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kg
2/11/2002 2:50 PM
weekend discoveries
a) partition noise in pentodes can be greatly reduced by decreasing cathode current.
b) a couple of small ferrite beads work very well in place of grid stoppers.
c) regular old 1n400x diodes (reverse biased) work very will in place of rk/ck in the cathode circuit,
approximating a fixed bias stage... difference is no time constant in the cathode circuit which
translates to quicker recovery from transients. LEDs also work very well and offer greater vf.
d) you really have to try inductors in a tone control situation. very sweet tones.
e) when you're diy, you can eliminate the "grid leak" resistor from the input stage of your amp,
and instead use the circuit inside your guitar to provide the necessary path to ground.
that's all for now. more later.
ken
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Dr.dr. KT88
([email protected])
2/11/2002 3:17 PM
Re: weekend discoveries
e) when you're diy, you can eliminate the "grid leak" resistor from the input stage of your amp,
and instead use the circuit inside your guitar to provide the necessary path to ground.
Hey KG, what, if any effect is there on tone of gain?
D.KT88
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kg
2/11/2002 7:27 PM
Re: weekend discoveries
in my particular circuit, it made a noticable difference in gain, and in LF output. the thing that
makes my particular circuit different is that i have a 6s45pi input stage, running at 35mA, and that
little firecracker needs a low DCR g1 circuit. so i had only 100k from g1 to ground in the amp.
such a low impedance input rolled off gain and LF. when i measured the DCR of my guitar pup, it
was low... around 20k iirc. i rarely back off the volume knob (which is a 500ka pot), so i just
eliminated the 100k and used the guitar itself as the return path to ground.
i suspect that the average 12ax7 gain stage with a 1m input impedance would not see much
change.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=167792
2/11/2002
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ken
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Shea
([email protected])
2/11/2002 4:13 PM
Re: weekend discoveries
"d) you really have to try inductors in a tone control situation. very sweet tones."
I'd love to. Have you had a chance to dig up your list of the inductances of those little trannies?
When you talk about using diodes in the cathode circuit, are you referring to preamp stages or the
output stage, or both? In a preamp stage, how do you translate the formulae for calculating gain,
output impedance, & frequency response when you're using a diode in the cathode?
Do you have a mini-schemo for the diode-in-the-cathode trick?
Shea
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Max
([email protected])
2/11/2002 4:32 PM
Re: weekend discoveries
Hi everybody!
Ken,my old boogie MKIIB was using rectifier diodes on the cathodes!But i don't know why,hence
the amp was fixed bias.May be like thoses fenders,mix between cathode and fixed?
Best regards.
Max.
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Dave H.
([email protected])
2/11/2002 5:13 PM
Re: weekend discoveries
I posted this but it was buried in Mook’s thread. I think the gain, output impedance and frequency
response will be the same as for a bypassed cathode resistor with an infinite value cap. i.e as if the
cathode were grounded to AC.
Dave
>>I haven’t tried it myself but you could get maximum gain and eliminate the cathode RC by using
a couple of diodes in series with the cathode to bias the 12AX7s. It works for power tubes too with
a zener. If your B+ is a bit hot you can knock a few volts off it and get a free bias supply. It can be
made adjustable by putting a pot across the zener with the wiper connected to the grid resistors.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=167792
... 2/11/2002
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Page 3 of 4
kg
2/11/2002 7:41 PM
Re: weekend discoveries
mini schemo:
b+
|
/
\ Rp
/
|
|
p
-------g
|
k
/
|
\
v
gl /
|
|
v 3 x 1n4007, cathode band towards ground.
|
|
gnd
v
|
gnd
When you talk about using diodes in the cathode circuit, are you referring to preamp
stages or the output stage, or both? In a preamp stage, how do you translate the
formulae for calculating gain, output impedance, & frequency response when you're
using a diode in the cathode?
so far, i've only used it in the preamp section.
as dave points out, the diode presents a very low impedance when forward biased--i.e. conducting-yet it retains the ~0.6v drop across it, so you don't need to use a bypass cap to increase gain. for
more bias voltage you simply series up more diodes. the diodes are very quiet when operated in
this way.
the only downside that i can see is that you have eliminated the DC NFB inherent in the rk of your
typical cathode biased stage, so that different tubes will have operating points that are more
disparate when diode biased... regardless of whatever tube you put it, in will have the same
effective -vg1, which may or may not put the vp where you originally designed it. on the other
hand, this merely serves to increase the differences between tubes as you swap, which can be cool
in its own right.
so far, i've used it on an ef86, a 12ax7 and a 6c45pi.
ken
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aron
([email protected])
2/11/2002 5:14 PM
Re: weekend discoveries
Cool stuff Ken!
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=167792
... 2/11/2002
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B) is used by Carvin feeding their input grid.
C) I think this is something that Jack Orman used in the FX community to bias FETs. When I put it
in, I thought I heard a more "animated" tone. I need to try this.
D) Yep! My old Pignose Crossmix had inductors for the tone controls - especially the mids.... very
sweet.
E) does anything bad happen when you unplug the guitar and for some reason don't have a good
ground contact with the switch in the input jack?
Thanks,
Aron
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kg
2/11/2002 7:44 PM
Re: weekend discoveries
E) does anything bad happen when you unplug the guitar and for some reason don't
have a good ground contact with the switch in the input jack?
actually i use a nonshorting 1/4" jack on the front pannel, and when i disconnect the guitar the
tube tends to pinch off its plate current, pretty much silencing the amp. i have seen no ill effects
due to this so far.
different tubes may react differently to this treatment. i'm using a 6c45pi input tube; if this one
doesn't go batty (running at 150vp, 35mA) i don't think any of them will.
ken
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=167792
... 2/11/2002