Alternative S.E.X.

[Dyna Saur]

Having built two S.E.X amps, one with the C4S,  MQ iron and cap upgrade, and the other one  just "lean and mean" and very basic  (I skipped the headdphone output and stayed with the stock output iron and plate chokes)  I decided it was about time to try some "Alternative SeX"...   This time, with a different PSU design, 13EM7s in place of 6DN7s, and using MQ BCP-15 plate chokes and EXO-45 output trannies. The power trannie is a toriodal Trafomatic 250-0002, with 190VAC @ 200 mA (actually closer to 215 VAC) and 26VAC @ 700 mA.  The volume control is a 23 step 100K stereo step attenuator, from Hong Kong, and bought through the e-place.  The chassis is a Hammond 12X 10 inch,  black powder coated steel.

The 13EM7 filaments are connected in series, with a 0.1 uF film cap across the 26VAC, and another connected from the junction between the two filaments to ground.  

PSU is full wave four diode bridge using MR4100s, and a CLCRC filter.

The sonic results are surprisingly good, very good sound quality and absolutely NO hum, ripple, buzz or background noise, to the point that I thought that it wasn't working properly, on initial power up and test, until I started playing music through it.  The toroiral power trannie is very effectively self-shielding, and with  no magnetic field coupling into the open-framed MQ iron.

Since they say that a picture is worth a thousand words, I'll save two thousand words here:

and under the chassis:

This project is actually  about three years old, but I never got around to posting uit until now...

It is currently serving as the bedroom sound system, driving a pair of "SEXy" speakers, and a rather modest, but decent-sounding  CD player Rad Shaq Optimus CD2460 which I snagged at a yard sale for $5. .

I also ended up building up a series-feed version of this design using some  James 5K OPTs, and fit it all into a 10X 6 X 2 steel chassis.     The rest of the circuitry is identical to thos one, except that the fancy step attenuator was replaced with two 100K volume controls - though I did since then, buy two 100K mono step attenuators to go in this one, some day (if they don't end up in yet another future amp build).

.

/ed B in NC

 

[Dyna Saur]

One may wonder what I did with the Speco OPTs and plate chokes from the upgraded SEX amp. Well, I never let anything go to waste, and came up with another parallel feed SET, this time using 12B4As, in a mutant edition of Jef Larson's "Abraxas" SET design.  The filaments are fed with 12.6VDC, and the HV PSU uses the standard two rectifier full wave voltage doubler. The power trannie was salvaged from a junker Heathkit audio generator carcass which I got on the freebie table at a swapmeet. Free is always good!  

I shall now save myself and y'all another two thousand words:

and a peek inside:

Power output is approx 1 WPC, and sounds very nice!  

This design has since led to the creation of  a "fraternal twin", which uses 6S4As instead of 12B4as, and it also uses some leftover Speco OPTs and BH plate chokes.   I managed to fit the OPTs and plate chokes under the chassis this time, for a cleaner looking  top deck. I thought I had an under the deck photo, but I couldn't find it in my files...

   

The chassis were leftover and scrapped prototypes from my previous employer, aluminum with a nice gray powder coating.   I ended up with several of these chassis, and have built other SE and PP amps into them.  Size is 9.5 X 10 X 1.5 inches, the smaller separate compartment was originally intended for a small 5V, 3A SMPS.  

The white binding post in the back is just a chassis ground.

/ed B  in NC

 

[grufti]

Great stuff!

 

[jrebman]

I agree.  One of these days I'll have to get around to building a 12b4a amp of some sort -- maybe even a PP.

-- Jim

 

[Dyna Saur]

From some more recent experimentation, I determined that the 12B4A runs very nicely with a B+ voltage of 245 to 250VDC, plate current between 25 to 27 mA, with a 1K, 5W cathode resistor, bypassed with a100-470 uF / 50V cap, and a 5K to 7K primary OPT, in either series or parallel feed. This seems to be a better overall  combination than used in my 12B4A parafeed, and its series feed sibling.  

The power output is around 1.5WPC  The VA tube is a 12AX7, in either design.  Interestingly enough the 1626 based "Darling" design  uses nearly identical  circuitry, voltages and resistance values, and I suppose that one could design and build a convertable version, which could accomodate either 12B4As or 1626s, using parallal-wired octal and 9 pin sockets for the output tubes.

The photographed 12B4A (and 6S4A) "Abraxas" knockoff ran with a higher B+ around 320VDC, RK of 2200 ohms and plate current around 18 mA, into the 16-22K impedance load.  THis delivers a little under a Watt per channel, around 900 mW.  

The 6S4As are  very happy with a 325VDC B+, 25 mA plate current and 390-400 ohm, 2A cathode resistors, bypassed with 100-470 uF / 25VDC caps, and feeding a 16-22K plate load.  The driver can be a 12AU7 or 5963, as the 6S4As require only about +/- 10V grid drive signal.  12AX7 will work, but its gain is a lot higher than needed in this circuit.
6S4As haev a truly "sweet" sound quality  of their own.

/ed B in NC

 

[Paul Joppa]

...
6S4As have a truly "sweet" sound quality  of their own.

Yes indeed - been a long time, but I've heard this effect in push-pull too. The EL-84 has a similar reputation, much more well-known though. For what it's worth, the 6S4 came very close to being the SEX amp tube of choice - I was looking at a lot of options at the time. It was about the last one to drop out.

 

[Grainger49]

Ed,

Those are impressive builds.  The chassis are just right, divided and cast.

 

[Dyna Saur]

Thanks for the kind words!  The aluminum in those prototype chassis is somewhat softer and thicker than that used in the production units, and it is very easy to work, drill and punch, and still more than sturdy enough for a durable build.
These particular chassis were designed for a 100 MB/S to fiber optic adapter / transceiver, and required rather substantial RF & EMI shielding / containment in order to meet special requirements.

I more recently built up a 6S4A PSET amp, using a 5963 SRPP driver.  Originally I used 12AT7s but they had too much gain, so I simply switched them out for the 5963s, without changing any of the circuitry, and the sound was much better.  The OPTs are 6.6K SE cehapies, but they sound pretty good.  I'll have to post the pix sometime, it is on a different size  cast-off prototype chassis, and came out looking pretty good, almost as good as it sounds.   Each of teh 6S4As has a 390 ohm, 2W cathode resistor, bypassed with 330 uF / 35V caps. The SRPP uses RA and RK of 1K, 1/2 W, 1% metal films. The 12AT7s drew around 2.1 mA each, the 5963s about 4.8 mA each.   12AU7s would also work just fine. I just wanted to keep the tubes all GE, made in USA.

I also have a 6S4A PP stereo amp  in somewhat delayed progress. I modified one of Shannon Parks' DIY-ST35 PC boards to work with 6S4As - required some etch cuts and short jumpers around the four EL84 locations, before installing the sockets .  It will be set up to operate with a B+ of 325V, each 6S4A set for 25 mA idle current (using  LM1317s as cathode CCS)  and OPTs yet to be determined, probably some small 10K CT PP iron I have, salvaged from an old 6AQ5 amp carcass.  

Of course no amp photo shoot isn't complete without a look under the  chassis, so here goes:

FWIW, branching off on a different subject, 6GK6s are functionally equivalent to 6BQ5s,  but with a somewhat different pinout .  These also have 10V, 16V and 29V filament versions, and they all sell NOS  for cheap money.  

For one of my 6B4G SET stereo amps, check out this posting, in "Gallery":

http://www.bottlehead.com/smf/index.php/topic,775.msg5468.html#msg5468

/ed B in NC