Megatron Electrostatic Headphone Amplifier

[kevin gilmore]

The input tube filaments for the T2 are also very close to the inputs.

grounding one side of the filaments takes care of the problem.

inside the tube the grid is a fraction of an inch from the filament anyway.

[Pirx]

Here is something indeed a bit different...

OMG, looks crazy

 

Don't you think that eight EL34 with no holes around them are going to be too hot?

Even when you put them on the top of the case there will be no air from bottom.

I would worry about temp. of PCB inside the case, temp of capacitors etc ...

[wink]

Nice porcelain sockets should fix that.

[spritzer]

Heat will be an issues but all the hot bits are on top of the chassis and I'm in Iceland after all...

[Pirx]

Heat will be an issues but all the hot bits are on top of the chassis and I'm in Iceland after all...

I realized you are making very nice sounding oven

 

Which schematic, from hundreds in this thread, is the right for your Megatron?

[Victor Chew]

Would the Megatron be too hot to operate without heat problems in the equitorial climate? Temperature ranges from 30C to 34C most of the year but a fews digits lower during the monsoon season, which is now.

[spritzer]

I don't think we ever posted a schematic for this amp, I've at least never seen it.  It just saves time to do the PCB's directly and we pretty much always know what the other one is thinking. 

 

The Megatron won't run any hotter than many tube power amps and all the really hot running bits are outside the chassis.  We will probably add some went holes to the board for the final version but I really don't foresee any issues. 

[spritzer]

All but ready now.  Goes to show I have all this HV stuff in stock but no 12V zeners...

 

 

The forum doesn't allow me to use the linked BB code for some unknown reason...   Here is the full version. 

Edited December 14, 2012 by spritzer

[Victor Chew]

Spritzer , thanks for the heads up on the heat issue.

 

How many transformers are we looking at for this "monster"? I believe 1 for the PSU; 1 shared for the input tubes ( L and R) and 1 for left output tubes and 1 for right output tubes? Total 4 transformers?  

[deepak]

I would not expect anything less from the Stax Mafia to go from easy to build electrostat amp to this

[crappyjones123]

forgive my ignorance but isnt the volume pot facing the wrong way? or are you planning on using one of those extension things all the way to the front panel?

[n_maher]

forgive my ignorance but isnt the volume pot facing the wrong way? or are you planning on using one of those extension things all the way to the front panel?

It gets the pot as close to the inputs as possible and uses a shaft extension to get to the front panel.

[spritzer]

How many transformers are we looking at for this "monster"? I believe 1 for the PSU; 1 shared for the input tubes ( L and R) and 1 for left output tubes and 1 for right output tubes? Total 4 transformers?  

 

There are 7 different filament supplies for this amp plus 3-4 power supplies needed.  I'll be using a BH transformer so that included two HV windings, a winding for the low voltage supply (+/-15V, the + one just for the indicator led's) and two filament windings for the output tubes.  That leaves the 4 filaments for each of the CCS tubes which will be handled by two transformers (two windings each) inside the amp chassis, directly underneath the tubes.  That leaves the 6.3VDC supply for the front end which will be fed off it's on supply.  I'm not going to bother with a separate +300V supply and just use a ~18K resistor to drop the HV line. 

 

Nobody said this one was going to be simple to build but this is always the problem when doing tubes properly.  Violating the cathode-filament barrier now could make things very interesting on a grounded supply...

Edited December 15, 2012 by spritzer

[n3rdling]

Question on tube matching for this amp: say, for the 6CA7, do we need 4 matched pairs, 2 matched quads, or a single octet?  12AX7?

[spritzer]

Matched pairs for everything is just fine.  The input tubes are 12AX7's and 12AU7's so matched pairs with closely matched sections.  With the EL34's you need two matched pairs for each output stage, output tubes matched together and the CCS tubes matched. 

[Victor Chew]

This would have to be a 2 box build. What would be the costs of building this amp excluding the cost of volume pot?

[crappyjones123]

Thanks Nate.

[spritzer]

This would have to be a 2 box build. What would be the costs of building this amp excluding the cost of volume pot?

 

I wouldn't even think of cramming this into a single box.  It can be done but it is a pain to mount toroid's upside down... 

 

As for cost, well that's not something we care about at all.     North of 1K$ at least, just the tube sockets are 100+$, tubes 2-300$, transformers another 250$ and the list goes on and on.  Still this is BH level of performance so it will wipe the floor with pretty much everything out there.  It will also look rather menacing...

[luvdunhill]

I think you need a single chassis. Something like this:

http://www.diyaudio.com/forums/showthread.php?t=225623

[JoaMat]

That leaves the 4 filaments for each of the CCS tubes which will be handled by two transformers (two windings each) inside the amp chassis, directly underneath the tubes.

 

Why do you choose to have those transformers in the amp chassis?

[spritzer]

Why do you choose to have those transformers in the amp chassis?

 

The potential on the filament is quite high so the transformer has to be close as it possible.  Remember that the cathode of these tubes sits on top of the output tubes anode so there is a lot of voltage there. 

[JoaMat]

Thanks for info. Is it OK to put the transformers right below the board or is some sort of shielding necessary?

[spritzer]

Should be just fine but I'll report back on any issues.  I'll be using the 80mm Galaxy cases so it will be a tight fit. 

[Victor Chew]

Spritzer, would it be possible to have a hole near each of the internal heater filament terminators for the wires to reach them? This is to keep the wires from spanning across much of the board to prevent humming. Perhaps my unwarranted fear of humming.

Am I correct that each of the 6CA7 will require 1.5A? If so, one transformer with 20va with 1.587A x 2 can feed 2 large tubes.

Am I also correct that each of the small tubes will require 500mA? If so, one transformer with 14va with 555mA x 4 can feed all the 4 small tubes.

I stand correct in respect of the above.

Edited December 27, 2012 by Victor Chew

[spritzer]

The internal filaments you are referring to are the output tube filaments?  They are referenced to the - power rail so a small trace will not cause any hum issues. 

 

The tubes are rated to 1.5A but there is always some variation so I always overspec for at least 10%.  I'm using 25VA transformers to drive them in this amp. 

 

Each of the front end tubes draws 300mA at 6.3V so 1.3A would be good for the front end.  I highly recommend DC for this role though. 

 

My Megatron boxes just arrived but I'll probably not get to do any construction for a while.  Other stuff to finish first...