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Svetlana 30th August 2006 07:22 AM

Anyone using an 813 in pentode?
Is anyone using the 813 in pentode?
The only pentode connected 813 schematic I found is here:

I was thinking of using a B+ of 2250 and G2 of 750 ...but what do you do for out put iron with a 20K P to P Z out...and it has to be hi-potted to 3k...

Also, would a 6sl7 drive them? or 2 x cv4058?

Miles Prower 30th August 2006 07:46 AM

Re: Anyone using an 813 in pentode?

Originally posted by Svetlana
Also, would a 6sl7 drive them? or 2 x cv4058?
I highly doubt it. The 6SL7 doesn't have a great deal of current sourcing capability, nor does it have a very high g(m) (1.6mA/V nominal). The 6SL7 was meant to drive loads with high resistance and low capacitance. That doesn't describe very many power final grid circuits, and certainly not an 813 even if it never goes into grid-positive overdrive. You're likely going to need either cathode follower grid drivers or a power MOSFET as a source follower.

Svetlana 30th August 2006 08:01 AM

thanks. :|

In this schematic:

they drive vt4c (PP) grids with 2 x 6j5' that not like half a 6sl7?

How aboout driving the grids with pentode connected el34's (B+650V, G2 330V) ?

Miles Prower 30th August 2006 08:15 AM


Originally posted by Svetlana
thanks. :|

In this schematic:

they drive vt4c (PP) grids with 2 x 6j5' that not like half a 6sl7?

How aboout driving the grids with pentode connected el34's (B+650V, G2 330V) ?

My opinion of that design:

That is so horrible on several levels that I'd ignore that. You also are just a bit confused there. The 6SL7 is a high gain dual triode that gets its high amplification factor by driving up its r(p). The 6J5 is a medium gain triode that is actually half of a 6SN7 (or darn close to it). 6SN7s do make good cathode follower grid drivers, and can also drive as common cathode voltage amps, which is how the Williamson used them. Higher g(m) (3.0mA/V) and current handling capability.

Svetlana 30th August 2006 08:28 AM


please help me, I have a PP vt4c, with a B+ of 1150, and I want to make the amp rock, but cant afford vt25's (drivers) or a 80ma nikel interstage...I tried a mosfet (powerdrive), but it destroyed the tone....

owen 30th August 2006 09:03 AM

C4M or EL84, or a ECL82 would do the job nicely - set up as a pentode... High(ish) gain, and high current capability, but you'd have to watch the plate voltages.

There's a C4M scheamtic around developed by Thorsten to drive a 300B - if it's like any of his other designs, it'll have tone in spades ;)

Just a thought,


wrenchone 30th August 2006 07:24 PM

To Svetlana - You have to be careful using a MOSFET follower to drive your output stage. If you use a a garden variety high voltage device, the capacitance can load down the previous stage. You need one with as low a capacitance/gate charge as possible. Check tubelab's web site for suggestions. The other alternative would be to use a different input tube with more drive capability - maybe both.
I've also been looking at vertical oscillator/amplifer dissimilar triodes like the 6CY7 or 6GF7A for driving a class A2 enhanced triode sweep tube SE amp. You get a small, high gain triode with fair transconductance and low plate resistance (at least compared to something like a 12AX7 or 6SL7) and a beefy high transconductance triode with a decent plate dissipation rating. The only drawback to this approach is the need for a negative bias supply to rein in the triode and provide the right DC-coupled bias voltage to the output tube. I also wouln't expect this approach to work if you need tens of milliamperes of grid current. In both of the instances I'm using this approach, the drive levels are pretty modest, or the tube is reasonably tractable, so I don't expect to neeed more than a milliamp or so of grid current.

Svetlana 30th August 2006 11:01 PM

Thanks Wrenchone,

I used the mosfet and set up as per tubelabs instructions.

I think I will try a DHT triode maybe 10y, 45 or 2a3 into a interstage.

While Im mortgaging my house to try that idea, I might play round with a pentode driver

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