1970 Silverface Twin Reverb upgrade questions.

Discussion in 'Amps and Cabs' started by Gabriel, Jan 23, 2012.

  1. Gabriel

    Gabriel Member

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    Hello!
    I own a 1969-1970 silverface twin reverb with oxford speakers with AA270 schematic(http://ampwares.com/schematics/twin_reverb_aa270.pdf) [What's the difference between AA270 and AA769?Apart the cap before the phase inverter) as far as I can see. I believe everything is stock apart the tubes.Tubes are 6L6 GC special design for Fender and preamp tubes has the same “design for Fender” stamp also.(Apart one philips ecc83 at slot v5).Unfortunately there was no tube chart info.

    I am willing to do some changes with that amp.I am not an amp tech and I don't have the ability to do it. I have a friend who is electronic engineer but not an amp tech. He is going to do all the soldering job and mods.But I have to tell him according to my research what he should do.

    But before I go forward I have some questions.So if anyone can help it would be appreciated.So here we go.

    1.Power supply filter capacitors: These are going to be changed with TAD AudioCap 20uF 550V and 70uF 350V. According to http://www.el34world.com/charts/fenderservice7.htm I should change the 1w carbon resistors with 3w metal film resistors.Is that ok?

    2.Electrolytic caps: All 7 will be replaced with Sprague Atom 25uF, 25V or TAD GoldCap-BP 25uF 25V.I have both but I am not familiar.So if anyone had experience with both his opinion is welcome.

    3.Tubes: I have a spare quad JJ 6L6 and will try on them but I was wondering if I can something better.Some people propose 7581A/PHILIPS.These are expensive but are supposed to be the ultimate(?!)
    Preamp tubes:A good adive on that section is needed too.

    4.Speakers: It has oxford with number 465-951 12T6-11.
    a)According the numbers on the transformers amp is made between 69-70.Thats according to producing time fender gives.According to the serial number (A21400 to A25600 – 1970, http://www.superiormusic.com/page195.htm) it is made in 1970 BUT according to that: http://www.webervst.com/codes3.html and http://www.webervst.com/codes.html twin reverbs were not supplied with oxford speakers after 1967(that's a question for some historians)

    b)I will change them for sure cause the one is ripped and glued very good...i am not pleased with the sound of them.Too muddy with cleans and like f*arting with drive pedals.
    I've listened to clips from jensen site and I really liked P12Q and P12N.
    From weber speakers they proposed me https://taweber.powweb.com/weber/chicago12f.htm cause fits me needs for clean and for smooth overdrive.
    On the other hand I see some others proposing speakers like Eminence Delta Pro A12 with 400watt that I feel it's overkill(or isn't).What is the tonal difference among 50-80w and 400w power?

    5.Do you think wiring should be changed?I don't intended to rewire all the amp :) but is there any spot that should be wired with better quality of wire?

    6.Bias Pot and voltage...
    I found many guides how to make the balance pot into bias pot. All of the guides just say to convert the AA270 to AB763.Doesn't matter that the voltage from the tubes doesn't match???

    7.All coupling caps will be replaced with sozo caps.(i've received today:) ) Shouldn't I? The only problem was that I could find a coupling cap in the phase ivnert of 0.001μf,i've picked a .001 400v SoZo Vintage Cap.What did you used instead?


    8.According to http://acruhl.freeshell.org/mga/main/silver_twin.html and the 7 steps above,there are only few things to change to blackface.My question is if that's the right way.Should I convert everything to blackface?

    9.read about the 3 prog power cord.Mine SF is european model.3 wires.The 2 of them are going to the ground switch and the 3 is grounded on the chasis. And from the ground switch it goes to power switch,at the fuse and at the 1pin of the V9 power tube.Is that ok?


    I hope i've not forgot anything.I'd like to thank in advance for the help and sorry for any english misunderstanding and my ignorance on technical matters. :munch
     
  2. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    I have a Vibrolux from that same era and all it really needed to go from rather old/tired to FREAKIN AMAZING sounding was new electrolytic caps. I did do a few other things to it, but the new caps are what really made the biggest difference. I happen to LOVE the 10" Oxfords (10L5s or 10L6s) though I have yet to hear a 12" Oxford that I really like; so I'm pretty sure I'd replace those. (It had the brown turd caps and pvc wiring that everyone hates...but man...this is one of the best sounding amps I've ever played through.)

    If your twin was mine, I would most likely switch the speakers to either Eminence Swamp Thangs or Cannabis Rex. I would absolutely replace the electrolytics and probably the dropping resistors under the cap pan.

    I might replace the caps in the tone circuit with the Sozos or some good Russian oil caps (if I had some laying around). Might wind up replacing some plate resistors if the originals were tired/noisy. These two steps may really not be necessary (but if you already bought the Sozos, you may want to try them to see how much--if at all--anything improves).

    The rest of this is not really necessary, IMO. I'd play it all by ear depending on what kind of tubes I found and how they ran in that particular amp.
     
  3. smolder

    smolder Gold Supporting Member

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    I'm a recent twin guy again... I'd leave the coupling caps unless they are leaking. Leave the two middle power tubes out and rebias with a single 8 ohm speaker (in my cases JBL D or K120). I don't really know what guys do with 80 or 100 watts.
     
  4. LarryN

    LarryN Member

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    Sprague are best for electrolytics given a choice. If the power supply resistors are in spec, don't bother changing. I'm not in the scientific/military spec resistor camp.

    JJ 6L6's would be fine.

    Old RCA 7025 in the V2 is my favorite preamp. The Phillips in V5 is wasted in the vibrato socket. See if it works good in V2. great tubes, usually.

    They used Oxfords well past 67. I like Oxfords OK, turn down the bass. Don't bother with the Delta Pros, with such an abundance of better choices.

    If you have a matched set of power tubes, the voltage required is the same. I'd convert to bias control.

    Sozos sound good to me, though unneccessary in this amp. Blue?

    P.I. cap, .001 to .01, depending on how much bass you like. I like .01

    I wouldn't blackface. I've done it several times and didn't care for the results. To much else is different in the wiring layout, components to make a difference I liked.

    Not sure about the 3-prong. If it's factory, it's probably fine.

    That's what I'd do.
     
  5. vibrostrat43

    vibrostrat43 Supporting Member

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    1. You'll be fine with the values and brand you proposed, but I'd say instead of the 70uf caps use 220uf ones with the same voltage rating.

    2. Either one will work just fine...go with what's cheaper.

    3. JJ's will sound good. The 7581A's will sound really good, and will raise the power of the amp by a few watts, which won't be that noticeable in volume, but will be in headroom. If the power tubes don't break up when you play wherever you play then I'd suggest you just keep the JJ's. If they do, and you don't want it then get the 7581A's, or if you want a different sounding overdrive from the power tubes then get the 7851A's.

    4.a) can't comment (sorry)
    b) Make sure the combined wattage rating of the speakers is at least 80w, but I'd go for at least 100w just to be safe when overdriving the power amp. I personally like Fane speakers, and/or Celestion speakers in my Vibrolux Reverb, but it's not quite a typical bright and mid-scooped Fender tone.

    5. There is one place I'd tell him to check...the filament wires that go to the power tubes (green wires that are twisted and go along over the tube sockets). Make sure that on all of the power tube sockets (the larger 8 pin sockets) the wires terminate on the same pin of each socket...that is the wire from pin 7 on the first power tube should go to pin 7 of each power tube...the same with pin 2.

    6. I recommend going from the AA270 to the AB763 for the bias circuit as well.

    7. Have your friend check whether the caps are leaking more than 1/4 a volt of DC, and only replace the ones that are leaking that much.

    8. If you have brought the bias circuit from AA270 to AB763 then you've done what's (imo) the most important step. The only other thing I'd recommend is removing the .002 ceramic capacitors that go across the pins of the power tubes.

    9. I always just bypass the ground switch myself, but it's not necessary.

    I also recommend using .022uf blocking caps to the power tubes as opposed to the .1uf caps. These are the two identical capacitors on the far left of the board (when looking at the amp from behind)
     
  6. Gabriel

    Gabriel Member

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    Hello! I'd like to thank you for the answers :)
    Some questions on your thoughts.

    1.
    @vibrostrat43 I've already have the caps and if i was to try i can't wait 220uf.But that's a good tip for the next time.What about the power supply resistors?
    @Leonc @LarryN Why should I or not change the power supply resistors?
    Is it better to be changed with higher value resistors?

    2.@vibrostrat43 @LarryN @Leonc I have both tad and sprague right now. I wonder in what way do it each of these brands affect the tone.Otherwise we much put some crocodile patches to test a/b before putting in at last.

    3.Ok about the amp tubes. What about the preamp tubes? Even though that can be changed anytime after the inside changes.But i would like to know in case i'd order amp tubes now to order the preamp tubes together.

    4.I am a bit confused to that. Speakers tone is something personal.A speaker of yours likes may be not in my likes.But i do believe that i need to buy the speakers before changing the guts of the amp.As matter picking the right speaker for me is hard.Cause i have to trust youtube clips. How alnico speaker reacts with pedal drives?
    @Leonc i will check out the speakers you've mentioned.
    @LarryN any suggestion?
    @smolder are JBL being choked easily?

    5.@vibrostrat43 thanks for the info. Just in my amp the wires are brown. We will check them.

    7.The coupling caps are sozo blue.I am not sure that i should replace the original after your posts. What can i use for the PI cap 0.001? I've already have a sozo vintage cap to use.But any other suggestion is welcome.

    8.@vibrostrat43 i will remove these caps. :)

    @vibrostrat43 What is the tone change with lowering the 0.1uf caps to 0.022?

    Thank you all again!!!!
     
  7. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    I don't think you'll notice much if any tonal difference between sprague vs tad filter caps in a TR. It's probably more a question of how well you believe they're made and whether or not they'll fit under your amp's capacitor pan. Sprague makes some 20uFs, for example, that will not physically fit under there.

    If the power supply resistors have drifted in value (test them) or look "tired" or worse, cracked, out they go. No DO NOT use higher value resistors. Or do you mean higher rated resistors (e.g. 10W instead of 5W)...hmmm...not necessary IMO.
     
  8. Gabriel

    Gabriel Member

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    Leon I am sorry if i've been misunderstood. I was talking about rated resistors replace 1w carbon to 3w film resistors.
    As far the spragues i was reffering for the electrolytic capacitors 25/25.Now it has mallory.

    Best Regards,
    Gabriel
     
  9. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Ah, okay Gabriel, got it. Still I don't think you'll notice much sonic difference between Sprague and some other brand for cathode bias caps as long as they have the same values. If you used very different values (e.g., 5uF instead of 25uF), boy, you'd really notice the difference, LOL! But one brand vs the other? Not likely to make a noticeable diff in that role.

    As for the dropping resistors in the filtering section, I'm not sure there's a strong argument against using 3W metal film or carbon film vs 1W carbon comp, certainly not from a tone perspective. I've seen quite a few amps "repaired" this way. I'd suggest that keeping the amp operating safely and properly is of bigger concern (re: choice of resistor rating and composition) in that part of the amp than tonal considerations. But there are more experienced techs than me here; let's see what they have to say on this subject.
     
  10. smolder

    smolder Gold Supporting Member

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    I still use carbon comp resistors in vintage projects and inparticular in those preamp sections. I tend more towards metal or carbon film in newer builds... and have stopped buying those in lower wattage. Most any tagboard has room for 1 watt and they are smaller than the equivalent carbon comp... so I pretty much only by the one size.
     
  11. LarryN

    LarryN Member

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    The metal films are more stout, but I've never seen a problem in many Fenders with what's in there, I'm a little hesitant to change what's not nccessary, for vague reasons :).

    Make sure you replace the filter cap in the bias section. It's a 50uf at 70v. Most people go with a 100uf at 100v, but I like to stay as close to original as possible for vague personal reasons, again. TAD vs. Sprague is probably not an issue. You probably won't hear a difference. Sprague has a solid rep, but I haven't used TAD's. I know Spragues have gotten difficult to find and may have changed some. 20's have always fit under my cans. Have they changed dimensions, Leonc? I haven't bought any for a few years.

    I forgot that your Oxfords were torn. I've played some original Twins with Oxfords that sounded great, but they're not hi-fi :). Pretty wide range of speaker tastes and choices these days. I like Eminence Red, White and Blues. I'd also look into WGS speakers for a Twin. They have some nice vintage Fender flavored models.

    The blue Sozos are worth trying at least, IMO especially to compare to the dull brown, if that's what's in there. Fooling with the .1 caps in the PI I've tried, but prefer the bass turned down instead, for my Twin tone. I'd get the amp working right first and then start experimenting with values changes one at a time to hear what's going on, if you must.
     
  12. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Yeah Larry - I tried buying Sprague Atoms 20uF about six months ago when I was re-capping an old Super and the damn things were about 4" long!!! They've also become really expensive.

    After a bit of research, I found that JustRadios.com sells some well-made electrolytic caps in 20uF that were notably smaller and less expensive. I've been using them since and have been very happy with the results so far.
     
  13. LarryN

    LarryN Member

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    4"? I wonder what's inside? Thanks for the tip on JustRadios.com. I'll check them out.
     
  14. Gabriel

    Gabriel Member

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  15. cratz2

    cratz2 Member

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    Not trying to discourage you from buying the 12Ls... they are awesome and if i didn't have to move the amp, would be my first choice. Having said that, most Silverface Twins are in the 75 lb range (34 kg)... those 12Ls weigh about twice what the Weber Michigans do which aren't identical, but definitely the same ballpark.

    Basically a SF Twin with 12Ls or Delta Pros = almost 100 lbs/45 Kg
    A SF Twin with Michigans = 75 lbs/34 Kg
     
  16. Gabriel

    Gabriel Member

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    cratz2 The weight isn't matter cause the amp won't be moved.And the difference is only 4.6.Kilos for the pair.I see that Michigan 12" Ceramic are 5 Kilos 11lb and EV12L is 7.3Kilos.The only good is that are far cheaper.

    2 xEL12L costs almost 500$(shipping included) with currency now (Euro/$-1.29)
    2 xMichigan+Shipping 320$ but i don't know if i will pay duties when be arrived.
     

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