[Engineer]

On Feb 24, 3:04 pm, "John" wrote:
RCA had a decent sounding amp in their manual. It has
6sn7 line stage with 6sn7 splitter and one 6as7.

I have bult a 6as7 p-p, otl and headphone amp.
and it makes a great preamp regulator. I use them when I make studio
gear.


I'll take a look... missed that one!

Anyway, back to the power transformer recycling notion mentioned
earlier...
I drew some load lines on the 6080 plate chcs to no great profit,
except to show over 100 VDC bias in the 250 volt and 30 mA operating
region.
So, I ran up a breadboard trial today with a 6080 feeding the primary
a 240 VAC CT MT (CT to B+, plates to either end) with a 13.8 VAC
secondary to 8 ohm load resistor, a "MT/OPT".
Resulting p to p impedance: 2420 ohms (see above.) I used a 1.8 kohm
WW resistor (as it was to hand, close enough) as a common cathode
bias. How to drive it? I did not feel like building a 6SN7 phase
splitter, nor taking a feed from another PP amplifier with the OPT
tubes removed (too many B+ issues), so I just used a backwards
connected PP OPT (the Hammond 125E) as a driver transformer and fed it
from the speaker output of a another amplifier. B+ and heater was
from a Heathkit bench supply. Sig. gen. to driver amp, of course.
First the operating points - no drive

B+ I(total, 2 sections) V cathode bias
150 30 mA 52.6
200 40 69.8
250 50 87.1
300 60 105.3

The last point started to look promising, but read on...
Then, at 300 VDC, I tried to drive it. BTW, the speaker level drive
to the 125E "OPT" remained undistorted (as checked on 'scope) over the
whole experiment.
I started at 400 Hz.
Very bad results!
The maximum visibly undistorted voltage I could get across the 8 ohms
load on the "MT/OPT" secondary was 2 VAC, for an O/P power of 0.5
watts. This was sustained from about 40 Hz to 5KHz.
I then added bias from some batteries between "MT/OPT" CT and ground
to get it a bit more into class AB. I took the bias up to 128 VDC
(fixed plus cathode R drop.) Quiescent total current dropped to 43 mA
but no more power achieved.
Tried 4 ohm load... worse. Max volts 0.6 VAC for less than 1/10 of a
watt.
Stopped for coffee and contemplation... and to send this in.
I might be worth trying the 125E OPT properly, but I must admit that I
am losing any affection I ever had for the 6080 outside of use as a
series power regulator- it sits there like a miniature two-bar
electric fire laughing at you as it delivers half a watt! g Of
course, not fair to judge it with a "MT/OPT" kludge!
If I get any further I'll post results.
Cheers,
Roger