[mideel]

I recently bought MrMajestic2 balanced EHHA kit and my build is going along nicely:

The build includes o22 PSU with 100VA 2x30V Trafo, 12.6V o11 heater with 25VA 2x15V Trafo. All gain resistors are socketed, for tweaking. I'm still undecided on the volume control. I'm thinking of using two shunted alps pots at first and then maybe doing something crazy ie. using 17 switches and a PGA2320. What ever I end up doing case work is going to be a pain

But at least I'm making good progress, the o11 just tested OK, I used 12V zener diode (measured 12.01V) and a 220kohm resistor to set the voltage at the output to 12.6V.

 

[johnwmclean]

Quote:

Originally Posted by mideel /img/forum/go_quote.gif I recently bought MrMajestic2 balanced EHHA kit and my build is going along nicely: The build includes o22 PSU with 100VA 2x30V Trafo, 12.6V o11 heater with 25VA 2x15V Trafo. All gain resistors are socketed, for tweaking. I'm still undecided on the volume control. I'm thinking of using two shunted alps pots at first and then maybe doing something crazy ie. using 17 switches and a PGA2320. What ever I end up doing case work is going to be a pain But at least I'm making good progress, the o11 just tested OK, I used 12V zener diode (measured 12.01V) and a 220kohm resistor to set the voltage at the output to 12.6V.

Congrats mideel, just curious about your transformer for the σ11, going from the AMB site 2x8V would be best for a 12VDC output. A 25VA might be a to a little low for balanced (depending on what tubes you intend to use). I ran into some pretty tight thermal figures for making a σ11 viable for heater use, looks like the σ11 may be a heater in itself

Someone correct me if I’m wrong on this!

 

[Horio]

Even though I am planning to use 6gm8's on this amp, I wanted to option to power 6h30's. The thermal issues I kept encountering with a o11 heater supply eventually led me to order a glassware H-PS-1 (shows up today). I'll be interested to see how it the o11 works out for you mideel.

 

[mideel]

25VA with 2x15V in parallel, each 6GM8 draws 330mA (6.3V). Two tubes in series will draw 330mA (12.6V), so 4 tubes will draw 660mA (12.6V). The transformer is rated at 25VA/15V=1.66A.
The 6H30 tube is rated: Voltage: 6.3V Current: 0.825 A, so I wont be able to use 6H30 with this transformer.

P = (Vin - Vout) * I [Vin and Vout measured with 47Ohm test load]
P = (20.25V - 12.6V) * 0.66A = 5.05W

"Since two paralleled MOSFETs serve each rail, each MOSFET will dissipate half the power."

Tj = Ta + (P * (Rjc + Rcs + Rsa))
Tj = 30°C + ((5.05W/2) * (3.3°C/W + 1°C/W + 3.7°C/W)) = 50.2°C

"if the two MOSFETs will be mounted on the same heatsink, then the full power dissipation figure should be used."

Tj = 30°C + ((5.05W) * (3.3°C/W + 1°C/W + 3.7°C/W)) = 70.4°C

With the 6H30 tubes I would get 2.5 current draw resulting in:
Tj = 30°C + ((12.625W/2) * (3.3°C/W + 1°C/W + 3.7°C/W)) = 80.5°C

And just to confirm my measured results with 0.26A current draw (the mosfet heatsinks measured in at 34°C after 10 minutes):
Tj = 24°C + ((2.051/2) * (3.3°C/W + 1°C/W + 3.7°C/W)) = 32.2°C

Unless there is an error with these calculations, the default heatsinks should be enough even for 6H30 tubes and I can always use a fan or two from the 12.6V to cool the case down, if the heat proves to be a problem.

 

[Horio]

I'm not seeing anything in your calculation. Runing 6gm8's versus the power hungry 6h30's really helps the thermal issues. Let us know how it turns out.

 

[rhester]

Have there been any one box builds of this or should 2 boxes be used to minimize noise? Ready to box up but want to make it as nice sounding as pssible.

 

[sachu]

Quote:

Originally Posted by rhester /img/forum/go_quote.gif Have there been any one box builds of this or should 2 boxes be used to minimize noise? Ready to box up but want to make it as nice sounding as pssible.

I have built a one box SE EHHAs thus far and have had no problems at all. I just made sure when i laid it out I had maximum distance from the transformer to the amp boards and signal wring (used shielded cables here). But on my main build no shieldded wires nothing, it just works perfectly. The caveat however is that I use orthos for listening. I did notice however it performed with the blackest of backgrounds with the AG K701 as well. But, when I had the JH13pro on loan and plugged it in, i could hear some hiss.

 

[sachu]

Quote:

Originally Posted by rhester /img/forum/go_quote.gif Have there been any one box builds of this or should 2 boxes be used to minimize noise? Ready to box up but want to make it as nice sounding as pssible.

Rhester, to answer your question, I think this is as compact an EHHA (with an S22, DC heaters and relay ) can be built. Case dimensions are roughly 3.5 x 14 x 7.

 

[mideel]

Just a small update on my EHHA progress, I got my o22 power supply assembled and tested with two 2x15V 25VA transformers (my 100VA transformer might be weeks away from arriving because of the ban on air flight over Europe). After trying several 12V zeners I got the o22 measuring at 29.2V, I assume this is acceptable for an EHHA, or should I try to fine-tune the o22 to exactly 30V? and would 50VA be enough for balanced build? (the recommendation for balanced b22 is 100VA, this would be temporary until I get the 100VA transformer)
I got the gain resistors matched down to 0.1%, all of the transistors to ~5%, but the mosfets are only matched to 10%

Is this close enough or would it be better to order more?

On a more positive note I finally got my eXstata boards populated

now I only need to make a DIY stax socket, since the ones I ordered won't be arriving anytime soon.

 

[sachu]

I think you might be able to get away with 50VA.

As for the DC voltage..you are fine..doesn't need to be exact 30V.

 

[aloksatoor]

I was powering up the boards today and noticed I failed on the first test. The voltages accross both boards for R3 and R4 are around 0.68V. Also I left the board running without the servo opamp for a while and noticed that my 2W resistor R32 is glowing red hot. It still measures 2.2 ohms though. Have I blown a mosfet? Does anyone know what could be off? I verified no shorts on the board.

 

[sachu]

I'd say replace those output resistors.

For checking the Mosfets. power down, put your multimeter in 200 ohm mode and measure resistance between the metal tab and the two outer pins. If any of these read short or less than a few couple of hundred ohms they need to be replaced.

 

[aloksatoor]

The Q8M both pins measure around 300 ohms. I guess that is ok?
and what about the voltage across R3 and R4? is 0.68V normal?

 

[sachu]

No it isn't..it should be double that. can you check to see if the resistors are 1k each?

I know this is stupid but, you are testing them with the tubes in?

 

[aloksatoor]

yep tubes are in. the heater is working as expected and the tube glows orange. I wired the heater ground to star ground (Sigma 22 gnd) and also inputs star gnd to the sigma 22 gnd. ANd also the resistors are 1K each. Only discrepancy is the R32 33 are the wrong wattage (1/2 watt instead of 2). I did not verify the GJA kit when it came i guess.