Vibroverb Re-cap Dilemma

[Ted]

I have a 64 AA763 Vibroverb in front of me.

It is clean. Immaculately clean. Could be mistaken for a new repro.
Doesn't look like it has been run for more than a few hours.
The owner said "do whatever needs doing".
He wants to use the amp rather than keep it as an investment.

Having checked all resistor values and deoxited some very scratchy pots, it sounds fantastic.
My question:

I pulled a one of the 25uF cathode caps and one of the 20uF power caps and they test OK for capacitance. Don't have an ESR meter.

Heat and old age kill electolytics. These have never been hot for any length of time, although they have been around almost as long as I have.

Should I replace those 53 year old caps? I asked the owner and he had no opinion.

[potatofarmer]

Yes, of course you should. They may seem fine now, but that'll be little consolation if they blow two months down the road. He's already told you he intends to play the amp, and these are wear parts just like tires on a car.

Give him the old parts back; he can decide if he wants to keep them for originality's sake if he decides to sell the amp at some point.

[pdf64]

I have a 64 AA763 Vibroverb in front of me...Doesn't look like it has been run for more than a few hours.
The owner said "do whatever needs doing"...He wants to use the amp rather than keep it as an investment....I pulled a one of the 25uF cathode caps and one of the 20uF power caps and they test OK for capacitance. Don't have an ESR meter.

Heat and old age kill electolytics. These have never been hot for any length of time, although they have been around almost as long as I have.

Should I replace those 53 year old caps? I asked the owner and he had no opinion.

Another thing that is bad for ecaps is sitting unused for years without any charge to keep them formed.
As you've no means of assessing the leakage and ESR of those old ecaps then best to just campaign change them all.
No prizes for guessing what the owner's opinion will be if the amps fails soon after you've returned it with a clean bill of health

[Leo_Gnardo]

Should I replace those 53 year old caps.

Helluva find Ted, those amps sell for gold dust money in the USA "because Stevie Ray Vaughan used one." Prior to that, you couldn't give one away.

As for the electrolytics, what pdf64 said, age and lack of use kills 'em too. Don't forget to refresh the bias supply filter cap while you're at it, and if it's rated 50V as many old Fender bias caps are, upgrade that to 100V.

In electrolytic caps, aluminum oxide grows on the foil in the presence of an electric field iow charging them is what keeps 'em alive. The oxide is the dielectric (non conductive) layer. An "old timer" electronics tech told me some decades back, "if you kept electrolytic caps charged up they'd last forever." As it turns out he was right.

[Ted]

Leo. This one came from the US quite recently. The guy paid (to me at least) an unreal amount for it. It is as you say "The Texas Flood Amp". I assume that was why my guy was prepared to pay so much for it.

Sometimes you know the answer to a question before you ask it. Sometime you chose where you ask the question very carefully. All those untouched solder joints...

[Enzo]

You just bought a classic car, whatever you think is cool, a 1932 Chrysler, a 1957 Chevy, a 1956 Thunderbird. Now would it really be an issue to keep the original tires on it? Would it be a deal to not find the original spark plugs in it?

[Chuck H]

Another way to look at it. Fender didn't sell it like that. They sold it with fresh e caps and tubes.

[eschertron]

I pulled a one of the 25uF cathode caps and one of the 20uF power caps and they test OK for capacitance.

Sometimes you know the answer to a question before you ask it. Sometime you chose where you ask the question very carefully. All those untouched solder joints...

I think you've answered your own question. By pulling the caps for testing, you've 'spoiled' the amp for its museum value. But that's OK because the client wants a player's amp, right? Fresh caps and any other desirable maintenance and he'll have that amp, in good playing condition, for the next 40 or 50 years.

[g1]

Assuming that modern electrolytics have the same longevity as those made in the golden era some half a century (or more) ago.
For myself, I'm very hesitant to imply anything like that to anyone.

[Chuck H]

Agree. I do think new caps may actually perform as well, probably better (?) than vintage design caps. I don't have a lot of repair experience but I've seen a higher 'catastrophic' failure rate with newer design caps. That may be a consequence of manufacturers chasing dollars with dimes, but all I know is that today I'd be much more suspect of a twelve year old Xicon or Illinois cap blowing it's guts but fifteen years ago I'd have been surprised if a twenty year old Mallory did that. That's just my experience though.

[Ted]

Thank you all. As an engineer I knew the answer already. I asked the question here because I knew I wasn't talking to audiophiles or guitar collectors.

So far as modern caps are concerned, you get what you pay for. A cap spec'ed to run 12000 hours at 105C is going to cost more than one of those Illinois caps they put in modern Deluxes, which are probably spec'ed at 30 minutes at room temperature. I'd expect that better modern QA techniques would give a tighter spread of characteristics. Although I'm sure that someone is going to argue that the high ESR of old caps was gave BF Vibroverbs their sound.

I generally steer clear of specialist "hand-made" caps on the basis that anything machine made for Sony is going to be subjected to more rigorous QA than something made in a hut half way up a Swiss mountain by some old dude who does things "like they did in 64".

[Leo_Gnardo]

Thank you all. As an engineer I knew the answer already. I asked the question here because I knew I wasn't talking to audiophiles or guitar collectors.

Thanks for asking, you know we have a fairly practical brain trust here. It's always a bit of a thrill to hear about the 64-65 15" Vibroverbs. Early 1982 a local pedal steeler had me work on two of his. One had a bad OT but he wouldn't go the money to fix it right. Later on that year while I was on tour, he sold them thru a local music store for $500, both. Had I known, would have bought 'em just for the kool factor, any old Fender is a treat. Turns out that would have been an amazing investment. Since then the buyer of one of them has brought it to me for servicing, and he's happy as could be with his amp.