Megatron Electrostatic Headphone Amplifier

[steefdebruijn]

please bear with me but I try to understand the trafo/cap thing on the GRHV supplies. 

By reading this thread and the Carbon one from which the GR supplies emerged as far as I can tell we have to consider

- 1.414 factor for rectified DC

- minimum of 20V drop voltage across the regulator

- 20% headroom on caps rating to assure long life 

- account for 10% max fluctuation of land line (230V here; governmental docs here confirm this)

 

For 450V output I calculate 450(output) + 20(drop) = 470VDC prereg minimum / 1.414 = 332VAC minimum + 10%(max line flux) = 365VAC nominal output from trafo @ 120mA (which is the current the Megatron draws I read correctly). 

For the prereg cap we should use the max values. So 365VAC nominal + 10%(max line flux) = 402VAC max * 1.414 = 568VDC max prereg DC. Accounting for 20% overhead on caps to assure long life we come to 680V rating on the prereg cap. 

Am I way too conservative here, missing something or clearly not knowing what I am talking about?

Steef

[mwl168]

You would want your trafo to be capable of delivering more than 120mA current per rail. I would personally spec at least 250mA per rail for Megatron use. I am sure there are others that would prefer going much higher.

Also, base on my own experience, 20V drop across the regulator may not be sufficient for 450V regulated output under load.

[mypasswordis]

Too conservative and you counted 10% line variation twice. Stick with 350-360vac transformer with 550V caps and you will be fine, as 600V+ caps are hard to source and they're probably very large. You can always measure your mains voltage to take some guess work out of it (be careful). 

YMMV but personally I find it unnecessary to go above 400V rails for any amp unless you are like pabbi1 and want to blow out your eardrums, or just like overkill. Going from 400 to 450V only increases headroom by 1dB anyways, and as you can see makes life harder. 

[steefdebruijn]

Thanks for your feedback. I agree to use a lot of trafo overhead but I just stated the trafo should be able to deliver 120mA @ 365VAC output (because higher current == lower voltage).

If 20V drop is too low how much should we account for?

I used the 10% variation twice because the land line (theoretically) can go as low as 10% below nominal and 10% above nominal. Maybe account for both 10% line overload AND 20% caps overhead is a bit over the top though.

So, if we get a 350VAC trafo and use 25V dropout (for example) we can handle max 4% lower land line before PS falls out of regulation (350V ×,96 (4% lower land line) ×1,414 (rectifier) −25V (dropout) = 450VDC). Right?

I reckon our land line is pretty stable so indeed should not be a problem.

About the cap rating: with nominal output of 350V on trafo we have nominal unregulated 494VDC on first cap. That is at 89% of its max voltage rating of 550V. 'The internet' states larger margins - 20-30% - for this but the common sense here is that's overkill?

With these numbers a 550V cap accounts for a max of 11% higher landline before it gets out of spec DC from the rectifiers. That indeed seems pretty unlikely.

 

If we can feed the Megatron with +-400VDC without problems that indeed may be preferable. Are there other changes necessary besides the PS resistors and the 21k voltage drop resistor on the Megatron board to get 300VDC for the little tubes (should be lowered from 21k to about 14k i guess)?

Steef 

[joehpj]

I use 325V/250mA secondary for 400V@20mA*4 for carbon. Megatron should be almost the same.

[JoaMat]

Megatron is probably 120 mA, that's plus 50 % compared to your Carbon.

[joehpj]

Then 330V@400mA should be enough. Thanks for correction. 

[steefdebruijn]

About load resistors to test the HV and LV sections of the PS. @kevin gilmore has stated somewhere in the Carbon thread

• 16K / 20W for the HV (per rail)
• 500R / 2W for the LV

Can someone explain (or are these only valid for Carbon loads)?

If I use 450V @ 150mA (to test with slightly higher load) I get values of 3K / 70W per rail (or two of 1.5K / 35W in series).

If I use 12V @ 800mA (somewhat higher load) I get values of 15R / 10W per rail

What am I missing?

Tia

Steef

[mypasswordis]

Probably not necessary to stress test the power supply, so some medium-ish load will do. Having massive heatsinks just for burning off heat from load resistors is also a PITA. As has been mentioned before, the current limiter prevents the PS from doing 150mA with the stock 5 ohm resistor. Also, for the Carbon the LV just powers the solid state front end. No tube heaters so much lower power.

[steefdebruijn]

Thanx @mypasswordis. 

I will use Kevin's suggested ones then. 

I have lowered the current limiter to 3.9R. Should that be enough? (how to calculate this value?)

Steef

[mypasswordis]

V=IR so I = .65/R roughly

[steefdebruijn]

Ha! I knew that formula but didn't know if and how it could be put to use here :-)

3.9R gives me 166mA then. Should be enough for Megatron

[THEOLOGOS SARAFIDIS]

Hello,

Please, how can i have the board of the Megatron amplifier? Is it in a form of a kit? 

[nopants]

no kit, also no active group buy

[Pars]

How to get a Megatron:
1) find Gerber files from Stax mafia site
2) download said Gerber file .zip file
3) send said Gerber .zip file to your favorite PCB manufacturer, ensuring correct specs (copper weight) and finish of your choice
4) find suggested BOM by reading this thread (also necessary for item 3, above)
5) order correct qty of parts determined in step 4 above from your favorite supplier(s)
Nothing to it...



Sent from my iPhone using Tapatalk

[JoaMat]

piece of cake

[DronEvil]

Hi everyone, 

I'm hoping you can sanity check these trafo specs for me before I pull the trigger on my order from Primrose Audio (SumR). I've read through this thread more than a few times to try and figure out exactly what I need for my megatron build. I'm using the v .2 meg boards and an older kgsshv psu board (the ones that have 2x 420-ct-420v and 1x 16-ct-16v inputs instead of the newer 2x 420/420 and 1x 15-15-ct). If you would like me to post pics of the board just let me know.  I know this isn't optimal, but it's all I have to work with unfortunately. 

Thanks in advance for your input! 

 

MEGATRON POWER

Power Rating 234VA

Secondaries

2x 420VAC-0-420VAC  @ 130mA

1x 16VAC-0-16VAC @ 500mA

 

MEGATRON HEATER FILAMENTS (2x 6CA7 Tubes)

Power Rating 51VA

Secondaries

2x 6.3VAC @ 4A

 

MEGATRON HEATER FILAMENTS (4x 6CA7 Tubes)

Power Rating 101VA

Secondaries

2x 6.3VAC @ 8A

 

MEGATRON HEATER FILAMENTS (2x 12AX7 & 2x 12AU7 Tubes)

Power Rating 10VA

Secondary

1x 6.3VAC @ 1.3A

[mwl168]

Doing some initial testing of my Megatron. This is the latest version from the GB with the 12.6v filament for the front end tubes. It's making music but I do have some more work to do yet.

The Antek AS-2T350 with its rated 350vac secondaries (sagged to 340vac under load) proven not enough to sustain the 450VDC targeted regulated voltage. The GRHV regulated rails sagged to about 433VDC under the load of the Megatron.  As such, the intended +300VDC for the front end measured only +264VDC. Also, the 2 Antek AN-0206  transformers I used for the CCS EL34 filament supply only measured about 5.75vac, within the 10% tolerance but a bit low for my liking. May need to explore other options. These with the main ac measured about 117vac. 

The output balance measured 2.8VDC and 0.7VDC and the offset measured -15VDC and -17VDC (L/R). 

A few points worth mentioning:

1. I use KSA1156 in place of the 2SA1968 (need to rotate the parts to observe the different pinout)

2. I use a pair of 7812/7912 in the LV part of the GRHV to supply the front end tube filament and the current sink. I mounted them on a 60mm tall heatsink and it appears to work fine for the jobs intended.

3. I am using the split GRHV boards with the CPC1117N delay. I used the 12VDC (with a series resistor to limit the current) from the LV through a toggle switch to manually control the HV delay. It makes for a bit of messy wiring but works fine.   

Back to work to adjust the GRHV for 425VDC rails and tweak the resistors in series of the B+ to provide 300VDC rail for the front end tubes.

EDIT: Forgot to mention perhaps the most important point - the front end tube filament supply connection: if you are using DC filament like I do, make sure you connect the ground wire of the filament supply to the ground pin (which is marked 12.6v on the board) and the 12.6VDC supply wire to the other connector which is NOT the one marked "12.6v". The one marked "12.6v" is actually connected to ground. 

 

 

Edited March 20, 2018 by mwl168

[Rossliew]

That's a beauty there!

[mwl168]

Did some fine tuning on my Megatron. Bring the regulated HV down to 430VDC rails and changed one resistor from 21K to 15K to supply 300VDC to the front end tubes. In my case the front end tubes draw about 8.5mA total.

Also changes the EL34 tubes around in an attempt to bring down the output offset. It's now down to 12VDC yet not as low as I would like. Have a few more sets of EL34 to swap and match to see how low I can bring the offset down to. The output balance is around 3VDC for both channels. 

The toggle switch on the back of the PSU controls the power to the CPC1117N relay on the GRHV and function as the HV delay switch.

Edited March 24, 2018 by mwl168

[chinsettawong]

That’s beautiful!

[Hopstretch]

Agreed!

[JoaMat]

Two different types of EL34 in there?

[jose]

EH 6CA7 and Tung sol EL34 maybe? 

 

[mwl168]

Matched quad of EH 6CA7 for the output CCS and matched quad of SED EL34 for the output gain tubes. I don’t believe the CCS tubes need to be matched to the gain tubes. Maybe Kevin, Birgir or someone can correct me if I am wrong.

Siemens 5814A for the 12AU7 position, Telefunken for the 12AX7. I was using Valvo ECC82 and Sovtek 12AX7 while I was doing early testing.

 

Edit: the output tubes and the CCS tubes do need to be matched to obtain the lowest possible DC offset and balance. Mixing the 6CA7 and El34 was a bad idea. 

 

 

Edited January 2, 2020 by mwl168