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Diving Into a 70s Silverface Twin- Help!

Discussion in 'Amp Tech Center' started by oldlefty, Oct 18, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Some amp manufacturers use them. Many times they will be under the end bell of the transformer. The one you have has been bypassed with a wire. I would get rid of it and the tag strip unless you find that it is indeed original equipment.

    If you want transformer protection, I think rob has some info for installing fuses.
     
  2. oldlefty

    oldlefty TDPRI Member

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    Isn't this pretty much exactly what's on the amp?

    [​IMG]
     
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  3. nash076

    nash076 TDPRI Member

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    Just want to point out that as much as people love the old blue caps, they aren't always fine.

    Keep them for now, but once the amp is up and running you're going to want to test them and see if there's DC voltage where it shouldn't be. If there is, they're leaking and as painful as it is, they simply have to go.
     
  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Well you got an interesting one here. It's got a hum balance pot on the back, that was the artificial center tap for the heaters. Never found a 100 watt schematic with that feature online, or anywhere else. Look at the 135 watt for it.
    Of course someone's been mucking about in there and added a couple resisters in the filament string just to mess you up now. You could just put two 100 ohm resistors off the pilot light and disconnect the hum balance pot. Plenty of older Fenders are done that way if you need to look at a layout.
    Then you got that thermo protector dohicky, don't recall seeing one of those but it is on a schematic. And of course some more add ons, that "lamp wire" ain't original.
    The two orange electrolytics in the bias supply aren't OEM either.
    Basically on an SFTR like this, order a full set of electrolytic caps, the filter caps, bias supply, go 100v-100uf for bias. I like F&Ts for those spots. For the cathode bypass caps Sprague 25v-25uf are fine. Toss them old white Mallory caps as far as you can, for five bucks they ain't worth trying to keep.
    Then add a bias voltage pot, the trim pot arrangement works great, it's easy too. I'll try to put up an image from a site with all common mods from all over the place, in one place. Glad someone finely did that. Wish I could remember the fellow here that first turned me on to it.
    Obviously you need that 470 ohm resistor on the power tube changed, do all four.
    Get all that done and clean up the last guys mess. Power it up no tubes and a light bulb limiter if it looks good pull the LBL and check voltages. (no point at all doing that with old electrolytics)
    Set the bias balance pot to it's "middle" point. Just measure voltage on each side and get it as close as possible. Then set the new voltage pot at its highest NEGATIVE number. That biases the tubes cold to start with.
    Got a tube tester? If not put one 6L6 in at a time with a light bulb limiter, if one is shorted the bulb will be bright and you won't hurt anything. (or just get four new 6L6s) If you got two good 6L6s that's ok too, I like four but two works fine.
    After that tube it up, set bias and play rock and roll!

    By the way if you use the shunt method for bias remember those meter probes are like a straight wire in that setting with like a half amp fuse in line. Screw up and that damned little fuse, that blows if you even get close to the wrong spot, WILL NOT BLOW if you're stupid enough to touch yourself with one probe, while the other is hooked to 450 volts.
    In the interest of helping y'all I tried that once while I was biasing an SFTR a few years ago. Just getting my hair to lay down right now. :)

    By the way, where all four 6L6s redplating or just two on either side? If the later then some damned fool turned the bias balance pot.
    How do you think I learned about bias balance pots in the first place?
    Bias voltage is very critical!
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes and no.
    The thermal protector is in place but... there is a wire in parallel with the thermal protector, essentially taking it out of the circuit.
     
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  7. avspecialist

    avspecialist Tele-Meister

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    I highly recommend looking at Uncle Doug’s video on youtube, he really does a great job going through this amp. Also, I really like RobRobs mod of adding bias adjust to the bias balance in this amp.
     
  8. oldlefty

    oldlefty TDPRI Member

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    Thanks. I've been spending a lot of time visiting with Uncle Doug lately. ;) And reading up with Cousin Rob.

    The electrolytics are here, bias trim pot arrived today, power tube resistors should be here Wednesday. Soldering iron is warming up.
     
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  9. oldlefty

    oldlefty TDPRI Member

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    Resistors arrived early so I'm going in there. Two more questions-

    I bought some 1N4007 diodes in case I needed them. Any sense just putting them in- or keep on hand just in case?

    I did notice some heat/age-related "changes" to the twisted wiring string over the main circuit board. Anything to be concerned about? I can't tell if they had started to melt together, have actually shorted, or just normal aging? If I need to replace, what gauge and good source?

    [​IMG]

    [​IMG]

    [​IMG]
     
  10. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Probably nothing wrong with those wires. Looks like someone got to them with a soldering iron though.
     
  11. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    I am not the amp guru at all, but I do love SF Twins and own a few with each one receiving the mods mentioned here (bias pot is a must). The one thing that I have experienced on each of my Twins has been problems with the rectifier diodes from the factory. There has been a lot of factory installed ones with incorrect ratings from what is found in the schematic. Two of my amps both failed due to diode failures, so I would suggest replacement of them as well now while you are in the chassis.
     
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  12. oldlefty

    oldlefty TDPRI Member

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    OK, four steps forward, two steps back.

    Got the filter caps in OK. Not beautiful, but OK

    [​IMG]

    Got three of the four 25/25 electrolytics on the main board OK. When I removed the V3 bypass cap I found the resistor under it burned in half. I can't read all the bands, but the first one is blue and I haven't located a 6XX resistor on the schematic. I see a 470 on the schematic coming from pins 3-8, but a 2200 on the layout?

    http://ampwares.com/schematics/100W_Mast_Vol_SF_Twin_Rev.pdf


    [​IMG]

    [​IMG]

    When I removed the electrolytic between V4 and the jack there was what looked like a jumper between pins 3 and 8, but when I bumped it it fell out and there was only insulation- no wire?? Strange. The schematic shows a jumper.

    [​IMG]

    [​IMG]

    This is the 12AT7 that was in V3 (I replaced it with a JJ before I first turned on the amp because it just didn't look right).

    [​IMG]

    Is that the "getter" at the top? Does it look normal? I've never seen one at an angle like that, but my experience is limited. It's a GE made in Japan. The resistor that's burned is also coming from V3.

    I have the grid stopper and grid resistors replace in two of the power tubes, and the bias filter caps in.

    Suggestions?
     
    Last edited: Oct 27, 2020
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  13. oldlefty

    oldlefty TDPRI Member

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    Any ideas on that toasted resistor? The schematic says 470, layout says 2200, neither match what looks like's there??
     
  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    WAG. 610R.

    IDK if fender used different reverb transformers. If the revb transformers had different resistances the cathode resistor value might change to keep a particular bias in the revb tube.

    I would put a 2k2 resistor in for cooler bias. Just 'cuz the reverb tube many times runs hot.
    Then...
    Run it to see if the reverb is up to snuff, then place another resistor across the 2k2 to make the bias hotter if you determine it might help the revb sound.

    EDIT: just looked at several other fender reverb equipped schematics/layouts. 2k2 seems to be the goto value.
     
    Last edited: Oct 29, 2020
  15. oldlefty

    oldlefty TDPRI Member

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    Thanks Lowerleftcoast. I went through a bunch of Twin schematics on EL84World and have found three that do have 680k V3 bias resistors, but they are all the later Twins (Ultralinear?) with the four-diode diamond bridge rectifiers (mine has the older 7-diode). I don't know if that matters?

    https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

    Going back through all the schematics it looks like almost all Twins (at least from Blackface on?) used the 022921 reverb transformer, and I'm guessing that's what matters? The earlier ones had the 2200 resistors, then they went to the 470 for a while, then to the 680.

    Could it be that they evolved through "flavors" of reverb as the Twin evolved, or would it be something more related to the hard-core electronics?

    FWIW, the schematic closest to mine has the 470 (but the resistor in mine looks like an original carbon comp 680). ??

    EDIT: Aha- going over the schematics again I'm noticing that all the 2200k resistors are paired with a 25/25 bypass cap- the 470 and 680 resistors go straight from pin to ground with no bypass cap around them. Mine didn't have a bypass cap either (at least as it came to me).

    I'm assuming that, just like with cars, production changes were made on the fly (and when the bin was empty) and would get caught up eventually with a new schematic?
     
    Last edited: Oct 29, 2020
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The blue layout shows the 2k2 without a cap.
    In any case the cathode resistor will bias the tube colder with 2k2 and hotter with 470R. It is possible the reverb would be a tad louder with the hotter bias. I have never noticed much reverb difference in the several Twins I have heard. They always sound like *that* Fender reverb. I bet it is not much of an issue.
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From Dozin dot com:
    Reverb Ch. Cathode Resistor (Rk) and Cathode Capacitor (Ck)

    These two components act on the pre-amp tube for the Reverb channel of the amp. Together they help control gain and frequency response for the stage.
    __________________________________________

    As I mentioned before the resistor will set the bias. The cap can set the frequency response but the 25uF cap allows all frequencies through. To reduce some bass, which is probably a good thing with the reverb signal, a smaller cap could be used. No larger than 2.2uF I would think.

    Again this can all be adjusted by clipping in resistors and caps to find *your* sweet spot.;)
     
  18. oldlefty

    oldlefty TDPRI Member

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    I did some digging around on the CSA thermal protector- thermistor. I was assuming it had something to do with overheat protection, but if (if) I'm reading it right it turns out it's more of a "soft start" device.

    https://music-electronics-forum.com...mps/maintenance-troubleshooting-repair/25993-

    Also interesting is the reference to "CSA", which stands for Canadian Standards Association- like the Canadian version of UL. And check out the back of my amp- CSA sticker and a Long & McQuade sticker- the large Canadian music chain. And lo and behold, this amp came from Buffalo (where I grew up)- pretty much a suburb of Canada ("you can see it from there"). So this must have been an "export model". If the thermistor was a CSA requirement that explains why most around here have never seen one?

    [​IMG]

    And since I still play hockey (Covid-permitting) at my "advanced age", that makes me happy. :D
     
    Last edited: Oct 29, 2020
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  19. tubegeek

    tubegeek Friend of Leo's

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    I suppose I must apologize, reading this, eh?
     
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  20. oldlefty

    oldlefty TDPRI Member

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    Just a bump for comments on the position of the "getter" on this 12AT7 from the amp. Does it look normal, or "collapsed" at a funny angle? It was in V3, which is also where the bias resistor was toasted. Related?

    [​IMG]
     
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