[wiatrob]

See also:

http://www.head-fi.org/forums/f6/soh...thread-390553/

 

[royalflush]

datasheet for 6dj8 indicates 365mAh max current. is the SOHA II capable of more than 300mAh tubes? I read omewhere there's a tweak you must do before it can.

 

[Mr.Duck]

Quick question about the DC servo. I havn't been following the SOHA II at all, so bear with me... but is the DC servo really needed? How about using a trimpot instead to offset the DC? Wouldn't that be a better way of doing it?

 

[holland]

^ thermal drift.

Servo is better than trimpots.

 

[Ferrari]

Quote:

Originally Posted by royalflush /img/forum/go_quote.gif datasheet for 6dj8 indicates 365mAh max current. is the SOHA II capable of more than 300mAh tubes? I read omewhere there's a tweak you must do before it can.

Use a 50VA transformer in stead of 30VA and change resistor R3P to 0Ω (use resistor lead snip for this).

Furthermore, to handle the higher current (up to 1.5A), the LM337 voltage regulator should be mounted on larger off-board heatsink.
The on-board stock heatsink is not sufficient for this task.

 

[ROBSCIX]

Quote:

Originally Posted by wiatrob /img/forum/go_quote.gif See also: http://www.head-fi.org/forums/f6/soh...thread-390553/

Thx for the link. I never seen this thread.

Got some more reading and research to do thx.

 

[royalflush]

Quote:

Originally Posted by Ferrari /img/forum/go_quote.gif Use a 50VA transformer in stead of 30VA and change resistor R3P to 0Ω (use resistor lead snip for this). Furthermore, to handle the higher current (up to 1.5A), the LM337 voltage regulator should be mounted on larger off-board heatsink. The on-board stock heatsink is not sufficient for this task.

Thank you Ferrari.

I'm assuming the negative regulator (H-) on the board is what you speak of. Where would be a good source for the 50VA tx? thanks in advance.

 

[jjazzyj]

I think Ferrari hooked up a switch to the R3P so he could switch to the lower/higher heater current on demand. Thanks for all the info on the tubes everyone. Now I just have to get some more tubes to try. It seems that FleaBay isn't the most cost effective place to buy NOS tubes. Anyone use vacuumtube.net?

 

[holland]

6DJ8 should work without any modifications (i.e. stock build). It's only 365mA. 6.3V heaters are in series, so the current is constant at 365mA. 6.3V+6.3V makes 12.6V which is what the heater regulator is regulating to. 12.6V tubes are in parallel, in that case the current is additive.

Many of us used 6DJ8s when we had stock configurations.

 

[Ferrari]

Quote:

Originally Posted by jjazzyj /img/forum/go_quote.gif I think Ferrari hooked up a switch to the R3P so he could switch to the lower/higher heater current on demand. Thanks for all the info on the tubes everyone. Now I just have to get some more tubes to try. It seems that FleaBay isn't the most cost effective place to buy NOS tubes. Anyone use vacuumtube.net?

NO, there is no switch on my amp. My balance SOHAII is set up for high current with a separate 80VA transformer in cooperation with 2 LM338 regulators (one for each stereo amp) for the heater circuits only. Another 80VA transformer serves the remaining low (±15V) and B+ voltage sections of the amp. By doing this way, the transformers can be run far under the limit of their max capacity.
Usually I prefer not to run transformers close to their rated maximum current ability (and there are reasons for that!!!).

The intention of the high current heater modification on my amp is to create more flexibility for any SOHA II equivalent tubes that require more than 300mA @ 6.3V each. Even current hungry tubes like 6N1P-EB/600mA, 6N6P-IR/900mA or 6N30P-DR/900mA (I have done that) can be rolling in without worrying about smoking amp.


Quote:

Originally Posted by holland /img/forum/go_quote.gif 6DJ8 should work without any modifications (i.e. stock build). It's only 365mA. 6.3V heaters are in series, so the current is constant at 365mA. 6.3V+6.3V makes 12.6V which is what the heater regulator is regulating to. 12.6V tubes are in parallel, in that case the current is additive. Many of us used 6DJ8s when we had stock configurations.

I don’t want to speak for runeight, but that is a bit beyond the design target of the maximum current draw from the heater supply.
The circuit is heavily tressed using the stock configuration that way.
It can work however, and of course you are free to do that on your own risk!

 

[wolf18t]

Quote:

Originally Posted by Ferrari /img/forum/go_quote.gif I don’t want to speak for runeight, but that is a bit beyond the design target of the maximum current draw from the heater supply. The circuit is heavily tressed using the stock configuration that way. It can work however, and of course you are free to do that on your own risk!

Already asked and answered in the past. IIRC The transfo upgrade, LM337 heatsink and R3P mod was for russian tubes support.

See post #369/360.
http://www.head-fi.org/forums/5236977-post359.html
http://www.head-fi.org/forums/5236987-post360.html

I didn't mod my SOHA II this way and I'm an happy 6DJ8 user. They are the best sounding tubes I have in my inventory with my Grado, no contest.

 

[royalflush]

what if I just short r3p and use a large enough heatsink for H- would that be sufficient enough to run 6dj8s tubes with the soha ii?

 

[holland]

Yes. That's what I use in my modified PS (for high heater currents). I'm currently running 12AU7s.

The resistor is to drop some voltage before the regulator. Either way, it burns displaces some heat from the reg to the resistor. A larger heatsink will handle that just fine.

 

[jjazzyj]

I'm 90% finished my SOHA II. I have crappy chassis making skills, though I was inspired by Zkool's amp when I built my chassis. Made lots of mistakes, managed to hide most of them. Trying to figure out what to do about the top. I have some hexagon perforated aluminum that I want to use, not sure how to make it look nice though. Here's some slightly fuzzy pictures.

 

[yzriver]

If I use 12AX7 in SOHA II, what modification I can do to the circuit? Can I change the gain by changing the NFB network?
I have a few 12AX7s, and don't waste them.
thanks.