From clear–(at)–cicnet.org Tue Aug 25 10:36:01 CDT 1998
Article: 123396 of alt.guitar.amps
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From: Admiral Ballsy
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Tue, 25 Aug 1998 02:29:22 GMT
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In article <6rsr8c$1a9$--(at)--nrp1.dejanews.com>,
In article <35E1A8CA.A56E85C--(at)--acstaff.wiscDOTedu>,
> Charles Thomas
> > I have a bit of a historical question.
> >
> > It seems to be almost unanimously believed that the “Blackface” pre-CBS
Fender
> > amplifiers sound light-years better than their Silverface counterparts (in
> > cases when actual CBS wiring and component changes took place).
> >
> > What I’m wondering about was the thinking behind making these changes. Was
it
> > cheaper to make the amps the CBS way? Was there a shortage of parts, labor,
> > know-how? Why would a company make changes to amps that are pretty
> > universally recognized as being a bad idea?
> >
I think the changes can be lumped into several categories:
1. Make the amps clean – get rid of distortion. I believe that this is the
motivation behind the changes to the inverter stage.
2. Fix problems caused by other changes. The PI changes also made the amps
sound thin. The coupling caps from the PI to the output tubes were in most
cases made larger, I think, to help remedy this.
3. Save money. The grid caps on the power tubes allowed sloppier (read:
faster/cheaper) lead dress. The bias balance let Fender use unmatched tubes.
4. Increase reliability. I think that the reason the SF amps went to 5U4GB
rectifiers is because good 5AR4s are a hell of a lot harder to make (read:
expensive). 5U4GBs are a more rugged. Devotees of Euro GZ34s may argue, but
an indirectly heated rectifier with such a low internal resistence is by
nature difficult to accurately manufacture and more susceptible to
shock-induced failure.
AB
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From ashinma–(at)–hem.ucalgary.ca Tue Aug 25 20:52:37 CDT 1998
Article: 123534 of alt.guitar.amps
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From: Scott Hinman
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Tue, 25 Aug 1998 18:23:46 +0000
Organization: The University of Calgary
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Admiral Ballsy wrote:
> I think the changes can be lumped into several categories:
>
> 1. Make the amps clean – get rid of distortion. I believe that this is the
> motivation behind the changes to the inverter stage.
Having black-faced a SF pro-reverb in steps, and tried really hard
to be objective all along the way, changing the plate resistors
in the PI section had very little effect, if any, on what I heard
out of my amp. At the time, I thought it maybe got a little louder,
but I didn’t install switches to go back and forth. This wasn’t
a really scientific sort of test. (One day I’ll go backwards
with it and find out in more concrete terms what thats all about).
> 2. Fix problems caused by other changes. The PI changes also made the amps
> sound thin. The coupling caps from the PI to the output tubes were in most
> cases made larger, I think, to help remedy this.
The coupling caps in the BF amps (Super, Pro, Vibrolux, Twin, Deluxe)
were 0.1 microFarad in the BF amps. They were the same value
in the early SF amps. Did they change later on? Perhaps in conjunction
with the master volume junk?
>
> 3. Save money. The grid caps on the power tubes allowed sloppier (read:
> faster/cheaper) lead dress.
I put switches in the grid caps on mine, and neither myself nor my
wife could hear any diff. between swithching them in or out. Even
during the middle of a loud sustained chord. (I also had it on
a scope at the time to look for oscillation, which there was none
of). I suspect that whether or not you hear any difference due to
the grid caps depends on what speakers/guitar you may be using.
For what its worth, the grid caps amount to installing a low pass
filter with a cutoff frequency around 7 kHz. Most guitar speakers
cutoff at around 5 kHz.
> The bias balance let Fender use unmatched tubes.
Well, yes.
>
> 4. Increase reliability. I think that the reason the SF amps went to 5U4GB
> rectifiers is because good 5AR4s are a hell of a lot harder to make (read:
> expensive). 5U4GBs are a more rugged.
In fact, some of the models that originally had 5AR4/GZ34 went the
other way. e.g. The BF AB763 Super Reverb had a GZ34 in it. The
“dreaded” AB 568, with all of the worst of the CBS mods, went with
the 5U4GB. Same with the Pro, and some others. They likely went
to the 5U4 for cost reasons alone ( a guess on my part). In any event,
the GZ34 is so close to solid state in performance, I don’t why
they didn’t just stick diodes in there. (Which they were doing
anyway with the BF Twin Reverbs).
As far as the early SF amps went, the only big difference in sound
that I heard when I BF’d my SF, was due to the partial cathode bias
they threw in there. And even then, it was just because they didn’t
use a big enough bypass cap on the cathode resistors. When I did my
amp, I rigged it with switches so I could short the cathode to
ground (i.e. BF), or switch 200 microF caps in parallel with the
cathode resistors, as opposed to the single 25 microF cap between the
two cathodes that Fender(CBS) used. Couldn’t tell the diff between
the 200 mF caps in parallel with the cathode resistors, or just shorting
the cathodes to ground. (Yes, I had a decade resistance box in there
to let me adjust the bias to the same level within a matter of mseconds
when I switched).
Anyway, my take on it is that the whole BF/SF thing is a little
overblown. (Of course, I’m talking early SF here, .. never played
with any of the master volume stuff). People will argue with me,
I know. But thats what my guitar and my speakers told me. Yours
may differ.
Best regards,
Scott H.
From clear–(at)–cicnet.org Wed Aug 26 11:10:51 CDT 1998
Article: 123615 of alt.guitar.amps
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From: Admiral Ballsy
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Wed, 26 Aug 1998 13:15:00 GMT
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In article <6rvjba$b4--(at)--s2.acs.ucalgary.ca>,
ashinma–(at)–hem.ucalgary.ca wrote:
> Admiral Ballsy wrote:
>
what thats all about).
>
> > 2. Fix problems caused by other changes. The PI changes also made the amps
> > sound thin. The coupling caps from the PI to the output tubes were in most
> > cases made larger, I think, to help remedy this.
>
> The coupling caps in the BF amps (Super, Pro, Vibrolux, Twin, Deluxe)
> were 0.1 microFarad in the BF amps. They were the same value
> in the early SF amps. Did they change later on? Perhaps in conjunction
> with the master volume junk?
Mea culpa! I meant that the PI INPUT coupling cap was made larger, .01 vs.
.001 in most cases. This *does* have a noticeable effect, it lets a lot more
bass through. Whether you like it or not it a matter of taste.
AB
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From john_g00–(at)–otmail.com Wed Aug 26 16:04:27 CDT 1998
Article: 123640 of alt.guitar.amps
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From: “johng”
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Subject: Re: Silverface…. WHY?
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Scott Hinman wrote in message <6rvjba$b4--(at)--s2.acs.ucalgary.ca>…
>Admiral Ballsy wrote:
>
>> I think the changes can be lumped into several categories:
>>
>> 1. Make the amps clean – get rid of distortion. I believe that this is
the
>> motivation behind the changes to the inverter stage.
>
>Having black-faced a SF pro-reverb in steps, and tried really hard
>to be objective all along the way, changing the plate resistors
>in the PI section had very little effect, if any, on what I heard
>out of my amp. At the time, I thought it maybe got a little louder,
>but I didn’t install switches to go back and forth. This wasn’t
>a really scientific sort of test. (One day I’ll go backwards
>with it and find out in more concrete terms what thats all about).
They do make a difference but mostly when the grid caps are still on the
output tubes. The larger values raise the corner frequency of the high
frequency rolloff. They will approximately double the gain of the phase
splitter as well.
>> 2. Fix problems caused by other changes. The PI changes also made the
amps
>> sound thin. The coupling caps from the PI to the output tubes were in
most
>> cases made larger, I think, to help remedy this.
>
>The coupling caps in the BF amps (Super, Pro, Vibrolux, Twin, Deluxe)
>were 0.1 microFarad in the BF amps. They were the same value
>in the early SF amps. Did they change later on? Perhaps in conjunction
>with the master volume junk?
I think he was talking about the couple cap to the *input* of the phase
splitter here. The cap change is always coupled with changing the grid
resistors. 1Meg/.001uF vs. 330K/.01uF. The 330K/.01uF lowers the frequency
response of the input to the PI. I found that the increase lower frequencies
getting to the output stage in a DR will cause it to start distorting a lot
earlier that it would with the BF values.
>> 3. Save money. The grid caps on the power tubes allowed sloppier (read:
>> faster/cheaper) lead dress.
>
>I put switches in the grid caps on mine, and neither myself nor my
>wife could hear any diff. between swithching them in or out. Even
>during the middle of a loud sustained chord. (I also had it on
>a scope at the time to look for oscillation, which there was none
>of). I suspect that whether or not you hear any difference due to
>the grid caps depends on what speakers/guitar you may be using.
>For what its worth, the grid caps amount to installing a low pass
>filter with a cutoff frequency around 7 kHz. Most guitar speakers
>cutoff at around 5 kHz.
7KHz? with 47Ks in the PI, the impedance of a 1200pF capacitor (not all grid
caps are 1200pF, but mine were) in parallel with the 220K grid bias
resistors is 47K at 2.2KHz. The gain of the PI section is half of its
unloaded value at this point because the 47K load is in parallel with the
plate resistors. At lower frequencies, the cap is a higher impedance and the
load becomes dominated by the 220k grid bias resistors. Without the caps,
the load on the plates is 220K all the time. I don’t see where you came up
with 7KHz?
With the plate resistors at 82K/100K, this point occurs when the impendance
of the capacitor (in parallel with the 220K) is equal to ~82K. This occurs
at ~1KHz, which is why they probably changed to the 47Ks in the first place.
If you have a decent quality speaker in there, the difference is *very*
noticable.
>> The bias balance let Fender use unmatched tubes.
>
>Well, yes.
>>
>> 4. Increase reliability. I think that the reason the SF amps went to
5U4GB
>> rectifiers is because good 5AR4s are a hell of a lot harder to make
(read:
>> expensive). 5U4GBs are a more rugged.
>
–Rectifier toob stuff snipped out–
>
>Anyway, my take on it is that the whole BF/SF thing is a little
>overblown. (Of course, I’m talking early SF here, .. never played
>with any of the master volume stuff). People will argue with me,
>I know. But thats what my guitar and my speakers told me. Yours
>may differ.
Did you change the speakers to match? If not, you really only did half of
the BF mod. Some of the changes (input to the PI) were made, IMHO, to
compensate for the lack of low-frequency response of the Oxford and Utah
speaker they were using at the time. If you don’t have a ‘Jensen’ style
speaker(e.g. Webers or Naylors), you’re not going to hear the majority of
the difference.
I have done both a 74 Deluxe Reverb and a 76 Pro Reverb of my own and the
differences were incredible. I would call it the difference between ‘alive’
and ‘sterile’.
That’s my experience.
–john greene
From clear–(at)–cicnet.org Wed Aug 26 16:04:43 CDT 1998
Article: 123663 of alt.guitar.amps
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From: Admiral Ballsy
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Wed, 26 Aug 1998 20:13:26 GMT
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In article <904149246.10239--(at)--etgate.RadioConnect.com>,
“johng”
> >
> >The coupling caps in the BF amps (Super, Pro, Vibrolux, Twin, Deluxe)
> >were 0.1 microFarad in the BF amps. They were the same value
> >in the early SF amps. Did they change later on? Perhaps in conjunction
> >with the master volume junk?
>
> I think he was talking about the couple cap to the *input* of the phase
> splitter here. The cap change is always coupled with changing the grid
> resistors. 1Meg/.001uF vs. 330K/.01uF. The 330K/.01uF lowers the frequency
> response of the input to the PI. I found that the increase lower frequencies
> getting to the output stage in a DR will cause it to start distorting a lot
> earlier that it would with the BF values.
>
AB: Yup, I was. But not all amps got the 330K; for example, Supers got the
.01 cap, but kept the 1M resistors.
> Did you change the speakers to match? If not, you really only did half of
> the BF mod. Some of the changes (input to the PI) were made, IMHO, to
> compensate for the lack of low-frequency response of the Oxford and Utah
> speaker they were using at the time. If you don’t have a ‘Jensen’ style
> speaker(e.g. Webers or Naylors), you’re not going to hear the majority of
> the difference.
AB: Speaker changes weren’t an across-the-board thing. Again, using the
Super as an example, I’ve seen the same alnico CTS speakers in amps dating
>from ’65 up to ’73.
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From ashinma–(at)–hem.ucalgary.ca Wed Aug 26 21:08:14 CDT 1998
Article: 123678 of alt.guitar.amps
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From: Scott Hinman
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Wed, 26 Aug 1998 12:30:18 +0000
Organization: The University of Calgary
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Scott Hinman wrote:
>
> Admiral Ballsy wrote:
>
> > 4. Increase reliability. I think that the reason the SF amps went to 5U4GB
> > rectifiers is because good 5AR4s are a hell of a lot harder to make (read:
> > expensive). 5U4GBs are a more rugged.
>
> In fact, some of the models that originally had 5AR4/GZ34 went the
> other way. e.g. The BF AB763 Super Reverb had a GZ34 in it. The
> “dreaded” AB 568, with all of the worst of the CBS mods, went with
> the 5U4GB. Same with the Pro, and some others. They likely went
> to the 5U4 for cost reasons alone ( a guess on my part).
Oops! Obviously it was pretty late when I wrote this. I didn’t
really say anything different than you did. I think what I was
getting at (who knows for sure) was that some of the early SF’s
(like my AB668 Pro Rev.) retained the GZ34. They didn’t move
to 5U4 till AA1069. Sorry for the confusion.
Regards.
Scott H.
From billbolto–(at)–EMOVE-TO-EMAILacslink.net.au Thu Aug 27 09:04:31 CDT 1998
Article: 123764 of alt.guitar.amps
From: billboltonREMOVE-TO-EMAI–(at)–omputer.org (Bill Bolton)
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Thu, 27 Aug 1998 10:49:04 GMT
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Admiral Ballsy
> I think the changes can be lumped into several categories:
>
> 1. Make the amps clean – get rid of distortion. I believe that this is the
> motivation behind the changes to the inverter stage.
As has been demonstrated previously in this newsgroup, the BF phase
splitter actually runs the tubes outside of specification.
The SF phase splitter changes runs them within their specification.
Its my opinion that give the various recorded statements about the
overall profile of Fender’s warranty experience with their amps as
production volumes increased, many (though not all) of the changes
that were made simply in response to both perceived and real issues
which would potentially reduce warranty costs.
Given that Leo never again became involved in amplifiers after leaving
Fender and seemed to be quite content to work on instruments, its
arguable that a number of amp changes would have occurred whether
Fender had been sold or continued under its former ownership.
Cheers,
Bill
From postmaste–(at)–riodeel.com Fri Aug 28 09:45:45 CDT 1998
Article: 123873 of alt.guitar.amps
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From: postmaste–(at)–riodeel.com (Ned Carlson)
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: 27 Aug 1998 23:38:18 -0500
Organization: Triode Electronics
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On Thu, 27 Aug 1998 16:17:24 GMT, “johng”
wrote:
>I don’t recall reading any discussion about the phase splitter running
>outside of specification. I’ll search DejaNews to see if I can find anything
>but right now I can’t think of how reducing the plate resistors would
>improve anything as far as operating point is concerned.
I think what he’s referring to is the fact that the 220K
grid resistors on the output tubes are indeed higher than
what’s spec’d for maximum DC grid circuit resistance
on a 6L6, with fixed bias.
Good 6L6’s will indeed work with a 220K grid
resistors, but cutting the value back to 100K
means one needn’t be quite as picky about tubes.
I imagine (and in fact I’ve of similar things happening
in the TV industry) that the people who supplied the tubes
may not have wanted to take duds back if the
tubes were used like that, or Fender was rejecting tubes
that wouldn’t work right with the 220K resistor,
and the supplier complained, or maybe it was some CBS
dork who saw that the value was out of spec & insisted it
be changed, whether it needed it or not.
Ned Carlson Triode Electronics “where da tubes are!”
2225 W Roscoe Chicago, IL, 60618 USA
ph 773-871-7459 fax 773-871-7938
12:30 to 8 PM CT, (1830-0200 UTC) 12:30-5 Sat, Closed Wed & Sun
http://www.triodeel.com
Your Start Page for Tube and Tube Amp info on the net…
http://www.triodeel.com/tlinks.htm
From mgarvi–(at)–anix.com Mon Aug 31 09:12:20 CDT 1998
Article: 124200 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: 31 Aug 1998 03:08:02 -0400
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>Scott Hinman wrote in message <6rvjba$b4--(at)--s2.acs.ucalgary.ca>…
>>Having black-faced a SF pro-reverb in steps, and tried really hard
>>to be objective all along the way, changing the plate resistors
>>in the PI section had very little effect, if any, on what I heard
>>out of my amp. At the time, I thought it maybe got a little louder,
> “johng”
>They do make a difference but mostly when the grid caps are still on the
>output tubes. The larger values raise the corner frequency of the high
>frequency rolloff. They will approximately double the gain of the phase
>splitter as well.
John,
True that gain can *theoretically* be increased by raising plate
resistance, with a perfect current source approaching the tube’s
mu. A couple things to consider, though:
First, loading of subsequent stages will defeat the purpose of
using a current source plate load. The bias supply resistors
will load the circuit anyway — even if there were something
to be gained by keeping plate impedance that high. BTW, the
bias supply resistors are the component that operates the
output tubes out of spec: High values possibly causing tubes
to ‘go gassy’ or even fail with (somewhat obscure) latchup modes.
Second, the increase in gain is not linear with respect to the
plate res value. The delta in gain slows after the plate load
surpasses the tube’s plate resistance. A 12at7’s plate res is
low enough that the silverface->bf change will not yield anywhere
close to a 2:1 improvement. In fact, the gain of the Fender
silverface split tail circuit is already fairly high, so I’d
expect a couple db or so. A tube with high plate res (12ax7)
would show much more variation in gain, of course.
>7KHz? with 47Ks in the PI, the impedance of a 1200pF capacitor (not all grid
>caps are 1200pF, but mine were) in parallel with the 220K grid bias
>resistors is 47K at 2.2KHz. The gain of the PI section is half of its
>unloaded value at this point because the 47K load is in parallel with the
>plate resistors. At lower frequencies, the cap is a higher impedance and the
>load becomes dominated by the 220k grid bias resistors. Without the caps,
>the load on the plates is 220K all the time. I don’t see where you came up
>with 7KHz?
I believe it was already pointed out that the 12at7’s plate
resistance is part of the source impedance equation. The rp is
probably around 11k.
MG
From larrys–(at)–ol.com Tue Sep 1 10:42:13 CDT 1998
Article: 124341 of alt.guitar.amps
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From: larrys–(at)–ol.com (LarrySB)
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
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>Not in all SF amps. Look at the Twin – the plate voltages drop by over
>50V at
>the change to SF.
I was looking at one the other day in fact, and the actual plate B+ was 496V.
The AA769 Schematic (from Pittmans book) does show 410v, which is a drop from
the AB763 schematic. That would support you contention about the earliest
Silverface Twins.
However, every other version of the Solverface Twin shows ever increasing power
supply voltages.
Every other model of Silverface amp shows a marked increase in B+ voltage,
especially in the later years.
—
Dr. Nuketopia
Compiling at this very moment.
Read the Blue Glow in Tubes FAQ at http://www.persci.com/~larrysb
Please note that your email is *not* spam in the subject line.
From clear–(at)–cicnet.org Tue Sep 1 10:42:27 CDT 1998
Article: 124401 of alt.guitar.amps
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From: Admiral Ballsy
Newsgroups: alt.guitar.amps
Subject: Re: Silverface…. WHY?
Date: Tue, 01 Sep 1998 14:03:15 GMT
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In article <1998090103510200.XAA1485--(at)--adder03.news.aol.com>,
larrys–(at)–ol.com (LarrySB) wrote:
> >Not in all SF amps. Look at the Twin – the plate voltages drop by over
> >50V at
> >the change to SF.
>
> I was looking at one the other day in fact, and the actual plate B+ was 496V.
>
> The AA769 Schematic (from Pittmans book) does show 410v, which is a drop from
> the AB763 schematic. That would support you contention about the earliest
> Silverface Twins.
>
> However, every other version of the Solverface Twin shows ever increasing
power
> supply voltages.
>
> Every other model of Silverface amp shows a marked increase in B+ voltage,
> especially in the later years.
When the MV came along, yes. But the AA270 circuit shows 405V; note also that
the power supply resistors between the 20mfd filters got larger, too, further
lowering preamp B+.
FWIW, I’ve got a ’71 on the bench now that runs ~440V, ref my ’65 that runs
about 480V.
AB
