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Re: single ended EL84 in pentode mode

Posted by Eli Duttman (A) on March 15, 2007 at 15:52:37

In Reply to: single ended EL84 in pentode mode posted by eedork on March 15, 2007 at 10:28:50:

Matt,

Zobsky is correct. For maximum linearity in pentode mode, screen grid B+ should be regulated at a fraction of the plate B+. Also, it is desireable to maximize (consistent with proper operation) the voltage differential between plate and screen grid. With those facts in mind, I suggest you use the Russian 6p14p-ev/EL84M, which is a 7189 equivalent that's rated for 400 V. on the plate. Look at the linked 7189 data sheet. Please notice that the screen grid should be 250 V. above the cathode.

In order to get the B+ rail voltage up, SS rectification is indicated. Use series connected pairs of UF4007s. Scan the Bottlehead Forum archives for RRSF. You SHOULD use a RRSF. CLC filter the rectifier O/P. 15 muF. is enough in the 1st position. The power trafo will run cool and you can squeeze a little more current out when the 1st filter cap. is relatively small. Follow with a Hammond 159Q choke. The bulk of the energy storage goes in the 2nd cap. position.

Google search for Maida regulator. That type of circuit will allow you to power both the 7189 screen grids and the driver triodes from the same supply.

Speaking of driver triodes, the 'X7 has too much gain and a gm that's too low. IMO, a BETTER choice is the 5965.

Have you given consideration to the NFB ratio that will be used?

Eli D.

• 7189   (Open in New Window)

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