[flipper]

On Thu, 1 Dec 2011 08:43:21 -0800 (PST), Ian Iveson
wrote:

On Nov 29, 5:22*pm, John L Stewart John.L.Stewart.
wrote:

Here is a successful auto bias cct used by Kevin O'Conner, published in
one of his books on guitar amps. Output is 4X 5881 running into a
Plitron OPT (toroid). The cathode current sampling resistor is 10R
driving point B on the schema. Each 5881 has an indepedant bias set cct.
Point A drives the OP tube grid bias.

Full loop NFB from the OPT secodary to the Grid 2 of the input dual
triode as is customary on these kind of amp.

Cheers, John

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John L Stewart- Hide quoted text -

- Show quoted text -

Hi John, good to see you're still here.

I see this thread is quite a close replica of one a long time ago.

I don't see how the Kevin O'Connor circuit works. It looks like the
intention is to force symmetry of the AC component of the current
sense signal by clipping both positive and negative-going excursions,
but hasn't the signal already been smoothed? If so, then the one diode
limits the DC drift but doesn't prevent it, and the other diode is
redundant.

I wish I was better at understanding transistor and opamp circuits.
Looking at the other end of the servo, I don't see how that open
collector works. Doesn't it need a pull-down resistor to the negative
supply? And there's a resistor with no value shown...where's Flipper?
He seems to know about solid state stuff.

I think your assessment is correct. For the diodes to work as you
presume they were intended to you'd need to isolate them from the
filter. Like, for example, stick another 100k between them and the
-input so they can clip and then you average the result. Not saying
that would be ideal, just an example.

The circuit's application is also limited because it depends on bias
being in the right range for the diodes although one might be able to
fix that with a gain buffer off the sense resistor. Well, you could
vary the sense resistor too but you usually want that as low as
possible and a buffer amp let's you do that and then gain to suit.
(That's 'modern times' solid state thinking, you see. What the heck,
throw in an opamp, they're cheap).

I presumed the open collector went into something else because it
doesn't do anything all by it's lonesome, unless he's depending on
grid leakage for some nominal current. Looks upside down to me anyway
because I'd end with a low impedance follower.


I guess Morgan got carried away with the elegance of his idea.
Clipping both ways isn't so clever, but it's a better solution I
think. The more the AC is chopped, the less the drift caused by
distortion.

Toroid in a guitar amp?

Ian