Silver face vs black face

[überfuzz]

Prologe
Every now and then there's a tinkerer asking questions about his, not sure we have any ladies here, silver face amp. There's some issue that has to be fixed. Quite often one of the helpers throws in a, oh and get it black faced while you're at it.

I never understood this craze for black faced amp, in comparison to a silver faced amp. In my view a black face amp is closer in sound to just any old combo amplifier whilst a silver faced amp often comes with a pure Fender tone. I have a black faces amp my self and I just love it, but I never hesitate to play a silver faced amp, never hesitate to use them while recording. With the right tubes and speakers they're just dandy, this goes for black faced amps to. Good tubes and speaker and your Fender will most likely sound great no matter if it's modded or not.

Comparison

Black face

• Earlier break up
• Linear tone
• Not so much of the Fender pull in the tone*
• Expensive

Silver face

• Late break up, if any
• Sheet hammock like tone, bass and treble
• Has the Fender pull*
• Often a bargain

*I don't have a good term on this, but a silver faced amp often comes with a tone that has an enormous push, oh seems I called it pull. Ah well, I guess you'll understand if you're ever heard it.

Ps, for all of you looking for a überdistorted amplifier, don't even go Fender in the first place.

[JoeM]

I dont think the differences are as great as you state, sonically speaking. I worked in a music store back in 70s, when SF amps were current production. Generally BF amps might break up a little earlier, but I've played great sounding SF amps and BF amps. (The later UL Fenders are really clean.) SF amps arent wired as neatly, my SFSR is a real rat's nest for example, but it does sound great. IMO the BF had nicer cabinets, I dont like the way the SF had the baffles done.

[Gibsonman63]

Blackfacing has become a smoke and mirror thing. Half of the Silverface Fenders I see advertised have had the "Blackface Mod". Most are nothing more than changing the bias to voltage adjust instead of hum balance and removing the snubber caps on the power tubes.

I have heard great sounding and mediocre sounding examples of both.

[überfuzz]

JoeM - Actually, I tried to make a case about the differences not being big enough to mod an amplifier. At least that's my view... It's often sufficient to just get good tubes, and speakers if it's a combo amp.

I agree on the wiring, I haven't seen that many seventies Fenders, but the ones I saw looked like they've been wired by chimpanzees. But fixing that is hardly a mod is it..?

[JoeM]

Yeah, my SF is near stock. But I did change the bias to adjust like Gibsonman63 mentions, rather than the bias balance it came with. But I dont consider that as BF'ing the amp. The stock PI values I prefer over the BF anyway.

[Gibsonman63]

A lot of the difference in tidiness of the wiring is the wire itself. The solid-core, cloth covered wire in the blackface amps really stays where you put it.

I went a little crazy on my SF Super Reverb. It was always a good sounding amp, but not great. I gutted it and pulled everything off the eyelett boards, then checked each componant for tolerance and wired the entire amp with cloth covered wire. Not for the faint of heart. In my opinion, it is a little warmer sounding and it breaks up slightly earlier and the tone is better balanced and doesn't get too trebly anymore.

[m-fine]

The main differences (twin reverb, other models vary)

1) The bias vs bias balance. No good reason not to fix this back to blackface bias adjust, or to have both. Bias balance only sucks.

2) Phase inverter
A) the 1m grids reduced to 330K in the Silverface years. This is not bad and was probably done to reduce resistor noise. Likely not an issue either way if you are not using carbon comp resistors.
B) The coupling cap (along with the grid resistors) allows more low frequencies to pass. A .001 with 1M grids has a -3db of 80 hz, the .01 with 330K's rolls off at 24 hz. A standard tuned guitar has the 6th string tuned right around 80 hz, but you may want the -3db point a little lower. Too low like 24 hz, and you are passing extraneous nois on to the PI and fron there to the power stage and speakers. My TR currently has the 330K resistors ans a .0047 coupling cap for a -3db of 51 hz. I think this works great.
C) The anode resistors (47 vs 82 and 100) and the cathode resistor (33 vs 22) set the gain and bias of the PI. This is probably where you are noticing some of the difference in breakup.

3) The resistors in the B+ chain alter the high voltage to the preamp section which also alters the bias slightly since the cathode resistors did not change.

4) The reverb driver cathode resistor was dropped WAY down from 2200 (with bypass cap) to 470 or even 270 (no cap). I have no idea why they would have done this, but mine had the 270 ohm resistor there and it cooked the tubes frequently. The BF spec had plenty of reverb, who the heck plays with it on 10 and wishes they had more?

5) On later models there was the awful push pull MV. If you want a MV there are several better options that will not suck the tone as much, or you can pull it out.

6) Some of the SF's have the extra caps which I think are mostly there to cover up potential issues from the sloppy wiring.

7) Finally, and this is not really a circuit change, some of the SF amps use lower quality caps. I am not a big believer in high dollar magic super mojo caps, but the cheap ceramics can have non-linearities and microphonic issues that affordable replacement Mallory 150's or Spragues, or a number of other brands will not have an issue with. For a buck or two a cap, I replace them.

Your mileage may vary, but if I am cracking the amp open for another reason, I am going to attack some or all of these

[greekie]

The BF amps also had the input wire (unshielded) twisted together with the wire from the volume pot, effectively creating a negative feedback loop that decreases treble response when the volume is turned up.

It has a small, but in my view important effect, that helps to complete the "Blackfacing" of later Fender-amps.

Jake

[hasserl]

C) The anode resistors (47 vs 82 and 100) and the cathode resistor (33 vs 22) set the gain and bias of the PI. This is probably where you are noticing some of the difference in breakup.

Several years ago on the old forum there was a good discussion about these differences and how the SF values are actually "more correct" for 12AT7 tubes than the BF values, which are more suited for 12AX7 tubes. In my experience the SF values do provide for less of the common Fender flatulence effect when the amp is turned up and pushed hard, and allow for greater non-linear operation of the power tubes. A well set up SF amp will sing better with less farting out and better power tube distortion than a comparable BF amp. My advice is to leave those SF PI values alone.

[m-fine]

Several years ago on the old forum there was a good discussion about these differences and how the SF values are actually "more correct" for 12AT7 tubes than the BF values, which are more suited for 12AX7 tubes. In my experience the SF values do provide for less of the common Fender flatulence effect when the amp is turned up and pushed hard, and allow for greater non-linear operation of the power tubes. A well set up SF amp will sing better with less farting out and better power tube distortion than a comparable BF amp. My advice is to leave those SF PI values alone.

I will have to go searching for that thread to see what people were saying and why. I solved the flatulence with the coupling cap ( 2. C) and different speakers. I am curious why the gain and bias of the PI would cause flatulence. It will impact when the PI clips and how, and to some extent how hard the output from the PI hits the power tubes, but that would be overdrive of all frequencies, not a farting out of the bass.

Also things that were not correct from the hi-fi standpoint are what made the classic guitar circuits sound good. Lets face it, most of the tone stacks are far from correct, and the 12ax7 cathode follower fender and marshall used on many designs is a hi-fi nightmare.

[JoeM]

The bias feed/grid resistors in SF amps are often 100k, (220k in BF), which I guess would reduce blocking distortion.

[hasserl]

Yes, it is the 100k bias splitter resistors that help to reduce flatulence. For an explanation of that see Aiken's paper on blocking distortion. i.e. "Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. "

Here is that discussion from the old forum, I had it saved (note, I cut and pasted relevant sections, there is a lot more in the thread):

From:
Doc @
Date: 9/11/2001 4:45 PM
Subject: Re: Bassman - AB165 vs. AA864 question
I recently completed a mod to a SF Bassman that was a little later than the AB165, but had similar power stage circuitry. They use a different arrangement of negative feedback, which doesn't look like the usual Fender loop going from the secondary to the phase inverter cathode. So that's the part they say to convert back to earlier, like AA864, specs. Also, the AB165 has a low gain final preamp stage that has local plate-to-grid feedback. In Andy Ruhl's article (Machine Gun Amps site) he mentions that this stage, although it was not Fender's design intention, results in a mild pleasing compression effect, so advises to leave that stage alone. Some have 1meg grid leak & 1meg Rf, others have 470k for both. Does this make it an
inverting stage with a gain=1?

One thing I'd like to mention about "blackfacing" the PI by changing its plate resistors from 47k/47k to 82k/100k: I was displeased with the higher value resistors. (The cathode string was also changed out.) My ears tell me that the 47k plate resistors and tail resistor network used in later SF amps results in a louder, cleaner signal than after BF'ing. My friend's unmodified AA371 bassman (47k plate res)is louder and stronger than my BF'd version. And his amp has the stock funky feedback arrangement. A few years ago, I rebuilt a SF Bandmaster reverb head. I left the 47k network and 330k grid resistors in the 12AT7 PI circuit, and basically recapped the amp without modifying much of anything in the preamp. (I did remove those partial cathode bias resistors in the 6L6 circuit, and converted to bias level adjust supply.) This amp had a wonderful Fender tone when the owner played his Telecaster through it. Everyone asked him about his amp. I guess I did something right without realizing it.

Although the earlier PI circuit utilizing 82k/100k plate loads with 1meg grid leaks has a higher ultimate voltage gain, it doesn't seem to provide the output stage drive capability for the AB2 power stage as the 47k (1watt) plate load/ 330k grids network. Another benefit with leaving the 47k's in there is you can plug in a 12AU7 directly and have it biased correctly. The 12AU7 lowers the maximum signal power because of its lower voltage gain, but seems to give the amp clean headroom and an unusual overdriven sound at max. Hard for me to put into words. You have to try it and listen.
So, for your amp, may I suggest you modify the bias supply and the NFB arrangement like BF amps (remember to reverse the green & black secondary leads on the speaker jack), but keep the plate & grid resistors in the PI at the lower impedance level. Decide what you want to do in the preamp for gain structure & tone shaping.

I purchased a new 1x12 combo cabinet, pine with black tolex & silver grill cloth, built to house a Bassman chassis. It'spretty cool to have a modified Bassman amp in a small package. The single 8ohm speaker makes the plates see twice the load resistance, but it sounds fine. Maximum power will be lower, but who cares. It's a good guitar amp.

Hopefully something in here will be helpful to you.
-Doc

P.S.- I like to leave the tube shield off the phase inverter tube. The tube runs cooler & lasts longer. At this high signal level, that stage doesn't need shielding anyway.
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From:
Ted Pimm @
Date: 9/11/2001 7:59 PM
Subject: Re: Bassman - AB165 vs. AA864 question
Thanks Doc! Some great ideas here. I have an AB165 modified the way you have described, but I have been running 100k plate load resistors in the PI. I am interested in trying the SF style of PI arrangement to see what tonal difference it makes, and I also have the 12AU7 sub to try! I am running a 4x10 (2 ceramics/2 alnico) for a 4ohm load, and I can tell you it is REALLY LOUD. A mismatch might actually be a good idea to lower a bit of volume. Thanks again for the fun ideas...
Ted.
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From:
Ray Ivers @
Date: 9/13/2001 1:19 PM
Subject: Re: Bassman - AB165 vs. AA864 question
Doc, I agree 100% about the PI load resistor change. IMO the 82K/100K or 100K/100K values, while appropriate for a 12AX7, have no business being in a 12AT7 or 12AU7 driver stage unless you're looking for a somewhat squashy, compressed sound. With a 12AU7, plate loads can be reduced even further for even more headroom and output tube drive, although eventually you'll end up with over 300V at the plates, of course. I like my stock AB165, but I'm not trying for anything like a sparkly clean sound out of it, and I bet I would like it even better with 33K - 47K PI plate loads.
Ray
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From:
KB @
Date: 9/13/2001 3:27 PM
Subject: Re: Bassman - AB165 vs. AA864 question
Thanks guys I have an AB 165 in shop right now that the bias has been converted to BF but the PI is 100k/100k and it breaks up pretty early at about 7 and the amp really compresses hard and gets mushy. I'm going to try Ray's 33k first then the 47k. I ran a Bass thru it and it sounded good until that point but these amps are Taylored for guitar. Has anyone made extra use of the unused 1/7025 ?
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From:
Shea @
Date: 9/14/2001 4:32 PM
Subject: More PI ideas / questions
Doc, I follow what you're saying about using the SF pi arrangement, but I need mine blackfaced to get the gain & distortion I want. Maybe I'd be happier using a 12AX7 together with blackface specs? On the other hand, sometimes a 12AX7 sounds more loaded down than a 12AT7, even using the blackface specs. The 12AT7 seems to push through clearer, while adding its own distortion.

Anyway, I'm getting the idea that a good design might be to develop all the gain you want before the pi, and use a 12AT7 or 12AU7 pi with SF specs to push more current through to the output tubes. That might give more room for gain & bass before you run into blocking distortion at the output tubes, although the DC blocking caps will still put a limit on how far you can go. It's like CBS almost got it right - for loud, clean amps they DID get it right (except for the bias circuit), but for the master volume models they needed to put more gain in the preamp.

I wonder how much more gain CAN I get out of a Fender preamp? I could change all the 1k5 cathode resistors to 820. Ray has told me that 220k plate resistors are uncool because they set the bias wrong (if I understood him correctly), so I'd be inclined to stick with 100k. I could reduce the value of the 3 meg resistor that leads to the reverb recovery tube, down to 1 meg or 500k. Will it be enough to rock?
Shea
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From:
Ray Ivers @
Date: 9/14/2001 5:01 PM
Subject: Re: More PI ideas / questions
Shea, It's quite possible to get the same gain with a 12AT7 as a 12AX7, especially when using lowvalue plate loads with both (in this case, you'll probably get more gain with the AT). You'll also be pushing the output tubes a bit harder, which will translate into more overdrive and crunch; does that sound like what you want? Plugging a 12AT7 into a 12AX7 circuit, especially a driver circuit, will not give you a true feeling for what a 12AT7 is capable of IMO, although it will function.

I'm actually a real fan of 150K - 330K plate loads with 12AX7's, provided that the tube is biased correctly for the new plate resistor; this is not really a drop-in mod. Changing all the 1.5K's to 820's and then putting in 220K plate resistors would be an example of what not to do IMO; you would end up with a strange bias point and asymmetrical clipping. I would use a 2.7K cathode resistor instead, and the 150K plate/3.3K cathode combination also looks like a winner to me in TubeCAD (370V B+).
Ray

[m-fine]

That shopuld give me enough info to find the thread, THANKS!

To the point though, which design (47's vs the 100/82) is better is going to depend a bit on which amp and what you are after. With a 80-100 watt Twin Reverb, "a louder, cleaner signal" may or may not be desirable. I certainly have no need for louder, and with all the extra head room, you can get plenty clean (IMHO) by backing of the volume a bit. But, if you are after the loudest cleanest amp, which is what the CBS engineers were trying to achieve in the 70's, yes, they did it right. For the most part they were not stupid, although the bias thing and cathode resistor on the reverb driver do make me wonder what they were smoking at times.

[m-fine]

The bias feed/grid resistors in SF amps are often 100k, (220k in BF), which I guess would reduce blocking distortion.

Mine were 68k on opposite sides of the bias balance pot, so an average of about 73k. I never swapped them for 220's since I initially added a bias control and left the balance inplace, but I ripped the entire bias section out recently when I installed the Vari-Watt FX100 kit which included separate bias controls and a LAR-MAR post PI MV which essentially replaces the resistors.

[hasserl]

That shopuld give me enough info to find the thread, THANKS!

To the point though, which design (47's vs the 100/82) is better is going to depend a bit on which amp and what you are after. With a 80-100 watt Twin Reverb, "a louder, cleaner signal" may or may not be desirable. I certainly have no need for louder, and with all the extra head room, you can get plenty clean (IMHO) by backing of the volume a bit. But, if you are after the loudest cleanest amp, which is what the CBS engineers were trying to achieve in the 70's, yes, they did it right. For the most part they were not stupid, although the bias thing and cathode resistor on the reverb driver do make me wonder what they were smoking at times.

But, what you also end up with is an amp that drives the power tubes harder and further into non-linear operation. I think that what the guys there were describing as cleaner and louder is the effect of reduced voltage gain at lower volume settings. IME as you crank the volume up past 5 up to the sweet spot around 7, and particularly if you are pushing the front end of the amp with a boost, the 47k PI will drive the power tubes harder than the 82k/100k PI will, and combined with the 100k bias splitters will remain flatulence free without blocking distortion. If you like "power tube" distortion the 47k PI delivers better than the 82k/100k PI does. Try it out for yourself and see.