grid stoppers with mosfet AB2

[imaradiostar]

I'm toying with building an amp that'll run some KT88's in class AB2. I'll use mosfet source followers to drive the output grids. I'd guess there will be a voltage drop across the grid stoppers when drawing grid current. How will this effect linearity while drawing grid current? What about preventing RF oscillations or other ill effects that grid stoppers usually help with?

thanks, hope this isn't a stupid question. It's been a long day so stupid questions tend to happen.

jamie

[defaced]

If you'd like I'll take some measurements for you. I'm running 4x KT-88 with direct coupled MOSFET buffers and 2k2 grid stoppers. My ears don't hear anything degenerative at full volume except what is likely my ears folding inside out.

[Merlinb]

I'd guess there will be a voltage drop across the grid stoppers when drawing grid current.

jamie

They will limit the current and therefore the max grid voltage. With a large value it will be no different from using a tube driver. E.g., if you use a 1k stopper, then when the grid starts drawing 5mA it will drop 50V across the stopper.
You can use a low value stopper (maybe even 100 or 47 ohms), but if you run into oscillation then you either need to think about neutralization (fiddly), or larger stoppers. But if you use larger stoppers then there really isn't much point using the FETs in the first place!

(Incidentally, Class AB2 is only useful for triode amplifiers. There is (almost) no extra power and certainly no extra volume to be gained from pentodes, as they can already swing the anode down to <50V)

[Steve Conner]

The grid stoppers do hurt linearity, as has been said.

But on the bright side, they also help to stop your awesome MOSFET powah from combusting the grids.

[imaradiostar]

Thanks for the info- this all makes sense.

I wasn't as interested in AB2 for power as I was for smooth output distortion with no grid blocking and the ability to use a 12ax7 + mosfets to drive multiple KT88's without any trouble.

defaced- how does your amp sound? What kind of preamp and transformers are you using?

jamie

[defaced]

Right now (it's a modular design and my preamp is buggy and needs to be removed for diagnosis), the the power chain is an Antek 4TK400 PT-> FWB -> 330uf -> 5H -> 50uF -> then 10k/22uf on down the power line. The signal chain is a VHT Deliverance preamp -> Cathodyne PI -> MOSFET Driver -> KT-88 -> Hammond 1650T.

As for sound, well, it's a very tight metal amp. There are some things I don't like about, but that's why it's still a prototype. I swear I can hear a difference from when I didn't use the drivers to now, not so much at lower volumes, but at high volumes now it sounds like it stays together. Honestly, I will have to find my dummy load and get a scope and look at the output. I plan to do because the MOSFET driver takes up space I could use for other crap but if it sounds better, then it stays.