What's up with this '67 AB165 Bassman head?

Discussion in 'Amp Tech Center' started by spagett, Dec 11, 2018.

  1. spagett

    spagett TDPRI Member

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    Hey folks, fledgling hobbyist amp tech here with my first dive into the wacky world of vintage tone. I picked up this craigslist-find blackface Bassman head for a reasonable price over the weekend. Owner believed it was a '67. Although it still had the original two-prong power cord, it recently had a cap job (including a fresh death cap!), and sounded fantastic when I tried it out. When I got home and pulled the chassis to swap in a grounded cord, I found a little surprise -- a fourth transformer!

    After poking around, I've deduced that the original power transformer was replaced with a Stancor PC-8422 at some point. BUT, there's another transformer tucked inside -- a tiny Stancor P-8610 with the green leads connecting to the rectifier board and chassis ground, and the black leads meeting up with the PT's primary at the fuse holder and power switch. Any idea what the purpose of this was? Should I rip it out track down a more 'stock' transformer?

    [​IMG]
     
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  2. Ten Over

    Ten Over Tele-Holic

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    The replacement PT doesn't have a bias tap, so they used that little PT for the bias voltage. If it plays good and you like it, leave it alone.
     
  3. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Some pretty unusual and Rube Goldberg stuff going on there from what I can see. But just because I'm OCD, you dont have to be I guess!
    Bias pot resistor mess
    That extra tranny due to improper replacement tranny.
    Series resistors on the bias board
    No resistor across the odd salvaged cap on the bias board?
    Replacement Output Tranny too I guess as the leads were slovenly extended.

    Is the rest of the inside that bad too?
    Still if it was dirt cheap what the heck!
    Have you already removed the power cord?
     
    Last edited: Dec 11, 2018
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  4. spagett

    spagett TDPRI Member

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    Awkward bias tap hack, got it! Thanks.

    Yes, I already yanked the cord. Here's another gutshot and under the doghouse... anything else jump out to you, schmee?

    [​IMG]
    [​IMG]
     
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  5. Ten Over

    Ten Over Tele-Holic

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    Yeah, well, sometimes you gotta use what you got. Fender is no stranger to twisting two resistors together--look at the 68K's at the input.

    There's no resistor across the bias cap in those amps.

    The bias pot is wired exactly like Fender and looks functional to me.
     
  6. clintj

    clintj Friend of Leo's

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    It looks like the main PT is half held in by fender washers, since the new one is physically smaller. If you look closely, two of the original mounting holes are empty. That's not going to hold up well if the amp tips over or gets dropped. That's a damaged chassis and PT waiting to happen.

    I'd spend the coin and swap in the proper PT, and clean up that wiring as well.

    It looks like the previous repair guy was competent in sone ways, and "creative" in others. The screen grid and dropping resistors are swapped for modern types, which I like.
     
  7. ranjam

    ranjam Tele-Holic

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    Not only is the bias supply a dog's breakfast, but to add insult to injury, it's that gawd-awful bias balance. The OCD in me would be redoing a lot of 'stuff' in there, including the weird parallel resistors in the power supply.
     
  8. Faceman

    Faceman Tele-Meister

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    I dont see anything out of line really on the power supply board other than a Frankenstein job with the components. You could completely rebuild that power supply board with the correct value components for around $50. Same with the bias supply board for around $10 in components. The transformers really aren't that much of an issue as long as the voltages are on point. IMO
     
  9. Ten Over

    Ten Over Tele-Holic

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    You mean the balancing resistors? I didn't want to discourage the kid because I think he has a pretty cool amp there, but I don't like those mismatched capacitors on the first filter stage. Who knows, maybe the guy calculated the appropriate balance resistors for a 47uF/100uF combination. Even if he did, I still don't like it and a pair of 100uF's with 220K balancing resistors would make me happy. The rest of the doghouse looks fine to me.
     
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  10. spagett

    spagett TDPRI Member

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    I appreciate all of your thoughts, suggestions, and gripes! I'm certainly not discouraged at all (at least yet!) -- I definitely bought this as a learning experience. Digging into a real project is definitely helping that stack of Kevin O'Connor books make sense :)
     
  11. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    As noetd, that's the bias...not rectifier...board.

    Plenty jump out to me:

    1. The orange filter cap is old and needs to be replaced. The blue Sprague atom looks to be 14 years old - might as well do it to, and I suggest replacing the others as well.

    2. The resistors and caps - for the most pat 0 that have been installed hav sloppily bent lads and very sloppy soldering work.

    3. A bias balance can be left *if* adjustable bias is added.

    4. There's so much sloppy confusion on the circuit board it's highly recommended that you take it to a qualified tech and have it "repaired" - the transformer situation handled in a proper manner, out of date parts replaced and the whole thing dealt with in a professional manner. Whoever did the previous work was obviously an amateur or an electronics repairman not really familiar with guitar amplifiers or audio circuits.

    I don't know what you considered a "reasonable price" but I fear it probably wasn't close to "reasonable" when the costs to fix all the hack work and perform needed repairs are considered - in addition to normal service that was either ignored or improperly done.

    I tell vintage amp shoppers to add $150-450 to their purchase price for "normal service" (cap jobs and normal maintenance - which is rarely done, although filter caps have a service life of 15-20 years) - the price variance depending on whether or not tubes ned to be replaced.

    But that does not cover "rehabilitation" of hack modifications. It really upsets me when an unknowing buyer brings me something like this and I have to tell them the repairs AND normal service may end up costing him 1) more than the purchase price, and 2) possibly more than the average vintage value of such amps in good condition.
     
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  12. Lynxtrap

    Lynxtrap Tele-Holic

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    True, but once an amp has been altered beyond "vintage state" like this, there's no reason to go overboard with the rehabilitation IMO. Once safety is taken care of, there is really no reason to change things that work.
    Or if the buyer is a hobbyist tech, as said in the TS, it could be a good project to learn on, going through the circuit and layout to restore it.

    Just need to decide what you want with it. Do you want it as close to stock as possible, or do you simply want a working amp? For instance, it seems like every resistor and most of the caps have been replaced on the main board.
    You could get carbon comp resistors and get some of the "vintage vibe" back, if you believe in that. It might also make the amp noisier.

    It's easy to look at an amp like that and judge it as FUBAR, but it might still sound and work as good or better than the stock amp. With obvious problems taken care of, of course.
     
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'd remove the death cap, add adjustable bias (see below), verify the new circuit board caps are the correct value, replace that Sprague power cap and verify the solder joint under the electrical tape. If you're the anal type you could clean it up but if it sounds good then enjoy it.

    https://robrobinette.com/Silverface_Amp_Mods.htm#Convert_Bias_Balance

    [​IMG]
     
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  14. keithb7

    keithb7 Friend of Leo's

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    This amp would go under the knife. I'd install a stock spec PT from Classictone. Remove the bias tranny. Clean up wiring. No black tape. All new F&T power supply caps. The original Whale cathode caps replaced. The bias board would bug me. A radial cap and those two resistors twisted together, replaced. Those two balancing resistors on the power supply cap 1. That central ground point drilled to that chassis near the bias pot, gone... This would keep me up at night. Once completed I would close it all up never to be seen again. But I'd take gut shots of it all cleaned up and show all the fellow amp geeks here. What have I become? It appears, the anal type.
     
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  15. Ten Over

    Ten Over Tele-Holic

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    He needs the Rob Hull version for this amp. May as well go with 4.7K to replace the 15K so that he can still bias tubes that are real dogs.
     
  16. Ten Over

    Ten Over Tele-Holic

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    If you are going to keep the Stancor PT, then you should secure it a little better. You could cut a plate to cover the entire hole and big enough for all four bolts, or maybe just an 1 1/2" piece long enough for the two floating bolts.

    If you are going to replace the PT, tubesandmore.com has the Hammond 290EX on sale for $75.60 until Dec. 18. Excellent transformer at an excellent price.

    Can you tell us what the OT is?

    I would also like to know what electrical skills you have and what equipment you have.

    How much money are you willing to throw at this amp?
     
  17. schmee

    schmee Poster Extraordinaire Silver Supporter

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    A few things. Much has been renewed but hey, nothing wrong with that really. You want to play it and want it quiet right? The Filter caps values look all over the map though, Right? But I've sometimes found it's not a huge difference ... I think they were intended to be 80/80/20/20/20 uf. The output coupling caps (right end-orange) seem small? But it's all about their rating I guess. Stancor Trannys are supposed to be very good, so....
     
    Last edited: Dec 12, 2018
  18. spagett

    spagett TDPRI Member

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    OT is an 022871, hooray for stock! Maybe it was just lazily rewired at some point?

    Wouldn't consider myself an absolute beginner, but certainly no expert. Comfortable soldering and taking multimeter measurements. I've built a couple pedals from kits, and about halfway through an AX84 p1-eXtreme build. Successfully installed a grounded AC cord last night bypassing the ground switch and death cap without frying anything, so that's a start, yeah? Really interested in learning the theory. RE: equipment, I'm a member of a local makerspace with access to all sorts of test equipment I'd love to learn how to use (scopes, power supplies, variac, etc).

    If I spend a couple hundred on components and learn a thing or two along the way, I'm thrilled. Not interested in paying anyone else to do the work though, where's the fun in that? This amp has way more value to me as a learning opportunity than a museum piece, which it will almost certainly never be. I'm going for "safe" and "sounds cool". Even then, "cool" is pretty subjective, and I really like fuzz pedals anyway.
     
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