Twin Reverb overhaul

[cdub66]

So, I'm looking at buying a mid-70's Twin Reverb from a friend, went to have a listen to it tonight. He hasn't been using it (don't know for how long, he's a bit of a collector, lots of other amps to use) so he put in a new set of 6L6's so I'd get off to a good start.
When I arrived, he had it apart again, trying to figure out a "hum" that starts after it warms up for a while. Didn't see anything right away, so we re-assembled and tried it out.

WOW.

That's all I can say. I was blown away-my 30w Line 6 Spider III amp will never do now. Played a short time, maybe 10 mins. no hum, started plugging to a few other amps to see how they compared (left the Twin turned on) After about 20 mins more the Twin started to do its hum.
He's going to figure out the problem and fix it before I buy it but I'm wondering what else I have to look forward to (potential) problem wise. I'm thinking a complete overhaul with new components (there sure is a mess of stuff in there) would be a good idea, but likely quite costly.
Should I be scared of this amp (that I REALLY like) and look for something newer? What would even compare?
Or has the freak problem reared its head at an opportune time and life will be sunshine and lollypops from here?

I know there's no way for you to tell, just looking for opinions from people more familiar with older tube amps than myself.

Any input appreciated! :food-smiley-004:

[Big White Tele]

A cap job is due, Parts are 75.00 for the top of the line Atoms. The hum could be a variety of things, grounding is a likely problem. A proper set of tubes, and proper bias, cleaning all the pots and tube sockets, retensioning the sockets, input jack cleaning etc etc. Basically a good tune up. Its not really a big deal but most likely not something you'll do yourself unless your a technical guy. The biggest problem is finding someone who can do it. Youll be looking at around 250.00- 350.00 with tubes, parts and labour depending on who does it. if you plan on looking around in there, make sure you drain the caps before poking around. Lethal voltage is present even after its unplugged. All that aside, Theres nothing like a Vintage Fender amp, and thats..........Priceless.

[cdub66]

Thanks BWT, I'm not going to back away from this one-if he replaces the caps in the process of fixing the hum that's good, if not I'll have it done when I can.

I still can't believe the sound this thing puts forth. sdsre

[suttree]

yah what BWT said. that should have you covered for the next 5-10 years anyways. and no, there's not really anything as good as an old fender amp that will cost you any less, at least.

[konasexone]

The caps don't necessarily need to be changed "au complet". A good cap meter like a Sencore will often reveal that a cap is well within spec years later. Don't overdue it in the cap department, its often unnecessary. Remember, your amp is old and things are brittle in there so be gentle. It might not be a good idea to gig with it but it may do just fine in the corner of the rec room. Make sure you have a good earth ground (three prong plug). Muck of that noise is due to ground loops (resistance in the ground paths). If possible, try to rewire all grounds to one location. Point to point solder boards are the best for doing this.

[Wild Bill]

The caps don't necessarily need to be changed "au complet". A good cap meter like a Sencore will often reveal that a cap is well within spec years later. Don't overdue it in the cap department, its often unnecessary. Remember, your amp is old and things are brittle in there so be gentle. It might not be a good idea to gig with it but it may do just fine in the corner of the rec room. Make sure you have a good earth ground (three prong plug). Muck of that noise is due to ground loops (resistance in the ground paths). If possible, try to rewire all grounds to one location. Point to point solder boards are the best for doing this.

Maybe I'm lazy but I never use my cap meter. For the price of a new filter if one's old and dead then screw it - I yank 'em all! To me, to change only one would be like changing only one brake shoe on my car. False economy.

Grounding schemes are not likely to be causing you hum. The stock wiring worked. Unless someone did a hack job the wires won't have moved around on their own!

No, with an amp that old the filter caps are the most likely culprits. You can easily tell. Filter hum is not the same frequency as guitar cord hum. It's 120 hz instead of 60hz, around a low B on a bass with extra bottom strings. If you plug in a cord and tap the end to inject some hum listen to the two notes. If the amp hum is higher pitched than the finger tap then it's the filters gone for sure.

There are a few other possibilities. More long shots but still possible. Sometimes the input jack contacts can get bent inside. They are supposed to short themselves out when nothing's plugged in, so that there's less hum while the amp is just sitting there. Most Fenders use a pair of 100 ohm resistors on each side of the filament line to ground, to kill filament hum. Over the years you might have had a shorted or wonky bad tube that fried these resistors. Everything would still work but there would be lots more hum. The cure is to replace the resistors. There are other parts that could do something similar.

Still, as I said these are long shots. For the age of the amp I would change out the filters. It takes as much labour to open up the amp to change one filter cap as all of them. The parts cost is much lower than the labour charge. To have them replaced one at a time will KILL you on labour costs! Remember, filter caps fail 'cuz they're made with a wet paste inside that slowly dries up over the years. When one goes the odds are the others are not far behind.

After the filter cap job the odds are the hum will be gone. If not, at least you've eliminated one possibility.

:food-smiley-004:

[Greg Ellis]

Speaking of capacitors, what's the deal with the $5 to $15 per cap prices on the Sprague Atom caps?

Mallory and similar brands cost something like 50 cents to 3 bucks or so.

Do they make the Atoms out of plutonium or something?

[Big White Tele]

I Just bought about 50 sprauge atoms from the tube store, 25@25 are 1.95, the filters (16@475)were 7.50. each. And great service by the way!!!

[Wild Bill]

Speaking of capacitors, what's the deal with the $5 to $15 per cap prices on the Sprague Atom caps?

Mallory and similar brands cost something like 50 cents to 3 bucks or so.

Do they make the Atoms out of plutonium or something?

The Atoms are made just like the old days. They're the same as they were in the 50's, which is why they are so much bigger for the same cap and voltage rating.

Mallory and Illinois brands are more modern.

It costs more to do a special run the old way. The volumes are much smaller, as well. After all, the number of tube heads is mice nuts compared to regular cap applications these days. The number of requests for OLD style filter caps is even smaller!

It's like asking for brake shoes for a '57 Chevy. You can still find someone who makes 'em but they won't be as cheap as a set for your 2002 Neon from Crappy Tire.

:food-smiley-004:

[Greg Ellis]

Thanks Bill,

So there's no practical difference in operation, it's just a fit and style issue?

I imagine the newer amps are designed with the newer caps, right?

[Wild Bill]

Thanks Bill,

So there's no practical difference in operation, it's just a fit and style issue?

I imagine the newer amps are designed with the newer caps, right?

Yep! You got it! They offer the Atoms as part of the 'mojo' trip, for those guys who don't know enough technicals to understand the brand of filter cap has no real-world sonic differences but think if they get the original style of cap they will get the original sound.

Easier than spending time practicing, I guess!:smile:

:food-smiley-004:

[nonreverb]

A quick question: does this particular Twin have the hum balance control? If it does and new tubes are in it, it may need to be adjusted after the amp has been running for a while...

So, I'm looking at buying a mid-70's Twin Reverb from a friend, went to have a listen to it tonight. He hasn't been using it (don't know for how long, he's a bit of a collector, lots of other amps to use) so he put in a new set of 6L6's so I'd get off to a good start.
When I arrived, he had it apart again, trying to figure out a "hum" that starts after it warms up for a while. Didn't see anything right away, so we re-assembled and tried it out.

WOW.

That's all I can say. I was blown away-my 30w Line 6 Spider III amp will never do now. Played a short time, maybe 10 mins. no hum, started plugging to a few other amps to see how they compared (left the Twin turned on) After about 20 mins more the Twin started to do its hum.
He's going to figure out the problem and fix it before I buy it but I'm wondering what else I have to look forward to (potential) problem wise. I'm thinking a complete overhaul with new components (there sure is a mess of stuff in there) would be a good idea, but likely quite costly.
Should I be scared of this amp (that I REALLY like) and look for something newer? What would even compare?
Or has the freak problem reared its head at an opportune time and life will be sunshine and lollypops from here?

I know there's no way for you to tell, just looking for opinions from people more familiar with older tube amps than myself.

Any input appreciated! :food-smiley-004:

[cdub66]

A quick question: does this particular Twin have the hum balance control? If it does and new tubes are in it, it may need to be adjusted after the amp has been running for a while...

Not really sure nonreverb, how would I tell? The friend I'm buying it from seems like he knows his stuff and I don't want to tell him what to look for (as I know zero about these things so far) If I knew what to look for I could point to it and ask "what's that do?"

Thanks everyone for the helpful ideas! :smilie_flagge17:

[konasexone]

Maybe I'm lazy but I never use my cap meter. For the price of a new filter if one's old and dead then screw it - I yank 'em all! To me, to change only one would be like changing only one brake shoe on my car. False economy.

Grounding schemes are not likely to be causing you hum. The stock wiring worked. Unless someone did a hack job the wires won't have moved around on their own!

No, with an amp that old the filter caps are the most likely culprits. You can easily tell. Filter hum is not the same frequency as guitar cord hum. It's 120 hz instead of 60hz, around a low B on a bass with extra bottom strings. If you plug in a cord and tap the end to inject some hum listen to the two notes. If the amp hum is higher pitched than the finger tap then it's the filters gone for sure.

There are a few other possibilities. More long shots but still possible. Sometimes the input jack contacts can get bent inside. They are supposed to short themselves out when nothing's plugged in, so that there's less hum while the amp is just sitting there. Most Fenders use a pair of 100 ohm resistors on each side of the filament line to ground, to kill filament hum. Over the years you might have had a shorted or wonky bad tube that fried these resistors. Everything would still work but there would be lots more hum. The cure is to replace the resistors. There are other parts that could do something similar.

Still, as I said these are long shots. For the age of the amp I would change out the filters. It takes as much labour to open up the amp to change one filter cap as all of them. The parts cost is much lower than the labour charge. To have them replaced one at a time will KILL you on labour costs! Remember, filter caps fail 'cuz they're made with a wet paste inside that slowly dries up over the years. When one goes the odds are the others are not far behind.

After the filter cap job the odds are the hum will be gone. If not, at least you've eliminated one possibility.

:food-smiley-004:

And that's what a good instrument like a Sencore will tell you. The tests can be made in circuit. You'd be surprised how long of a working life a cap will have in an amp that is used regularly. I recently fixed an old Gibson amp for a harp player out here with the exact same problems. This thing had been made in the ? 50's maybe and the caps were still good. Ground loops were the culprits here. You'll often find that the hum does not always originate from the supply. Pull the preamp tubes and leave the power tubes in and listen to see if that hum persists. It often doesn't.

[nonreverb]

Not really sure nonreverb, how would I tell? The friend I'm buying it from seems like he knows his stuff and I don't want to tell him what to look for (as I know zero about these things so far) If I knew what to look for I could point to it and ask "what's that do?"

Thanks everyone for the helpful ideas! :smilie_flagge17:

It will be located on the back panel. Sometimes on these amps, the pot is toast which will be evident if adjusting it has no effect...:smile:

[cdub66]

So, I think I'd like to buy the filter caps and help install them, best way to learn about my new amp! :banana:
Suggestions on where I should be shopping for this kind of stuff? Any brand better/worse? Also important-which ones do I need? There are 2-350 volt, and 3-500 volt I believe, but I can't read the other value (10UF, 20UF etc.)
How's this look? http://cgi.ebay.ca/Fender-Pro-Super-Twin-or-Reverb-Cap-Kit_W0QQitemZ330232143751QQihZ014QQcategoryZ10171Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

Again, thanks for all the help! :smilie_flagge17:

[toby2]

If the hum is not there when you first turn the amp on , I would say that
caps are not the culprit . Often a tech will tell a customer to replace the
caps just because they are old .

[Wild Bill]

If the hum is not there when you first turn the amp on , I would say that
caps are not the culprit . Often a tech will tell a customer to replace the
caps just because they are old .


My experience has been different. After the amp has been turned on for a while old filter caps can heat up. This aggravates the dried up paste electrolyte inside and makes hum louder.

I would replace the caps simply because when they're that old they're likely to go soon anyway. As I said, it's like having 50 year old brake shoes in your car. When the caps were first installed the industry expected they'd last 10-15 years if you were lucky. Guarantees only ranged from 1-5 years. Recapping was a normal part of maintenance for tube equipment.

Replacing them will eliminate one area of possible problem. Odds are the amp will then be good for another 15 or more years anyway.

The reason I replace them all is that if I didn't I'd feel like I was setting my customers up to cheat them! When caps start going they tend to fail within months of each other, since they would be the same age to start with. You're talking $5-10 per filter cap. Just opening up the amp, replacing a cap, checking voltages and such on general principles and closing the amp up again is at least an hour of labour. My shop rate is $50. Lots of guys charge more.

If I change the caps only when they actually fail then I will end up charging the owner the same labour each and every time. Most amps have 4-6 filter caps or more. Over a year or two that's a big bite! If I do them all at once it's only an extra 5-10 minutes and probably would fall into the same amount of labour time anyway. To do it the first way would bother my conscience.

It's different if you do your own amp work. Your labour is paid in love so who cares if you keep opening up your amp for yet another cap failure? When you have to pay a tech, wouldn't you rather deal with someone who helps you avoid extra expenses?

Meanwhile, back to the hum! If your tech friend is a good one I'd just let him handle it. Certainly he should have a 'scope and that can dramatically speed up the process. We can give you tips to try till the cows come home and it would only be luck. We've covered the easy stuff! It's like trying to help someone who knows nothing about cars to get it to start, when you're limited to email msgs! After you've told him to check that there's gas in the tank, the battery's not dead and the wires haven't fallen off the spark plugs there's not much more you can do.

There is one more long shot. I don't recall if you said if you replaced the output tubes. If one was weak and mismatched to the other after an hour or so of warmup it could get even weaker, making the mismatch even worse. A BIG mismatch can cause hum.

Anyhow, if your friend is a real tech there's not much you can tell him that he wouldn't already know. It would only aggravate him!:eek:

:food-smiley-004:

[Big White Tele]

Many many reports of Illinois caps failing in fender amps. Sprague and F&T seem to be the highest regarded replacements. You might be able to get cheap ones at Canadian Tire..

[cdub66]

Many many reports of Illinois caps failing in fender amps. Sprague and F&T seem to be the highest regarded replacements. You might be able to cheap ones at Canadian Tire..

The Sprague Atoms are on the way!!!

[cdub66]

Me again..new filter caps are in-sounds great, but....still the hum. Started after maybe 15-20 minutes or so. Seemed like the reverb and tremelo weren't working either (not much anyways).

Don't think I mentioned previously the sound is coming through the speakers (as opposed to an electrical hum such as a fluorescent light makes). also, it increases with the volume knob (either one) up to 5 then quietens down as you keep turning it up.

Gonna recheck the grounds again I guess, any more ideas?

Thanks for all the help everyone! :food-smiley-004:

* nonreverb-no hum balance control to be seen *