tubegarden wrote:
Hi RATs!
So, Mr. Numbercruncher, are you going to try Mr. Blumlein's idle
current balancing "garter" circuits?
Or are you so sure we are crazy you can't waste a few hours hearing
some good news?
He did document the circuit before WW2, you know, just not everyone
thought better sound was worth the huge investment, in your case four
resistors and two caps per channel 
Happy Ears!
Al
PS Yes, I think my time and the solder are free
The Garter circuit has Rk for each tube twice the normal value and cross
couples the bias R to try to balane Ia of each OP tube. Its a brave
attempt to equalise Ia dc for each tube and it is **slightly** better at
doing this job than the conventional method of using one Rk and Ck for
each tube. The better sound you hear is due to more equal Idc in each
1/2 of the OPT primary. Where you are supposed to have 50mA per tube and
have 45mA in one and 55mA in the other, you have 10mA of dc diference,
and I leave you to work out the dc field strength in Tesla and maybe it
makes the OPT saturate with ac at a higher F than it was designed to
cope with. Ac Saturation F is proportional to the applied ac voltage
amplitude as well as inversely proportional to frequency, and core Afe
and primary turn numbers. More turns and iron and less applied acV =
lower Fsat. With dc, the saturation is worse with more turns unless the
iron is gapped, or low µ iron. Pardon, but I am adressing the issue far
too briefly, but many OPT manufacturers only pay lip service to the
rules so clearly spelled out in RDH4. Most OPT used in many commercial
amps have been designed by accountant bean counters to reduce the copper
and iron to the barest minimums, Quad-II and many Leak models are
examples. RDH4 gives a formula for core caused distortions in a dc
balanced OPT but hasn't got much on OPT with specific dc offsets.
Tremain's Audio Encyclopedia has more to appall the mind about THD/IMD
with increasing amounts of dc offset. I've often heard Quad-II with
hopelessly unbalanced Idc, 90mA in one KT66, 40mA in the other, and real
crap sound. Placing a Garter circuit in there would improve things, but
you'd loose 30V of B+ headroom, and the Garter won't help class AB op
much. So there is better than the Garter. CCS for each cathode plus
something to limit Ek rise during class AB, or my system of Dynamic Bias
Stabilisation at my website, etc.
In one pair of Quad-II amps I re-engineered I have cathode bias with
each cathode with Rk = 470 ohms plus 1,000uF bypass. There is a
transistor LTP to drive a pair of red LED to indicate dc balance and an
adjustable trimpot to balance the applied slightly negative grid bias to
each OP tube. There is a small circuit board to do this monitoring job
plus active shut down protection if Ia goes too high for longer than 4
seconds. I can say to you this works better than thre Garter ever will
to ensure the best sound and to give amp owners peace of mind about the
condition of their amps and output tubes. As long as each LED shines
with equal brightness, Idc balance is very close. And if OP tubes drift
or a fault develops, there is immediate indication that something is
wrong with the biasing, and blooming bad biasing is the most common
problem in tube amps.
Patrick Turner.