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Friday, March 15, 2002 12:03:32 PM
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dl
3/14/2002 2:10 AM
Diode instead of bypass cap?
Can a diode be used instead of a cathode bypass cap?
Would it make more gain?
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kg
3/14/2002 12:48 PM
Re: Diode instead of bypass cap?
use the diode forward biased... ie put cathode to ground, anode to tube cathode. string more than one in
series to adjust forward voltage drop (Vf). use LEDs for larger Vf... it goes proportionally to wavelength, so
infra-red diodes have the lowest drop, then ROYGBIV after that. orange have about 1.9Vf, red about
1.8Vf. regular Si PN diodes like 1n400x have about 0.6Vf; germanium about 0.3Vf.
use this diode string in place of a resistor/cap combo. note that the bias voltage will NOT change with
different tubes, so the operating point will shift as you tube-roll.
it will give you about the same gain as a well-bypassed cathode resistor/cap combo.
ken
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Henrik ([email protected])
3/14/2002 1:21 PM
What's the pay-off?
I'm just curious as to the effect of such a substitution.
Cheers!
Henrik
From:
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kg
3/14/2002 2:06 PM
Re: What's the pay-off?
a) no RC constant in cathode circuit. no stored charge effects to slow recovery from transients and
overload. no shift in bias point as stage is driven into heavy even harmonic distortion. full gain from DC
up--no pole/zero frequency response deviations or phase shifts. no large value electrolytic cap to dry out
in 20 years.
b) vg1 bias point remains exactly the same regardless of which tube is plugged into socket.
c) eerie glow eminates from beneath chassis if LEDs are used. provides visual indication of cathode
current.
d) no one else really uses them, so they're cooler by default. :)
ken
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Friday, March 15, 2002 12:03:32 PM
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Henrik ([email protected])
3/14/2002 2:17 PM
Re: What's the pay-off?
Thanks Ken!
What happens to bass response with this setup, and how do you make adjustments to it, which would
otherwise be dealt with through changing cap-values?
H.
From:
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kg
3/14/2002 2:27 PM
Re: What's the pay-off?
h,
as far as tweaking frequency response via different Ck values, you can't do it!
if you want to roll off your LF do it at the coupling cap. if you want to add LF shunt some HF to ground, or
use some series inductors.
ken
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Gus ([email protected])
3/14/2002 2:41 PM
Re: What's the pay-off?
I have used an led in a tube microphone circuit. Its a green High Brightness about 1.85V drop. It sounds
ok its a test microphone. I will have to do a before and after recording to Know if it sounds better or worse.
From:
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Scott Swartz ([email protected])
3/15/2002 6:18 PM
Re: What's the pay-off?
no one else really uses them, so they're cooler by default.
Not to be a smartass, but because people might be intersted in the schems, I know I saw a Marshall
schem that used a diode in place of Rk, I think JCM800 or 900, something later like that.
This was written up in the TubeCAD journal too, I don't remember which issue, it could be found.
Don't you love it someone can only remember part of the story? ;)
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Scott Swartz ([email protected])
3/15/2002 7:06 PM
Re: What's the pay-off?
Friday, March 15, 2002 12:03:32 PM
here's a link, they probably used it elsewhere too:
http://www1.korksoft.com/~schem/marshallamps/2205_pre.pdf
From:
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StevieP
3/14/2002 2:19 PM
Re: Diode instead of bypass cap?
use this diode string in place of a resistor/cap combo.
I don't know a lot about tubes so maybe these are pretty basic questions:
If you replace the resistor/cap combo, how does the circuit pass ac signals?
Don't you still need the cap in there?
How does the diode string provide the same resistor based current control for Q point set-up?
From:
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kg
3/14/2002 2:37 PM
Re: Diode instead of bypass cap?
i don't mind basic questions!
If you replace the resistor/cap combo, how does the circuit pass ac signals? Don't you still
need the cap in there?
the diode has a very low bulk resistance when it is forward biased (i.e. conducting). as a result, as long as
current is flowing through it, it acts a bit like a voltage source... doubling or tripling the current through the
diode will change the forward voltage drop VERY little. in essense, it does not need to be bypassed.
there is a bit of signal present at the cathode but it is in the mV range--bypassing with a cap in my
experience has been unnecessary and superfluous.
in terms of setting the q point, hook up your stage normally, with Rp from plate to b+, Rg from g1 to gnd,
and substitude various strings of forward biased diodes into the cathode circuit. measure the plate voltage
at each iteration, and if you're looking for headroom pick the one that places Vp at B+/2 (or just pick the
one you think sounds best). try swapping a few different tubes in the socket, and see how much the plate
voltage changes. you may need to pick a middle of the road value to compensate for tube characteristic
variations.
if you need a higher bias voltage, you can also consider zener/avalanche diodes, but those must be used
REVERSE biased (cathode band to tube cathode) to net the rated breakdown voltage. forward biased
they are just like any other si pn junction diode, i.e. ~0.6 Vf.
ken
From:
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Shea ([email protected])
3/14/2002 7:10 PM
Re: Diode instead of bypass cap?
"if you're looking for headroom pick the one that places Vp at B+/2"
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Friday, March 15, 2002 12:03:32 PM
So Vp = Vak ?
Shea
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kg ([email protected])
3/14/2002 7:27 PM
yup, Vp is voltage of plate wrt cathode, aka Vak.
.
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kg ([email protected])
3/14/2002 10:00 PM
here's a nice little chart
you're looking for the steepest slope.. "red" seems to have it.
ken
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