Bruce, 

I'll have to try out some of those tone
stack configurations on one of my
blackface Fenders. 

Lately, I've been favoring 250p(SM),
.047 bass, and .022 mid with a 100K
slope resistor. 
I used to use .022 for both bass and
mid, but found the tone too be too
thin-sounding. Changing the bass cap
from .022 to .047 seemed to improve
the bass and low-midrange response
and bring out more of that woody,
strat neck-pickup tone. By the way, I
use a .01 cap (instead of the stock
.001) as input to the phase inverter, if
I modify the tone stack cap values. 

I've been meaning to try
experimenting with lower slope
resistor values, such as 91K and
82K. It sounds like trying out a 330p
treble cap might also be a good thing
to try out. I'm always interested in
ways to get more sweetness, warmth
and fatness out of blackface Fenders,
while still retaining some clarity and
spank. 

I know it can be difficult to describe
in words, but what tonal qualities do
you like about the result of the 330p,
.022, .022, 85K-90K combination?

* * * * * * * * * * * * * * * * * * * * * *
Yes Jim! 
I forgot to mention the last coupling
cap to the phase inverter. One of the
most important ones to consider too.
Make it a great cap! No junk. 
I always bump that up to at least a
.01uF O' drop or some other
polypropylene cap of some kind. 

The difference in tone with these
mods are almost exactly as you
describe. 
I think the clarity is retained by
controllng how much of the low-lows
get to the phase inverter. 
So I don't let the preamp stages
develop too much energy below
100Hz in the first place. 
If I have an amp that needs a bit more
fill in the upper bass and low
midrange, I install a .033uF to .047uF
cap too but never more if I've
bumped the overall preamp gain. 
The caps you choose are especially
good for an average Strat player, Jim.

This also works real well in BF
Deluxes too but the 6V6's grids can
get hit too hard and fold up so I use
the standard tricks to limit any grid
current. 
But, I find that if you don't limit the
sub bass lows, 
all the BF amps can get a wee bit
farty at full power using a hot neck or
any powerfull pickup, so a little
descretion is advised. 
A SM .0018uF to .002uF installed
between the treble pot wiper and the
volume control is a good fix for that
too. This will limit the low rumble
sub bass freqs by about -3dB to -6db.

Change the 820 feedback resistor to
about 2K5 and replace the 47 to 100
ohm long tail ground resistor to about
240 - 360 ohms and bypass it with a
.5uF to 2uF cap. Yes that was two uF
not .2uf. 
Too much will brighten the amp too
much, so use your ears alot while
doing this or any mod! 
Also, it is important to remember to
tune the amp while keeping a bass
player and or keyboard player in
mind. 
Most tuners get a little carried away
with the low end freq response. The
guitar player and amp sound GREAT
all by themselves in a practice room
but then it ends up stepping all over
the bass player's low mids and the
overall band sound is a little muddy,
nebulous and un-groovey. 
You can't just tune the thing for a guy
who plays all by himself in his living
room every time. 
Well, if the guy is a thrasher, drop
most of the mids and bump the highs
a bit and make the amp very
powerfull in the bass end. They love
it. 
An interesting small subtle tone trick
to darken up the sound a little.. use a
120K to 180K plate load resistor
with a small 110pF to 1000pF SM
cap across it on the first preamp
section. 
This "can" take some of that annoying
sizzle off the top end while
increasing the gain to fuzz out the
amp a bit. 
Do what is needed to keep the tube
biased at around .001a to .0015a and
about a 5uF/10uF cap. 
Another thing to try, with the reverb
BF amps, is to lower the reverb
mixing resistor to about 2M2 to 2M7
and use a 27pF to 47pF SM cap
instead. 
If your not really into the vibrato
sound..... 
Replace the 50K vibrato pot with one
that has a push pull switch so you can
lift this pot from ground when you
don't want it. Big gain boost. 
Or, use a 100K resistor on top of the
50K pot for a little more signal to the
last stage and still have a little
vibrato left over. 
That will give the last section a nice
little bump. 
On my 50 watt Crusader amps ( my
version of a single channel hot
rodded Super Reverb) I don't use any
vibrato, so I use a fixed 100K to
180K resistor in the circuit to
ground.. 
Check a stock SR to see what I
mean...where the signle splits after
the dry/wet mixing resistor/cap
combo to the vibrato section and
tube. 
As far as clarity and spank... 
I don't use much more then 5uF to 10
uF bypass caps in the preamp circuits
anymore. 
And regardless of what I've said in
the past or anyone else says, I use
NO ceramic caps or Metal Oxide
resistors in ANY tone circuit or
audio coupling circuit. 
A little less gain in the PI/driver and
then turn the volume control up higher
will crunch it up a little too. 
The Svetlana 6L6 is my choice of
weapons at 440v and 34ma. 
Well there goes another novel.....
sorry folks. 

Bruce 
Mission Amps 
yes, I still build the BlueStone
amps.....etc

* * * * * * * * * * * * * * * * * * * * * * * * *

I'm also a big fan of the vibrato
circuit disconnect mod - not only a
significant gain increase, but a real
tonal improvement as well! 

I too, never understood why Fender
decided to design their blackface
amps with a preamp circuit that
generates massive bass response and
then use a .001 cap on the phase
inverter to limit the bass response,
after the fact. 

I think the main areas for
improvement with the tone we often
get from the stock blackface circuit
are: (1) bass is too "tubby" sounding;
(2) midrange is a bit nasal; (3) treble
is a little harsh and grainy; (4)
overall tone is on the "tinny" side.
(Not that the stock tone is at all BAD
- it's still WAY better than that of the
"clean" channel of just about any
modern channel-switching amp,
IMO). 

A lot of players make futile attempts
to remedy these shortcomings only by
swapping tubes and paying big bucks
for NOS tubes. I believe the real key
to improving the tone is doing what
we've been talking about: changing
cap/resistor values in a few places,
upgrading caps to better quality
types, and disengaging the vibrato
circuit's 50K pot. (And combining
these mods with the use of good NOS
tubes in a few of the sockets can
yield UNBELIEVABLE tone!). The
real trick here is convincing
customers that the stock Fender sound
(even if the amp is in perfect working
order) can be significantly improved
upon. 

By the way, I'm curious - what is
your objection to using metal oxide
resistors in the signal path? What
about metal film?

* * * * * * * * * * * * * * * * * * * * * * 

I don't really have a strong objection
to the Mtl Ox or ceramic core
resistors... 
The carbon resistor vs anything else
resistor thing has been raging for a
while (and I hate to admit it), but,
after a long period of not using ANY
carbon comps, I built the last three
amps using just carbon comps and all
three had a new very nice subtle silky
hi freq rolloff that I really loved.
Very stringy. 
The three amps sounded very lively
but not so brittle or glassy with the
treble turned way up. 
As I replaced the carbon comps one
at a time with Mtl Ox or other
ceramic core resistors, I found that it
made no real difference in any circuit
other then the circuits that actually
passed audio to a next stage. 
When I'd use the ceramic core
resistors, that "zing" on top of all the
high freq notes would start coming
back in. 
I think you could use the ceramic
core resistors in the plate supply OK
too but the slope, mixing or
contouring sections sounded a tiny bit
smoother and less brittle when they
were the carbon comps. 
I know this sounds dumb too, but I'd
swear that if I used 1 watt carbon
comp resistors instead of 1/2 watt
ceramic core resistors, it made even
a bigger difference. 
I'd love to hear others opinion or
experience in this. 
I never thought I'd hear myself say
that, but there it is! 

By the way Jim, what you described
in your reply above was an excellent
discription of how to super tweak an
average mass produced amp. 
I don't even bother building two
channel 50 watt Fenderish amps
anymore! 
I never see anyone really using the
normal channel. 
And, as Steve A says, the
vibrato/reverb channel is now the
abnormal channel and it is the hot
one. 
Especially after all the tweaks. 
A good example of how the whole is
greater the sum of it's parts. 

Bruce

* * * * * * * * * * * * * * * * * * * * * *

I know this sounds dumb too, but I'd
swear that if I used 1 watt carbon
comp resistors instead of 1/2 watt
ceramic core resistors, it made even
a bigger difference 

Actually I can probably add some
credibility to this statement. Back in
'85 or so when I worked for a large
aerospace company, I had access to a
lot of expensive equipment and one
of my favorites was a HP impedance
analyzer. The $24,000 kind. I was
having a lot of trouble getting some
45MHz designs to have the gain I
wanted and one day I decided to see
what resistors looked like at higher
frequencies. 

I found that carbon comps, which
was the majority of our stock in the
lab, were terrible. 1K's were going
reactive (capacitive mostly) at 4 or 5
MHz. The higher the resistor value,
the lower the frequency. Carbon
films were a lot better with Metal
Films being the best. 

So when you get to the 100K to 1Meg
range, you're probably getting quite a
few extra picofarads thrown in as
well. I wish I would have taken notes
back then but all I cared about was
that I needed to use carbon films at
the least for the work I was doing at
the time. We restocked the lab with
carbon films immediately after that. 

I would go as far as to surmise that
with the larger volume of the higher
wattage resistors, this effect would
be greater. Noise always seems to be
a problem with comps though. 

--johng

* * * * * * * * * * * * * * * * * * * * * * 

That makes me feel a bit better John. 
I'm going to drag my LCR out and see
if I can measure any difference in
capacitance. 
I bet the inductive reactance is
climbing too and combined with the
additional capacitance of the carbon
comp resistor, it is acting like a tuned
choke. Sorta like those RF plate
chokes on high HF or VHF
transmitters. Hmm... I just had a brain
fart! 
My gut is telling me that the carbon
comps have some weird sliding
impedance bump upwards as the freq
goes up and the high freqs are seeing
a higher impedance then the lower
ones, thus a high freq attenuation. 
We should try a reisistor in parallel
with a very small (few ma) 50uH to
500uH plate choke and see what
happens. 
I'll grab a DMM and my HP signal
gen and push some 5kHz-20kHz freqs
through some various resistors and
see what comes out the other side. 
The jury is still out on the plate load
resistors, but, I think the ceramic core
resistors being more quiet is a great
trade off anyhow. 
For you folks out there that have not
had a chance to tweak some amps....
this is real anal stuff and don't
concern yourselves, that you'll never
get a great tone unless you do all this
nonsense!! 
This is mostly for us lousy players
who are gear heads and get off on
petty techno-woof-woof. 

Bruce