MiniMutt - a Hybrid-Mu Driven DynaMutt-II for the SCA/ST-35

[Geek]

Hi Folks,

Had some time to work on a little challenge given to me - DynaMutt a SCA35.

The circuit is easy, finding room in the dang cabinet will be the challenge, LOL!

One of the biggest obsticles was the power available from the little transformer. With the phono stage in place, every mA of heater current is accounted for and the HV secondary is pretty much maxed as it is. So this led me to use but one tube for the driver+phase inverter per channel.

Rather than doing a pentode+triode, like a 6BL8 or 6GH8 or something, I decided to do a fully balanced route and make a mu-follower with MOSFET tops. This also satisfied part of my challenge with the amp was balanced inputs.... when there's no room for transformers.

Given that it's a balanced design, the PSRR will be superior and we can chop out "uneccessary bits" of power supply. I have a SDS cap board for this thing that will be modded a bit to suit.

Speaking of power supply, we do need a -V supply for the CCS... it just would not function satisfactory any other way. What we do have room for is a little Tamura PCB mount, 6VA, 36V transformer. Rectified and filtered, we have -50V (more given we aren't going to use anywhere near its 170mA rating). Since this is available, I went fixed bias on the output tubes. We're getting 15W from a lazy pair.

I did try this with individual and common cathode bias and we were getting just over 1/2 that amount of power clean.

An interesting point of note is the anode follower return points - unlike the DynaMutt-II ( http://geek.scorpiorising.ca/contrib/Ge ... utt-II.png ), returning the anode feedback to the MOSFET sources caused all sorts of grief. Returning them to the 12AT7 anodes made everything come together as it should.

Once the trimmer is adjusted for best overload symmetry, the unit is insensitive to most decent-life 12AT7's you drop in... Philips, DuMont (made by ?), Brimar, Mullard, RCA... it's happy.

Output bias... on a sinewave, there's no visible crossover distortion weather biased cold (17mA/tube) or hot-hot-hot (32mA/tube) and power output does not suffer, nor compression point. I plan on starting at 27mA and let my ears chose the right point.

Cheers!

http://geek.scorpiorising.ca/contrib/Ge ... utt-1a.png

[fiftyfivefilms]

...will you be doing boards for this?....I imagine many (myself included) are owners of now Transformer-less SCA-35s after harvesting the trannys to build Shannons ST-35 design ( a great one I might add), resurrecting these carcuses sounds like it might be the ticket.
cheers,
Kirk

[Geek]

My board guy is having difficulty getting it all to fit. As it stands, there will be one main board to fit in the hole, two axilliary boards for the cathode CCS' + power supply.

Therefore I am considering abandoning this as an SCA project and making an ST-35 one out of it.
(I'll still use the SCA as a /dev/ setup)

Cheers!