From ms26262–(at)–indspring.com Tue Jul 22 12:01:47 CDT 1997
Article: 57134 of alt.guitar.amps
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From: Randall Aiken
Newsgroups: alt.guitar.amps
Subject: Re: bass notes “farting out” on BF Super Reverb
Date: Tue, 22 Jul 1997 01:57:54 -0400
Organization: MindSpring Enterprises
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Xref: geraldo.cc.utexas.edu alt.guitar.amps:57134
J wrote:
>
> In <33d22c44.75984--(at)--ntp.cajunnet.com> epguidr–(at)–ajunnet.com writes:
> >
> >I’ve got a ’65 super reverb, with 4 new Weber VST p10r’s. The amp
> >sounds incredible, as most BF SR’s do. However, I was playing at an
> >outdoor jam the other day, and trying to get some good volume out of
> >the amp. It exhibited the usual (in my experience) Fender symptoms:
> >high volume bass-farting.
> >
>
> The source of your bass flatulence is probably grid conduction
> in the output tubes. I have the same problem with my 69 Super.
> For a long time I thought it was the stock CTS speakers bottoming
> out but a fellow on this NG (Mark Garvin) suggested it could be
> grid conduction. I took the chassis out and set it up on my
> bench with speaker and guitar plugged in and an AC coupled scope
> on one of the power tube grids. When I struck a chord on the
> bass strings with the volume on 8 I could see the signal from the
> phase inverter clip just at the onset of farting. Whats happening
> here is when the PI signal goes more positive that the 6L6 cathode
> the grid starts to conduct and clamps the signal at the cathode
> voltage. Yep, our wonderful old Fender has diode clipping at this
> point. Let me note that everything in the amp circuit is “correct”.
> I made the SF>BF conversion but it did not cure this problem. I’m
> not sure what to do about it other than not playing that loud.
> Anyone have any suggestions?
>
> Joe
Oh, boy! My favorite subject to harp on has reared it’s head once
again…(no, not cathode followers, although I’ll work those in later
on…!).
Your amp is likely suffering from what is commonly known as “blocking”
(not the RDH4 definition of blocking, but the more esoteric Crowhurst
definition). Blocking occurs when, as you mentioned above, the output
tube grids are driven to the point of conduction. Since the previous
stage is a relatively high impedance source, and, to make matters worse,
is AC-coupled, it cannot maintain enough drive to the output tube grids
to maintain the signal once the output tube goes into grid conduction.
The grid of a tube normally presents a very high impedance load to the
driving source when it is biased in the negative region. When the tube
grid bias voltage nears zero, or goes positive with respect to the
cathode, its input impedance drops drastically, and it starts drawing
current from the source. It behaves similarly to a forward biased diode
at that point. If the driving source is capable of providing this extra
current that is needed, the tube will continue to amplify normally and
put out additional power. This is commonly known as class AB2
operation, where the 2 suffix indicates grid current being drawn during
a portion of the cycle, as opposed to class AB1 operation where no grid
current is drawn.
Now, the problem with your standard resistance-capacitance coupled
amplifier stage is that the onset of grid current makes this forward
biased diode out of your previously high-impedance grid circuit. This
results not in diode clipping, as mentioned above, but rather diode
_clamping_, which is the cause of both the transient distortion, and an
increase in another type of distortion known as crossover distortion.
What happens is, the forward biased diode clamps the tops of the grid
waveform to a relatively fixed point. Since the previous stage is AC
coupled to the grid, the tops are fixed at the clamp point and the
entire waveform then shifts downward as gain is increased, pushing the
amp more into class B operation, with a resultant increase in crossover
distortion.
Since each tube in a push-pull class AB or class B pair amplifies only
part of the waveform, the two halves are summed back together in the
output transformer secondary to produce the full waveform. If the amp
is pushed farther into class B operation, the resulant waveform will
have it’s center “cored out”, producing a dead zone, or flat spot, at
the zero crossing of the waveform. This occurs because the center
portion of each half has been pushed down into the cutoff portion of the
tube’s operation. This is known as crossover distortion, and too much
of it can sound really bad.
In addition to an increase in crossover distortion, if the time constant
of the AC coupling is large enough, and the transient waveform is of low
enough frequency, a transient distortion known as blocking occurs. This
happens when the transient signal quickly pushes the clamped grid
waveform down far into the cutoff region, and there is a finite time
that is required for the grid waveform to recover and float back up to
the correct bias point once the transient signal is removed. Until the
bias comes back to the correct point, the output stage is effectively
cut off for a major portion of the signal. This results in a choppy,
“farting” sounding distortion.
Fender amps are particulary susceptible to this because of the large
values of coupling capacitors on the grids of the power tubes (0.1uF).
Blackfacing your Super Reverb can actually make the problem worse,
because you change the grid bias feed resistors from 100K to 220K, which
increases the time constant of the AC coupling to the output tube grids.
You will note that most Marshalls use 0.022uF coupling capacitors and
100K resistors, which gives a much faster time constant. In addition,
the preamp stages have a much more rolled off low frequency response.
This is why they sound tighter when played wide open.
Negative feedback can exacerbate (I’ve always wanted to use that word!
feedback loop opens up, and the gain of the phase inverter stage
increases by the amount of degenerative feedback that was formerly
present. This increase in gain pushes the amp even farther into cutoff.
What can be done to fix this problem? I only know of a few solutions,
and any of them will change the sound of your amp to a certain degree.
Fenders sound the way they do because of the design values, and anything
you do to change them will take away from some of that characteristic
tone. Having said that, here are the solutions:
First, reduce the value of the coupling capacitors. Try progressively
lower values until you find one that reduces the problem, but doesn’t
rob you of too much low end.
Second, reduce the value of the grid bias feed resistors. This has the
unfortunate side effect of attenuating the gain of the signal from the
phase inverter, which may or may not be that noticeable.
Third, increase the size of the so-called “grid stopper” resistor, which
is usually a 1.5K – 5.6K resistor in series with the output tube grids,
and is usually soldered directly to the grid pin as a parasitic
oscillation prevention measure. Increasing the size of this resistor
will limit the amount of grid current that can be drawn and reduce the
clamping effect quite a bit. This modification has the unfortunate side
effect of rolling off the high frequency content of the amp. Depending
upon the type of output tube, you can probably go up to around 50K –
100K max before the high frequency loss becomes too noticable.
(Remember, most guitar speakers don’t do too much above 4KHz anyway).
Another potential problem with this method is that the output tube is
rated for a maximum grid circuit resistance, beyond which it will become
unstable due to changes in grid voltage caused by the voltage drop
across the grid resistor from small grid currents. This value is
published by the manufacturer typically as two values, one for
fixed-bias operation and a higher value for cathode bias operation,
since it is self-correcting to a certain extent. The sum of the bias
feed resistor and the grid stopper resistor should not exceed the
maximum stated value for the grid number 1 circuit resistance.
Fourth, add a DC-coupled cathode follower between the phase inverter and
the grid of the output tubes, with the cathode follower cathode resistor
returned to a high negative voltage, and the grid bias applied to the
grid of the cathode follower. This effectively isolates the output tube
grid circuit from the phase inverter and its associated AC coupling, and
provides a very low-impedance source for the output stage. This will
prevent the output stage from going into grid clamp, and will eliminate
the long time constant of the AC coupling. This method has the
unfortunate side effect of requiring an extra tube and completely
ruining the value of your vintage amp, so it is best used only on new
designs. 🙂
Fifth, limit the signal level to the output stage at full volume by
adding either a resistor across the phase inverter outputs (after the
coupling caps) or by adding series resistance on each side between the
coupling caps and the grid bias resistors. Adjust the attenuation so
the output stage starts to clip only after the phase inverter starts to
clip. If you can prevent the output stage from going too far into grid
clamp, you can minimize the problem.
Sixth, reduce the amount of negative feedback used, or remove the
negative feedback loop entirely. Generally you will have too much gain
in the output stage once you remove the feedback loop, so you will
probably want to reduce the signal level as indicated in the fifth
method outlined above. This method has the unfortunate effect of
rendering any existing presence control inoperable. If you must have
presence, add a “treble cut” pot across the phase inverter output as in
the VOX AC30, and increase the treble in a previous stage to give you
some extra treble to work with.
Seventh, look for the problem in the preamp stages. Blocking can (and
does) occur just as bad on RC-coupled preamp stages that are overdriven.
The solution is similar. Add large value series grid resistors (100K –
470K), reduce coupling capacitor values to the minimum required for the
desired low frequency response, or
bounding circuit to prevent the grid from being driven too far.
That’s it. If you aren’t completely bored or confused by now, and would
like to read further about this, do a Deja News search under my name on
the old non-current newsgroup database and you’ll likely run across some
articles I wrote a year or two ago on this very subject.
Hope this helps,
Randall Aiken
raike–(at)–indspring.com
