Pentode driving a SE triode? any experience?

[rvrazvan]

A few people suggested me to use a pentode in the driving stage of a set amp(6c33).
My initial goal was to use only triodes but this has it's own list of disatvantages. Usuing small double triodes means haven at least 2 stages..or a SRPP(wich is a for of push pull?).
Ho to pentodes sound in the driver stage? Is it a good trade to have just one pentode driving stage vs 2 triode stages?

 

[brady]

Hi R. just an opinion. DHTs require sizeable amount of current, and low Z drivers --- both not usually a characteristic in pentodes. but then again, only way to find out is to try ....hehehehe

 

[kevinkr]

Something like a D3A, C3G or 7788 running at around 15 - 20mA into a low resistance plate load might do the job fairly well.

 

[the_manta]

Hi R. just an opinion. DHTs require sizeable amount of current, and low Z drivers --- both not usually a characteristic in pentodes. but then again, only way to find out is to try ....hehehehe

That counts for small signal type Pentodes like EF86.

A Pentode's characteristics are very similar to a MOSFET which can (I think nobody will deny) drive loads with a very low Z and swing huge currents.

And thats the Point that matters, the transconductance and the allowed cross current. That's the interesting property about pentodes, you don't quiet need more parameters, couse of the very high Zi. Triodes are much more complicated because of their Anode control ratio "punching" through the grid.

EF86 has a transconductance of only 2 mA/V. As a pentode, its Zi is very high and is mostly determined of the Anode Resistor.
But the 6Э5П for example has a transconductance of over 30mA/V and takes currents up to 100mA. This small russian pentode is a great tube for a very low price. (http://www.cqham.ru/tubes/soviet/scan/6ee5p.gif)
This thing is like 15 EF86 in parallel. And you can indeed drive nearly everything with it. Problem is the max. anode voltage of 250V. But when using an Anode choke not even that is a Problem.

As kevinkr said, there are many other high gm tubes wich are used succesfully to drive DHT's and sound in my ears better than staged Triodes. Those staged drivers have significant amaounts of high order harmonics wheras a single Pentode driver's harmonics mostly shows K3 and K2.
Here two 300B examples with pentode drivers (german): 300B-SE-Amp von Thorsten Loesch and SHM 002 SE

Hi R. just an opinion. DHTs require sizeable amount of current, and low Z drivers --- both not usually a characteristic in pentodes. but then again, only way to find out is to try ....hehehehe

Describing SRPP would fill pages here. Short reply:Yes, you can see it as Push pull but I like the view as bootstrapped grounded cathode amplifier. Lower tube is standard anode follower and upper tube helps its Anode to swing the voltage. Read more about it at John Broskie's page.

And yes, you have to try explaining won't convince ears.
Have fun

 

[DF96]

Lower tube is standard anode follower and upper tube helps its Anode to swing the voltage.

Not quite. SRPP lower valve is standard grounded cathode, not anode follower. Upper valve helps to supply more current to the load. The SRPP and grounded cathode/bootstrapped CF are closely related, as you say.