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SOHA II Builders Thread - Page 61

post #901 of 1665
Well, it's not a Ferrari - but I'm tuning up some casing for the Colorado Meet...

post #902 of 1665
where are the sides? are you going wood?
post #903 of 1665
Quote:
Originally Posted by holland View Post
where are the sides? are you going wood?

In the paintshop... pretty plain metal case with top vent holes. This build is the high current version, production PCB (B+@~96V), all nichicon caps, Obligato couplers, R4L/R bypassed with 1u MKP caps. A few muse caps from the parts box here and there.

It is really quite amazing sounding, especially driving my modded YH-3. We have taken a few detours lately with the CTH and EHHA (worthy ones to be sure) but this amp holds it's own with RCA 5963 Black Plates and 6H23's. I am going to set it up with matched Tungsram 12AU7s for some weekend listening with the Orthos - probably the setup I'll bring to the Colorado meet.

It is DEAD BLACK silent with no input at full volume with either tube voltage - careful wiring and shielding pays off for these builds...
post #904 of 1665
Sounds great, wiatrob. I'm still modding my original. I've got a Tungsram 12AU7 in my modded SOHA 2. It's pleasing, but odd at the same time. The highs are too smooth. It's a pleasing tonality though, but it just seems too artificial to me. I'll have to run it in longer and see if it changes.

I think you should join me in HV land.

Can you remind me what is in the high current version? I can't remember except for higher heater capacity, which I have done as well. Did you change the buffer bias for more current too? I haven't done that, it gets hot enough for me as it is and I think 100mA is enough for my needs.

I'm glad yours is dead silent. I don't think mine is at full volume, but it's quiet enough for me. I find the tubes have the largest impact. It's hit or miss with respect to noise. It kind of sucks when you go through a box and find more than a few that hiss or buzz or a microphonic.
post #905 of 1665
The only mods are for the heater current. I have been using "Premium" (I paid someone to go through the box and select low noise, matching tubes in my SohaII's -I have the 5963s, 12BH7s and the Tungsrams) saving me the disappointment at somewhat-much greater expense!

I flirted with the idea of higher voltage B+, as well as doubling the heater circuitry capacity to support 6H30 tubes, but I like the way this is sounding now so I will stick with Alex' sig mantra: Don't mess with anything that is working.
post #906 of 1665
Quote:
Originally Posted by holland View Post
I modded my SOHA II prototype to run a B+ of ~241V and 219V on the plate.
And no firework and smoke at the current mirror section???
post #907 of 1665
x2

Mine is probably the last stock build left in here (300mA max). I've stayed with 5963s for a while with great results, also a pair of tungsram ECC82s (hungary) which are actually pretty good.
post #908 of 1665
Quote:
Originally Posted by Ferrari View Post
And no firework and smoke at the current mirror section???
None.

B+ is 241V, but after the cap multiplier it drops down a bit, I forget how much but I'm running @ 10mA per tube so it's quite a bit more than the standard 2mA configuration. The drop across the mirror is effectively the same as it was before the B+ was raised to ~241V. If they were to blow up, they should have blown up @ 90V too. They are floating relative to the plate. That's about the depth of my knowledge. There may be points during power up transients where the max ratings are exceeded, but that should happen @ 90V as well. Because of that, I assumed they floated well, and since you ran @ 120V, I figured it was all good.
post #909 of 1665
Quote:
Originally Posted by wiatrob View Post
I will stick with Alex' sig mantra: Don't mess with anything that is working.
LOL, that's just anti-DIY. I ignore it quite well.
post #910 of 1665
Quote:
Originally Posted by holland View Post
LOL, that's just anti-DIY. I ignore it quite well.
As long as you can FIY (Fix It Yourself) that's fine. And we know you can.
post #911 of 1665
Quote:
Originally Posted by holland View Post
None.

B+ is 241V, but after the cap multiplier it drops down a bit, I forget how much but I'm running @ 10mA per tube so it's quite a bit more than the standard 2mA configuration. The drop across the mirror is effectively the same as it was before the B+ was raised to ~241V. If they were to blow up, they should have blown up @ 90V too. They are floating relative to the plate. That's about the depth of my knowledge. There may be points during power up transients where the max ratings are exceeded, but that should happen @ 90V as well. Because of that, I assumed they floated well, and since you ran @ 120V, I figured it was all good.
I indeed ran my proto amp at 120V B+, but I modified the current mirror as well, using high voltage transistor in stead of BC560C.
Good to know it works for you, without firework.

On a side note, I’m listening to my balanced SOHA II via AKG K1000 while typing this, it sounds damn good.
I can confirm that a well-constructed balanced SOHA II can drive AKG K1000 at a very loud level (the wife starts complain) without any sign of distortion or other shortage.
post #912 of 1665
I see. Thanks. I'll keep an eye on it. The mirror is only dropping ~19-20V which is within it's tolerances. The max is 45V collector-emitter. As long as the plate doesn't go to ground, I think it'll be OK.
post #913 of 1665
I just finished populating the SOHA II board. I hooked up the transformer according to the Inlet wiring, as soon as I turn the switch on the transformer buzzes for a few seconds and the fuse blows. The first time I noticed I reversed the Hot and Neutral according to the inlet wiring diagram. I fixed this and the next two times the fuse has blown. Now the last two fuses weren't slow-blo fuses, but they still shouldn't have blown in my opinion. Any ideas as to what is going on? Do I have a shorted transformer? I need to add, that I've checked and double checked for solder bridges, and I don't have any. I've also checked for short circuit between PS test points and I get a resistance value for each one.
post #914 of 1665
jjazzy, sounds like there's a short of some kind. Have you taken any voltage readings from the tx filaments?
post #915 of 1665
I don't have the amplifier section even hooked up yet. J1, J2, J3 remain unstrapped.
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