6L6 speech amp issues

[Steve W8TOW]

Kinda wondering,
I just did some mods to my speech amp (this is a "HIFI" SA by heath)
that now the output "common" leg isn't grounded...and I am running
balanced output to the modulator....
THis amp had a neg feedback circuit from the 8 ohm tap back to the
cathode of a 6SJ7....now, with the "common" leg ungrounded, there is
nasty HF oscillation....
I removed the feedback and all works well now....
any thoughts?
Also, anyone tried triode connected 6L6's as drivers?
73 steve 8tow

[Bacon, WA3WDR]

Are you using the 25V or 70V output as the balanced output, or maybe the 16 ohm?  If you have a tap that is in the center, maybe you should ground it, and do the feedback more or less like before.  But if you are giving more voltage to the feedback network, you would want to increase the series resistor value, and if there's a capacitor across that resistor, decrease its value.

Do you use transformer coupling in the modulator too?  Maybe you should just re-ground the output connection as before, and let the inter-cabinet line be unbalanced.  It shouldn't matter if there's another grid transformer in the modulator deck.

I never tried 6L6s as triodes.  I did try push-pull 6L6GC with unbypassed resistors between the supply and each screen grid at about 350V DC plate and screen supply.  5.1K to 5.6K in series with each screen made them much more linear.  There is a little reduction in screen voltage at audio peaks this way, so increasing the screen supply voltage about 50V, and using slightly more control grid bias, would be good.