Any reason why the SOHA doesnt' use low volt tubes?

[AudioCats]

I am just wondering why a SOHA uses high voltage tubes like 12au7....... since there are some car tubes still available, such as the 24V ECC86, is there a reason not to use the car tubes? Their price is not that much higher than the 12AU7 (my 12AU7's average to $12 each NOS -- the Amprex and Mullard brought the average price up -- while the ECC86's NOS are like $15 each...).

Thanks for any info

 

[dsavitsk]

http://www.ecp.cc/meha.html

That project never really took off, but I think the concept was pretty sound.

The 12au7 has a lot of distortion. As the voltage gets lower, this distortion increases. But, the distortion is of the type that is "tubey" sounding, so people seem to like it. And, to be fair, the distortion decreases with a CCS on the plate, so this probably helps some.

 

[AudioCats]

outstanding! thanks dude! that link has quite some good info......

I will probably get some ECC86 coming, the only thing that was holding me back on a tube amp project was the high voltage, kind of hesitate to get into it (got zapped too many times in this life already)

 

[dsavitsk]

The 6gm8 is a pretty good sounding tube and fun to play with. It is also very similar to a 6dj8, and you can sub the 6dj8 in and run it at low voltages without any issues.

As for high voltages, you certainly shouldn't work with them if you are not comfortable. But, here is a good thread on high voltage safety practice that might help: http://www.diyaudio.com/forums/showt...threadid=30172

 

[AudioCats]

I am comfortable working with high voltage, have been around the HV a little too much that was how I had the chance to get zapped....

but 24V is a lot nicer than 110, that is for sure

The real reason was I don't have any 110-110 isolation trans and I don't want to just run the wall 110 directly to a diode bridge...... I guess one way to get around the problem is use 24v AC wall trans (which also provides heater power), then use a small 24V trans inside the case to jack the voltage back up to 110AC, but that way there is quite a bit of voltage change once the music starts (due to the high internal resistance of the small trans) ........the original intention was to make cathod followers for 600 ohm headphones, so there will be alot more than just 5mA current draw, more like 40mA for two tubes.

Have fun

 

[ericj]

iirc one of the factors in chosing the 12au7 type was that it has Just Enough gain, about twice as much as the 12ae6a, iirc.

Also, while there were only about 4 actual factories cranking out 12ae6 tubes, there are many, many 12au7 types out there, many of which sound pretty good.

And with the higher B+ of the SOHA design, in addition to more gain, the SOHA is less likely to go into clipping than the millett.

I like my millett hybrid, but with inefficient cans it just doesn't have enough gain sometimes.

 

[AudioCats]

Thanks ericJ........

doesn't the millet use a 6922 (ECC88, which has mu of 33 while the ECC82/12AU7 is only 17)? or are we talking about different millets.....

 

[tomb]

Quote:

Originally Posted by AudioCats /img/forum/go_quote.gif Thanks ericJ........ doesn't the millet use a 6922 (ECC88, which has mu of 33 while the ECC82/12AU7 is only 17)? or are we talking about different millets.....

Umm ... no offense, but I'm not sure what ericj is referring to. The Millett uses three types of tubes - each with different gain. The 12AE6 has the most gain - 14. The actual Millett circuit ends up with a little negative gain, but Amb tested it out to a gain of 12.4 with a 330ohm load:
http://www.ibiblio.org/tkan/audio/millett.html

That's more than enough for any high-impedance headphone. The revMH Millett suffered from a bit of a lack of current leverage with the original monolithic buffer stage, which effects low impedance cans more - but that was mostly addressed with Steinchen's original DB's. The other tubes used are the 12FM6 and the 12FK6. The 12FK6 has the least gain.

Of course, the MAX is way more powerful than either.

 

[dBel84]

You could always get your hands on an 8416 (the only ones I have seen are amperex and cheap

). It is a 12.6V version of the 6922 and worth a shot as a drop in for the SOHA..dB

 

[dsavitsk]

Quote:

Originally Posted by dBel84 /img/forum/go_quote.gif You could always get your hands on an 8416 (the only ones I have seen are amperex and cheap ). It is a 12.6V version of the 6922 and worth a shot as a drop in for the SOHA..dB

Shhh ... Don't let the secret of this tube out ...

 

[AudioCats]

now the 8614 price is gonna go WAY up.....

seems like there are less than 4 such tubes on ebay at any given time, if more than two people really want these nice little Holland amperex thingy the price will probably go up in a hurry....

 

[ericj]

iirc runeight says the gain of the 6922 is too high for the SOHA.

If you actually read the design threads over at headwize, they say that one of the problems they had originally was finding a tube with enough, but not too much gain.

Post 11 in the original SOHA thread, mains_hum says:

Quote:

I did try some other 12.6Vac valves (ECC series) but the *gain* was way too high. But the main issue was to pick a a valve that's not particularly good ( equals *cheap* and easy to get) yet the datasheet says possibly works at lowish anode volts, then try to compensate for other nasties that introduces. That's why the anode load is a CCS, yup, runeight sorted that.

Post 702, runeight:
Quote:

Also, without in any respect trying to dissuade you from building this with a 6922, you are almost certainly going to have too much gain. This may limit your ability to adjust the volume control. And with so much gain I highly recommend that you use the protection diodes for the opamp.

 

[AudioCats]

Thanks, can anyone provide me a link about basic tube design calculations? I am still confused about the mu and S thing, is mu used for voltage gain design and mu used for current output design?

If the 6922 has too much gain, is it possible to step it down between thhe tube and buffer? (R5 is 1M, can't you just use two 500k and take the buffer signal from the middle?)

have fun

 

[TzeYang]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Umm ... no offense, but I'm not sure what ericj is referring to. The Millett uses three types of tubes - each with different gain. The 12AE6 has the most gain - 14. The actual Millett circuit ends up with a little negative gain, but Amb tested it out to a gain of 12.4 with a 330ohm load: http://www.ibiblio.org/tkan/audio/millett.html That's more than enough for any high-impedance headphone. The revMH Millett suffered from a bit of a lack of current leverage with the original monolithic buffer stage, which effects low impedance cans more - but that was mostly addressed with Steinchen's original DB's. The other tubes used are the 12FM6 and the 12FK6. The 12FK6 has the least gain. Of course, the MAX is way more powerful than either.

Bolded: Are you talking about the same db used in the older millett hybrid? Like the one posted on diamondstar.de?

I thought that particular db was designed by ppl? why the "steinchen's db?"

Before anyone flames me, this is merely a question. I don't mean to offend anyone.

 

[tomb]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif Bolded: Are you talking about the same db used in the older millett hybrid? Like the one posted on diamondstar.de? I thought that particular db was designed by ppl? why the "steinchen's db?" Before anyone flames me, this is merely a question. I don't mean to offend anyone.

No problem, but if you notice, "diamondstar.de" is Steinchen's web site.

It is correct to credit PPL with the original ground-breaking Diamond Buffer design - included in the PPAV2. However, Steinchen designed the specific implementation for the Millett and there are several important differences. These include parallel pairs of output transistors and other items. Most important, he designed the whole thing to fit in the ridiculously small space allowed on the revMH Millett board - approx. 3/4" x 2-1/2" for both channels! This involved the creative use of SMD components and creative stacking of output resistors and transistors - a truly accomplished feat of engineering and design. (You must be familiar with this, since Millett DB's are in your sig.)

Finally - again as you note - he also created an outstanding web site that still serves as the primary reference for Diamond Buffer design and operation. It includes available output transistor reviews, biasing details, and many tweaks learned from what must be hours of first-hand experience and development.

Give PPL his due for the original creation of the Diamond Buffer circuit, but I have no problem referring to the Millett's version as "Steinchen's DB's" and will continue to do so.