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Man o'Blues
Page 1 of 3
([email protected])
10/9/2001 1:50 PM
12AX7 Gain and Phase
I'm interested to know how I can rig two triodes for maximum output and phase reversal.
I tried the parallel config, which flips the phase but doesn't seem to get me that much more gain
than using a single section. Can anyone tell me how to tweak this configuration for more output, or
if there other ways to hook up two sections to achieve max gain and phase reversal? Thanks!
From:
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KB
([email protected])
10/9/2001 2:42 PM
Re: 12AX7 Gain and Phase
Uncreasing the plate load resistance and decresing the Cathode resistance will give you more gain
and of course bypassing the Cathode resistance will increase it further. Plate Voltage is considered
but trying to get to much gain at once is not recommended as noise becomes a factor especially
with high plate loads and small Cathode resistance sources. That's why most amps use the step by
step gain stage topology to get a more even balance. The Tone stack is going to suck up some gain
so you 'll loose some there and another reason why a gain stage is added after that circuit. As for
the inverted phase any Common Cathode circuit will cause phase but Cathode Followers will not
but have slightly less gain than 1.,
Kerry B.
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J Fletcher
10/9/2001 2:49 PM
Re: 12AX7 Gain and Phase
I think you could do it by taking the output from the first triode plate, and apply it to the second
triode cathode, with the second triode's grid at AC ground.The output is then taken from the
second triode's plate.
Don't know what values of cathode resistor, or exactly how to configure the second triode
though....Jerry
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Ray Ivers
([email protected])
10/9/2001 3:16 PM
Re: 12AX7 Gain and Phase
Unless you're willing to try one of the more 'exotic' configurations (cascode, SRPP, mu-follower,
etc.), IMO it's hard to go wrong with a gain stage direct-coupled to a cathode follower. With a 350
to 400V B+, you could use a 270K plate resistor and a 1.5K cathode resistor on the gain stage with
a .5 to 3 uF bypass cap, and a 120K resistor for the CF's cathode resistor. This would give you high
gain from the gain stage combined with a fairly low CF output impedance that could be used to
drive a tone stack, etc.
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144389
... 10/10/2001
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Page 2 of 3
Ray
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Man o'Blues
([email protected])
10/9/2001 6:57 PM
Re: 12AX7 Gain and Phase
Ray, is this basically the same configuration as V2 in a tweed bassman, except with the component
values you specified?
If not, are there any schematics I could look at that have the setup you mention?
Thanks!
From:
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Subject:
Ray Ivers
([email protected])
10/9/2001 8:55 PM
Re: 12AX7 Gain and Phase
Yes, this is basically the 5F6A V2 setup, except with the gain stage tweaked for additional voltage
gain and the CF cathode resistor adjusted slightly to compensate for the new grid voltage.
The Marshall 6100 amp's V5A/B preamp stages are a very similar circuit (gain stage is 220K/2.7K,
with a 100K on the CF). This setup biases the CF a lot hotter than my circuit values would, but still
well within 12AX7 limits.
Ray
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Man o'Blues
([email protected])
10/9/2001 9:24 PM
Re: 12AX7 Gain and Phase
Thanks Ray! Some more Q's, if ya don't mind...
The circuit I am experimenting with injects the reverb wet signal at the 2nd grid of the LTPI and I
have a whole dual triode for reverb recovery purposes. With both triodes in parallel, the reverb is
not quite wet enough, with B+ at 350V, a shared 820 ohm cathode resistor bypassed with 5uf.
Hence the quest for more gain, without phase reversal.
[ Note: I do notice that while the reverb is not lush, it is however, noise free (possibly the
paralleled sections contributing to noise reduction?) I also notice that the reverb injected at the PI
does not hazy up at loud volumes and sounds much clearer than using the standard BF method, so
I'm kinda liking this hookup! ]
My very limited understanding of CF's is that they don't add any gain, but they do convert the
output to low impedance. Are there any problems using your CF configuration when the next stage
is an LTPI? Can I expect a much wetter reverb?
Thanks again!
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144389
... 10/10/2001
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Ray Ivers
Page 3 of 3
([email protected])
10/9/2001 11:45 PM
Re: 12AX7 Gain and Phase
It's usually possible to reverse the reverb output polarity by reversing the wires from the
transducer to the RCA tank output jack; it's a bit of a hassle, but at least it's an option. I assume
you're getting reverb phase cancellations and a nasal reverb tone with the opposite polarity reverb
connection to the LTP?
As you discovered, the parallel connection of both halves of a 12AX7 does not result in much of a
gain boost by itself. It will result in lower output impedance, and possibly less noise, depending on
circuit resistance values.
The LTP 2nd grid input is high impedance, so the CF's low output Z won't really be needed - unless
the CF output will be driving a Fender-value 100K reverb pot. You could just connect the gain
stage's plate output through a cap directly to the 2nd grid, and have half a 12AX7 to use elsewhere
- provided that you use a 500K or 1M reverb mix pot to the PI input, and not the 100K. The reason
that Fender used the 100K pot value (fixed attenuation of Vibrato-channel dry signal) doesn't exist
in this design.
Ray
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Subject:
Man o'Blues
([email protected])
10/10/2001 1:54 PM
Re: 12AX7 Gain and Phase
I assume you're getting reverb phase cancellations and a nasal reverb tone with the opposite
polarity reverb connection to the LTP?
Adding a second gain stage caused instability (not sure, but sounds like motorboating)the second I
turned the reverb pot past 0. Assuming this is a polarity issue, I wonder if reversing the reverb
tranny's secondary leads would cure this?
At any rate, you're saying that a single triode for reverb recovery driving a 500K-1M reverb mix
coupled to the PI grid #2 should be sufficient for a wetter signal, so I'll certainly try this out
tonight.
Don't know exactly what I'm doing, but I'm sure having fun!!
Thanks again Ray.
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=144389
... 10/10/2001