I used a hammond 1645 output transformer (30w 5000ohm plate to plate) my B+ around 320
VDC.
I built mine like the 1956 RCA manual (page 276) with a few execptions:
my power transformer was a 240AC full wave bridge rectified by 6A10. I used two 193d
inductors (hammond 8H@150mA )and four 33uf 630v poly caps (solen) and .1 uf on the end of
the power supply. I use a 5vdc industral switcher tweaked to 6Vdc for the heaters. I split chasis
my amp the power supply is on one chassis and the amp on another. This lets me tweak the
power supply separately from the amp and it helps with controlling the magnetic noise the
power supply emits . I use jones connectors for my power interconnects.
In most of the tube push pull amp the output stage acts really like a buffer, the voltage gain is
happening before it in the tube predriver stage before the pase split . In the 6as7's case,
needs a really high impedence input with about 60-80% of what signal you want the output
comming out of it. and the grid loading the 6as7 reflected back to the phase splitter should be
isolated by some type of balanced driver stage. And last, it needs the medium-low
impedence load of 2500 ohms per plate to establish the idle current without causing
transformer saturation. I go about 2-3 times over on power rating of the output transformer to the
circuit to achieve low transformer saturation and better bandwidth. its about 12 watts of amp.
I was going to expirament with using a signal transformer 600:50k ct (sowter 9062) grounded
center tap sec. and two stages of john brooskie's akido per signal path (one two tube 6sn7 for
the signal + and one two tube 6sn7 for signal - ) as a balanced amp. Otherwise, to cut parts
count down, a simple 6sn7 or 6sl7 common cathode will fit the bill. If you dont have any of those
octals try 12ax7, 12ay7, 12au7 9 pins.
this is the link to RCA amp above:
http://www.kbapps.com/audio/tubemanual/276.html
"Engineer" wrote:
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