Hey Pat, I just tried a 6SN7 as an input stage
to my previous 12AT7 concertina: looks so good at 100KHz
I can spot the small notch in the peaks that my signal
generator has. The total gain when using 480V B+ is
exactly 15, in other words 3V in to get 45 V peak to peak
output. I'll try a higher gain triode next and see how things
shape up.

Just to confirm things, if my datasheet for the
4X150 wants 48V peak grid drive, then that is equal
to having two 48V outputs from the phase splitter?.

Regards
Mark Harriss

Ross Matheson wrote:

Mark Harriss questioned:

: I mean if I don't need the extra power they provide
: what would make them advantageous?.

Loosely bouncing off this to where the RSGB 3rd ed. is talking about SSB
linear amplifiers, and Choice of Class of Operation, p.322, an approach I like

"Before proceeding to a practical design, the question of whether the
amplifier is to be operated in class AB1 or AB2 must be resolved. An advantage
usually claimed for class AB1 is that, as grid current does not flow at any
time, it is possible to drive the stage with a voltage amplifier incapable of
delivering power, and that a regulated bias supply is un-necessary. There is
little or nothing in the first point; any driver, no matter how heavily
swamped by a resistive load, ought to be able to deliver the fraction of a
watt required to drive the amplifier to full input in class AB2. The second
point is valid, but potentially dangerous. In most cases it is virtually
impossible to guarantee that the grid will not be driven positive from time to
time, and when this does happen the distortion will be excessive. Even if the
intention is to operate an amplifier exclusively in class AB1 it is desirable
to design the driver, input, and bias circuitry as if operation in the
grid-current region were intended. The extra complication in construction will
be a small price to pay for effective insurance against high distortion if the
stage is slightly overdriven."

Radio Society of Great Britain "The Radio Amateur Handbook" 3rd Ed. '61-'62.
__

OK, they are talking about an RF Single SideBand linear amp here - but I
thought it was worth sharing, in the context of designing amps with RF valves,
and possibly worthy of a considered transposition to audio amp driver design.

Perhaps someone else may have already commented but I think the distortion
one worrys about in the case of an SSB Linear Amp is the frequency sidebands
developed out of channel as the concern here. That is a serious problem for the
RF guys so that it is a primary consideration for them. Cheers, JLS