[tubegarden]

flipper wrote:
Just wanted to mention some bias circuits I'm working with in case
anyone is interested and/or has comments.

The problem statement is I want as close to a perfect current balance
through both sides of the OPT primary as I can get.

Approach 1 is to opamp servo the current. Bias on one side is set by a
conventional negative voltage source to the grid. Cathode current
though each tube goes through individual 10 ohm sense resistors that
are filtered and opamp compared, with the opamp's output providing the
(negative) grid bias voltage to the second tube. So, tube 1 is
'manually set and tube 2 is servo 'd to follow. Spices great.

It does require opamp supplies but that's not a problem, per see; just
more stuff to make. With low ohms sense resistors opamp offset can
become significant (I selected the OP07C for that reason). Bias
voltage must also fall within the opamp's voltage range, at least for
direct drive, which can be a problem if a large negative bias voltage
is needed, but one could probably get around that with a voltage
offset and the opamp driving a +- deviation around it.

Next...

The original idea used in my 6EM7 PP breadboard was a cathode current
mirror. Tube 1 goes into a cathode resistor, pretty much as normal,
which feeds on into a current mirror with the other side of the mirror
feeding the second tube thereby forcing it to the same current. Both
are bypassed so it's the average (idle) current.

That one behaves much like a conventional bypassed cathode resistor in
that it charges up just like one would expect under positive grid
drive conditions. The advantage, of course, is that the OPT primary
currents are tightly matched.

That one, btw, forced a leaky 6EM7 to 'behave' when on the current
mirror output side as it simply forced cathode V to whatever was
necessary to overcome the grid leak.



Which comes to the third version because I'm looking at doing positive
grid drive on a triode strapped 6GK6 PP stage. So, I replace the tube
1 cathode resistor with a P channel MOSFET acting as a voltage source
to tube 1's cathode. I.E. MOSFET source to the cathode and fixed V ref
(bias V) to the gate with the drain going on into the current mirror
as before (mirror bypassed)

In this version tube 1's cathode is fixed at Vbias by the MOSFET and
cathode V on the other tube is set by the current mirror (that NPN's
collector) forcing it to match current to tube 1.

Advantages are it's simple with no negative supplies, no opamp
supplies, no opamp voltage compliance problems and it doesn't 'charge
up' like the previous current mirror design The downside is, like any
cathode bias arrangement, it eats some B+ voltage.

A note on charge up. In fact, both the opamp filter caps and MOSFET
current mirror bypass caps do 'charge' (to the average current at the
time) but they charge the same on both sides so the bias voltage (on
average) doesn't 'change' (in both cases any asymmetry causes an
initial upset but both types force a re balance with the time constant
set by the bypass/filter RC.)

So, what do y'all think?


Hi RATs!

http://www.tubecad.com/2005/May/blog0046.htm


I never went beyond Blumlein's Garters.

They seem to work OK

Happy Ears!
Al