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RE: Pentode Driver Comparison 6C6, EF37, 717A, etc.

Posted by rtsang (A) on February 14, 2009 at 05:13:32

In Reply to: RE: Pentode Driver Comparison 6C6, EF37, 717A, etc. posted by lovetube on February 14, 2009 at 02:59:04:

I have tried various plate resistor. From 220k to currently 47k. In my experience, for a pentode, the plate resistor only affects the gain and output impedence, not the sound (assuming the impedance is enough to drive the next stage). Thorsten once wrote that chose the lowest plate resistor that will give you enough gain. The output impedance of the pentode driver is nearly equal to the plate resistor. A 47k resistor allows me to have just enough gain to use a passive preamp. I think the amp would be -1db at 20kHz on 1W output. Mind you, that is a PX4, much easier to drive than a 300B.

I totally agree with you that the screen voltage has a immense impact on the sound. Look at the plate curves, an increase in the screen voltage will push the knee of the plate curve to the left (The area which the plate curve turns 90 degrees). So the plate voltage will not swing into the distorted area of the "knees" of the plate curve.

I also think that the plate current will affect the sound, mainly the current delivering ability to drive the miller capacitance of the SET.

In my EF37A driver, I tried to maximize both parameters, which in the end were limited by the max. plate dissipation.

The 717A has a better current handling ability, but the maximum anode voltage was lower, limiting the maximum voltage swing. For a 300B, we need a +/- 70V peak to peak to drive it to max power in A1. The 717A maximum plate voltage is only 180V, where as the EF37 is 300V. On the other hand, the max. plate current of the 717A is 7.5mA where as the EF37 is 6mA.

The WE310A or 6C6 are even worse than the 717A or EF37.

My current favourite is the EF36/EF37/EF37A. Not just the sound, but the red colour coke bottle with a grid cap on the amp just look nice. Some day, I will try and find the silver or gold coated version from Mullard, just for the looks!

My next try will be on using those super pentodes like E810F, E180F and D3a. I am just a bit intimidated by their high transconductance and their tendency to oscillate.

Anybody has any experience on these "super pentodes"?

P.S. The C3m and C3g family is another choice, only that I do not have any loctal sockets. Yamamoto recently use the C3m as the driver of their new 300B amp.

Raymond

Edits: 02/14/09 02/14/09

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