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Diving Into a 70s Silverface Twin- Help!

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

  1. oldlefty

    oldlefty TDPRI Member

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    Hi Guys- pretty much my first post here, although a long-term lurker and Fender user. Had a lefty Jazzmaster back in the 60s, have a hybrid tele (American Standard neck, Warmoth blonde body, Callaham bridge, 4-way switch), a Squier Precision bass and a CV Jazz bass (along with about 15 other guitars and 5 amps). I have a red knob Champ 12 with some mods and I built the BYOC Tweed Royal amp from their kit (along with about 10 of their pedal kits).

    But the reason for this post is that the bass player in my old high school garage band gave me his non-working silverface Twin Reverb (non-working like a decade or two ago- stored in the basement since then). The transformers all date from late '76, it has a MV with pull boost. It had a blown fuse which I replaced. Symptoms are- when you turn it on it's "fine", but when you flip it off standby you get sound through the speakers for about 30-45 seconds, then breakup, then a hum builds up and the power tubes start to redplate.

    I put together a current limiter with a 150 watt bulb and when powered through that the tubes don't redplate, but when you switch from standby to power the current limiter bulb glows much brighter. You get reasonably good sound through the amp (at reduced power), all the controls work except reverb, most pots are scratchy.

    I pulled the chassis and it looks pretty much stock! The only visual flaw I can find is a cracked (screen?) resistor on V9 (the 470 one).

    OK, my first time really troubleshooting an amp (and it's a TR....). So is the cracked resistor "the chicken or the egg"? Did a failed power tube cause it, or could it just fail. Is that a reasonable cause for the redplating, or something else going on? I haven't gone on to any voltage checks yet. I have the capacitor discharge probe I made when I built the BYOC amp.

    Also- I'm assuming at least some (most?) of the caps should be changed out on this 45 year old amp? Filter caps for sure, most of the big blue molded board caps? Anything else that should get replaced due to age? I love soldering!

    My objective with this amp is to 1.) Get it running right, 2.) Most likely Blackface it, 3.) Possibly do the Jerry Garcia mods to it (although I already have the SMS preamp so there's really no need to go that far).

    Thanks for the help!

    [​IMG]

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    Last edited: Oct 18, 2020
  2. Whatizitman

    Whatizitman Friend of Leo's

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    If the current limiter is glowing bright, there’s a short. I wouldn’t power it up again. I think my first task would be to check the power transformer?

    A blown fuse is never a good sign. It’s meant to blow when current is out of whack due to something being wrong, to protect the rest of the circuit.
     
  3. Boreas

    Boreas Tele-Afflicted Silver Supporter

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    Great find! Start by changing out the caps and see where you are.
     
  4. Whatizitman

    Whatizitman Friend of Leo's

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  5. W.L.Weller

    W.L.Weller Tele-Holic

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    I'd want to test the power transformer before spending $ on filter caps & output tubes. Blown fuse + heavy current draw means something is still wrong.

    Unless you're dead set on getting it working again, no matter the cost.

    The rest of the capacitors don't necessarily have to be replaced (unless they're leaking DC or the value is way outside tolerances.)
     
  6. Whatizitman

    Whatizitman Friend of Leo's

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    Yup. Don’t spend a cent on parts until you know if the transformer and power chord grounding are good.

    But some good visual inspection is in order. Compare pics of your chassis to good known ones via google.

    For instance... I hope I’m mistaken. But that looks like speaker wire running from the power and standby switches to a terminal strip next to the PT. That... strip... is not stock. Check the ground on that mess. :eek:
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From your description the PT is OK. You don't know whether the broken screen resistor is shorted, open, or out of spec. It makes it difficult to say what may be happening there.

    You may be able to rule out a power tube problem. The amp will work on just two power tubes. Either the two inner tubes or the two outer tubes can be removed. Of course remove the redplating set, or the broken resistor set.;)

    The OT secondary is 4ohm.(2 - 8ohm speakers). When you run on two tubes, ideally you want to use an 8ohm speaker on the 4ohm OT secondary. So disconnect one of the speakers.
    This will allow the reflected OT primary to match what the tubes want to see. The twin reverb OT is stout enough to handle a mismatch but you don't have to have a mismatch.

    The blue blob or blue turd caps are not bad caps. Keep them in unless they are defective.
     
    Last edited: Oct 19, 2020
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  8. BigDaddy23

    BigDaddy23 Tele-Holic

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    There appears to be some random jobs done to that amp - the terminal strip between the switch and the fuse, and the single resistor on one leg of the heater string (to the pilot light). Get all that tidied up and stock to the schematic first. That screen resistor is buggered! It could be the problem, or a symptom of another problem. Whatever happens, the filter capacitors will need changing so I would start making a parts list now (filter caps, cathode bypass caps and the bias cap). You'll get it working with the help of folks here.

    Follow LLC's advice to see if the tubes are the issue (I agree that it appears the PT is fine) - if not, I'd be looking for shorts and or open components in the power section.
     
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  9. Jon Snell

    Jon Snell Tele-Holic

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    I would suspect an output valve and also check the negative bias voltage on the output valve grids (with no valves in) is correct before adding valves.
    The main smoothing capacitors are old but will not cause red plating, they stay charged up for some time at lethal voltages!
     
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  10. Wildeman

    Wildeman Tele-Meister

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    Yikes, classic symptoms of wasted power caps! Don't turn that on no more:eek:
    Sweet amp btw, congrats, now show it some love.
     
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  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Agree with Jon Snell. Check the negative bias voltage with power tubes out before proceeding.
     
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  12. schmee

    schmee Doctor of Teleocity Silver Supporter

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    "Also- I'm assuming at least some (most?) of the caps should be changed out on this 45 year old amp? Filter caps for sure, most of the big blue molded board caps? Anything else that should get replaced due to age? I love soldering!"


    OK, the problem is likely the filter caps and/or those 470 ohm resistors on the power tube sockets. I see one is broken and it happens often. I have bought 2 Bassman heads super cheap because they were "not working" and that was all that was wrong with them. Replace with 2 watt resistors.

    Replace the filters, replace those resistors. Give it a try. You may have a dead power tube though.
    Eventually you should replace the little white caps on the board (coupling caps) but those wont cause an amp to do what you are experiencing usually. Many people go to 25uf 50V instead of 25uf 25V on those.

    DO NOT replace those big blue caps, they are fine.

    Get yourself some flux, I like No Ko Rode, it helps a ton in heating old circuit solder.

    Also, don't jump to BF'ing the amp until it's working well as it is now. There some issues at times just blindly "blackfacing" amps.
     
    Last edited: Oct 19, 2020
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  13. Sax-son

    Sax-son Tele-Holic

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    Filter caps(if original) are almost a guarantee that they will be bad after all these years. Check all the grid resistors(that is where I seem to have the most problems with SilverFace Twins. However, once they are up and running, they are a real workhorse. I have a worn No. one power tube socket that the tubes don't fit in correctly and if I am not careful, they will blow the resistors. I still love them, even the 135 watt ultralinears.
     
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  14. D'tar

    D'tar Friend of Leo's

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    Negative bias is the most important voltage, IMO. With the amp in standby and on the bulb limiter, measure your bias voltage at all of the output tube grids. Adjust the bias balance pot so the voltage is the same at every grid.

    You have there a bias balance set up. If one were to think otherwise and attempt to adjust the bias. You could unknowingly unbalance the tubes and cause redplating.

    You surely need to replace the electrolytic caps, bias caps and bypass caps. As previously mentioned. Leave the blue ones.

    Please get it stock and working before attempting anything further towards bkack facing. You will find it likely sounds amazing!!!

    I do however recommend adding a bias adjustment pot to the bias circuit. This would allow you to adjust AND balance the bias.

    https://robrobinette.com/Silverface_Amp_Mods.htm#Convert_Bias_Balance
     
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  15. Sax-son

    Sax-son Tele-Holic

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    If your are not really up on amp building or electronics, I suggest taking the amp to a good tech and having them overhaul the amp. You will have a lifelong friend as a result. I have had (3) in my lifetime and although heavy and louder than god, they are still one of my favorites. I have a Silverface 135 watt twin, one of the last built in 1981 that I still play through religiously. They just have that big clean tone. I fitted it with Weber Chicago's and really tightened up the sound. Good tubes and reasonable maintenance, you will have a friend for life.

    You don't have to play it maxed out either. I usually have the volume on 3 or 4 and it is plenty loud. Add a decent overdrive pedal and you are good to go.

    Caution: Don't let anyone try to convince you to hot rod or modify this amp. I drank the cool aid once and bought into that idea and completely ruined a good amp. Now I don't go near that "con job" within a country mile.
     
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  16. oldlefty

    oldlefty TDPRI Member

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    Yikes! Thanks for all the advice, you guys are great.

    First- my intent is to get it up and running as is, any possible Blackfacing would be down the road sometime, maybe....

    Second- that "thingy" on the transformer- anyone know what it is or does? It looks like OEM, not an add-on. That board is attached directly to the transformer. As Whatizitman said it looks like speaker wire feeding it from the fuse and switch? Here are some closeups:

    [​IMG]

    [​IMG]

    Anyone ever seen something like it? (amp is a '76-ish).

    Third- As BigDaddy mentioned, it looks like there has been at least a little work done to the amp (resistor at the pilot light, maybe the terminal strip at the transformer?). What about the bias caps on the power supply board? Would those orange Sprague Atoms be original?? If not, leave or replace?

    Since that 740 resistor on the output tube is cracked I'm assuming I should replace all of them while I'm doing it? Is there any "mojo" reason to stick with carbon comp, or is it better to update in this application? And is current practice to position them outside the socket as opposed to across the grid stoppers?

    I'll do the filter caps and those resistors, check the bias and report back. Thanks again!
     
  17. andrewRneumann

    andrewRneumann Tele-Holic

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    WAG: thermistor?
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I can't make out what *that thingy* is. I believe it is a thermal protector. It appears to be bypassed. This terminal strip is not a stock item. AFAIK, Fender did not use square drive screws either.
    I see a thermal protector on this schematic:
    http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg

    The small terminal strip seems to somewhat replace the artificial heater center tap. Possibly a LED pilot light was contemplated? Anyway, I would put that back to stock or add a humdinger pot.
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Replace the bias cap(s).

    The orange caps have probably replaced the faulty first filter cap(s). They will be redundant when you replace the filter caps. Get rid of them when you put things back to stock.

    Replace all of the 470R on the tube sockets. CC is of no benefit in this position. Hang the new fireproof 470R 2W to 5W off the side if you wish. (I would replace the 1k5 grid resistors also. Others may disagree.)

    I assume you want to keep this amp. If so, I would replace the white cathode bias caps. Again, others opinions will vary on this suggestion.
     
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  20. oldlefty

    oldlefty TDPRI Member

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    Yeah- that's exactly what it looks like! One wire to the fuse, the other to the switch. I'd seen that on the schematic before but didn't recognize what it was "hardware-wise". Since most don't recognize it does that mean it was only used for a short while? Thanks!
     
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