WE91A Pentode Questions?

[sgerus]

Hello,

I planning on building a WE91A clone for my next project and need help deciding on which pentode to use.

The practical side of my brain says 5693 (Pros: octal socket, 6.3V heater)

The exotic side of my brain says WE310A or WE328A (Pros: like the original and it looks cool)

The high tech in says Siemens C3M (Cons: not really WE91A clone, 20V heater, loctal socket….. But can run at high current to avoid “skew rate” issues)

So what would you folks recommend?

My second question is what current to run the 5693/310A at.
The WE91A is set at 2mA plate current.

From what I can tell from the datasheet, running this tube at 5ma (plate) produces a lot lower 2nd harmonic distortion, at the cost of a lower Av.

Any recommends on the operating point of the 5693/310A?

[kevinkr]

To complicate issues even further how about D3A, and C3G? LOL

I imagine the original bias point of the 310A was chosen deliberately to cancel some of the 2nd harmonic in the output stage..

[pmillett]

I'd go with a 6J7G. Basically the same as a 5693 in a top-cap "G" bulb. And a lot cheaper than a 310A.

Pete

[AndreasS]

Or a 6C6, 77... (6-pin socket like the 310A, but 6,3 Volt-heater)

Andreas

[ianc13]

Or you could go with the C3O - 6.3V version of the C3M.

They can be found, there is a batch of 10 for sale on eBay at the moment but they are rare and the guy who is selling them does not take PayPal (Germany based) only bank transfer.

[sgerus]

Thanks for the replies, this is all great info.

The 6J7G is PERFECT
You have the same look as the 310A, which is what I really wanted and all of the pros of a 5693 (octal, 6.3 V heaters, lower cost)

-Scott

[jeff mai]

The first WE91's indeed used a 6C6.

I tried a bunch of these pentodes as 300B drivers. My favorite was the 6J7G. I preferred it over the C3m. Two recommendations:

Use an OC3 to regulate the screens.

And use the smallest plate resistor you can get away with on the pentode. I had a fairly high output CDP at the time and I think I still had enough gain with half the size of what WE used.

Don't forget to recalc the operating point if you change the resistor! And up the plate current a little, but not too much. One thing I found is that generally I liked pentodes that liked to run at 3-5ma plate current better than those running 10ma and above. An exception was the 6SG7. It sounded really nice at 10ma, but you must choose an operating point that stays well away from the remote cutoff region.

I only tried octals and the C3m. I got bored and went onto something else before the miniatures made it to the table.

[sgerus]

Jeff, What is an OC3?

[the_manta]

It's a voltage stabilizer tube.
See here

Right name is 0C3, not OC3. The first letter specifies the heating voltage as we all know. And because a voltage stabilizer has no heater, it has accordingly zero heater voltage. This explains the Zero as first letter.

Pentodes need a very tight screen voltage. It should be "locked" to the cathode. This can be done with a big cap, or better with a regulator valve. Unfortunatly these can be noisy.

Regards, Simon

[jeff mai]

Originally posted by the_manta
Right name is 0C3, not OC3

Yes, of course it is. Typo!