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R.G.
3/13/2002 10:01 PM
Carbon Comp Distortion Calculation
This is going into the next update of the Tube Amp FAQ along with about a zillion other things, but
I thought you'd like to see it.
=================================================
FAQ: ... I would like to hear opinions concerning the use of carbon composition resistors.
Obviously, the vintage amps we all love had them, and they certainly have their share of mojo, but
they used them in the 60s because that's what was available.
Now there are lots of options. It's my understanding that they drift a lot, compared to their stated
values. What's the story on using carbon comps to replace otherwise operating resistors in a
modern amp, or in rebuilding a vintage amp to play through where reliability and low-noise
operation are as important as the tone?
If anyone can shed some light on the pros and cons of the carbon comps, I will appreciate it. Is this
purely a "mojo" thing, or is there some science involved?
What about metal film resistors? Used in almost every "modern" hi spec amp today including
Soldano and so on....
R.G.: Carbon comps have excess noise, high drift, high pulse power, and high variability. They also
have a high voltage coefficient of resistance (as well as a high thermal coefficient of resistance temperature drift).
That means that the resistance actually varies with the voltage across the resistor. That induces a
certain amount of second harmonic distortion only in the case where the signal across the resistor
is large - in the 50V and up range to be really audible.
In only those conditions, you can hear a certain amount of "sweetening".
You also hear the excess noise and drift. Us them sparingly only where your personal ears tell you
that they're useful. For instance, use metal film for low noise in input stages, and carbon film in PI
and output stages.
From an electrical circuit point of view, especially as regards noise, all resistors connected to a tube
that handles signal are in the signal path. They don't have to be in series between tubes or used as
a plate resistor to be in the signal path from the standpoint of noise contribution.
Only resistors with both large voltages across them -and- large signals in those resistors can offer
the carbon comp sweetening distortion. That leaves out grid resistors of all kinds (low voltages
across them), all bypassed resistors (no AC signal across them), and first-stage preamp plate
resistors (signal is too small).
Since the distortion is small, it's going to be most noticeable in the plate(s) of the phase inverter
and the tube just before the plate inverter. Earlier stages are too early - not enough signal - and
too sensitive to noise because of the later amp gain.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
... 3/22/2002
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I'm always amused at people who advertise putting carbon comp resistors in their 9V powered
effects to give them some kind of magical vintage sound. Urban legend is tough to kill, though.
In my own rough and ready estimation, you have to have at least 50V of DC and about a 50V or
more signal on a resistor to get the sweetening distortion. Obviously, 50V is not a hard and fast
dividing line, just where I judged it to be significant. Here's some web references and an example
calculation
==============================================
From www.metechinc.com:
The Voltage Coefficient of Resistance-VCR-describes the reversible change in resistance due to
increased voltage gradients. ... CAUTION: VCR (like TCR) is not a linear property. VCR's can
increase significantly when measured at higher voltages. Testing in your application is always
prudent.
(translation: the very proportionality of resistance itself varies with the applied voltage; worse, the
amount of variation gets larger at larger voltages too.)
From http://www.texascomponents.com/7%20reasons/Reason%207.pdf
... Reason 7. Non-Measurable Voltage Coefficient As mentioned earlier in our section on resistor
noise, resistors can change value due to applied voltage. The term used to describe the rate of
change of resistance with changing voltage is known as voltage coefficient.
Resistors of different constructions have noticeably different voltage coefficients. In the extreme
case the effect in a carbon composition resistor is so noticeable that the resistance value varies
greatly as a function of the applied voltage.
From http://www.riedon.com/handbook_default.htm:
Voltage coefficient is the change in resistance with applied voltage. It is associated with carbon
composition and carbon film resistors, and is a function of the resistor's value and its composition.
A typical carbon comp resistor voltage coefficient can be seen at
http://www.irctt.com/pdf_files/IBT.pdf - which shows carbon comp at 0.005%/volt for that
company's products. Another was 0.008%/V
From http://www.irctt.com/product_selector/irc/frames3.cfm you can find carbon film at
0.001%/V.
So - for a 50V swing, the change in resistance will be .25% in CC, 0.05% in CF, compared to
unmeasurable in metal film, wirewound and bulk metal foil. That is, with 50V across it, the small
signal resistance is 0.25% lower than it is with 0V across it. But what if we're feeding the resistor a
DC current with enough AC imposed onto it to make the *signal* swing 50V? The high end of the
signal will be 0.25% smaller than the low end of the signal.
That's pure second harmonic, by the way. You probably can't hear it as other than a sweetening or
fullness.
I'd have to do some more calculation to relate the change in resistance to straight % distortion,
you get the idea. At 9V, even carbon comp sees only 0.045% change, pretty neglible. At 100V, you
get 0.5% nonlinearity, and at 200V, 1% nonlinearity.
I guessed at an arbitrary point below which I think the voltage coefficient distortion would be
negligible. The exact cutoff point is arguable, but I think we'd all agree that there is some range
below where the distortion is not noticeable, and above which it has an audible effect.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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The numbers are for IRC CC's. They'd be a lot worse from other makers, maybe.
From:
Date:
Subject:
Wild Bill
(wildbillcostello@sympatico.ca)
3/14/2002 2:15 AM
Re: Carbon Comp Distortion Calculation
At last! A truly scientific basis for discussion abut carbon comp mojo!
As for .25% giving an audible "sweetening", I'm afraid that my fingers just don't have much
sweetening at all! :)
---Wild Bill
From:
Date:
Subject:
R.G.
3/14/2002 4:16 AM
Re: Carbon Comp Distortion Calculation
I was truly relieved when my thumbnailing turned out to produce results that correlated with
available info, and also gave a good way to predict where carbon mojo could and couldn't be had.
From:
Date:
Subject:
Gino
(giorfida@drifteramps.com)
3/14/2002 3:04 PM
Re: Carbon Comp Distortion Calculation
... in other words, changing out those NOISY carbon comps as plate resistors on the first and
second stages of old FENDERS isn't going to do a dang thing to the tone -- esp in the first stages,
whree a 'trickle' of noise would sound like a waterfall after being amplified thousands of times over
througout the circuit....
VERY cool article.....
From:
Date:
Subject:
R.G.
3/14/2002 3:57 PM
Re: Carbon Comp Distortion Calculation
You certainly ought to get the most bang for the buck latest in the amp. PI plate resistors should
be good.
This is really good news for people making their own amps or modding. You make the front end
quiet with metal film and get tube squish from CC's in the later stages where the noise doesn't hurt
you so much.
R.G.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
... 3/22/2002
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Mike Conner
Page 4 of 16
(rmconner80@hotmail.com)
3/15/2002 4:03 PM
Re: Carbon Comp Distortion Calculation
Before changing those noisy plate resistors, check their value. If they're off spec (didn't they have
like 20% tolerance or something) or have drifted, it might be possible that the off spec value is
contributing to the uniqueness or tone of an amp.
Replacing a drifted 150K or 80K plate resistor with a 100K might have some effect, I guess (maybe
not?). Just replace the drifted one with as close a value as possible if you don't want to change the
amp.
Mike
From:
Date:
Subject:
Anon
3/14/2002 10:52 PM
Re: Carbon Comp Distortion Calculation
What about the AC effects? How do various resistor topologies behave to "guitar frequency" signals
in a high impedance circuit?
From:
Date:
Subject:
R.G.
3/14/2002 11:30 PM
Re: Carbon Comp Distortion Calculation
What about the AC effects? How do various resistor topologies behave to "guitar
frequency" signals in a high impedance circuit?
There's always more research to be done - you might have a real winner of a topic to dig into
there ... I'm still drinking my victory beer from digging this one out 8-)
My personal guess is that for the 40Hz to 7kHz of signal that comes out of a typical guitar pickup,
you'd have a huge difficulty measuring any frequency effect on any of the common resistor
technologies.
There are really only three possibilities for frequency effects. Those are
(1) spiral cut carbon film
(2) spiral cut metal film
(3) wirewound
In all these cases, the effect is from inductance of the conductor and distributed capacitance.
However, the inductance of a spiral cut resistor is easy to measure. Just hang a high quality cap
across it to make a parallel RLC circuit and find out where it resonates. The inductive effects can be
calculated in a straightforward manner from that.
I would bet money that you won't see audibly significant effects below 20kHz, and probably not
below 100kHz. The inductive effect gets worse with higher resistor values which need more turns
of resistive material to get a long enough conductor string. Carbon and metal film resistors are
used in noncritical radio circuits, and they couldn't be if the inductances were very high.
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Carbon comp will be a relative winner there, though. Because of the highly distributed nature of
the granular contacts in the carbon, there are many paths, and CC's are relatively resistive to high
RF ranges - out where we can't hear the difference 8-).
R.G.
From:
Date:
Subject:
Alexander
(Alexander@Retrodyne-amps.com)
3/15/2002 12:46 AM
Certain Mfr.'s are gonna wanna kill you...
Thanks, R.G !
Science triumphs over 'mojo'/urban legend again.
I'm sure half the builders here have an amp that contains metal film resistors and metallized
polypropelyne caps and they sound GREAT. But there's always that nagging doubt...bottom line:
Trust your Ears ! If it sounds good, it IS good.
We all oughta chip in and publish this post in Vintage Guitar & Guitar Player magazine to help
straighten out the superstitious masses !
Damn, I just found an old electronics warehous that has THOUSANDS of old .5 & 1 watt CC
resistors !
Thanks again for the continued enlightenment.
Alexander
Retrodyne Amplification
From:
Date:
Subject:
b
3/15/2002 5:10 AM
Re: Certain Mfr.'s are gonna wanna kill you...
Where is a good online source that sells CC resistors? Mouser?
From:
Date:
Subject:
Anon
3/15/2002 2:43 PM
Re: Carbon Comp Distortion Calculation
Thanks for bringing up this subject, RG. It's long overdue for serious study.
Eddy currents are generated when current flows in a conductor. No one has ever checked out the
Eddy current effect on resistors. Another science project!
Many metal film & Carbon film resistors are made by depositing the material on the outside of a
ceramic cylinder. We know that as frequency rises, the signal moves to the outside of a conductor.
But at WHAT frequency this begins to occurr, and the effects of phase shift & harmonics, is
anyone's guess.
In an AC circuit, a resistor is not a resistor. It's an impedance.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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Anon
From:
Date:
Subject:
kg
(ride5000@ride.ri.net)
3/16/2002 9:32 PM
quick'n'dirty skin effect...
well, there's no ONE frequency at which skin effect starts.
it's ALWAYS there in varying degrees, since every conductor creates an electromagnetic field when
current flows. the greater the number of electromagnetic flux lines surrounding the conductor, the
greater the inductance of the conductor. this is exactly how chokes work.
because the center of the conductor has more lines of flux surrounding it, the center has more
inductance. therefore, at higher frequencies, the impedance at the center of the conductor is
increased, which pushes the AC signal to the edges.
the skin effect is therefore a decrease in effective conductor cross section, and correspondingly an
increase in effective wire resistance. this increases power loss in the conductor and can lower the Q
of an LCR resonant circuit.
this is not to be understood as a simple increase of impedance due to self-inductance (which would
be largely reactive, and not very resistive), but rather an increase in power loss and dissipation due
to higher effective resistance of the conductor itself.
now here's the key: the skin effect is only an issue if the increase in effective wire impedance has
some negative effect on the circuit... i.e. it is a wideband circuit which has frequency components
spread out such that some experience the lower impedance, and others the higher impedance, and
this is undesirable. in other words, the skin effect becomes an issue when the CHANGE in
impedance becomes too large for the circuit in question.
ironically, the THINNER the conductor is, the less skin effect is an issue, since the CHANGE in
impedance will not be so drastic... the thinner conductor has a higher impedance (well, higher DCR
really) to begin with, so the change in impedance does not alter the circuit operation that much.
in the same way, tubular conductors are used to minimize the change in impedance due to skin
effect, since they have no center which would be a low impedance at LF and a higher impedance at
HF. their impedance therefore remains more stable across an extended frequency range than a
solid conductor's would.
foil or strap conductors are preferred in this respect as well, since their edges will not be encircled
by so many EM flux lines, and will thus carry more of the current.
the extreme is the litz wire, which consists of multiple conductors insulated from one another and
woven together in such a way that, over the length of the conductor, each strand has the same
"distance" from the center, and thus will be encircled by the same # of lines of flux. constructing
the wire in such a way makes sure that each strand will carry the same current.
furthermore, because each individual strand is of a much smaller diameter than the effective
aggregate diameter, the frequency at which the skin effect starts to become an issue is pushed
outside of the passband of interest. it is used extensively in RF coils to preserve high circuit Q, but
is a very expensive construction technique.
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IMHO, skin effect is REALLY HONESTLY not an issue worth obsessing over at audio frequencies.
ken
From:
Date:
Subject:
Anon
3/16/2002 11:32 PM
Good stuff, KG!
Good stuff, KG. I still find myself pondering over phase relationships in a complex wave form.
(Current leading/lagging voltage, etc.)
Knowing that resistors, capacitors and inductors all contain an L/C/R component can be mind
boggling. This is where comparing the sonic difference of resistors has brought us.
I can't wait to see RG's final report on the matter. I used to think it was all "hogwash" until I
started experimenting with solid wire & CC resistors in tube & SS audio circuits. Using this
technology, were really making instruments, not "perfect" amps.
Anon
From:
Date:
Subject:
anonymous
3/20/2002 9:44 PM
Re: quick'n'dirty skin effect...
IMHO, skin effect is REALLY HONESTLY not an issue worth obsessing over at audio
frequencies.
I think we can be more strident than this.
Look here:
http://www.signalintegrity.com/news/skineffect.htm
If you chug through the math, you will find that at audio frequencies the skin effect depth is
greater than the "typical" conductor diameter!
IOW - it doesn't amount to squat.
From:
Rob Mercure
Date:
3/21/2002 4:59 PM
Subject:
(rmercure@mikrotec.com)
Re: quick'n'dirty skin effect...
Anon, Ken,
I'll second (or third) that notion - the "skin effect" is mostly just used to "skin" yuppie hifi nuts
wallets - IMnsHO.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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Rob
From:
Date:
Subject:
Doug B.
(jzmc@mail.marist.edu)
3/15/2002 10:46 PM
Re: Carbon Comp Distortion Calculation
R.G. Would the wattage of the resistor have any effect on this?
- Doug B.
From:
Date:
Subject:
R.G.
3/18/2002 4:05 AM
Re: Carbon Comp Distortion Calculation
Probably. Now that I think about it, it's got to be the per-unit voltage stress across carbon granules
that makes for the variation in resistance. With a higher wattage resistor, there's a lot more
resistive filling, so the per-unit voltage stress is probably smaller.
It's a theory, anyway. I bet if I looked back at the AB and IRC data sheets I could find out.
R.G.
From:
Date:
Subject:
moocow
3/22/2002 9:07 AM
Re: Carbon Comp Distortion Calculation
Not surprisingly, the relationship between resistor power rating and distortion is described in the
Radiotron Designer's Handbook, 4th Edition. It turns out that smaller resistors do exhibit greater
distortion than larger resistors. These effects were known many years ago and this is why Fender
started using large 100K plate resistors in their amps. They were trying to reduce distortion by
using resistors with a lower distortion coefficient.
Distortion in carbon composition resistors that is caused by large voltage swings was discussed on
Ampage a few years ago. Since then, I've been using 1/2 watt carbon compostion resistors in plate
circuits only and metal film everywhere else in the signal path. Now that everyone else has caught
on, I can admit that I've been greatly amused by those who feel the need to use 1W and 2W
carbon composition resisitors. By using larger than necessary carbon composition resistors, you are
shooting yourself in the foot by reducing the amount of distortion induced by the resistor while
retaining the undesirable characteristics (noise, drift, etc.) inherent to carbon composition.
From:
Date:
Subject:
kg
(ride5000@ride.ri.net)
3/22/2002 2:24 PM
Re: Carbon Comp Distortion Calculation
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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By using larger than necessary carbon composition resistors, you are shooting yourself
in the foot by reducing the amount of distortion induced by the resistor while retaining
the undesirable characteristics (noise, drift, etc.) inherent to carbon composition.
not really.
the larger bodied resistor has more thermal mass and more surface area, and will not get as hot
for a given power dissipation--therefore it will make LESS noise, since the noise power, P (in
watts), is given by P = kTf , where k is Boltzmann's constant (in joules per kelvin), T is the
conductor temperature (in kelvins), and f is the bandwidth (in hertz). the bottom line is that using
a heftier resistor is NOT completely devoid of benefits.
additionally, there are certain spots where the increased dissipation rating may be demanded by
the circuit. i agree that these spots are not that common, unless you design your own circuits and
push things up a notch... case in point: in my preamp, i burn about 5 watts in both the input tube
AND its plate load! ;)
ken
From:
Date:
Subject:
R.G.
3/22/2002 4:53 PM
Carbon Comp Design for Distortion
Hey, this is good. So we can start listing the tricks for designing for Carbon Comp tone mojo.
So far we have:
1. high voltage across the resistor is necessary (in the range of 50 or 100V on up)
2. large signal swings across the resistor are needed (ideally, a large fraction of the static DC
voltage}
3. only positions in the amp that have both high DC voltage and wide signal swings as in 1 and 2
will give you enough resistor distortion to benefit from; other places should be chosen for low noise
and/or economy.
4. resistor power rating should be the minimum needed to work for a reasonable life in the circuit
to maximize resistor distortion. Maybe a good guideline is that the dissipation should be selected to
be as close to two times the average dissipation as possible.
5. as a corollary to the power guideline,we should be prepared to replace CC's every few years as
the life at high temp makes them drift and get noisy(-er).
I'll go sort this into the Tube Amp FAQ.
R.G.
From:
Date:
Subject:
moocow
3/22/2002 8:52 PM
Re: Carbon Comp Design for Distortion
I'd like to comment on #3. I support using a carbon composition resistor for a first stage. Someone
might want to use an effects pedal to intentionally overdrive the first stage, so now there would be
enough signal swing on the first stage to allow the Carbon Composition Tone Mojo to take place.
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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As for #4, the benefit of using carbon composition resistors drops significantly for power ratings
greater than 1/2 watt. Here are the numbers from Radiotron Designer's Handbook where the
percent change is resistance of a 1M carbon composition resistor is measured for a given drop in
applied voltage:
1/4 watt: = 2.1%/200V
1/2 watt: = 2.5%/300V
1 watt: = 1.3%/500V
2 watt: = 1.5%/500V
For a given change in voltage, the 1/2 watt resistor changes its value 3 times more than the 1 watt
resistor. These values are for a 1M resistor and the changes for a 100K resistor should be even
less. I suppose there might be some benefit to using 1- or 2-watt carbon-composition resistors, but
I would suggest that the amplifier should be designed to use 1/2-watt plate resistors. By the way,
the JAN-R-11 spec for carbon composition resistors limits the voltage coefficient to .035% per volt
for 1/4 and 1/2-watt resistors, and .02% per volt for 1-watt and above.
I'm also wondering if it is useful to use carbon composition resistors in a phase splitter. My
understanding is that second order harmonics generated by the phase splitter and following
circuitry are canceled out by the push-pull output section. Also, I'm superstitious about purposely
introducing distortion inside a feedback loop. In any case, the phase inverter equations for the
typical Fender amp show that it is already unbalanced, even if the triodes and resistors are
perfectly matched.
However, I would guess that a split-load inverter would react differently to carbon composition
resistors. The cathode output would not be affected by a change in its load resistor, but the plate
voltage will. The second harmonic is generated only on one side of the push-pull circuit, so it
shouldn't be cancelled out.
Of course, if we want more second harmonics from our amplifiers, we could just lower the plate
supply voltage and bias the triode closer to cutoff. But where's the Mojo in that ?
From:
Date:
Subject:
kg
(ride5000@ride.ri.net)
3/22/2002 11:34 PM
Re: Carbon Comp Design for Distortion
However, I would guess that a split-load inverter would react differently to carbon
composition resistors. The cathode output would not be affected by a change in its load
resistor, but the plate voltage will.
how do you figure?
the current through both cathode and plate resistors must be the same. if they are both the same
resistance value, that means the same DC voltage drop occurs across both, and the same goes for
the AC signal. that's how the concertina works in the first place!
ken
From:
Date:
R.G.
3/23/2002 1:41 AM
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Re: Carbon Comp Design for Distortion
the current through both cathode and plate resistors must be the same. if they are
both the same resistance value, that means the same DC voltage drop occurs across
both, and the same goes for the AC signal. that's how the concertina works in the first
place!
I guess that more to the point, both resistors have max and min voltages across them at the same
time, so the effects add. That means you're getting twice the voltage change across the resistors,
but also twice the resistor to spread it over. I think this one comes out to be the same distortion
per signal level as the same signal size across a single resistor - except that you can use lower
dissipation resistors since the voltage is split...
In any case, I think the distortion will show up.
R.G.
From:
Date:
Subject:
R.G.
3/23/2002 2:27 AM
Correcting myself...
See my response to moocow on the split load. I think he's right - a split load will only show half the
effect, on the plate resistor. The cathode resistor is running as a follower, and the follower doesn't
much care what the resistor is, within reason. A 1% resistor change won't change the output by
anywhere near the resistor value change. Essentially all the resistor distortion will be on the plate
resistor.
R.G.
From:
Date:
Subject:
R.G.
3/23/2002 2:06 AM
Re: Carbon Comp Design for Distortion
I support using a carbon composition resistor for a first stage. Someone might want to
use an effects pedal to intentionally overdrive the first stage, so now there would be
enough signal swing on the first stage to allow the Carbon Composition Tone Mojo to
take place.
I suspect that if you're overdriving the first stage enough to cause the big signal swings, the tube
distortion itself might well overpower the resistor distortion. It's a good place for some further
work. In any case, we can definitely say that using them in the first stage will enhance the excess
noise they carry.
On the other points...
Here are the numbers from Radiotron Designer's Handbook ... I would suggest that the
amplifier should be designed to use 1/2-watt plate resistors.
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Or, looking at the numbers, even 1/4W where those can be used safely - it's change per volt is
even larger. The trick seems to be to use the lowest dissipation rating that doesn't excessively
impact the resistor's life. And that's what rule #4 says.
However, it does point to a quantizing effect that might explain some subtle differences in
amplifiers. If a resistor is used at just the hairy edge of its safe voltage rating, the effect is
maximized. Depending on the design of the amp, dropping the voltage to a stage to get the
dissipation down just enough to use the next lower power rated resistor might let you get more
resistor distortion by stepping to the next lower size.
the JAN-R-11 spec for carbon composition resistors limits the voltage coefficient
to .035% per volt for 1/4 and 1/2-watt resistors, and .02% per volt for 1-watt and
above.
... if we could find JAN qualified resistors any more except in out of the way crevices 8-)
My understanding is that second order harmonics generated by the phase splitter and
following circuitry are canceled out by the push-pull output section.
No. The output stage PP setup only cancels even harmonics generated within that stage. If it
comes from the PI, it's not cancelled.
Also, I'm superstitious about purposely introducing distortion inside a feedback loop. In
any case, the phase inverter equations for the typical Fender amp show that it is
already unbalanced, even if the triodes and resistors are perfectly matched.
Superstitions aside, the feedback around the stage reduces the internally generated distortion by
the feedback factor. That cooks out the carbon comp mojo you worked to get in there. This may be
one (more) reason why the AC-30 sounds so different, even clean - there is no feedback loop to
reduce resistor (or tube) distortion where most of it is generated.
The cathode output would not be affected by a change in its load resistor, but the plate
voltage will. The second harmonic is generated only on one side of the push-pull circuit,
so it shouldn't be cancelled out.
I answered KG's (great initials, those!) note first, and now that I read your post in detail, I do tend
to agree. You're right, the cathode output will be much less affected by the resistor change, as it's
acting as a cathode follower with unity feedback, so the actual resistor value is not what is driving
the output. On the plate side, though, the resistor value is all that's there. The resistor change will
only be generated by the plate side.
However, there's an offsetting mechanism in the design. Split load phase inverters can only exist
where the output tubes only need a smallish voltage to drive them, like with the EL84. If you did a
diffamp PI, the EL84's would still only need the same amount of drive, and so the voltage effect
would be limited to the same size as in the plate side of the split load - well, maybe 3db more,
since it's generated in two plates.
Which leads to another possible design-for-resistor-distortion trick. In the modern, enlightened
design concensus developing here, we deliberately don't massively overdrive grids because of
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blocking distortion. However, massive, almost-full-power-supply swings are what maxes out the
resistor distortion, and would also max out the tube's inherent nonlinearity.
It seems like what we might want to do is to quickly blast the signal up to close to a 100V swing,
then divide it back down with resistors to an appropriate level for the next grid. The distortion is
made in the plate resistor swinging massively, and stays in the signal that's divided down to be
amplified up in the next stage again.
For EL84's you only need maybe 24V p-p to turn them
fully on or off. Why not make the signal level in the PI driving them be 100V, then do a 4:1 divide
down to drive the grids?
I'll have to go inspect my navel for an hour or two to let this digest a bit. This seems like another
advantage of dividing down between stages beyond simply stepping out of blocking. It lets you
max the soft distortion in the driving stage without stepping over into clipping.
R.G.
From:
Date:
Subject:
Mark Abbott
(abbottmark@hotmail.com)
3/16/2002 7:58 AM
Re: Best places for Carbon comp resistors.
Dear RG
As always, great work!
I suppose I'm asking for the short answer here, where would you use them?
I gather you would replace the 82K, and 100K resistors in P.I. circuit, but would you replace any
other resistors in the P.I. circuit?
Would you replace the bias resistors with carbon comp resistors, when the amp is being pushed I
seem to recall that you have about 96VAC across them.
Again thanks for your help.
Yours Sincerely
Mark Abbott.
P.S. I'm intending to do the change to resistors in my Boogie Mark 1/S.O.B. I own, and for the
record the original Boogies came out with Carbon Comp resistors.
From:
Date:
Subject:
R.G.
3/18/2002 4:13 AM
Re: Best places for Carbon comp resistors.
I suppose I'm asking for the short answer here, where would you use them?
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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Page 14 of 16
The short answer has to be "wherever the signal swings are biggest. The plate resistors for the PI
are the prime candidates. The stage just before the PI is the next best.
Grid resistors of all stripes are least likely, because the actual resistance there matters almost not
at all. Case in point - the grid bias resistors for the output stage. Yep, there's the same voltage
there as on the PI plate resistors. However, the actual resistor value could change around a lot and
not affect the signal. On an AC basis, this is in parallel with the PI plate resistor and rp, so the
value of the bias resistor can't change the value much - it's got to be much larger than the plate
resistor for signal transfer reasons.
Likewise, cathode resistors shouldn't matter, as there's low voltage across them (in preamps,
anyway) and they're usually bypassed. No big signal swings.
Cathode followers also should not exhibit the CC distortion. There can be a big signal swing, but
the impedance of the cathode is so low that the actual resistor value doesn't affect the signal
much. The distortion effect is swamped.
Hey. I didn't know I knew this much about this 8-)
R.G.
From:
Rick Erickson
Date:
3/18/2002 8:40 PM
Subject:
(ampdog@eskimo.com)
Re: Best places for Carbon comp resistors.
The best place for carbon comp resistors is in the trash can imho. :)
RE
From:
Date:
Subject:
Anon
3/18/2002 9:25 PM
Re: Best places for Carbon comp resistors.
You are absolutely right. When my employer threw away 1000 audio vacuum tubes, countless Allen
Bradley CC resistors and Sprague capacitors, Germanium transistors, sockets, fuses, etc., I went
straight to the trash can! :D
From:
Rick Erickson
Date:
3/20/2002 8:03 PM
Subject:
(ampdog@eskimo.com)
Re: Best places for Carbon comp resistors.
Hey - I'm a dumpster diver too. :)
Sounds like a good score on most of that stuff. Depends on why they were thrown out though...
fwiw - I just removed three noisy 2W carbon comps from a Matchless HC-30 preamp section. (plate
resistors) I knew I would be seeing some soon. These beloved amps are destined to become
expensive niose generators in time. The good news is they can be easily cured, depending on your
definition of easy. (terminal strip wiring can be a mo-fo to re-do neatly) The bad news is I'm
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
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Untitled Document
Page 15 of 16
having a hard time finding a source for 2W carbon film resistors, which is my prefered
replacement. Anybody have a good source for these? I may have to go to metal-oxide if carbon
film supplies dry up. :(
RE
From:
Date:
Subject:
R.G.
3/20/2002 11:09 PM
multiwatt film resistors
Multiwatt film is hard to find.
Have you tried wirewound?
R.G.
From:
Date:
Subject:
PaulC
(p.cochrane@worldnet.att.net)
3/21/2002 1:30 AM
Re: multiwatt film resistors
I just did that to a DC30 last week. Replaced a few 2 watt CCs with 1W CF. The 2 watters were
overkill trying to deal with the noise in the first place. I like to think I can catch the little details in
tone, and all I caught was a little less noise ( also minus the crackle that was the problem). The
tone seemed dead on. Makes me really want to dig into this voltage level thing that RG is pointing
out.
From:
Rick Erickson
Date:
3/21/2002 3:53 PM
Subject:
(ampdog@eskimo.com)
Re: multiwatt film resistors
Replaced a few 2 watt CCs with 1W CF. The 2 watters were overkill trying to deal with
the noise in the first place.
I agree with that! Even 1/2W cf's would work fine for most of the circuit, but I do try hard to
maintain the look and feel of the original amp as much as possible. 1W cf's would look o.k. among
the 2W cc's imo, and can still be found in the Mouser catalog iirc. If I owned one of these
(Matchless) I would want to replace ALL the cc's with cf's. What a pain that would be...
RE
From:
Date:
Subject:
Bruce /Mission Amps
(missionamps@aol.com)
3/21/2002 4:21 PM
Re: multiwatt film resistors
What ohmic value of 2 watters did you need for that amp Rick?
I have about 90,000 carbon comp resistors in stock, (yes 90k) and some of them are 1 and 2
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Untitled Document
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watters.
Myabe I have a few that will work.
Bruce
From:
Rick Erickson
Date:
3/21/2002 3:47 PM
Subject:
(ampdog@eskimo.com)
Re: multiwatt film resistors
Have you tried wirewound?
I have a few 2W ww's in low resistance values, mainly for SS amp repairs. They are ugly
rectangular ceramic cased little things that just don't look "right" in a vintage or modern-vintage
amp. I may have to adjust my attitude if that becomes the only available choice.
RE
From:
Date:
Subject:
anonymous
3/21/2002 9:12 PM
2W Carbon film
Ric, I get 2w c/films from RS Components. http://rscomponents.com
Do a product search using "2w carbon films" and you'll see 'em
HTH
Pete :)
From:
Date:
Subject:
PeteR
(valveart@zipworld.com.au)
3/21/2002 9:13 PM
Re: 2W Carbon film
That was me (Damn cookies)
Pete
http://www.firebottle.com/fireforum/fireBB.cgi?forum=ga&thread=173522
... 3/22/2002