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Fender 75 conversion to Super or Deluxe Rev.

Discussion in 'Amp Tech Center' started by Dreadneck, Apr 22, 2021.

  1. Dreadneck

    Dreadneck TDPRI Member

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    How hard would it be to "convert" or rather rebuild a Fender 75 amp to a Super Reverb or a Deluxe Reverb?

    I know they are very different and I am unsure of how much of a trouble the ultra-linear transformer would cause.

    Would it be a "gut it completely and build from ground" scenario or is it possible to work with some of the existing parts?

    The reason I wonder is because I've been trying to get it up and running for ages but the damned thing just won't work... Now I have grown tired of it and thought to rebuild it completely to a friendlier modell.
     
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  2. Wally

    Wally Telefied Ad Free Member

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    The 75 can be a very good amp, ime. There are a couple of simple changes that make it a much better amp, but then it has to be running to make improvements. Have you let a tech work on it?
     
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  3. Wally

    Wally Telefied Ad Free Member

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    And...the basis for the amp is the AB763. What is the problem with the amp?
     
  4. Musekatcher

    Musekatcher Friend of Leo's

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    Seems like it would be much easier, to trouble shoot and fix it? Or, even easier to drop it off at a good tech, and have it fixed? But, I've been thinking of swapping the 15" baffle on mine for 4 10" Jensens, for a more Bassman/Super Reverb thing. Gotta clean out the garage first before tackling it.
     
  5. Dreadneck

    Dreadneck TDPRI Member

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    No, I have been working on it myself. But perhaps a more experienced tech would be able to solve it.. It blows the main fuse instantly when turned on.
    Something in the output section is drawing loads of current. But the output transformer seems fine and I've tested with known good tubes.. I've rebuilt the power section. No change. Rebuilt the output section. No change. Gone through the preamp section loads and loads of times... Nothing.. It eludes me every time.

    What changes would that be? I have had hopes for it before, so I'm curious to hear!
     
  6. Dreadneck

    Dreadneck TDPRI Member

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    That was my hope, but it has proven to be a tough nut to crack.. I am looking around to find another victim who would like to tear his/her hairs out over it ;)
     
  7. Jowes_84

    Jowes_84 Tele-Meister

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    What Wally says. The last two years on this forum I learned to listen better:)
     
  8. Jon Snell

    Jon Snell Tele-Holic

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    Not a lot to go wrong in the power input stage that would blow the mains fuse.
    Removal of the output valves ... does it blow fuses?
    Remove the rectifier ... does it still blow fuses?
    With the amp in standby ... does it blow fuses?
    Disconnect the main smoothing capacitor ... does it blow fuses?
    That's about it. Nothing else except the mains transformer but if it is not blowing fuses or getting hot with no rectifier in place, it is probably OK.
     
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  9. schmee

    schmee Doctor of Teleocity

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    Are you using slo blo fuses?
     
  10. Wally

    Wally Telefied Ad Free Member

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    Pull all of the tubes. If the fuse blows, you then know that the problem is in the PT, the rectifier diodes, or the filter caps. Lift the HT from the rectifier section...does the fuse blow? If so, you probably have a bad PT primary. If not, then you look at those rectifier diodes and the secondary windings....then the filter caps and bias cap. It is tech time. That voltage lowering circuit to the output section is not a simple thing. What would a non-functioning Fender 75 worth? Not much, imho. If it were near me, I would probably want to make an offer, but it would not be a very ‘generous’ offer in some people’s opinion. A person has to buy with the worst case scenario in mind...how much money and work will it take to repair an amp with unknown problems? Add the worst case scenario to whatever the cost of the amp. One doesn’t want to end up upside down on a Fender 75, which despite my favorable assessment of these amp do not command much in the marketplace. When they are working well, they can be bargains, imho.
     
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  11. Lynxtrap

    Lynxtrap Tele-Holic

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    IMHO these are very different from the AB763 and can’t be expected to sound anything like a blackface amp. Disregarding the bells and whistles, the basic topology of the clean channel is certainly Fenderish, but still has more in common with the late 70’s UL amps.
    http://www.stratopastor.org.uk/strato/amps/twoseriesfenders/fender_75_schem.pdf
     
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  12. Musekatcher

    Musekatcher Friend of Leo's

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    I'll admit, I'm not going to rely on my own troubleshooting skills with something this deep, but, there really isn't a lot of mystery here. You should be able to isolate the first branch of input or output, then is it the primary or secondary circuit, and so on. You keep bracketing down, till you implicate a half dozen or less components/connections, and in the worst case, just replace them all. As long as its not the transformers, we're talking dollars, not hundreds. Even if it is a transformer, still under $100 to replace.
     
  13. Wally

    Wally Telefied Ad Free Member

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    yes, these have ultralinear output. The clean preamp is basically an AB763 circuit if you leave the bells and whistles alone....much like the UL SR. If a person wants, that can be changed to a non-UL output. I have no problem with the cleans of a 75 when it is running correctly. There are changes that make the drive circuit yield better overdrive, ime.

    But..if someone wants to gut one of them and build something else, that is their personal choice. I would advise against building a new circuit with those transformers until one knows that the transformers are good. I will also observe that if one cannot get this amp going, one might not want to proceed with a build. Dead amps are the things which teach the most important lessons, perhaps. Next on that list would be amps that makes sound but have problems. How much would I give for one of these amps if it came to me today for sale and I know the history??? $135....max. I am not giving say $300 for an amp that for all I know needs both a PT and an OT, tubes, all new electrolytics—-this is a given for me, some preamp tubes, and a lot of hours of work...unless I can sell it at a PROFIT of some sort.....for more than a functioning of that model will bring.
    Oh yeah, if one is going to build with those transformers, one will be taxed to get the power supply to function at BF levels. One will also have the challenge of making that UL OT sound just like a BF amp. I say this amp needs to be into operation as a healthy 75. Then, if one cannot find a way to make sound as good as the last one I had sounds, then sell it and buy a BF SR???.
     
  14. Musekatcher

    Musekatcher Friend of Leo's

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    I'm looking at the OP's geographical challenge. He may not have great access to these heavy amps, so fixing what he has might be a better option, versus options for us state side folk. Mine's healthy, has a great clean next to a '70 Twin, and a new SS22. For me, the drive channel works best as a volume boost from the clean channel, plus maybe a drive pedal. That's how I use mine right now. If I were looking at repairing mine for $250, versus selling it for $135 and spending $800 on a SR, I think I keep and repair the 75? ymmv.
     
  15. JRapp

    JRapp Tele-Holic

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    A healthy Fender 75 is a nice-sounding amp. Is it a pre-CBS DR? No. No many amps are. But is an excellent clean platform for pedals and all of that. I sold mine to a friend of mine who plays surf music and it sounds very good. The right 15" speaker makes a big difference.
     
  16. Dreadneck

    Dreadneck TDPRI Member

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    Thank you for all the input!

    I'll try to answer you all at the same time here :)

    As you pointed out @Musekatcher the geography is against me with this amp. There are not many 75's over here and even less people that know something about them. We have lots of Fenders just not 75's.
    However I do have a replacement OT (made by Hammond though) that would fit right in. I haven't tried it out yet because I wanted to make sure nothing else was the reason behind the issues.

    Yes I use slo-blow fuses.

    I'll try to give the full history here:
    I got the amp dirt cheap. (A warning sign right there) But it worked, kind of unstable and noisy though.
    At this time I had been into amp repair for about a year or so and had successfully repaired amps. Among them a Fender Dual Showman Reverb, a Marschall JCM900, a Studiomaster Lead... (don't remember the full name at this moment, but it was based on a Mesa Boogie).
    Since then I have repaired other amps as well but the 75 has been put aside for a long time now.
    It is a tricky amp. There is soooo much cramped into the chassi that the rats nest itself could cause numerous issues (I guess you guys know all about that though ;) )..

    Anyway, the noise and hum was bad so I did not want to play it without giving it some service.
    The first thing I noticed was that the previous owner had put a 10A slo-blow fuse in the socket where there should be a 2.5A fuse................
    Naturally I tried it with the correct fuse. Poff! Fuse blew..

    I opened the amp up and could right away spot fishy wiring and components. The electrolytics looked so so. I changed them all to specs acording to the schematic. Likewise I measured resistors and caps. Some good some bad.
    Replace, rinse, repeat..
    There seemed to have been attempts to mod the amp. I think I was able to spot them all though and have restored the amp according to the schematic.

    I have managed to get it running with all but power tubes.
    If I put limiting resistors before the rectifier I can get it running with power tubes.
    I have measured both the PT and OT and they seem fine. But I won't rule anything out.

    The fx jack is troublesome. I can't get it to behave without shorting it with a tele plug.

    Also the voltages measures according to what is expected from the schematic, both in high and low power.

    My guess is that the output section is what is causing the fuse to blow.. I have to look in to it further.


    I hear what you are saying, makeing a DR or SR from it might be more work than what I would want. Plus killing a 75 as well.

    So I'll be going for getting it up and running.

    Honestly it IS a good practise amp for tech skills. But if I can't get it to work after a few more attempts I will leave it with a more experienced tech. But then I guess it will be a keeper no matter what I think of the sound ^^ I have heard one in a store once though and I liked it a lot!


    Oh and before anyone asks. Yes, I have the pedal for channel switching connected ;)
     
  17. Wally

    Wally Telefied Ad Free Member

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    measuring a transformer might not reveal every problem. Also, did you measure from each winding to ground? W I ding can short to ground while still showing proper continuity within the w8nding.
     
  18. Musekatcher

    Musekatcher Friend of Leo's

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    That's a game changer, not having a basis for what's right, wrong, etc. I once took a Twin in a box of parts to a knowledgeable tech. He was able to piece it back together, because he knows those circuits blindfolded. He only charged me for a re-tube. I suspect, taking something obscure like the Lead 75 in for re-construction, is a research project, and gonna take some time, which is $$, not counting parts. My 75 is dead stock, unmodded, and an easy re-cap, re-tube service. At this point, yea, yours may not be worth correcting, if you have alternatives. I wonder what other Fender chassis might fit that cab? Shipping a chassis from the states might not be that bad.
     
  19. Lynxtrap

    Lynxtrap Tele-Holic

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    Depends on what we mean by "basically AB763", I guess.
    If we start looking at component values and other details, there are almost no identical parts of the circuit, as far as I can see.

    Much of it could of course be "blackfaced" just by changing and removing some resistors and caps.

    I agree that these sound pretty good clean, and that there is no sense in rebuilding it not knowing if the transformers are good.

    Otherwise, I would say these are good candidates for modding or rebuilding. It's not PCB, right?

    If the transformers are good, you could build almost anything in there.
     
    Last edited: Apr 23, 2021
  20. Wally

    Wally Telefied Ad Free Member

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    yes, the input resistors are lower value..more input signal hitting the input preamp gain stage. The cathode circuit values on the preamp cathodes are the same....1.5K/25mfd. 100K plate resistors. The tone stack has the same slope resistor and tone cap values. The topography is the same. Yes, there are some differences in some few areas...but it is based on the AB763 amp, ime.
     
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