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FDP Forum / Amp Mods, Repairs, and Projects / It followed me home - can we keep it? (1972 Fender Twin project)

jeff_free69

US

Mar 7th, 2019 05:22 PM   Edit   Profile  

Greetings FDP-ers! Tube-newbie here, seeking wisdom and advice.

I recently came across a 1972 Twin Reverb, kicked to the curb along with some old furniture. Skeptical-me says it must be completely shot, but how could I leave it there to get tossed into a garbage truck? If nothing else, its a cool speaker cabinet.

Back in the day a bandmate had the exact same amp. But more on that later.

I'm very comfortable with electronics, measuring components, soldering etc. I have no real experience working inside a tube amp, but do have a healthy respect for hi voltage capacitors etc.

After a bit of research (checking the SN# on the chassis and transformer) I'm fairly certain it was built in early 1972. Its a Silver Face with a Master volume (no push /pull), Utah speakers and casters. I found several schematics online, the closest being one labeled "CBS... 100 WATTS RMS AMP W/REVERB AND VIBRATO" . drawing 010182 RevC (shows Master Volume / NO pushpull)

The overall condition is ok, just VERY dirty and dusty. I'm guessing its been sitting buried in the basement for a decade or two.
I've started doing some cleanup and exploratory surgery.

Pulled the speakers and they're intact. hooked them to another amp - they work ; no rubbing voice coils etc.

Tubes:
The preamp tubes are a mishmash of rca, and (mostly) unbranded. So obviously (cheap) replacements
All 4 power tubes are Ruby "matched" (interesting that 2 are slightly larger !), so obviously also replaced.

Pulled the chassis, also looking OK

Pulled off the 'doghouse' lid and did a voltage check on the big filter caps (dead as doornail) and just for good measure, a discharge (I haven't even plugged it in yet). Found that the (2) 70uF/350V have already been replaced, also Ruby parts.

Heres where I found a problem: one of the original big blue Sprague Atom 20uF caps had a broken connection - the leg pulled right out of the solder joint. Note that when I pulled off the lid, there was some unexpected sticky foam added between the lid and the caps (why?). I was gentle, but I had to pry it off, so its not impossible that this pulled the cap up.

So whats next? It seems in reasonable shape and has been serviced somewhere along the way.

I'm thinking I'll:
Resolder that cap
Remove the death cap
Deoxit-ize the pots, tube sockets and pins
Muster the courage to plug it in and see what comes out - music or smoke.

If there are no disasters, I'll make the invesment and see how far I can take by replacing the other caps and tubes, etc
Maybe even "Blackface" it , if thats feasible for this build.

Any other low hanging fruit or gotchas to watch for before powering up?
Any pointers greatly appreciated.

Thanks,
Jeff



Roly
Contributing Member
**********
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Whitehorse Canada

I don't get out much
Mar 7th, 2019 09:02 PM   Edit   Profile  

"Pulled off the 'doghouse' lid and did a voltage check on the big filter caps"

You state that you have not powered up the amp...(that's a good thing).
How does one do a voltage check on the doghouse caps without powering it up?
Are actually referring to ESR and microfarad measurements of the caps?

pdf64

UK

Mar 8th, 2019 07:33 AM   Edit   Profile  

Perhaps jeff_free69 was verifying that there wasn't any residual charge remaining in the caps?

I suggest to build a light bulb limiter and use it until you're sure any of the nastier gremlins have been evicted.
If you do, there's a good chance you'll be glad you did; there's no worse feeling than when vintage / expensive transformers release horrid smelling fumes as they give up the ghost, when it could have been avoided by having the automatic current limiting that a light bulb limiter provides.

LBL build thread

jeff_free69

US

Mar 9th, 2019 10:23 AM   Edit   Profile  

Yes, that's what I meant (checking for residual charge).

I like that idea about the light bulb limiter, makes perfect sense for the first startup. I should have all the parts laying about. Might have to scrounge around for a large incandescent bulb, though

I gather that after the intial surge the bulb should be very dim; a bright bulb means a short in the Amp, correct?

Thanks!

I didn't have access to the Amp Garage file, but found some equivalent videos (Uncle Doug on Youtube)

External link

(This message was last edited by jeff_free69 at 12:29 PM, Mar 9th, 2019)

pdf64

UK

Mar 10th, 2019 01:37 PM   Edit   Profile  

Yes, expect the LBL to show fairly bright at initial power up, then go fairly dim.
The bulb wattage will need to be appropriate for the size of the amp, eg use a lower wattage bulb for a lower wattage amp.
If it shines at high or even full brightness continuously, then yes, that indicates a short somehwere in the amp.

Uncle Doug's LBL would be quick to build but doesn't look safe; wiring should be enclosed.
See robrob's site (linky) for a much better and safer build, for hardly any more time, trouble and cost.

Halogen type bulbs are incandescent too, should carry on being widely available for some time yet, and they should work the same as regular old fashioned filament type bulbs.
Also look in thrift shops, and for 'special purpose' bulbs; as long as they fit into your box's socket, many types will be fine.

Regarding your amp, I strongly suggest a slight mod, to fit 1 ohm cathode resistors for each power tube. The 1 ohm resistor just needs to go between terminal #8 on the power tube sockets and 0V chassis common.
The mV across each resistor (at idle) allows you the assess whether the bias voltage is ok for that individual tube. They all run a bit different.
If the amp is then fed a constant test tone, the mV then allows the condition of each power tube to be assessed, easy to identify if one of the 4 is weak, as it's mV won't rise as much as the others.
So they are a valuable tool and can be used to save a lot of time and money when checking an amp.

robrob LBL

(This message was last edited by pdf64 at 03:45 PM, Mar 10th, 2019)

Bubbalou
Contributing Member
**********
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USA

THE LOW END OF UPPER TEJAS
Mar 10th, 2019 10:42 PM   Edit   Profile  

I built a current limiter years ago that I still have. I used a Halogen bulb and put a piece of 1/4" thick glass in front of it so it didn't blind me due to a direct short in an amp. I use a variable voltage transformer(Variac) so I can bring the voltage up slowly. I put an analog voltmeter and ampmeter on the front of the box with a 110vac outlet on front also to plug the amp into..

pdf64

UK

Mar 12th, 2019 01:09 PM   Edit   Profile  

Great job, that's a pro grade power up box!

FDP Forum / Amp Mods, Repairs, and Projects / It followed me home - can we keep it? (1972 Fender Twin project)




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