Fender BF/SF lead dress: feedback
Robert Angst
while digging into the lead dress issues with not so recent Fender amps
I found some threads on shielded wire in SF Fenders and the capacitance
introduced by that.
Looking closely at the BF lead dress I have the impression that quite
often signal carrying wires with opposite phase (e.g. input jack to 1st
grid and 1st plate to tone control) are twisted or run parallel in close
proximity. Does this not equal a frequency dependent feedback for each
of these stages? Wouldn't that effect the tone at least as much as a
small low-pass from a shielded cable? What could be the benefit of the
BF approach?
Thanks,
Rob
PMG
I haven't done enough comparisons. And I own a limited amount of
Fenders.
In the BF/SF eras, the my amp with the best lead dress work is my '65
Bassman. Way downhill from that, is my '68 Bassman. Even farther
downhill (understatement) is my '73 Princeton Reverb. The only one I
recall having any shielded cable in it is the PR. Not surprisingly,
the '65 Bassman is the amp with the best mojo. Wire dressing? Or
parts? IMO, it's probably a combination of the two.
I imagine that people go overboard with shielding. Seems to be a
substitute for good craftsmanship.
Now, if I felt more ambitious, I'd rip the wiring out of my '68
Bassman, and rewire it like my '65 Bassman, and using solid core cloth
insulated wiring. But I don't feel like taking on the project.
Besides, if I ever rebuild that amp, I'd rather rebuild it as a Tweed
Bassman.
Pete
--
Oh look, the exploding circus is coming to town.
One night only. --Brak's Dad
Rich Koerner
This is a good one.
Robert Angst wrote:
>
> Hi,
> while digging into the lead dress issues with not so recent Fender amps
> I found some threads on shielded wire in SF Fenders and the capacitance
> introduced by that.
As a design consideration, layout location of key signal control components on the front panel,
parts board, and chassis location of active devices (vacuum tubes)... HUM is most often in the
forefront of the mind of the designer, with the problems created lead dress, as an after thought.
If both layout and lead dress were initial design considerations throughout, the applied practice
you mention below, would not be all that necessary, to address the problems of lead dress created by
improper layout.
In the world of amplifier designers, you can tell the men from the boys, by the presence,... or
better yet, the absence of such practice.
Especially, when it involves the HIGH GAIN guitar amplifiers, demanded by tone mad musicians who
expect perfect amplifiers in an imperfect world.
> Looking closely at the BF lead dress I have the impression that quite
> often signal carrying wires with opposite phase (e.g. input jack to 1st
> grid and 1st plate to tone control) are twisted or run parallel in close
> proximity. Does this not equal a frequency dependent feedback for each
> of these stages? Wouldn't that effect the tone at least as much as a
> small low-pass from a shielded cable?
The answer to BOTH your questions is YES!!!!!!!!!!!!!!!!!!!
With respect to your second question. That depends if the problem frequency resides in the audio
range of human hearing, or if produces an artifact in the audio range of human hearing.
However the question remains, could proper layout eliminated such practice early on in initial
layout design stage of development. Here again, the answer is YES.
> What could be the benefit of the
> BF approach?
That would be the same as in any electronic amplifier.
It's either an act of prevention, or an act of cure.
From many years of working on vacuum tube amplifiers, you can see the lost art of vacuum tube
amplifier design go down the crapper.
For example, just take a tour through these pages and see for yourselves.
http://timeelect.com/vinamps.htm
http://timeelect.com/gtampidx.htm
How is it that on these two pages, those old amps from the years gone by, sound so killer when
compared to those over-kill high tech amplifiers with their over populated circuit boards.
Compare this, http://timeelect.com/mar2k.htm
To this, http://timeelect.com/eh-150.htm
That EH-150 has the most KILLER natural gain, and shits on anything found in the store today. It's
an amazing amp.
<thinking>
There are very few around today who understand the fine details of proper design practice in high
gain vacuum tube circuits. It's a lost art today when it was common practice years ago.
To those today who don't truly understand this, it becomes the land of Voodoo and Witchcraft.
Regards,
Rich Koerner,
Master Witch Doctor at
Time Electronics.
http://www.timeelect.com
Specialists in Live Sound FOH Engineering,
Music & Studio Production,
Vintage Instruments, and Tube Amplifiers
PMG
wrote:
WTF Rich!! You just killed my mild interest in Hughes & Kettner amps!
Ok, that wasn't the one that interested me, but...
>How is it that on these two pages, those old amps from the years gone by, sound so killer when
>compared to those over-kill high tech amplifiers with their over populated circuit boards.
>
>Compare this, http://timeelect.com/mar2k.htm
>
>To this, http://timeelect.com/eh-150.htm
>
>That EH-150 has the most KILLER natural gain, and shits on anything found in the store today. It's
>an amazing amp.
>
>
><thinking>
>
>There are very few around today who understand the fine details of proper design practice in high
>gain vacuum tube circuits. It's a lost art today when it was common practice years ago.
>
>To those today who don't truly understand this, it becomes the land of Voodoo and Witchcraft.
>
>
>Regards,
>
>Rich Koerner,
>
>Master Witch Doctor at
You'll undoubtedly be burned at the stake. Like the rest of your
kind. Don't worry though, you'll be given a trial. Maybe even a last
cigarette.
Wämp
Rich. I'll wonder about this and watch the thread. I understand it
took Leo lots of tries in the practical balance between getting it and
the cure. The AGA has helped me begin to see / understand some of these
things. I still wonder about the best choice for chassis material and
magnetic eddies. Things like the place ment of chokes, irons and all.
Also when is human hearing the cursor? Can there be effects before we
can hear, them, that dictate what we do hear? Neatness of dress, best
I've seen are Sun's , HiWatts and a Sove Tek (yup impeccable neatness in
a Sov Tek)
I get the feeling the BF stuff went for a more quiet reliability than
the very early 60s stuff which look like fire boxes, but the tone stacks
cheapened. The most hideous lead dress, I have seen, was in the special
order 73 DR I use.
I still wonder why heaters are supplied the way they they are.
Currently I'm tearing out some wiring in a circa 1911 (not 45) Craftsman
home. The stuff from 1911 is holding up better than the stuff from the
50s the stuff from the early 70s is ok, as yet. Often the neutrals are
running more than 4' apart! Naturally who neens a ground? GFI bath.
Peace Chris
Phil S
"Rich Koerner" <ri...@timeelect.com> wrote in message
news:43330F5C...@timeelect.com...
>
>
>
> This is a good one.
>
And the reply was at least as good as the question!
Thanks, Rich. On-topic reading has been mighty scarce here in the last few
weeks.
Regards,
Phil
Wämp
go. What does one paint on there? Thanks Chris
Robert Angst
Thanks for the input everybody.
I'd like to get back to my original question. When I was writing about
reversely phased wires running in parallel I was referring to a '65
Vibrolux, an amp that is definitely regraded as good sounding. Is this
possibly because of the neg. feedback introduced by the lead dress for
-each stage- of the preamp?
Damn, so little components and still so much to learn ;-)
Rob
Wämp
Rich Koerner
I have been changing my software over here and didn't see some posts pop up in some threads I had
posted in.
Robert Angst wrote:
>
> Thanks for the input everybody.
> I'd like to get back to my original question. When I was writing about
> reversely phased wires running in parallel I was referring to a '65
> Vibrolux, an amp that is definitely regraded as good sounding. Is this
> possibly because of the neg. feedback introduced by the lead dress for
> -each stage- of the preamp?
Gee, this is a rough one to answer without having it on the bench to verify the present function of
the amp.
You see, one has to verify is the present condition of operation is, or is NOT, dependent of this
negative feedback/cancellation created by physical lead dress.
In other words, is it prevention, or cure for any oscillation from a positive feedback to any of the
input stages that make up the amplifier. Be it local or global.
One test is to lift and separate the leads and see if the amp breaks into oscillation. If the amp
does not break into oscillation, then the lead dress is an act of prevention.
With the amp on a RTA (real time analyzer), with pink noise on the input, now you return to the
original lead dress, while noting any change in the RAT.
If there is change on the RTA, then the lead dress does impact the sound of the amp. If there is no
change on the RTA, the lead dress has no impact on the sound of the amp.
Now, going back to the undressing of the lead dress. If all hell breaks loose with all kinds of
oscillation, then the act of the lead dress is now a cure for such. Which would indicate improper
layout to some degree.
<thinking>
OK, so you have a Fender amp where all the normal lead dress is in place, all the components have
been updated, and the amplifier is still acting up with the gremlins that are causing problems from
Voodoo Land.
The first things you do is remove the Rev Pan, the Rev driver tube, and the tremolo tube. This
removes the Rev circuits as a possible door for gremlins to walk in through, and gets the tremolo
out of the picture too.
The B+ will rise on the remaining stages, giving them more gain, and making it easier for the
gremlins to do their Mac Nasty.
Now, verify if the gremlins have left, are in either channel, or both.
Chances are, they didn't leave.
If they left one channel, pull that pre amp tube, and see if the leave the remaining channel. If
they did, either the two pre amp stages are talking to each other, or the problem is at the input of
the mixer stage for the Rev and Trem.
If they didn't leave the remaining channel, the door they enter can be either of the first two pre
amp stage control grids for that channel.
If the remaining channel happens to be the Normal channel, pull the mixer tube, and see if the
gremlins leave. If they left, the open door is the control grid of that stage, or the control grid
of the Rev return line.
At this point, if the gremlins are still active, all you have left is the Normal pre amps stages,
the phase inverter, and the output stage to play with the lead dress.
To find which wire that talks the loudest, use a scope set for max sensitivity, watch the trace as
you bring it in proximity of the wires. Big trace deflection, big mouth gremlin talking on that
wire. Do this with a sine/square gen with the amp on a test load.
> Damn, so little components and still so much to learn ;-)
It never stops!!!!!!!!!!!!!!!!!!!!!!!!!!!
Regards,
Rich Koerner,
Wämp
vanity press, I bet you do better than break even. Peace Chris
Robert Angst
cool post. But I don't have problems with oscillations. I was originally
referring to a posting of you where you described dramatic sound changes
after ditching some shielded wires in a SF Fender.
My point (or better: question) is, that this may not have to do with the
capacitance of the cable (twisted wires have one, too, and the -3db
frequency of the filter formed by that cap is quite high..) but some
feedback that is not there with the shielded cable.
I know, kinda heretical since it is contrary to the common believe that
the 'purer' (that means not filtered or altered by feedback) the signal
is, the better. But then again we are not talking about good or bad, we
are talking about "the" sound ;-)
Cheers,
Rob
Rich Koerner schrieb:
Rich Koerner
Robert Angst wrote:
>
> Thanks Rich,
> cool post. But I don't have problems with oscillations. I was originally
> referring to a posting of you where you described dramatic sound changes
> after ditching some shielded wires in a SF Fender.
Yes, shielded wire will attenuate highs. Some types or brands, more than others.
Keeping in mind, all shielded wire is not created equal.
Some wire is designed for RF (radio frequency) and some for AF (audio frequency).
Any attenuation is a change in tone.
> My point (or better: question) is, that this may not have to do with the
> capacitance of the cable (twisted wires have one, too, and the -3db
> frequency of the filter formed by that cap is quite high..) but some
> feedback that is not there with the shielded cable.
It's possible. Both, involve capacitance to various degree.
With the shielded wire, the capacitance involves the singular wire. However, in the case of lead
dress, stray capacitance due to proximity, does effect multiple wires. That, with respect to each
wire in proximity, to each other, and the chassis ground. Not only are the inter wire capacitance's
present, the signal levels of each wire greatly impact the out come.
Keeping in mind, that not all phase cancellation is due to an exact 180 degree phase shift between
grid and plate. You also have the additional phase shift caused by the coupling caps. Which, is not
constant throughout the frequency range of amplifier operation, and will cause peaks and valleys in
the frequency curve of the amplifier.
There are less variables involved in the use of a shielded wire, in spite of the attenuation from
wire capacitance. Correction for the attenuation can sometimes be corrected in a later stage in the
amp.
> I know, kinda heretical since it is contrary to the common believe that
> the 'purer' (that means not filtered or altered by feedback) the signal
> is, the better.
Yeah, that seems to be the point of view many can not achieve anymore.
But, that would be the goal.
Here again, is such practice an act of prevention or cure.
> But then again we are not talking about good or bad, we
> are talking about "the" sound ;-)
Yes, and because of that, proper electronics convention is tossed out the window.