‘66 Bassman: Incoming Cap Job

Discussion in 'Amp Tech Center' started by Luthier Vandros, Feb 24, 2020.

  1. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Some time aftter rescuing a Mojotone 5e3 kit build for this cool cat, I received a text from him asking to bring his 66 Bassman head in for some filter cap lovin’. What cool amps! I’ll be posting up some pics for you goons as soon as I get into it.

    I need your thoughts and prayers for the 61 I’ve been working on. The filters arrived tonight and they’re going on. Send me some good vibes and juju, so the tubes and coupling caps will behave. Please and thank you.
     
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  2. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Alright! Just picked up the amp from the customer. I’ll take you goons along on this journey. We’ll be visiting our filter pack friends under the dog house and converting this AB165 to his preferred version, the AA864. She’s a little rusty and crusty. We’ll see what we can do about that.

    Pics soon.
     
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  3. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Replace those 470 ohm resistors on the tube sockets with metal oxide, for some reason they like to give it up on Bassman's... IME.
     
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  4. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Yep. Someone was in this chassis prior and that was done. This thing is a bit of a terminal strip graveyard. Looks like someone modded it a few times.
     
  5. Paul G.

    Paul G. Friend of Leo's

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    The best thing (in my opinion) to do with those AB165s is minimal preamp stuff -- remove the .1 ceramic across the bass channel plate resistor and the 500pf on the normal channel. Otherwise I like the preamp, especially with that nice third stage on both channels.

    On the power amp, change the bias balance to true bias -- most of these come biased way cold with the balance control. Remove the 220K resistors from the 6L6 plates to the PI output (and all the wiring, you don't want 440 VDC sitting on that terminal strip with no place to go until something brushes against it). This is a local NFB to cut gain and distortion on what was ostensibly a bass amp. Now the amp can really sing.

    Some people redo the NFB to AA864 specs, but it's a pain -- you have to cut or remove an underboard wire to put a tail resistor in. I don't do it and have no complaints about the result.

    Check the Coupling caps between the PI and power tubes, they seem to go bad a lot.

    Bias to 60-65% of max plate dissipation and you've got a great guitar amp.
     
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  6. -Hawk-

    -Hawk- Friend of Leo's Silver Supporter

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    I see quite a few people mod the bass channel to make it useable for guitar, sometimes even adding a gain stage. Ever done that?

    I actually just got one of these - a 68.
     
  7. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Customer wants the full conversion. I like your approach better hah.
     
  8. Luthier Vandros

    Luthier Vandros Tele-Holic

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    I started at 8:30pm and just looked at my phone. It’s 2:21am and I’ve just finished cleaning up after the project.

    The fog of war!

    I do love this customer, so I take care of him. He texted me with: ”Can you recap my ‘66 Bassman?“

    *arrived with bag of parts, schematic, and a smile

    Come on in.

    The recap became an AA864 conversion. Then I noticed the other things that needed attention. The scratchy pots, sketchy partial recap, and suspect solder joints. Then I had to clean up and rearrange some of the grounds the other tech created during a 3-prong cord conversion. Had to clean this thing up.

    What an experience. Getting the
    subterranean wires removed was easier than I thought. The toughest part was all those 3/4 component junctions and tracing the circuits with the meter. Even then, the eyelet construction really makes things easy. No, the hardest part was removing the PI coupling caps; the internet lore had me sweating a bit when I hit those blue moldies’ joints with the iron.

    I’ll put fire to it tomorrow and will see what kind of bias range I’ve got at the pot. He’s got some mullards and sylvanias up front and a mismatched pair of 6l6’s out back. Gotta love these little rescues. Pics soon.
     
    Last edited: Feb 26, 2020
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  9. Paul G.

    Paul G. Friend of Leo's

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    Even with the conversion, you didn't remove the 3rd stage from the normal channel, right?

    Also, if you move the NFB to AA864 specs, I'm pretty sure you need to reverse the output transformer to plates. You'll know right away.

    I personally think the AA864/AB165 hybrids are among the greatest guitar amps ever!

    Bottom amp is mine, top one is one I did for a buddy of mine. He seems to love it.


    2 bassmen.jpg
     
    Last edited: Feb 26, 2020
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  10. Luthier Vandros

    Luthier Vandros Tele-Holic

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    The conversion is true to the AA864 schematic. I did switch the leads at the output jacks.

    If this were my amp, I’d be keen to try your suggestion. Sounds like she’d be full of nasty goodness.

     
  11. Paul G.

    Paul G. Friend of Leo's

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    If you want a distortion amp, you can mod the bass channel, lower the bypass cap value, then you can use the unused half of the 12AX7 to add a fourth(!) gain stage to the channel.

    Personally, I think if you want a fuzzy, buzzy amp, you should just buy one and let the Bassman do that fat, juicy, singing, kinda sorta but not quite clean, glorious sound.

    Personally, I think it would be a better world if guitar players would back off on the distortion.
     
  12. -Hawk-

    -Hawk- Friend of Leo's Silver Supporter

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    Fair enough, but it seems like mods are very popular on these amps and the bass channel is essentially useless as it is (aside from jumpering the two channels). Was just curious if you’d done it - thanks!
     
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  13. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Oh, no. I’ve got pedals for that job. I think what the Bassman does is amazing. It’s something I’ve never beheld in person until recently.

    That distortion comment. The more grays I find in my do, the less gain I tend to use. There’s a poem in there.

     
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  14. Paul G.

    Paul G. Friend of Leo's

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    There is a .01 ceramic disc capacitor straddling the plate resistor. Remove this, your bass channel will wake up, be less dark, less woofy.
     
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  15. Luthier Vandros

    Luthier Vandros Tele-Holic

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    I’ll see what customer would like to do. Will suggest the PaulG mod heh.
     
  16. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Update. She biased right up with HUGE range on the pot. Just a little hiss from the old microphone preamp tubes and plate resistors. Going to chopstick around a bit before I call the customer to pick her up.
     
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  17. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Some final pics of the journey and the lovely time I spent with this amazing amp. The owner may visit TDPRI, so I won’t post the before pics. He’s learning so much and his soldering and tech skills are growing by leaps and bounds.

    Such a lovely patina. The Statue of Liberty would be jealous.
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    The jaws of life
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    [​IMG]

    So, you can’t access the switch for proper cleaning? Back the upper screw out and you have a nice port for your contact cleaner and lube.
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    I love European tubes for this
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    [​IMG]

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  18. Paul G.

    Paul G. Friend of Leo's

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    If you did a pure AA864, this has already happened. This was a AA165/AB165 thing, to make the bass channel "bassier" because a bass amp should sound woofy. At least that's what the CBS engineers decided.
     
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  19. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Right on. Forgot to mention that build of yours is beautiful.
     
  20. Paul G.

    Paul G. Friend of Leo's

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    Last edited: Feb 26, 2020
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