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Man o'Blues
12/10/2001 10:53 PM
Using the cathode to mix in a second signal
Hi all. I've been looking at several schematics of amps that use a triode's cathode as an input for a
second signal and started wondering how this works, and how I might apply it in my homebrew.
For example, the old Ampeg reverb amps e.g. Reverborockets would typically feed the reverb 'wet'
output from the reverb recovery tube into the grid of the last triode gain stage while feeding the
dry signal into the cathode, on top of the cathode resistor. In fact, these amps would use a 22K
resistor to bridge the cathode of the first gain stage to 'mixer' triode's cathode, so it looks like the
cathode can be used as an input or an output. This seems like a cool way to mix and maybe less
'lossy' compared to using, say, BF grid fed topology with the 3.3M/10pf isolation resistor?
And in tweed Champs like the 5E1 and 5F1, -ve feedback is injected on top of the cathode resistor
of the last gain stage.
What are the pros and cons of isolation resistors/grid mixing versus grid + cathode method for
mixing two inputs? And how does the gain and phase relationship of the cathode fed signal
compare to the grid fed signal at the output? Any insights would be appreciated. TIA!
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Jan
([email protected])
12/11/2001 2:37 PM
Re: Using the cathode to mix in a second signal
Just getting you on top again. This sounds really interresting.
Jan
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Ben N
([email protected])
12/11/2001 4:18 PM
Re: Using the cathode to mix in a second signal
Yeah, I want to hear about this, too. Sounds like this could be a fruitful approach for 2-channel
amps with the "Normal" or "Bass" channel converted to tweed Bassman, for mixing the two
channels without interaction between the bf channel and the tweed tone stack.
Are the experts out there?
Can anyone point to a specific schematic showing this in use?
Ben
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kg
12/11/2001 4:44 PM
Re: Using the cathode to mix in a second signal
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=156122
12/11/2001
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problems/issues:
cathode node is low impedance, approx. 1/gm in parallel with rk. obviously you cannot use the
cathode to inject a signal if it is bypassed with a cap, or if a traditional fixed bias arrangement
(cathode grounded, -ve voltage on g1) is used. the lack of bypass cap limits useful gain from the
stage.
the signal input to cathode will be amplified out of phase with that input to grid. i.e. the grid input
inverts phase, the cathode input does not. the signal gain at cathode will be slightly higher than
that at grid (max gain @ grid is mu, max gain at cathode is mu+1)
the absolute best way to mix channels w/o interaction is a virtual earth mixer. unfortunately you
must give up a gain stage to impliment it. actually, you CAN rig it up to give you SOME gain--just
not the maximum you would expect from the stage.
ken
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Ben N
([email protected])
12/11/2001 4:56 PM
Re: Using the cathode to mix in a second signal
Thanks, Ken.
the absolute best way to mix channels w/o interaction is a virtual earth mixer.
Not familiar. Is this something that can be done with the shared triode stage in a SF Bassman
without harm to the Normal channel sound, leaving 3 stages for a mock-tweed preamp
(gain+gain+cf) in the Bass channel? Alternatively, can it be done with a MOSFET?
Ben
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MBSetzer
([email protected])
12/11/2001 5:41 PM
Re: Using the cathode to mix in a second signal
Well, there wasn't much response when I saw this, but here's my 2cents anyway,
I don't know all about this, but take a triode for instance.
The idle current depends on the bias difference between the grid & cathode, typically the cathode is
more positive than the grid and the triode idles with nominal standing plate current.
The plate current remains steady as long as the difference in voltage between grid & cathode are
steady. If either one of them varies, the plate current will respond accordingly, developing the
amplified output across a load connected to the plate.
Typically the cathode voltage is constant and the grid varies with incoming signal. Works the other
way too, a grounded grid amplifier varies only the cathode relative to the grounded grid, this is
seen on some Music Man output stages.
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=156122
... 12/11/2001
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Still what is occurring is the difference between the grid & cathode voltages are what is being
amplified. Neither electrode necessarily has to be held constant for reference, simultaneous signals
could be applied and the plate output would represent the differences in those signals only.
Signals coming in on the grid will be inverted when the amplified version appears on the plate.
Signals coming in on the cathode will not be inverted so they are in phase with the plate output.
Seems like it could be a way to mix out-of-phase signals plus produce gain at the same time. I
doubt if this is new, there's probably a little something about it in RDH4 ;)
Depends on where you get your signals from as to what kind of input would be appropriate for you.
Imagine again a typical triode gain stage with only 1.5K to 2.2K for a cathode resistor. The grid is a
high-impedance input, so signals coming from a high-impedance source, like the plate of a
previous 12AX7, do not have their amplitude sucked down by encountering a low resistance to AC
ground before they have a chance to get into the tube and be amplified. In fact even inside the
tube there is no way for voltage on the gid to find its way to ground very easily, that's why its a
high-impedance electrode. The amplitude of the previous 12AX7 output is therefore nearly fully
seen at the grid of the next tube without being *loaded down* by a load that is too heavy for the
12AX7 to provide adequate current to maintain the amplitude, in other words the *heavy* load is
the one with fewer ohms.
OTOH, on that same typical triode, there is such a low impedance from the cathode to ground
(1.5K to 2.2K max) that a few volts of AC signal coming in on the cathode from the same previous
12AX7's plate will only produce millivolts difference between the grid & cathode, most of the
amplitude developed beforehand has been lost as it went down the toilet through the 1.5K resistor
to ground. The 12AX7 has a high impedance plate and just can not deliver the current needed to
maintain the amplitude it is capable of when the load is only 1.5K to 2.2K. So for cathode input,
the resulting amplification seems really weak compared to the grid input, from a high impedance
preceding stage.
However, if you had a lower impedance tube driving the mixer triode's cathode input that might be
all it takes to combine out-of-phase low-level signals. For instance, you might be able to tweak the
preamp 12AX7's just right to where you are producing two separate out-of-phase signals with
different voices you want to mix. One could be applied directly to the grid of the mixer tube, the
other one could be applied to the grid of a low-impedance cathode follower like a 12AU7, then the
cathode output of the 12AU7 could be used to drive the low-impedance cathode input of the mixer
triode.
Hope this helps,
Mike
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Dave H.
([email protected])
12/11/2001 4:44 PM
Re: Using the cathode to mix in a second signal
For example, the old Ampeg reverb amps e.g. Reverborockets would typically feed the
reverb 'wet' output from the reverb recovery tube into the grid of the last triode gain
stage while feeding the dry signal into the cathode, on top of the cathode resistor. In
fact, these amps would use a 22K resistor to bridge the cathode of the first gain stage
to 'mixer' triode's cathode, so it looks like the cathode can be used as an input or an
output.
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=156122
... 12/11/2001
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The cathode can be used as an input but it is low impedance compared to the grid. In the Ampeg
amp the dry signal from the vol pot is buffered by the cathode follower action of the other triode
before being fed into the low impedance of the ‘
mixer’cathode through the 22k resistor. The
impedance looking into the cathode of the ‘
mixer’will be about 1k so there will be a 20:1 signal
attenuation due to the 22k/1k divider which is nearly the same as the 4M7/220k in the Fender but
as the impedance is lower the 10p compensation cap isn’
t required.
Dave
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Mike D.
([email protected])
12/11/2001 7:05 PM
Re: Using the cathode to mix in a second signal
For Reference:
http://www1.korksoft.com/~schem/ampegamps/reverb_rocket_12r.gif
V2-A:
Can be viewed as a cathodyne phase splitter with unequal output levels.
Cathode output will be much smaller than the plate output.
Low output impedance at cathode, high at plate.
V2-B:
Reverb signal to grid, attenuated dry signal to cathode.
Operates in both "Grounded Cathode" and "Grounded Grid" modes.
Compare to the Fender Circuit with high value series resistance:
http://www1.korksoft.com/~schem/fenderamps/princeton_reverb_aa1164_schem.gif
Advantage of Ampeg Method:
Avoidance of the high value series resistance, resulting in lower noise than the Fender Method. (In
theory, anyway)
Disadvantage of Ampeg Method:
Less overall gain, neither half of V2 can have the cathode bypassed.
In this case, it doesn't look like Ampeg was going for a lot of gain since they put a 6SN7 here.
Grounded Cathode:
Grid = Input, very high impedance.
Plate = Output, high impedance.
Output is phase inverted.
Greatest gain when cathode is at AC ground
Grounded Grid mode:
Cathode = Input, low impedance.
Plate = Output, high impedance.
Output is not phase inverted.
Greatest gain when grid is at AC ground. Gain is comparable to Grounded Cathode.
The 2nd half of a long-tail pair is grounded grid.
Combining the two modes:
Mixing Stage:
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=156122
... 12/11/2001
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Inputs fed to both grid and cathode.
They should not be the exact same signal.
Low input impedance at the cathode.
Neither the grid nor cathode should be bypassed with a cap to ground.
NFB connection:
NFB fed to cathode.
Preamp signal to grid.
NFB should be in phase with the grid signal.
Ratio of the feedback resistor and the cathode resistor sets the amount of NFB.
Does not use a cathode bypass cap.
Source of NFB should be low impedance: output transformer; cathode follower, etc.
http://www.firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=156122
... 12/11/2001