"Blackfacing" a 100W silverface Twin?

Discussion in 'Amps and Cabs' started by AlRob, Jan 17, 2008.


  1. AlRob

    AlRob Member

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    Has a anyone had any experience with "blackfacing" a silverface Twin? Mine is a 70's, 100W with a master volume. It's a nice amp but the clean tone has always sounded a bit brittle to me. I was wondering if one of the mod kits available might help richen the tone of the amp and get rid of the brittle sound. Any other suggestions to help the sound of this amp? Thanks alot for your help.
     
  2. meterman

    meterman Member

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    Basically the further you get away from the original circuits the harder and more costly it is to "blackface" a Fender, according to several techs I've spoken with. The addition of the master volume also coincided with other less obvious circuit and component changes which basically need to be 'undone'. By the late 70s the transformers had changed to an ultralinear type and the wattages increased (70w Supers, 130w Twins) and these amps are so far off it's not really worth it IMHO. NMV Fenders from pre-'73 are the easiest to convert back and a good tech would charge around $100 for it. BF Twins sound great but unless volume can be pushed up around 4-5 the tone will still not be optimal for that amp. You might have better results with a Pro or Super Reverb...
     
  3. DGG

    DGG Member

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    I've done it on a 69 TR and it was a complete overhaul. Essentially ripped out all the guts, cleaned everything up and built the AB763 circuit. Took some considerable amount of time to build and installed the best parts. It sounded great afterward.
     
  4. AL30

    AL30 Member

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    Yes, blackfacing your twin will help take out the "brittle" clean sound. I have a '72 w/ a MV and converting it did just that. Before the mod I had to use a pedal when I was running clean just to take the edge off. It will warm things up a bit for you and I would recommend it if you aren't bonding with the SF.

    You shouldn't need a mod kit. Just compare some schematics. A quick search for the mod should also reveal a lot of information. You can order parts from numerous sources - Hoffman, Mouser, Digi-key etc.

    http://www.hoffmanamps.com/

    AL
     
  5. hasserl

    hasserl Member

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    Yeah, it's not hard to do and the MV actually gives you an added benefit. The stock MV is not very useful, but it's easy to rewire this to a post PI type MV that makes the amp much more versatile. SInce the location on the amp already exists you don't have to drill a hole into a classic amp to add it, so this is a benefit. Don't listen to the naysayers, this is a worthwhile project and a cool amp.
     
  6. John Phillips

    John Phillips Member

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    +1.

    IMO the best of all worlds is a 'halfway' circuit with Blackface/early Silverface circuitry but retaining the master volume, rewired as a defeatable post-PI MV. The location and type of the pot are ideal for doing this, as hasserl said.

    The only thing to be careful of is that you then don't accidentally push the MV knob in and defeat it when you've got the channel volume fully cranked or you'll instantly become Marty McFly and end up in the next room ;).

    These amps are LOUD :).
     
  7. AL30

    AL30 Member

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    hasserl, do you a schem of the MV you're talking about or at least an amp that uses it? Thanks.


    John if he's got the 100 Watt SF model it does not have the Push/Pull MV. Just a standard pot. I believe the MV's started on the twins around 1972 and went to Push/Pull around 1976. The push/pull's (to my knowledge) are the 135Watt UL models.

    Marty McFly HA !! :crazy

    AL
     
  8. pfflam

    pfflam Silver Supporting Member

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    I love that brittle sound.

    But I would say go for it .. .. study and research and then warm up the soldering iron . . . i did my first 'blackface' to a, um, a blackface Bassman, and the best part about it is having an amp that you yourself have made what it is . . . I like that in all endeavors though, and, it demystifies some of the spookum and adds some more in places where it should be
     
  9. Dana-L

    Dana-L Supporting Member

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    Running 12AY7s in place of the 12AX7 valves in the early preamp positions should warm things up a bit.

    Cheers,

    -Dana
     
  10. John Phillips

    John Phillips Member

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    No, definitely not. I do have a '72 100W here with a non-pull MV, but the pull circuit came in probably shortly after that and pull-boost 100Ws are far more common than non-pull ones in my experience. This is the first non-pull MV one I've seen in a long time. It must be one of the very first MVs actually, since it has a 'tail' logo too.
     
  11. AlRob

    AlRob Member

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    Thank you all for the replies.
    John, hasserl, What does "post-PI" mean?
     
  12. AlRob

    AlRob Member

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    Mine is a 100 watt with a push-pull MV (which sounds really bad in the pull position). I'm not exactly sure of the year. The guy I bought it from in 1988 said it was from the 70s
     
  13. AL30

    AL30 Member

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    Really. Hmmm? That's interesting. Maybe it's a location thing. Although I do think the push/pull seem to show up a bit more I have seen MV models. I have a '72 (it's a Quad not a straight twin but the same circuit) with a standard MV.

    So your "No definitely not" comment was referring to 1) not all push/pull being UL models or 2) the year when these were introduced. Not arguing - just curious.

    PI stands for Phase Inverter


    AL
     
  14. John Phillips

    John Phillips Member

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    Both. There are three variations of 100W Twin - non-MV, MV without pull boost, and MV with pull boost. In my experience the MV/non-pull-boost is by far the rarest. It could even be that both changes occured in '72, I don't have a definite date for the pull boost. As far as I can remember all the non-pull MVs I've seen - not more than a handful, I remember wondering if the first one I saw was a mod - have been '72s.

    I can understand why they would change it quickly - if they did - since the non-pull-boost model really doesn't have enough gain to make the MV worthwhile. (But putting it after the PI instead does since the PI distortion is then included.)

    Later 1976 is the correct date for the change to the 135W Ultra-Linear model though, yes.
     
  15. AL30

    AL30 Member

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    Thanks John. That's good info.

    I just did a quick search - someone had quoted the Fender book as saying the push/pull came around 1974 and the 135W UL a few years later.

    AL
     
  16. John Phillips

    John Phillips Member

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    Quite probably correct, I may just not have seen any '73s! Or they over-ordered pots in '72 so all the codes say that into 1973 as well (I usually look at the pot codes for dating rather than the transformers for some reason) - just as they did with guitar pots in 1966 and you find these right up to 1970.
     
  17. 2x6L6

    2x6L6 Supporting Member

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    Definitely do the mod. I used Weber's book, "Desktop Reference of Hip Vintage Amps" as a guide, but there are several "how-tos" on the web - they're all basically the same - change the bias from balance to blackface level adjust, resistors and caps in the PI section, etc (working from memory here - more to it than that but nothing requiring an EE degree). I always refer to this as "blackfacing" (using the quotes) because (according to Weber) unless you also swap out at least one of the transformers, you haven't gone all the way to blackface voltages, which is part of the BF tone.

    Also, as many other folks have, I went with a mixed circuit, as I found the blackface PI coupling cap value to be brighter than I preferred, so went back to the SF value. But overall I found the "blackface" mods I did to my SF Deluxe and Vibrolux Reverbs to be pleasing. Made already great-sounding amps sound even better/different.

    You might want to keep a log as you go - I found it very handy to record each change I made, including results of play-testing the changes as I went. Also did this with cap comparisons - i.e. between Orange Drops and Mallory 150s. It's been an indispensible reference when I've done additional work on the amps, or just wanted to remember exactly what I did and what the results were. This is how I did it:
    http://www.alaska.net/~jimbeck/guitars/Vibrolux Reverb Maintenance and Mod Log.htm

    Funk's book has a couple cool mods to the Fender tone stack that are also worthy.
     
  18. hasserl

    hasserl Member

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    Um, there are a few different post PI MV's you could use. The simplest to wire up is the "crossline" type, as used in Matchless amps. It's not the best sounding MV in many cases, because it needs a well balanced PI. But I've installed one in a Super Reverb that works, well, Super. ;) Seriously, it works well in these amps because they do have well balanced PI's, and that is one area you don't want to screw up when "Black Facing" the amp. (see below) Also, this type is easily defeatable, which John mentioned. Since the pot already has a push/pull switch, you just incorporate that to take the pot out of the circuit and the amp will act as if it is not even there. You will have a complete non-MV amp. However, just engage it with the switch and you have the MV available too. It's really a trick setup and it's surprising to me it hasn't been more widely embraced.

    post phase inverter Putting the master volume after the phase inverter allows you to take advantage of the clipping of the phase inverter that is THE sound of classic overdriven amps. People talk about power tube distortion, that is rare. What they mostly think of as power tube distortion is actually the PI clipping. Putting a master volume before the PI prevents this, and gives all pre-amp distortion. Putting it after (post) adds the PI clipping to the preamp distortion, and gives 90% of the sound of a hard driven amp. It's not quite the same as a non MV amp cranked up, but it's close. And since most of us can't dime a non MV amp where we play, it adds versatility.

    My advice when black facing a silver face amp is to be careful with the PI section. You can change out the tail resistor, and the grid resistors, but leave the 47k plate loads alone. Those 47k plate loads are actually better for working with a 12AT7 and the amps sound better with them in place IMO. It will drive the power tubes harder with less farting out and less blocking distortion. There is not much else different between these SF MV models and the classic AB753. The preamps are identical. Just clean up that funky pull boost section, and master volume, and be careful with the PI. ANd I actually like the bias balance pot. You can add a bias level pot if you want to make bias jobs easier. But it's not a big deal to change bias resistors, IMO, and I like the balance function of the stock setup.
     
  19. JDJ

    JDJ Supporting Member

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    I owned a '72 Twin w/ push/pull master volume. It had JBLs.
     

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