Which Component(s) determine how hard the PI drives the power tubes ?? 18-watt lite

[crunchman]

I'd like to lessen my power tube drive in an 18 watt lite IIb.

[MWJB]

Easiest method is to change from a 12AX7 to a lower mu tube, like 12AT7, 12AY7, 12AU7. These tubes draw more plate current, so ensure that your plate resistors are of an adequate W rating (2W to be safe). You might even like a 5751/12AT7 in V1?

Alternatively, you might like to try adding a NFB loop, by lifting the ground connection to the PI tail & the out of phase PI triode grid cap & inserting a load resistor (100ohms - 4.7K value isn't critical, it's the ratio). From the junction of the load resistor, cap & PI tail run another resistor to the hot OT secondary, start with one around 15-20x larger than the load resistor for an 8ohm speaker. This loop can be made switchable.

If the NFB loop doesn't grab you, going larger on the PI tail will make the amp a little looser, at 47K it's quite large already, maybe try 56K/68K? Don't enlarge the PI tail AND add a NFB loop, it will probably be overkill.

Lastly changing the 470K grid load resistors down to 220K will also make the amp a little less aggressive.

Do any mods one at a time, so you can reverse any you don't like.

[crunchman]

Thanks for the input mwjb, looks like I have 56k on the PI tail, are the 470k's PI or power tubes? I think nfb is out, its a pcb amp, schem listed on post below.

[MWJB]

470Ks are grounded from the power tube grids. Sorry, should have made that clearer.

[crunchman]

ok, next to the 8.2k grid stoppers, I'll have to order some parts and try. Thanks for the help !