page Eli Duttman's Simple p-p with 5965 MQ EXO-173 and 6AQ5

[Bandersnatch]

Guys,



Well first off say you used something like 220K grid resistors and 0.1uF caps and a tube phase splitter. Well as the signal approached 60% that 220k resistor is going to start to draw current and rebias the tube.

So with an AC induced grid current, when does this AC current start changing the voltage across the choke?

Something like the 173 removes the need for the phase splitter and also has a low dcr which means you will get considerably more power out of the design, because when the grid starts to draw current the tube won't rebias.

Gordon, the design is still AC coupled, there is no getting grid current through it, just like any other cap coupled circuit. Now since the cap is bigger you've got some more current available as the cap discharges.
cheers,
Douglas

 

[Wavelength]

Douglas,

Have you looked at grid current at clipping? This is something I pay allot of attention too. The grid current is not from the driving stage it's from the output stage. Even though this current is very small it appears as DC current in a negative fashion therefore increasing the bias voltage and cutting off the tube.

Even though this is a small current, over a large resistor it can present several volts which inturn, especially with an EL84 rebias the tube down and create early clipping.

The DCR of the 173 is very small and therefore pushes the tube much better than a phase splitter and large grid resistors would.

Thanks
Gordon

 

[Bandersnatch]

Douglas,

Have you looked at grid current at clipping? This is something I pay allot of attention too. The grid current is not from the driving stage it's from the output stage. Even though this current is very small it appears as DC current in a negative fashion therefore increasing the bias voltage and cutting off the tube.


Thanks
Gordon

Yes I have looked at in considerable detail. You say that at 60% the grid current comes in in qty enough to disrupt things. What is it doing at 50%, at 70%? So if the signal goes from 40% to 70%, how do you figure it is DC?
cheers,
Douglas

 

[Wavelength]

Douglas,

Yes there is significant at 50, even 40% and much more at 70%.

I put my Agilent 6.5digit meter in current mode in line with the grid resistor put it too ac measure the current then put it too dc. Pump an AC signal in either with my Prism dScope III, my MAC/PC or precision generator and look at the results.

That am I have made over 1000 tube amps since the 70's.

Heck sometimes these companies send me new tubes they are trying to bring to market and even at steady state they draw excessive grid current. This was especially the case with many of the 300b's made in ex soviet areas and one of the reason's you are seeing more and more the reference to gold grid wire. They are trying to decrease the plate spacing to lower the Rp and increase the output capabilities without looking at the ramifications elsewhere. It's an interesting comparision for when I talked to WE, GE and other engineers back in the old days to some of the Sovtek engineers of today.

Thanks
Gordon

 

[Bandersnatch]

Douglas,

Yes there is significant at 50, even 40% and much more at 70%.

Sounds like a description of AC to me. So how does the choke deal with AC?
cheers,
Douglas

 

[Bandersnatch]

hey-Hey!!!,
Here is some old discussion on the 173's performance:
http://www.audioasylum.com/forums/tubediy/messages/4/48463.html

It does indeed work well with a low impedance drive. After some more experimentation with the interleaving idea, I came to the conclusion that the simpler, higher inductance and gapped core with balanced drive was a better sounding solution.
cheers,
Douglas