adamasd

6AS7 fixed bias
« on: July 19, 2005, 08:24:03 PM »
I have been experimenting with the 6AS7 as a push pull amp and I noticed short magazine article I have on it says, "Must Not Be Used in Fixed Bias". I have looked and I can not find  why this is. I have no plans to run them fixed bias, but I am curious why they say not to. They also said they should never be run in parallel, but I already did that, it worked, nothing started on fire and I am still alive. Any reason while they say not to use it parallel?

thanks
adam
so there


PRR

6AS7 fixed bias
« Reply #1 on: July 19, 2005, 10:25:59 PM »
6AS7 varies too much tube-to-tube to use a fixed bias and get usable results in production.

Yes, for DIY you can adjust the bias for each tube, and watch it for the first few days.

Parallel operation is quite common. Again you face the problem of poor matching. Use a significant cathode resistor on each tube, and check for reasonable current sharing.

6336 is essentially a double-size 6080/6AS7, hand when you have a "two 6080 job".

6c33 is a grossly oversize 6080 from another land, and currently cheaper than 6336 even with its special socket. 6c33 is a single-triode, not twin-triode like 6080 or 6336.

IMHO, there are few audio chores where a 6080 is the best choice. If not for the awkward cathode, 2A3 or 6B4 is still hard to beat. 6EM7 has a 2A3-like triode plus a bonus hi-Mu triode, and is cheap. 6L6 is not a bad triode, and sure is available anywhere.

adamasd

6AS7 fixed bias
« Reply #2 on: July 19, 2005, 11:52:24 PM »
Ok, that clears things up, I figured it would be something simple like that. The only reason I am useing the 6AS7 is because I have them, along with all the parts to make an amp with them. I wanted to play around with triode amps, Started with 6S4a and 171a, but wanted something a little more powerful.  I am just playing with the tubes I have and learning. No need to buy any tubes for this, especially when I have probably around 300+ tubes to play with.

thanks,
adam
so there

Larrchild

6AS7 fixed bias
« Reply #3 on: July 21, 2005, 07:12:08 PM »
Some tubes will slowly achieve thermal runaway like an old pnp when fixed bias is used. Self bias tracks this phenomonon out.

another possibility.

CJ

6AS7 fixed bias
« Reply #4 on: July 21, 2005, 07:46:28 PM »
looks like a servo tube. amp factor 2,   Ip=125 ma. weird.


must have major grid current or something.
If I can't fix it, I can fix it so nobody else can!
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Larrchild

6AS7 fixed bias
« Reply #5 on: July 21, 2005, 08:33:59 PM »
they were a very popular regulator.

i saw the coolest link im still looking for.
The guy took his 70's pioneer transistor receiver and gutted the power amp and put 2 6AS7 SE output stages inside.
Now thats determination!

PRR

6AS7 fixed bias
« Reply #6 on: July 22, 2005, 12:14:11 AM »
> looks like a servo tube. amp factor 2, Ip=125 ma. weird.

It was THE voltage-regulator pass-tube. A 7905, but without the reference and error-amp; essentially the big transistor in a 7805 to which you added the rest of the reguator.

2A3 was used before, but it has a fairly high plate drop for a given current. For a similar cathode, you get lower plate drop with a lower Mu. But if Mu is too low, the drive swing becomes insane. So there is a practical limit.

For a conventional audio amplifier, using some rules of thumb for R-C coupled drivers, and assuming the driver eats the power amp supply voltage, the minumum Mu is about 5 for a fix-biased tube, maybe 4 for self-bias (since some voltage is dropped in the bias resistor).

For the common voltage regulator configurations, for semi-constant output, for reasonable efficiency, you don't have to swing the grid such a large proportion of supply voltage, and it does not have to be linear enough to listen to. So the pass-tube Mu can be low, 3 or even 2. That (plus the bigger heater) lets a 6AS7 pass current with much lower voltage drop than a 2A3. (Also the insulated cathodes mean you may not need a dedicated heater winding.)

A 6AS7 is a poor choice for audio (don't let that stop you). The grid drive is enormous, requiring heroic drivers. The self-bias losses are also enormous. Fixed bias reduces those losses, but the very low Mu means that if the current is wrong, you need a BIG change of grid voltage to correct it. That plus loose specs and apparently sloppy production (it is "only a pass-tube") means you can't predict the bias accurately, and two tubes won't match except by accident.

And in my estimation, it wasn't any kind of brilliant design for 1949, it was a toss-off. They beefed-up an older low-Mu triode, loosened the grid for lower Mu, and called it "done". They could have made it a little more efficient even with the older technology. They could have made it more linear; the techniques were well known in the 1930s. They didn't have to make it very good to sell enough to be profitable, so they didn't bother.

As an audio amp, you will probably run out of drive before you swing the 6AS7 to its limits.

It is much more linear up around 70-100mA than it is down below 30mA, because it was made to live at high current (if you only wanted 30mA, there were cheaper tubes).

6336 is the much-improved version, improved for power. Twice the heater power and cathode area and plate dissipation. Lower Mu, which requires a harder-working driver (anything big enough to need 6336 should not gripe about driver costs).

Banks of 6AS7/6080 were used in precision oscilloscopes, and apparently in military radar systems. Some switched to a lesser number of 6336. It seems the USSR decided to make a monster 6AS7 for similar chores, hence the 6c33.

> must have major grid current or something.

I don't think so. The maximum grid resistance is actually lower for 6550 or 8417 than for 6AS7. It seems to work fine with 470K grid resistors. Anyway, even large grid current in a large grid resistor does not have a huge effect on a tube with such low Mu and such large operating bias.

analag

6AS7 fixed bias
« Reply #7 on: July 22, 2005, 08:48:06 AM »
Speaking of 6AS7 my man John Broskie has an article that include this tube as a cathode follower in his great sounding aikido amp. I use this buffer in most of my tube pre outputs...much better than the White cathode follower soundwise.

http://tubecad.com/2005/July/blog0051.htm

Analag
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.

kambo

Re: 6AS7 fixed bias
« Reply #8 on: August 14, 2019, 10:53:41 PM »
Speaking of 6AS7 my man John Broskie has an article that include this tube as a cathode follower in his great sounding aikido amp. I use this buffer in most of my tube pre outputs...much better than the White cathode follower soundwise.

http://tubecad.com/2005/July/blog0051.htm

Analag

there is the topic i have been looking for :)




 

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