Quote:
Originally Posted by Ian Iveson[_2_]
On Nov 29, 5:22*pm, John L Stewart John.L.Stewart.
wrote:
'Ian Iveson[_2_ Wrote:
;943887']Following the 10c thread, I checked my slightly musty copy of
the
quintessentially English Morgan Jones, "Valve Amplifiers", Newnes
1999.
I was curious because I thought his circuit uses a zener to restrict
the upswings of the signal from the cathode-current sensing resistor.
Using a fixed reference isn't ideal if the bias current is adustable,
because then the zener clipping point won't necessarily be correct. As
it happens, he doesn't use a zener. This is what he does instead.
First, he uses a 1R current sense resistor at the cathode of the
valve, followed by an opamp set to a gain of 100. This minimises the
effect of this part of the circuit on the operation of the valve. Each
10mA of cathode current thus results in 1V output from the opamp.
He generates two reference voltages, one twice the other, from a
convenient +ve voltage using a pot and two fixed resistors. The upper
reference is used together with a transistor/diode clamp to limit the
upswings of the signal from the opamp. A low-pass filter then smooths
this to a steady DC before it is compared to the lower of the two
reference voltages, using a second opamp.
The ensuing difference signal is then used to alter the grid voltage
via a second transistor.
Because the signal to the filter is always limited to twice the set
bias level, its AC component remains more or less symmetrical when the
valve cuts off during AB operation, so it smooths to a steady DC
level. With a triode, the servo will tend to very slightly raise bias
current during extended periods of AB, which is better than reducing
it as others tend to do.
That's two opamps, two transistors, a diode, and a bunch of resistors
and caps to maintain the fixed bias of one output valve. Morgan
doesn't use the circuit in any of his amps AFAICS, so there's no
example of how the scheme is extended to the one or more other output
valves.
It wouldn't be necessary to replicate the whole circuit for the other
valves, because they need to be set at the same level, to balance the
transformer current. The set reference voltages could be made common,
and the rest of the circuit replicated. Alternatively the unclamped
signal could be smoothed and used as the reference for the other
servos, which then wouldn't need clamps.
It's possible that reading the above will not result in a working
diagram in your head. The actual circuit does work. Buy the book. It
is British and we are short of money.
Some thought, in the details, must be given to stability. Each servo
is a feedback circuit, and all are related in various ways via various
delays to themselves and to each other.
Ian
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 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
|
Hi Ian & Others- Yeh, I'm back again after another 5 year stint in hitech sales. But I guess for the last time at my age!
Kevin O'Connor is adventurous & perhaps does some things others may not approve of. Thus a toroidal OPT in a guitar amp. No problem with servo bias holding the lid down.
This one not as elegant as that found in the Brook, easily the best I've seen in my very long career. It is unique & sure makes a lot of sense. How else to get 30 Watts at low distortion out of a pair of 2A3's in AB2?
I got interested in servo bias some time ago while doing my thing with 6AS7/6080 PP circuits. Some may remember the bootstrapped driver schemes with & without the UL OPT. But the heat from the cathode bias resistors under the chassis is a serious problem. So lets try somekind of servo bias. Perhaps this coming Winter.
I've been busy on the bicycle (like Patrick), more than 4000 Km since May 1st. And a lot of physical labor here on the acres. I would rather be outside than slaving over a hot soldering iron!
For anyone who would like a complete schema of the Kevin O'Connor guitar amp referenced email me direct as follows-
jh dot stewart at sympatico dot ca
Gotta go now. Today its time to service the summer equipment & put it away till Spring! And here is that easy to drive 6AS7/6080 amp for the curious.
Cheers, John