"Patrick Turner" wrote in message
...
wrote:
Thats what I wanted to hear!. As long as the AC is balanced. (most
important)
I wasn't sure if the DC should be balanced also.
If the LTP is a twin triode, or a low µ triode like the 12AU7, 6CG7,
6SN7,
then you should find the dc balance very close with most tubes that
test well.
Maybe you have a 6SL7, or 12AX7, 12AY7, 12AT7,
and the gain is higher, so expect more difference in Ea for the
two triodes.
Usually is never more than 10V difference.
The biasing of each LTP triode shouldn't use greater than 470k grid
R
lest the tiny grid current that flows develops a voltage which can
effect the
quiescent anode volts, ( Ea ).
Direct coupled LTPs like in a Mullard 520 etc have the first triode
direct coupled to the anode of the input tube, then a 1M or more
is taken to the second triode grid which is grounded via a 0.1uF or
suchlike.
Sometimes there is a slight dc voltage across the 1M, so this dc
unbalances the
LTP.
BTW, for the Mullard circuit to be properly ac balanced
while a common cathode R of say 22k is used from cathodes to 0V,
the usual way to achieve balance is to adjust the value of the DC
carrying RL
to each triode until balance is achieved.
This usually upsets the dc balance which does not matter much
in most amps because the LTP will clip well after the output stage
clips.
But the LTP with a CCS allows much easier setting up with equal RL
values
which are cheaply available at 1% tolerances or better.
Its the RL values which automatically balance the LTP with a CCS
that makes it such an attractive gain block.
Just make sure you have a high impedance meter for
measuring the outputs and dcV values; it should be
better than 9M input Z to the meter.
Most digital meters are like this and read ok between 20Hz and 1
kHz.
Above 1kHz, digital meter signal volt readings roll off due to the
capacitance
of the meter.
Patrick Turner.
I've nearly completed a "spares box" 10 watt amplifier design (EF86,
6SN7, 2 x 6V6) that uses a "Mullard 5-10" EF86 circuit (120K, 2,2K,
100 ohms, etc), directly coupled (108 VDC) to g1 of the first 1/2
6SN7, with a CC circuit in cathode. The latter is an MJE340
transistor with a 5.6 Kohm emitter resistor and a +25 volt (approx) DC
reference to the base for 4.3 mA CC (2.15 mA for each 1/2 6SN7 with
33Kohms on each plate.)
DC balance is close to perfect and drive to 6V6's very balanced.
Getting 10.4 watts RMS max but still tweeking things. I plan to post
a schematic on the binaries later for comments/criticism.
Cheers,
Roger
Thanks
"Patrick Turner" wrote in message
...
wrote:
Anyone know of design info for this type of splitter.
I have one up and running on the bench using a 12AX7 @ 300V &
1.2mA each
side
but I can't seem to make it DC (150V / 168V) balance. Should I
worry
about
the DC balance if the AC
is balanced? In info apprecaited.
Thanks
RonL
Please find an example of an LTP with CCS in the amp at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm
If the RLs to each side of the pair are equal, output signal also
will be.
But the dc anode voltage won't ever be exactly the same, since no
tubes
are
exactly
matched for bias conditions.
The tubes could be quite different, one could be a 1/2 12AU7, the
the other a 1/2 6SN7 triode and as long as the RLs are equal
and the CCS is an extremely high resistance, such as in the
schematic I have refered you to,
ac balance will be ok even though dc balance is not good.
But using dissimilar tubes to each 1/2 of an LTP
results in 2H distortion that is normally cancelled away when
each
1/2 is matched to the other in terms of Ra, gm, and µ.
Patrick Turner.