LS50/GU50 Both Grid Drive

[smoking-amp]

Anatolyi: "I am going to try a double-drive such a way: 1..2..3..4..5.."

George: "OK, why not one typical P-P driver with each output feeding a pair of mosfet drivers (4 total). Each mosfet has independent gain and DC bias adjustments."

Ken: "driving G1 from the lower 533R"

I'm smelling success now.

"It's impressive that anyone still living knows Lewis Carroll..."
I thought it was Rudyard Kipling who spoke about "taking the road less traveled..." I guess they didn't have GPS back then.

Don

[oldeurope]

Hi Tom,
note the 22K resistor in series to g1.
It is false to say they are tied together.
See post #6 why this isn't possible with the 807.

Kind regards,
Darius

[smoking-amp]

sorta OT: Say Darius, I recall you were looking for a tube with a Mu of 1 a while back. I just ran across a data sheet that mentions just that (bottom 1st page):

http://www.mif.pg.gda.pl/homepages/f...093/6/6AH6.pdf

But probably rather high rp with this tube. Could try g3 drive with some low rp pentode instead.

Don

[Tubes4e4]

Hi Darius,

note the 22K resistor in series to g1.

I did so.

It is false to say they are tied together.

No, it is not, and you would know and agree if you would have done some grid current measurements for such conditions yourself.

Regards,

Tom Schlangen

[Wavebourn]

Originally posted by kenpeter


If you find yourself needing to adjust the overall amount of UL,
resistive plate to plate (output plates back to the concertinas)
feedback might be handy.

Great idea, thanks!

[tubelab.com]

Maybe you mean the schematic as attached below.

That is the one. I played with this several years ago and wasn't impressed. The effective Mu of the "triode" wired tube is quite high. It likes to oscillate! You need to tweak the 22K resistor value for best linearity. Too low and your driver gets mad, too high and the output stage distorts. You need a resistor on the screen too. This circuit was conceived before mosfets were popular. We can do better today.

I'm smelling success now.

You might be smelling success, but I am smelling the fine aroma of burnt parts. Lets see...... this one is Allen Bradley, 1969......

[kenpeter]

Which component do you most expect to burn up, and why?
Or are you referring only to the older class B amp?

As for oscillations: I always thought the cause was unterminated
resonant stubs. Low impedance on one end, high impedance on
the other, and a quarter length "whip antenna" of some resonant
frequency. Then you abuse the high voltage/high impedance/low
current end of that resonant antenna stub as one of the control
elements of a tube or FET, and let it pick up stray garbage from
the amplified output.

Or high impedance both ends, and resonant on the half wave...

[tubelab.com]

Which component do you most expect to burn up, and why?

It was a poor attempt at humor. But I do have a history of toasting parts. Sometimes you just know something is going to fry and it works great. Other times you have this great idea, and the fire gods just dance all over it.

As for oscillations: I always thought the cause was unterminated resonant stubs.

Sometimes it is just stray capacitance resonating with leakage (and other unwanted) inductances being excited by the abrupt spike created when an output tube is driven to cutoff. Some circuits just want to oscillate. Some circuits may exhibit some oscillitory instability without the user being aware of it. I tend to "sniff" my tube circuits with an RF spectrum analyzer while driving them to clipping. My full time job is transmitter design, so it is just a habit.

[smoking-amp]

"Allen Bradley, 1969......"

Yup, was a good year for phenolic. Someone should come out with scented resistors. Boutique parts ya know. Hyacinth or Honeysuckle, maybe persimmon. Would probably sell a bunch.

Don

[oldeurope]

Originally posted #24by Tubes4e4
...
No, it is not, and you would know and agree if you would have done some grid current measurements for such conditions yourself. ...

Originally posted #26by tubelab.com


That is the one. ... You need to tweak the 22K resistor value for best linearity. Too low and your driver gets mad, too high and the output stage distorts. ...

Thanks to tubelab.com

Kind regards,
Darius