Megatron Electrostatic Headphone Amplifier

[wink]

But.... but.... Stax has been running underspecced tubes from the start......   It sort of works.

 

I guess that if Stax had done their amps right in the first place, there'd be no need for the KGBH, BHSE, DIYT2, KGSS, KGSSHV, etc.

[spritzer]

There is using something that is under spec and then using something which is nothing like the intended tube, far lower voltage and a different bias point. 

 

Most of the Stax amps are great to excellent but hamstrung by what they can do.  The 007tA I'm using right now is awesome simply by changing the tubes to 6S4A's. 

[chinsettawong]

I'm sure you guys are not talking about using 6c33 for a Stax amp or are you?  I really like the heat and the shape of 6c33. 

 

Wachara C.

[RiStaR]

Yes, 6c33 is just awesome in what Andy is doing.  Using 6P3S-E in a 900V amp is just untold amounts of stupid.  Just because some idiot on the net claims this tube is similar to the EL34 doesn't make it so.  It's a lower spec version of the 6L6 and is not compatible.

Hi spritzer,

Not disputing the conservative ratings on the mil tubes - I did know that going in. I thank you for caring enough to speak up about it though, even if its abrasive.

Best,

[justin]

someday, someone will build the gilmore mig amp w/ 6c33. just needs a servo and dc output protection. then people will replace all the other parts w/ a 6sn7 and a coupling cap

 

http://gilmore.chem.northwestern.edu/kgdt1.gif

 

http://gilmore.chem.northwestern.edu/kgdt2.gif

 

for the balanced version, you've got >125W of solid state heatsinking to do, 125W coming off the plates of the 6c33, and 166W for the heaters. plus power supply heat. so, easily 500 watts of heat

Edited December 7, 2013 by justin

[Victor Chew]

Kevin,

 

Looks interesting. Any chance this will be available in pcb form in the near future?

[spritzer]

I'm sure you guys are not talking about using 6c33 for a Stax amp or are you?  I really like the heat and the shape of 6c33. 

 

Wachara C.

 

The max voltage of 450V does mean it isn't terribly useful for that role.  That and this tubes peculiarities (long warm up time) means it doesn't make a whole lot of sense.  Now the KT150 would be fun to tinker with if you like heat... 

 

Hi spritzer,

Not disputing the conservative ratings on the mil tubes - I did know that going in. I thank you for caring enough to speak up about it though, even if its abrasive.

Best,

 

I for one don't care if you blow up your amp, that's your problem entirely.  What I do care about is people reading that and thinking this tube is a sub when it clearly isn't suitable in any way.  This amp is supposed to use El34/6CA7 and no other tubes.  Not KT88, KT77, 6550, KT66. KT90, KT120 or KT150. 

 

someday, someone will build the gilmore mig amp w/ 6c33. just needs a servo and dc output protection. then people will replace all the other parts w/ a 6sn7 and a coupling cap

 

http://gilmore.chem.northwestern.edu/kgdt1.gif

 

http://gilmore.chem.northwestern.edu/kgdt2.gif

 

for the balanced version, you've got >125W of solid state heatsinking to do, 125W coming off the plates of the 6c33, and 166W for the heaters. plus power supply heat. so, easily 500 watts of heat

 

I love the "HS!" markings Kevin put in there... 

[FrankCooter]

The output  tube I've always wanted you guys to play with is the El156. It's a clear step up from any of the El34 or KT88 types.

[FrankCooter]

I'm sure you guys are not talking about using 6c33 for a Stax amp or are you?  I really like the heat and the shape of 6c33. 

 

Wachara C.

It would be a freak show, but if you were really motivated, something similar ( but with much higher voltage ratings)  that could probably be made to work is the 13E1.

[kevin gilmore]

The output  tube I've always wanted you guys to play with is the El156. It's a clear step up from any of the El34 or KT88 types.

trivial and almost instantaneous

 

buy some tubes, and use these

http://www.ebay.com/itm/1PC-EL156-TO-KT88-10Pin-to-8Pin-Vacuum-Tube-Convert-Socket-Adapter-/221235762073?pt=Vintage_Electronics_R2&hash=item3382add399

 

requires slight rewiring to add g3.

 

as for the 13e1, those are brutally priced

Edited December 9, 2013 by kevin gilmore

[spritzer]

Perhaps a better and far cheaper option would be the GU50.  Dirt cheap and supposedly very reliable... 

[kevin gilmore]

edit: the adapter wires the tube as a tetrode, so it would be completely plug and play.

[chinsettawong]

GU50 is nice!    

[justin]

let's not go all singlepower on the tube rolling

[luvdunhill]

let's not go all singlepower on the tube rolling

Zing!!

[kevin gilmore]

sure why not do the singlepower tube rolling. as long as someone else does it.

 

I have a large box of singlepower tube rolling disasters. el360, and other things

that cannot, or should not work.

 

The EL156 is something that actually should work. Subject to a very large bottle

sitting on a very flimsy adapter. What bad could happen.

[spritzer]

let's not go all singlepower on the tube rolling

 

I do have some SP tube adapters here....   

 

More interested in just designing new amps for these tubes, not simply dropping them into old circuits. 

[justin]

this is the guide to singlepower tube rolling

 

[spritzer]

When I got my ES-1 I looked up some rolling guide and I stared at it in disbelief. 

 

Turns out Woo are just as bad a SP when it comes to the rolling...

[FrankCooter]

I'm not a fan of rolling different types of tubes in commercial amps ( or somebody elses established DIY design) either. At the very least, you've randomly altered the designs established operating points. At worst you may be making dangerous electrical demands on the amp that it was never designed for. For example, the EL156, while a great tube, takes less drive and biases up substantially different from an EL34.

 

DIY is another story. Part of the fun, as long as safety and  electrical parameters are properly accounted for, is to throw anything and everything that might roughly work into a circuit and experiment with different operating points. Most of the time the results are predictable and disappointing, but every once in a while you stumble on to something exceptional.

[justin]

I'm not a fan of rolling different types of tubes in commercial amps ( or somebody elses established DIY design) either. At the very least, you've randomly altered the designs established operating points. At worst you may be making dangerous electrical demands on the amp that it was never designed for. For example, the EL156, while a great tube, takes less drive and biases up substantially different from an EL34.

 

no, no...the singlepowers were auto-bias 

[kevin gilmore]

that kid is a little to old for singlepower tube rolling.

 

for the megatron, since the tubes are stacked, and the grid bias really is pretty

close, it really would work.

[Jon L]

sylvania 12bh7 (it just sounded better than all the au7s i tried);

OT. Which version of Sylvania 12BH7 is that, black/grey plate, 50's/60's, and O/D getter?

[spritzer]

no, no...the singlepowers were auto-bias 

 

All amps are auto bias for most of HF... 

[FrankCooter]

Since we've strayed into tube rolling, here is a list of  the tubes I've tried as outputs for a simple two stage transformer coupled electrostatic amp. Absolutely no science or engineering here. Impressions are totally subjective and may have absolutely no relevance to any other design. Caveat, disclaimer, etc., etc.. All tubes were run in triode mode with 400V on the plate, and  all  were run in a linear portion of the curve,  Besides the output tubes, the rest of the system stayed the same.

 

EL84/7189 -Russian 6P14P and NOS RCA 7189

EL34 -generic Chinese, German RFT, Mullard XF-2, 60's Japanese Mullard copies

6550/KT-88 -vintage Tung-Sol 6550, vintage GEC KT-88, new production Gold Lion KT-88, briefly also tried the "Treasure" Chinese KT-88.

7591 - vintage Westinghouse, also tried the 6GM5, which is a 9 pin version of the 7591

7403- 50's Tung-Sol and Bendix

6P21S - A Soviet era directlly-heated 4 element tube ,

2E22- another directly-heated  4 element tube some claim can substitute for a 6550/Kt-88

EL156- 60's Telefunken

6BX7 and 6BL7- RCA

paralleled 6SN7GTA

845 - generic Chinese

 

Output transformers for all but the 845 are purpose-designed Electra-Print 1:1+1 with a 3.3k primary (below optimum for several of the above tubes) The 845 transformers do not have a step up in the secondary and have a nominal 5k primary.

 

Disappointments first:

 

The EL84's, the 6SN7's, the 6BL7/6BX7 were not up to the job. All could be made to clip on peaks and otherwise sounded constriained and flat. The low primary impedance of the transformer may have been a factor.

 

The directly-heated 2E22 and 6P21S. Major dissapointment here. Not bad, but no DHT magic. Transformer primary slightly low, but I think it may be more a factor of running the screen grids way beyond thier 220V ratings. I don't have a problem with running screens substantially beyond thier ratings from a safety standpoint, but I've been told that this will distort the triode cuves.

 

All the KT-88's and 6550's. They vary, and they aren't bad, but they all seem a bit less refined and musical than a good EL34.

 

The good:

 

Mullard EL34 XF-2.  The most musical of the large power tubes, incrementally better (but a worthwhile increment) to any of the 6550 types. Honorable mention to the Japanese Matsushita Mullard copies - I can't tell the difference

 

The clear all  around winner is the Telefunken EL156. This is the finest non-DHT power tube I've ever experianced.  It has a large, open, yet very detailed and precise presentation that sets it apart from any of the others. Perhaps a little cooler than the Mullards

 

The surprize of the bunch is the 7403. This is a regulator tube with a plate cap. You're on your own using it for audio purposes, but it seems suitable as a 6550/KT-88 substitute and it sounds comparable to the best of the 6550's.

 

The 845 is different breed of cat from any of the above and deserves seperate consideration. Hopefully, I'll get to share it with some of you.

Edited December 10, 2013 by FrankCooter