Shunt reg for the SEX 2.1 and other mods

[johnsonad]

I'm happily listening to my modified SEX 2.1 (set up for Stax headphones) though the base is small given all that I've done to it and I'm thinking of moving to a 12x12" base to accommodate and clean it up some.  With the base change this opens it up to other mods.  Currently I'm running CLC in the PS using a pair of SEX 2.0 PF chokes and a film output cap in the finals.  This made an audible improvement and I'm wondering if it would further improve with a shunt regulated power supply and DC on the filaments.

Here are some of the basic questions.

1. Can the 2.1 PS transformer handle converting to a shut regulated circuit in the present split design or would I need to move to a dual transformer circuit? 

2.  Given number one, would a VR tube be easier or go all the way and use a SEX C4S board and a separate shunt reg tube maybe in a CL configuration (I'll have the space for it)?

3.  To add more complexity, I was thinking of swapping out the 6DN7 for a 6SN7 as I don't need the power as this is a headphone only amp and the Stax work on voltage swing anyway (100v P-P = 118dB).  Maybe a single 6SN7 for the driver for both channels allowing for tube rolling and a single 6SN7 GTB for the output for both channels.

4.  And then some DC for the filaments to help lower the noise floor even further though, with the larger chassis, this may remove any residual hum that's there.

Thoughts?  Is it worth the effort?  Thanks guys!

Aaron

[Jim R.]

Hey Aaron,

I may be wrong, but the s.e.x. Already has DC filaments (heaters,actually.)

-- Jim

[mcandmar]

The S.E.X. already has DC heaters so there isnt much you can do there.  Referencing the center of the DC supply to ground helps remove a lot of hum if you have not already done that tweak. Second point of attack for hum reduction is the HV supply, i found a 10H choke sorted that out rather nicely. I have documented both in my rambling thread..

I had thought about shunt regulators too for the HV supply, but i don't think there is the voltage headroom  to allow for that, certainly not with the solid state setups i have used in the past which will need 30-50volt overhead to operate properly. I also have no idea of the current limit of the HV transformer as that may limit options too. Again the Salas and K&K regs i have used need to sink at least 20ma more than the load.

I also briefly tried the 6SN7 in the S.E.X, admittedly i didn't modify the operating points i just plugged it in and started testing, the results was so awful it didn't last very long.  If you want to use 6SN7's just start again and build another amp around them, don't ruin a perfectly good S.E.X. amp

Sorry if this is all sounding a bit negative.  Here is another idea, use the iron and convert it into a stereomour as i believe they used the same iron, or even better just buy a Stereomour II and modify that into a headphone amp. I know it sounds crazy but i am currently building a "big tube" speaker amp for 2A3/45/300b style tubes as i had never used them before.  Currently using the Russian 6C4C tubes which are a Russian 6A3/6B4G, which is a 6v version of the 2A3.  I am heating mine with Rod Coleman regulators via dual filament transformers, the rest of the amp is basically a dual monoblock setup, two mains transformers, HV supplies, shunt regulators and CCS loads for the driver tubes. For the output tubes i am using a bunch of Bottlehead/magnequest iron i picked up second hand so its not a million miles away from a Stereomour. The thing sounds so good, and is so quiet i am currently using it as a headphone amp with my HD800's.  It is hands down better than the S.E.X amp, and almost as good as my shunt regulated 6C45 based amplifier. The 6C45 has a hair more detail and resolution, but the 6C4C sounds more natural and realistic. I recon when i put some money into quality coupling capacitors it will trump it and become my main headphone rig.  Surprising really as that was never the intention when i started building it.

[johnsonad]

Thanks for the corrections and input guys. I've got another amp in the works that should be a better fit. The SEX just sounds so good that I was wondering if I could make it even better 😊

[Jim R.]

I'm of the opinion that the less you do to the s.e.x., the better. Such a nice amp as it is with very little room for even minor improvements.

Ok, so, I'm going to build mine on a larger chassis -- 10" x 12" only because I have that nice Bombe style spalted birch base and a nice 0.125" copper plate that fits the base. The other reason for a larger chassis is that with a few more vent slots, cooling may be a bit better, which can never hurt. Mine will see double duty, but will probably be mostly used as a headphone amp. I recently picked up a pair of HD-600s (I've owned the 650s and 800s in the past, but I think the 600s are more my style (especially with the Wy Wires "RED" cable upgrade.

It will also do very nicely in my main listening room system when I really need a pure triode sound.

-- Jim

[Paul Joppa]

The SEX power transformer has no current headroom above what the amp uses. The best approach would be to turn it into a monobloc, using the spare tube  to make two shunt regs, one for the output and another for the driver.

You would still be left with a ~10% reduction in available plate voltage, which means around 20% less power.

The 6SN7 has a much higher plate impedance, which might easily lead to a loss of high frequencies into a capacitive load (electrostatic headphones).