From reaike–(at)–x.netcom.com Mon Dec 4 10:40:28 CST 1995
Article: 3920 of rec.audio.tubes
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From: reaike–(at)–x.netcom.com(Randall Aiken )
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Fender SR–silverface vs. blackface circuit
Date: 4 Dec 1995 00:02:45 GMT
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In <49t2t6$i0--(at)--ountain.mindlink.net> michael_ken–(at)–indlink.bc.ca (Mike
Kent) writes:
>
>I converted an silverface AB568 Super Reverb (mixed cathode/fixed
>bias) to the generally more desired blackface AB763 fixed cathode
>circuit. Sure, the amp is cleaner and more powerful now, but… I
>think I preferred the complex harmonics of the AB568 circuit!
>
>I have two choices–do a bunch of Weber mods to the AB763 to fatten up
>the sound, maybe lose the vibrato and negative feedback, etc.; or,
>give the AB568 circuit another try.
>
>I’d like to hear some opinions about the AB568 circuit, beyond the
>tube book lore about how lousy the silverface amps were. Mark Garvin
>has said in the past that he suspects the phase inverter circuit as
>being a problem, rather than the mixed bias configuration on the
>output tubes. Can anyone comment on their experience with
>modifications to the AB568 circuit? If the silverface circuit is so
>lousy, please explain why. There are so many potential modifications
>to try out on any circuit, I’d like to settle on which basic
>circuit to work on, rather than waste time going down the ‘wrong’
>branch of the decision tree.
>
>reaike–(at)–x.netcom.com (Randall Aiken ) wrote:
>
>>As to a “class A” amplifier sounding better and having “touch
>>sensitivity”, the reviewers are usually talking about a class AB
>>cathode-biased amplifier and calling it a class A amplifier. Due to
>>the negative feedback action of the cathode bias resistor and the
>>shifting of operating point with signal level, these amplifiers
>>generate a richer series of harmonic overtones when driven hard.
>
>That’s what my ears heard when I tried out the AB568 mixed
>cathode/fixed bias over the AB763 circuit. Yet everybody avoids AB568
>like the plague. Why?
>
>–Mike.
>
I haven’t compared the two circuits side by side, so I may be off-base
here, but I tend to agree with Mark Garvin.
There are a couple of points of interest, though. The AB568 uses
separate bias resistors that are bypassed with one capacitor from
cathode to cathode. This creates a degenerate feedback arrangement by
coupling the two cathode signals together for AC signals, and would be
the same (for AC signals) as using a single, unbypassed cathode
resistor for both tubes. A far better approach would be to use a
separate capacitor for each resistor from cathode to ground, or a
common resistor of half the value bypassed with a large capacitor to
ground. I don’t know why this is never mentioned in the books.
In addition, let me clarify my statement about cathode bias shifting of
operating point. Cathode bias actually causes a smaller shift in
average plate current from zero signal to max signal than fixed bias
due to the “self-correcting” action of the degenerative feedback. The
voltage drop across the resistor will increase to compensate the
additional average plate current drawn by the class AB amplifier. This
changes the bias point of the circuit with changing plate current. The
average current changes because of the rectification effects of class
AB operation on the cathode waveform. Bypassing the cathode resistor
prevents degeneration at signal frequencies, but there will be a
greater average current due the asymmetry of the rectified cathode
waveform. Large transient waveforms will create greater distortion in a
cathode biased amplifier.
Mark’s point about the differences in the phase inverter circuit is a
good one. The 47K resistors put the tube on a completely different
spot on the plate curves, and lower the gain. In addition, there will
be a difference in the balance of the two plate waveforms, since the
gain at one side of the phase inverter is larger than the other side,
but the plate resistors are of equal value.
In addition, there is a rather large 2000pF cap from grid to ground on
the output tubes. This will roll of the high frequencies quite a bit,
contributing to a difference in tone. They were probably added to
prevent oscillations, so be sure to check for this if you remove them.
The AB568 circuit has a higher plate voltage; it is probably biased a
bit colder than the AB763 circuit to keep the plate dissipation down.
This can increase the amount of crossover distortion and change the
tone.
In conclusion, contrary to what the “experts” with the tube books say,
I think Mark was correct when he stated that the phase inverter
differences probably contribute more to the tone differences than the
150 ohm resistors. Also, don’t believe everything you read; if it
sounds better to you with the “bad” circuit, by all means, use it.
Also, has anyone tried a direct comparison of the two biasing methods
with the same phase inverter circuit and no grid capacitors? Perhaps
you could do this, Mike, and get back to us with the results. It seems
to me that if cathode bias is universally preferred over fixed bias, a
point in between the two would be better than fixed bias alone.
Randy Aiken
reaike–(at)–x.netcom.com
From timtub–(at)–ol.com Tue Dec 5 15:36:52 CST 1995
Article: 3955 of rec.audio.tubes
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From: timtub–(at)–ol.com (TimTube)
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Fender SR–silverface vs. blackface circu
Date: 5 Dec 1995 03:18:46 -0500
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Mike,
It has been my experience that the AB763 has throatier low mids with
smoother top end than the silverface versions. I would continue with the
course you are on but start messing with different output tubes (my
favorite in a Super Reverb are the Sylvainia 6L6GC with the single getter)
and bias points. Also try a bunch of tubes in the first gain stage, until
you find one that you like. Preamp and output tubes vary like crazy. I
would definitly keep the 5U4 (not the GZ34 as in the AB763) in that
circuit to keep B+ voltage down, also.
You can try disconnecting the feedback line. Some guys like this and some
don’t. This is usually a very dynamic mod, but can also introduce a bunch
of noise
One last thing would be to put a replace the 100k resistor on the first
gain stage plate with a 220k. This will certainly brown it up quicker.
Good luck with the tone quest, Tim
From SRSN81–(at)–rodigy.com Tue Dec 5 15:42:27 CST 1995
Article: 6147 of alt.guitar.amps
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From: SRSN81–(at)–rodigy.com (Joseph Pampel)
Newsgroups: alt.guitar.amps
Subject: Re: Fender SR–silverface vs. blackface circuit
Date: 5 Dec 1995 17:35:54 GMT
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reaike–(at)–x.netcom.com(Randall Aiken ) wrote:
>resistor for both tubes. A far better approach would be to use a
>separate capacitor for each resistor from cathode to ground, or a
>common resistor of half the value bypassed with a large capacitor to
>ground. I don’t know why this is never mentioned in the books.
– It’s in several of the old “Hi-Fi” design books actually. I’d always
wondered about the differences of using seperate Rk’s or using 1 Rk of
half the value (approx) to bias an output stage. Always assumed the amp
makers used 1 resistor to simplify construction, but then tying the
cathodes helps to balance the stage also I guess.
>Mark’s point about the differences in the phase inverter circuit is a
>good one. The 47K resistors put the tube on a completely different
>spot on the plate curves, and lower the gain. In addition, there will
>be a difference in the balance of the two plate waveforms, since the
>gain at one side of the phase inverter is larger than the other side,
>but the plate resistors are of equal value.
I also agree on the Phase Inverter. The amps I’ve modded allways
benefitted from the earlier PI circuit. I think the lower (47K) Rp’s
were about lowering the output imp of the PI to keep the amp cleaner
longer, and also to maximize the Va on the tubes (less resistor, less
drop). There are probably a bunch of more complicated reasons but those
are the ones I can think of. 🙂 If you like a lowr gain PI circuit, you
acn always use the AB763 type PI but raise the “tail” resistor from 27K
up to 47K or whatever suits you. Or if you’re a gain hound you can drop
it to 6.8K and just go nuts..
>150 ohm resistors. Also, don’t believe everything you read; if it
>sounds better to you with the “bad” circuit, by all means, use it.
I couldn’t agree more. All that matters is whether YOU like the way it
sounds.
>Also, has anyone tried a direct comparison of the two biasing methods
>with the same phase inverter circuit and no grid capacitors? Perhaps
>you could do this, Mike, and get back to us with the results. It seems
>to me that if cathode bias is universally preferred over fixed bias, a
>point in between the two would be better than fixed bias alone.
>
A buddy of mine has run his Princeton Reverb both ways (Fixed and cathode
biased) and he likes the cathode biased arrangement a lot better. He also
uses a Deluxe Reverb OT to get more oomph from the amp. Another friend
converts all his BF amps over to cathode bias to drop the power and also
because he likes the sound and feel a lot better. He just converted over
a BF Bassman head that I had converted to 5F6-A specs to cathode bias and
the output dropped to about 40W and he says it got much sweeter sounding.
And it sounded pretty ok to begin with. I know it’s harder on the tranny
twins, but I think it can be done fairly reliably as a retrofit if one is
careful.
Joe
From mgarvi–(at)–anix.com Wed Dec 6 10:19:07 CST 1995
Article: 3998 of rec.audio.tubes
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Fender SR–silverface vs. blackface circuit
Date: 6 Dec 1995 06:23:49 -0500
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I’d like to say up front that if it sounds like I’ve disagreed with
Randy Aiken on a couple of the following subjects, it’s really not
the case. He just manages to touch on subjects that interest me,
so my opinions are a bit more…er…’detailed’.
So, Randy–I’ll see your 100-line post and raise you 100 lines!
>> Mike Kent writes:
>>
>>I converted an silverface AB568 Super Reverb (mixed cathode/fixed
>>bias) to the generally more desired blackface AB763 fixed cathode
>>circuit. Sure, the amp is cleaner and more powerful now, but… I
>>think I preferred the complex harmonics of the AB568 circuit!
>>
>>I’d like to hear some opinions about the AB568 circuit, beyond the
>>tube book lore about how lousy the silverface amps were. Mark Garvin
>>has said in the past that he suspects the phase inverter circuit as
>>being a problem, rather than the mixed bias configuration on the
>>output tubes.
Hi Mike, I’ve tried to elaborate on the reasons (see below).
I was originally referring to the fact that the phase inverter
was changed at the same time, so some blame may have spilled over.
The 47k phase inverter tends to have a clean, defined sound.
Perhaps perceived as thinner than the older 82k/100k circuit.
Keep in mind that the AB568 cathode resistors are probably
developing under 5 volts of bias. 2 x 150 ohms is equivalent to a
shared 75 ohm cathode resistor…not a huge value. And in comparison
to true cathode-bias circuits, probably not a huge component of the
sound (but still a factor).
>>Can anyone comment on their experience with
>>modifications to the AB568 circuit? If the silverface circuit is so
>>lousy, please explain why.
{ comment left in, cause I’d also like to hear responses to this. mg }
>>reaike–(at)–x.netcom.com (Randall Aiken ) wrote:
>>
>>>As to a “class A” amplifier sounding better and having “touch
>>>sensitivity”, the reviewers are usually talking about a class AB
>>>cathode-biased amplifier and calling it a class A amplifier. Due to
I’ve heard some that some of the same ‘reviewers’ said that only
class AB amps have “touch sensitivity”. Quotes preserved here
because I think they are referring to bias shift, which has been
discussed in your separate thread on blocking. Applies equally
well to preamp and power tubes, of course.
Obviously, there will be differences in the effects (both pro and con)
in a cathode biased amp, since the output tubes will try to resist
(no pun) the bias shift.
>>>Randy:
>>>the negative feedback action of the cathode bias resistor and the
>>>shifting of operating point with signal level, these amplifiers
>>>generate a richer series of harmonic overtones when driven hard.
>>Mike Kent:
>>That’s what my ears heard when I tried out the AB568 mixed
>>cathode/fixed bias over the AB763 circuit. Yet everybody avoids AB568
>>like the plague. Why?
>reaike–(at)–x.netcom.com (Randall Aiken ) wrote:
>I haven’t compared the two circuits side by side, so I may be off-base
>here, but I tend to agree with Mark Garvin.
>There are a couple of points of interest, though. The AB568 uses
>separate bias resistors that are bypassed with one capacitor from
>cathode to cathode. This creates a degenerate feedback arrangement by
>coupling the two cathode signals together for AC signals, and would be
>the same (for AC signals) as using a single, unbypassed cathode
>resistor for both tubes.
I’ve heard some good pro’s and con’s regarding the shared resistor.
On one hand, some claim that the resistor helps the output tubes to
‘stay in balance’. But think about it: the resistor is half the
value that it should be for a single tube. So if one tube were
pulled out, the remaining tube will be running at double the normal
current. Not good.
This can occur in normal circuit operation: tubes age, one starts
hogging current, the gap widens, etc. Not exactly ‘keeping them
in balance’ is it?
>A far better approach would be to use a
>separate capacitor for each resistor from cathode to ground, or a
>common resistor of half the value bypassed with a large capacitor to
>ground. I don’t know why this is never mentioned in the books.
Intuitively I would have said the same. But I’ve found that the shared
cathode-to-cathode cap is surprisingly effective. No, I don’t use it,
but I tested it. I’d agree though–best avoid it if possible. I don’t
see much merit unless you’re building a million amps.
>In addition, let me clarify my statement about cathode bias shifting of
>operating point. Cathode bias actually causes a smaller shift in
>average plate current from zero signal to max signal than fixed bias
[interesting comments deleted for brevity. Read Randy’s article if
you missed this].
>Mark’s point about the differences in the phase inverter circuit is a
>good one. The 47K resistors put the tube on a completely different
>spot on the plate curves, and lower the gain. In addition, there will
>be a difference in the balance of the two plate waveforms, since the
>gain at one side of the phase inverter is larger than the other side,
>but the plate resistors are of equal value.
Yes, there is indeed a slight imbalance. But there’s a reason that
Fender stayed with the two 47k’s. At least if my math and programming
is correct. (I got tired of running the same long series of calcs and
wrote a C++ program to confirm math results–all caveats apply)
I believe that given the low value plate resistor and relatively high
shared cathode resistor, the ‘ideal’ value for the south-side plate
resistor is 49K or so. This was done with textbook-perfect values
for 12at7 plate res, mu, etc. Maybe merits jumping to the next std
5% value (51k) but maybe not. Fender certainly thinks about buying
volume of one given part, so I’d imagine that helped tip the scales.
>In addition, there is a rather large 2000pF cap from grid to ground on
>the output tubes. This will roll of the high frequencies quite a bit,
>contributing to a difference in tone. They were probably added to
>prevent oscillations, so be sure to check for this if you remove them.
Ah, another interesting Weber-focal-point. He claims that they will
shunt lots of treble. Looks logical, right? If you look back at
source impedance and see 47k resistors…or even 82k/100k, then
yes–it would start shunting freq’s higher than 1600hz or so. But
the 11k plate resistance of the 12at7 driver will change the picture.
Figure close to 8Khz at -3db. Certainly not as much a factor as it
may appear. And no worse than the common 470pf cap across a 12ax7’s
load resistor.
Part of Weber’s focus on this cap may be:
1. He never really tested it.
2. Caps to ground are just taboo.
3. He routinely replaces 12at7’s with 12ax7’s
Regarding the latter: he *does* recommend this frequently. And the
12ax7’s plate res is significantly higher, so the rolloff would be
noticeable.
Brings up another question: Why replace the 12at7 with a 12ax7?
My math and ears agree on this one: using a 12ax7 cannot boost gain
very much in the long-tail driver circuit. Probably less than 1 db
at best, even with the 82k/100k circuit.
BUT with the 47k circuit, a 12ax7 will actually LOSE gain. Yep, try it
and post comments, please. The other voodoo factor in play is that
12ax7’s are a bit ‘beefier’ sounding than the more concise, analytical
12at7. I’m vegetarian, though, so I’m staying with 12at7’s for phase
inverters. Well, until further notice, anyway.
By the way, I routinely remove the .002’s but not just to avoid treble
rolloff–I try to keep induced phase shifts within a feedback loop
to a minimum. Always a good idea. Again, contrary things in play:
best not to have any phase shift > 180 degrees. But if a high frequency
is going to shift by 180 anyway, best to make sure the open-loop gain
at that frequency (probably ultrasonic) is well below unity. Consider
the latter kinda tough. We’re talking about a relatively high open
loop gain.
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
Along with Weber’s and others’ recommendations to change Fenders back
to the old style phase inverter goes this caveat (posted for the
general public, since R.A. knows chapter and verse):
Fender changed the phase inverter values for a reason. Tube manuals
specify a max dc path to ground from the 6L6 control grids at around
100k. Too much resistance can cause degenerative effects or ‘runaway’
in the output tubes. The 47k plate resistors are merely an artifact–
the REAL objective was to get the bias-feed resistors down to 68k
(or 100k in some circuits). This low value of bias feed would have
loaded down the previous phase inverter circuit anyway.
>Randy:
>In conclusion, contrary to what the “experts” with the tube books say,
>I think Mark was correct when he stated that the phase inverter
>differences probably contribute more to the tone differences than the
>150 ohm resistors. Also, don’t believe everything you read; if it
>sounds better to you with the “bad” circuit, by all means, use it.
Thanks, Randy. My own posts aside (of course), I have to say that
I’ve seen more real knowledge of tube amp circuitry on this news
group than I have in most ‘mod’ books. Though I have to admire
the nuts to just go ahead and send it to the publisher.
>Also, has anyone tried a direct comparison of the two biasing methods
>with the same phase inverter circuit and no grid capacitors? Perhaps
>you could do this, Mike, and get back to us with the results. It seems
>to me that if cathode bias is universally preferred over fixed bias, a
>point in between the two would be better than fixed bias alone.
I’ve tested it, but not at very high volume. My interest at the time
was to see if I could tell the difference between the sound of the
fixed and semi-fixed bias. I hooked up a switch. Not much difference,
but at the time I couldn’t blast it. I suspect that published comments
on the mixed-bias circuit have limited basis in reality, but I’d
still like to hear from others about this.
By the way, has anyone read the new Vintage Guitar mag? A new Weber
mod: tie the two output tube cathodes together and put a single 10 ohm
resistor from there to ground. This is to help balance the tubes.
He emphasises that this is NOT the same as the 150 ohm Fender resistors
since they are separate.
Did I read his article correctly? 10 ohms?
Regards,
Mark Garvin
From reaike–(at)–x.netcom.com Thu Dec 7 10:40:27 CST 1995
Article: 4016 of rec.audio.tubes
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From: reaike–(at)–x.netcom.com(Randall Aiken )
Newsgroups: rec.audio.tubes,alt.guitar.amps
Subject: Re: Fender SR–silverface vs. blackface circuit
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In <4a3ug5$gt--(at)--anix2.panix.com> mgarvi–(at)–anix.com (Mark Garvin)
writes:
>
>I’d like to say up front that if it sounds like I’ve disagreed with
>Randy Aiken on a couple of the following subjects, it’s really not
>the case. He just manages to touch on subjects that interest me,
>so my opinions are a bit more…er…’detailed’.
>
>So, Randy–I’ll see your 100-line post and raise you 100 lines!
Heh..heh…it’s all in good fun and in the best interest of everyone
who reads these to have more than one opinion…I just wish more people
would join in on these 10K posts – makes good bedtime reading!
I don’t have time for a feature-length response, so I’ll just add my
two cents worth in where I think it belongs…
>I’ve heard some good pro’s and con’s regarding the shared resistor.
>On one hand, some claim that the resistor helps the output tubes to
>’stay in balance’. But think about it: the resistor is half the
>value that it should be for a single tube. So if one tube were
>pulled out, the remaining tube will be running at double the normal
>current. Not good.
>
>This can occur in normal circuit operation: tubes age, one starts
>hogging current, the gap widens, etc. Not exactly ‘keeping them
>in balance’ is it?
Yes, but think of the money they saved in AC30’s not having to buy four
200 ohm 5 watt resistors, but just one 50 ohm? 🙂 And besides, Groove
Tubes stay matched for the life of the tubes, don’t they??
>>A far better approach would be to use a
>>separate capacitor for each resistor from cathode to ground, or a
>>common resistor of half the value bypassed with a large capacitor to
>>ground. I don’t know why this is never mentioned in the books.
>
>Intuitively I would have said the same. But I’ve found that the
>shared cathode-to-cathode cap is surprisingly effective. No, I don’t
>use it,but I tested it. I’d agree though–best avoid it if possible.
>I don’t see much merit unless you’re building a million amps.
>
Look for my updated post on this…two resistors are necessary because
they changed over from a bias adjust pot to a bias balance pot, and the
tubes need separate cathode resistors to develop different DC bias
voltages to achieve balanced currents in the output transformer.
The cap couples the two cathodes together for AC signals.
>
>>Mark’s point about the differences in the phase inverter circuit is a
>>good one.
>>balance of the two plate waveforms, since the gain at one side of the
>>phase inverter is larger than the other side, but the plate resistors
>>are of equal value.
>
>Yes, there is indeed a slight imbalance. But there’s a reason that
>Fender stayed with the two 47k’s. At least if my math and programming
>is correct. (I got tired of running the same long series of calcs and
>wrote a C++ program to confirm math results–all caveats apply)
>
>I believe that given the low value plate resistor and relatively high
>shared cathode resistor, the ‘ideal’ value for the south-side plate
>resistor is 49K or so. This was done with textbook-perfect values
>for 12at7 plate res, mu, etc. Maybe merits jumping to the next std
>5% value (51k) but maybe not. Fender certainly thinks about buying
>volume of one given part, so I’d imagine that helped tip the scales.
>
Yes, the 47K may be right for the lower plate impedance of the 12AT7,
but I was referring to the imbalance between plates of the long-tail
pair irrespective of tube type and plate resistance. Any tube used as a
long-tailed pair will have an imbalance at the plates due to the
different path gain from the input grid side. A constant current source
will reduce this (the relatively large resistor acts as a poor current
source), but generally the two plate resistors are made slightly
different, or adjustable. I haven’t run the numbers on a 12AT7 vs a
12AX7, so I don’t know if they are less sensitive to the imbalance
because of the lower mu and rp, but the gain equations have both those
variables as well as the plate load resistance in them.
>>In addition, there is a rather large 2000pF cap from grid to ground
>>on the output tubes. This will roll of the high frequencies quite a
>>bit, contributing to a difference in tone. They were probably added
>>to prevent oscillations, so be sure to check for this if you remove
>>them.
>Ah, another interesting Weber-focal-point. He claims that they will
>shunt lots of treble. Looks logical, right? If you look back at
>source impedance and see 47k resistors…or even 82k/100k, then
>yes–it would start shunting freq’s higher than 1600hz or so. But
>the 11k plate resistance of the 12at7 driver will change the picture.
>Figure close to 8Khz at -3db. Certainly not as much a factor as it
>may appear. And no worse than the common 470pf cap across a 12ax7’s
>load resistor.
>
You are absolutely right, the equivalent source resistance taking into
account the internal plate resistance in parallel with the 47K’s and
the 100K grid bias feed resistors (plus the 1.5K grid stopper) is about
9.7K, which gives about an 8kHz corner frequency, and most speakers are
pretty far down by this point.
>Part of Weber’s focus on this cap may be:
>1. He never really tested it.
>2. Caps to ground are just taboo.
>3. He routinely replaces 12at7’s with 12ax7’s
You forget, this is the same guy who says coplanar traces on a PC board
affect the sound too much because of their capacitance (around a few
pF, if that?), and that’s why point-to-point wiring sounds better.
I’ve used PC boards at 50MHz and more without having too much rolloff;
I don’t recall guitars having harmonics that high that you need to
preserve…
>Brings up another question: Why replace the 12at7 with a 12ax7?
>My math and ears agree on this one: using a 12ax7 cannot boost gain
>very much in the long-tail driver circuit. Probably less than 1 db
>at best, even with the 82k/100k circuit.
>BUT with the 47k circuit, a 12ax7 will actually LOSE gain. Yep, try
>it and post comments, please. The other voodoo factor in play is that
>12ax7’s are a bit ‘beefier’ sounding than the more concise, analytical
>12at7. I’m vegetarian, though, so I’m staying with 12at7’s for phase
>inverters. Well, until further notice, anyway.
>
I’ll have to think about this and get back… 🙂
>By the way, I routinely remove the .002’s but not just to avoid treble
>rolloff–I try to keep induced phase shifts within a feedback loop
>to a minimum. Always a good idea. Again, contrary things in play:
>best not to have any phase shift > 180 degrees. But if a high
>frequency is going to shift by 180 anyway, best to make sure the
>open-loop gain at that frequency (probably ultrasonic) is well below
>unity. Consider the latter kinda tough. We’re talking about a
>relatively high open loop gain.
You have to roll the gain off at the high end somewhere in the forward
path; but, along with the compensation cap’s rolloff does come a phase
breakpoint….(we won’t go into the evils of negative feedback
compensation here!)
>
>Along with Weber’s and others’ recommendations to change Fenders back
>to the old style phase inverter goes this caveat (posted for the
>general public, since R.A. knows chapter and verse):
>
>Fender changed the phase inverter values for a reason. Tube manuals
>specify a max dc path to ground from the 6L6 control grids at around
>100k. Too much resistance can cause degenerative effects or ‘runaway’
>in the output tubes. The 47k plate resistors are merely an artifact–
>the REAL objective was to get the bias-feed resistors down to 68k
>(or 100k in some circuits). This low value of bias feed would have
>loaded down the previous phase inverter circuit anyway.
