Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sonny ReVerb on October 10, 2017, 04:21:56 pm

Title: MOSFET screen regulation
Post by: Sonny ReVerb on October 10, 2017, 04:21:56 pm

I'm thinking of using 7591a power tubes with a 330-0-330 PT. That should yield a B+ of 450-460VDC (SS recto). The datasheet for the 7591a says plate 550V max, so that should be fine, but screen 440V max. The 'Typical Operation' section gives an example of plate 450V / screen 400V. If I'm using a choke, the screens will be close to the plate voltage.

Can I use RG's MOSFET B+ reducer (http://www.geofex.com/Article_Folders/mosfet_folly/mosfol4.gif) to drop the screen node (B+1) 50V and leave the plate voltage as is? I read some old threads about zeners, etc. I also saw one where Steve mentioned 'fixed' VVR in the screen node. I haven't seen any of these implemented in any schematics, though. Anybody tried any of these methods? Any recommended MOSFETs and/or zeners for RG's design?

Title: Re: MOSFET screen regulation
Post by: PRR on October 10, 2017, 04:38:32 pm

> Can I use RG's MOSFET B+ reducer to drop the screen node (B+1) 50V and leave the plate voltage as is?

Not as-drawn. It reduces everything.

The advantage of a choke is low voltage loss. Here you seem to want to lose voltage. IMHO, lose the choke. 2K screen feed (perhaps 1K common to a filter cap and individual 1K in each screen lead) will drop 20V at idle, putting you on 440V; and 60V at max, reaching the typical 400V needed for full power.

Title: Re: MOSFET screen regulation
Post by: 92Volts on October 10, 2017, 04:57:41 pm

I'd agree with PRR to just use a resistor. The "problem" is varying voltage to your screens (likely doesn't matter) as well as your preamp, which might matter depending on your preamp and power supply design.

If you decide it needs consistent voltage, zeners should be fine. I think that MOSFET is used because it's dropping voltage for the whole power supply, which involves a lot of current. It's easier to get beefy MOSFETs than zeners, but for screens or screens+preamp only, that's not as challenging.

Title: Re: MOSFET screen regulation
Post by: kagliostro on October 10, 2017, 04:58:19 pm

BTW ....

http://el34world.com/Forum/index.php?topic=20467.msg215610#msg215610 (http://el34world.com/Forum/index.php?topic=20467.msg215610#msg215610)

at that time PRR was so kind to sketch this for me (but there the difference between Plate and G2 voltage was greater)

http://el34world.com/Forum/index.php?topic=20467.msg217908#msg217908 (http://el34world.com/Forum/index.php?topic=20467.msg217908#msg217908)

(Many thanks again PRR)

Franco