Recommendations for salvage amp build

Discussion in 'Shock Brother's DIY Amps' started by mtglick, Aug 1, 2020 at 2:07 AM.

  1. mtglick

    mtglick TDPRI Member

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    Hi, all--

    Posted this in the tube amp forum, was directed to your particular corner by another user. Longtime lurker who has benefited heavily from info and opinions here, generally in equal measure. Have a question for the amp builders around here. Ultimately, I'm looking for advice.

    I bought a nonfunctioning Fender Frontman 212r combo for next to nothing, predominantly because it had a couple of speakers in it that I could use in another cab. Either I was going to figure out how to repair it, or I was going to scrap the whole chassis and finish a different amp build in the cab.

    So, there's apparently a reason these things are DNR at Fender. Tried replacing the two big caps, plus all four transistors, and anything that looked burned. Reflowed half the board, all the pots, etc., and I'm still getting 26v DC at the speaker outs, which ate a test speaker, and one of the resistors keeps frying anyway. It may be an easy repair for some, but it's above my limited experience and toolset, especially when there are other bad ideas out there I can waste money on.

    Finally decided this morning that I'm done--this board is going in the trash, and the other project I thought would go into this cab is half an inch too long for this cab, and needs a different cab layout anyway.

    So, this leaves me with an interesting dilemma. I now have an empty, open-back 212 cab, in Fender silver and black. I have the chassis, and the 100w PT.

    Any ideas what a novice builder with some experience in amp repair could potentially do to make use of the chassis, PT, and cab? I have a Mojo 5e3 small parts kit in my pile, but that was intended for a different idea. Open to other kits/builds that are about the same level of difficulty as a 5e3 or thereabouts. Any thoughts?
     
  2. Jorgen83

    Jorgen83 Tele-Meister

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    Well it’s not as simple as a 5e3, but that cabinet looks like a twin reverb...
     
  3. Wally

    Wally Telefied Ad Free Member

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    The PT will not be suitable for a tube amp build.
     
  4. Nickfl

    Nickfl Tele-Afflicted

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    It would be interesting to hear any good uses anyone has come up with for SS amp power transformers. I've got a couple of them in my junk drawer that I'd like to find a use for some day.
     
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  5. tubegeek

    tubegeek Friend of Leo's

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    Doorstop.
     
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  6. mtglick

    mtglick TDPRI Member

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    Ok. PT going into the bin with the board, unless some ship captain has a need for ballast.

    I'm open to the Twin idea. I'm open to lots of ideas, really. What I'm hoping for is somebody to point me in the direction of a kit or two that could potentially work in the chassis I've got, and that worked for their early builds, or resources along the same line. I'm concerned about trying to design something myself, because I will kill somebody, probably me, and would prefer not to.
     
  7. dougsta

    dougsta TDPRI Member

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    What ac voltage do you get from the secondaries of the pt?

    I picked up a 100watt Peavey solid state combo to use as the base for a valve amp project, the pt has 2 secondaries, 36-0-36 and 20-0-20, these could be used for HT and bias (voltage multipliers to get what you need)
    You then just need to buy a heater supply transformer and an output transformer.
     
  8. muscmp

    muscmp Tele-Afflicted

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    have you thought of just using it as an extension cab? i ran into similar situation with a crate SS amp and trashed the board but use the two celestion speakers in it as an ext. cab. sounds great.


    play music!
     
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  9. glenlivet

    glenlivet Tele-Holic

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    Beat me to it.....for a 2x12 cab....way to go. If you wanted to get fancy....you could chop the top, or spin the bottom 180, so that the control panel points to the "back" and the speaker baffle (now the "front") has a nice clean look.....
     
  10. Nickfl

    Nickfl Tele-Afflicted

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    I have a transformer out of a solid state tabletop radio that I use as a power supply when testing the winding ratio on unknown output transformers. But yeah, other than that...
     
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  11. mtglick

    mtglick TDPRI Member

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    Thought of that, it's my fallback position at the moment. Would probably just cut and cover something with tolex (or bend a piece of aluminum trim) to fill the control opening if I go that way. But right now I'm feeling optimistic enough to explore options and see where it takes me.

    I've got a bunch of EL84 and 6v6-based amps already. My bigger amps are a touch too loud for, well, being in the same room with them. Following on another member's advice, anybody have any experience building a middle-watt blackface clone kit with reverb they'd be willing to recommend?
     
  12. mtglick

    mtglick TDPRI Member

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    Ok, did a bit of research, a little listening, and I'm leaning in the direction of a BF Pro Reverb circuit for this chassis and cab. I can find schematics, but very little in the way of kits or shortcuts--is anyone aware of a pre-collected kit (with layouts) for the BF Pro?
     
  13. Wally

    Wally Telefied Ad Free Member

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    Build a Super Reverb circuit with appropriate transformers for a Pro Reverb. Note that BFSR used a lower value midrange tone cap than did other circuits based on that AB763 reverb amp circuit. If I were building, the amp would have a dedicated BF channel with the .047mfd midrange cp and one channel with a totally different approach...but with effects on both channels. I would build the Bf channel in the Normal end of the chassis with a 6.8k ohm midrange pot. According to those who hold greater knowledge than do I that is a mid pot set at 5...good enough for me for a BF sound. In the other channel that has the TMB stack, I would use a 250k ohm mid pot to boost things....this would be the hotter channel with less scoop....different tone stack from the plate of the input gain stage to the controls. Ommv...
     
  14. muscmp

    muscmp Tele-Afflicted

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    no back panel and no chassis so i just use it as is. works well and i have too many other projects to do. ha!

    play music!
     
  15. glenlivet

    glenlivet Tele-Holic

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    IMG_20191204_204303961.jpg

    Frontman 25R that I chopped the bottom off, spun it 180 and glued back on (with some aditional chopping of the sides...)
    I think it turned out pretty well......

    not sayin' you should or need to....just sayin' you *could*
     
  16. printer2

    printer2 Poster Extraordinaire

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    Beat me to it. Schematic says +/-32V at the rectifiers, +\- 42V dc. So that is 80V right from the start. Triple that and you have 240V. Not a heck of a lot for a 6V6, but a 6CW5 will play just nicely at that voltage. The Datasheet says 25W in P-P but I would think it would be more in line of 18W as it was designed to be a lower supply voltage version of the EL84. But you would need to have a separate supply for the heaters. Or do you? The 6CW5 comes in a number of variants, a 6V, 8V, 10V and 15V heater versions. Two 15 is series is pretty darn close to 32V. But then the power transformers has an unbalanced draw. Not the end of the world but it almost calls for another set of 15CW5's on the other winding to end up with a 30-40W amp. Would need a 3-3.6k Output transformer per pair. Easy enough to do a 6.6k transformer that has multiple output impedances. Hey, could make a stereo amp. And the tubes are cheap for NOS. But maybe using a new PT is simplest.

    Anyway, I think I caused enough trouble here for now.
     
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  17. tubegeek

    tubegeek Friend of Leo's

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    THE TRUTH IS OUT THERE
     
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