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Circuit differences between later Fender amps

Discussion in 'Amp Tech Center' started by itsGiusto, Dec 10, 2020.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I'm trying to understand the circuit differences between some of the Blackface/Silverface fender amp models. Some of them seem close to identical! What's the difference between these 2x6L6 models?

    • Bandmaster
    • Pro Reverb
    • Vibrolux
    • Super Reverb

    And what's the difference between these 4x6L6 models?
    • Showman
    • Dual Showman
    • Twin
    All of these seem to feature the same tone stack, LTPI driving 6L6s, tremolo, etc. There are some differences in whether or not there's reverb in a few, but not all.

    And one additional question - I'm wondering, is there an amp which is exactly like a blackface twin, but a 2x6L6 instead of 4x6L6?
     
    Last edited: Dec 10, 2020
  2. oldlefty

    oldlefty TDPRI Member

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  3. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    The circuits are indeed very similar, that's why Fender stopped using a unique circuit code for each amplifier. The speaker complement is primarily what sets these amps apart. Check out the Fender Amp Field Guide! http://www.thevintagesound.com/ffg/

    For your last question, probably the Pro Reverb. 2x6L6GC in a 2x12" combo. Sub in a solid-state rectifier if you want to make it a little more like a Twin...
     
    Last edited: Dec 10, 2020
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  4. Dacious

    Dacious Poster Extraordinaire

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    The tagboards on all in a given year would be the same. Individual years you would find differences - the non-reverb Bandmaster didn't change much if at all. Ditto the Vibrolux Reverb, except it went 5U4 rectifier in Silverface, and later pull boost.

    The others - they went 40-70 for the Pro, Super and Bandmaster Reverb losing the tube rectifier and getting higher voltages with ultralinear output transformers. And 100-135 watts for the Twin, Showman and Dual Showman.

    Some minor components like snubbing caps on tubes to cure oscillation and values in reverb and trem changed allegedly to quieten them down. The phase inverter was rebalanced, again in search of cleaner sounds.
     
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  5. itsGiusto

    itsGiusto Tele-Holic

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    Cool, yeah, seems like a good candidate. Though the OT for the Pro Reverb is only a 4k primary, which is not what I would have expected. Double the Twin impedance would have made it a 4.2k OT. It seems that a lot of other push-pull 6L6 Fenders use a 4.2k OT, so I wonder why the Pro Reverb uses a 4k.
     
  6. Dacious

    Dacious Poster Extraordinaire

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    DCr isn't the only determinant on a large amp. I had a 70 watt Pro Reverb. It was very big and clean. The OT was much bigger than the 40 watt non-UL unit from the earlier units. The ultralinear part made the amp very much like a little Twin with no breakup.
     
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  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    4k and 4.2k are close enough not to matter in practice. Plenty of Twin OTs are just 2k. :)
     
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  8. Wally

    Wally Telefied Ad Free Member

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    In your first group of four, the Bandmaster has no reverb and therefor one less gain stage in the Vib channel. The Super Reverb has a larger output transformer than the other three. The Sr is punchier, louder and breaks up later and differently than do those smaller OTs. The Bandmaster rReverb, introduced in the SF era, became more like the combo Pro Reverb and Vibrolux
    Reverb. And...Vibrolux...the BF Vibrolux is also a non-reverb amp in a a 1 x12 format. The Vibrolux Reverb was introduced in 1964 in a 2 x 10 format and is virtually the same amp as the Po Reverb but for the speaker size. the Super Reverb also use a different midrange capacitor...smaller...than do the other amps in that group. in short, your question encompasses a large subject over the full lifespan of the handwired BF/SF Reverb amps. Of the 6L6 Reverb amps, the Vibrolux Reverb is the only one that did not undergo large changes...no master volume and no Ultralinear treatment. The VR is my favorite BF/SF fender Reverb amp. Ommv...

    fwiw, the BF Pro (non reverb 1 x 15) and the AB763 VibroVerb (1 x 15..in effect the stablemate to the BF Pro) fit into your first group and run the small OT format.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    • Showman
    • Dual Showman... two speakers so a different OT than Showman. Mid pot added to tone stack of the Vibrto Channel. The coupling cap to the PI was doubled. PI plate resistors and tail resistor changed. Reverb added.


     
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  10. itsGiusto

    itsGiusto Tele-Holic

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    I'm debating between trying to build a Twin Reverb head from scratch, vs buy a local mid-70s Dual showman reverb (head cab missing) for $700. Wiring looks messy inside, as mid-70s SF amps typically are. But maybe it'd be nice to have the Dual Showman Reverb as a jumping off point, instead of building from scratch.

    Whatever I do, I'm going to want to only run 2x6L6. As far as I understand, I think I can pull 2 tubes from a Twin or Dual Showman, then just run the speaker at 8 ohms instead of 4.
     
    Last edited: Dec 11, 2020
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Building a Twin or Showman might be expensive. Those transformers will set you back $...
    Pulling the 2 tubes is barely noticeable. I have a Showman (8ohm). I have built a 2x10 guitar cabinet with inefficient speakers to try and quiet it down (2 tubes 16ohm). It is still loud. lol.

    I like the Iconic look of the Showman. A local studio had this '64 Blonde Showman near the entrance. Every time I would walk in, it would catch my eye and I would think of Dick Dale playing down at the Balboa Pavilion. Anyway I was showing the studio owner a guitar amp I just built. I could tell he was gassing for it but couldn't justify another amp purchase to the wife. I told him I would trade it for the "Big Blonde" in the corner. He jumped at the offer. Both of us thought we got the better end of the deal.
    I am glad I got the 1x15 tone ring cab with it, but I can't see moving that cab around. It really isn't as heavy as it looks but it is bulky.

    I had been wanting a bass rig and hoped the Showman would be loud enough. It did not disappoint when I tried it with a 2x10 Yamaha 4ohm bass cab at a church gig. It delivered that warm tube bass sound. Add some compression and/or EQ and it can sound like a SS Bass Guitar amp.
    So, I purchased a couple of Bass speakers. I have not built the bass cab yet, but that cab will run with all 4 tubes at 8ohm.
    I have found the Showman to be versatile. It is known for the Dick Dale sound. With a speaker change it does the Fender clean thing. It takes pedals well, and it works as a bass amp. Win win win.

    Good luck on the Twin build or Showman purchase.
     
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  12. James Knox

    James Knox Tele-Holic Silver Supporter

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    Ive seen those pics. Do you get to experience it? I got to meet and chat with Dick when I worked at Fender. What a character, lol.
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Just once. I was young in those days. To young to get there on my own. He was playing most weekends there. And yes, he was loud. I never met him.

    I loved that venue. Big dance hall. Windows with water all around. Nothing like it.
     
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  14. Wally

    Wally Telefied Ad Free Member

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    I saw Dick Dale here in Lubbock in the mid-‘90s.....200-300 people venue. Yes, loud. No surf outside the windows, though...oh, no windows!
     
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