Fixed bias, screen fed, self inverting output stage.

[MWJB]

Copied & paste from the "Theory & Design" thread, as I anticipated this as a harp-centric design...

"Well, we gave it a workout yesterday....it "works/functions", a little splatty sounding when pushed, but I think that achieving a better balance via the screen grid resistor value will help (using 3.3K at the mo', will go a little larger). Imbalance as it is, is "livable with", not as bad as amps specifically designed to skew the PI balance. Output & dynamics are greatly improved over cathode baised, self split, fed by the cathode resistor (which is obviously no great surprise). The application I'm anticipating for this is as a harp amp, it's looking promising, lots of volume before feedback compared to the other fixed bias, PI driven, amps we compared to, giggable for sure already, but I think that there's better to come with a bit more fine tuning, perhaps 90% there?

Today 01:03 PM #5 MWJB
grid resistors upped to 4.7K each (535v at plates, 2v dropped accross screen grid R's at idle), better balance, still some mild see-sawing in AC from one 5881 grid to the other (+/-) 20-25%, but balance is improved over a greater sweep of volume, up 5W to 41W (not clean RMS rating) at balanced AC. Further listening tests to follow, but turning out easier than anticipated! There's got to be a catch? "

[MWJB]

Screen grid resistors now down to 4.1K, 0.002uf cap to ground from 'in-phase' tube's screen. Balance is spot on at 30W output, satisfactory at 40W output. One of the catches is that as the AC imbalances, so does plate current under signal...& that it might take a little while to settle on screen grid resistor value, you need to dial in at the volume/power output the amp is likely to be used at, rather than just where you get the most power...because if you can only play at max power, it will make your ears bleed! If your amp is already louder than you need, then this kind of set up might not be for you...could give a smaller amp longer legs.

At lower volumes, the available sweep on the tone controls, allows you to restore playable tone, but as you get louder you need to back off highs & increase bass as usual, as things can get pretty aggressive.

Compared to 6G#, fixed bias, PI fed (which I previously thought was as good as high power, stage harp amps get), SFSI (Screen Fed Self Inverting) is much more touch responsive, punchy & louder. It makes the stock set up sound sluggish & stiff. I have a SPDT (switches hi voltage end of PI coupling cap for the OOP tube from PI plate resistor, to 'in-phase' tube's screen grid) so that I can switch between the 2...well, the 3 - it was only after I fitted the switch realised it was an on/off/on, so I get SE operation too.

[arnenym]

With a little help from my...

Here is the schematic for bassmans and super reverbs.. Just PI and Power tubees


I've not tried it on my BM but self split sure sound good on a 6V6 PP amp !