mA.

I am debating trying a regulated power supply for 6S33S and I am looking for your advices on what other modifications I can try before I build an enclosure.

 

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RE: SET and PSET with 6S33S, posted on May 5, 2015 at 14:51:30
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
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.

 

RE: SET and PSET with 6S33S, posted on May 5, 2015 at 15:06:40
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
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.

 

RE: SET and PSET with 6S33S, posted on May 5, 2015 at 15:45:01
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
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.

 

RE: SET and PSET with 6S33S, posted on May 5, 2015 at 16:37:32
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
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. :-)

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 08:24:44
big muck
Audiophile

Posts: 162
Joined: December 31, 2000
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.

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 08:54:53
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
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?

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 12:30:03
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
Actually, the bias will pop up by about a volt, which is not insignificant.

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 12:44:38
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
Can you please explain how the bias can pop up?

 

Four Cree Schottky Diodes in Series, posted on May 7, 2015 at 15:24:06
Maxamillion
Audiophile

Posts: 856
Location: New Jersey
Joined: May 26, 2006
Are what I use under the 6E5P driver in my 833C amps. Gives just about the right voltage for this application, too. Sounds great!

 

RE: Four Cree Schottky Diodes in Series, posted on May 7, 2015 at 16:34:04
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
Interesting! Do you have a cap on the input? A schematic would be helpful.

 

Schematic Enclosed, posted on May 7, 2015 at 18:00:18
Maxamillion
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Posts: 856
Location: New Jersey
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No cap on the input. Not even a cap between the stages or in the cathode of the power tube.



 

RE: Schematic Enclosed, posted on May 7, 2015 at 18:06:56
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
Speechless! :-) I bet it sounds wonderful! Thank you for sharing.

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 19:10:37
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
Run the numbers for bias voltage assuming the input is shorted.

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 20:53:50
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
If you set up the amp properly, there is 0V on the input. What can happen if you short the input?

 

RE: SET and PSET with 6S33S, posted on May 7, 2015 at 21:35:51
big muck
Audiophile

Posts: 162
Joined: December 31, 2000
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.

 

RE: SET and PSET with 6S33S, posted on May 8, 2015 at 07:48:54
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
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.

 

RE: SET and PSET with 6S33S, posted on May 8, 2015 at 08:23:00
mstolpner@gmail.com
Audiophile

Posts: 25
Location: Seattle
Joined: June 4, 2009
This trim pot is used to set 0V on the input. You can change the 12.1K resistor to adjust the bias if needed.

 

RE: SET and PSET with 6S33S, posted on May 8, 2015 at 09:45:51
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
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.

 

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