63 Ampeg B15NC re-cap

[frankeg]

Merry Christmas everyone! I just got an old Ampeg B15NC which appears to be all original and clean. I have not turned it on yet because I want to change the power plug and re-cap it first. I am concerned about the higher line voltages today and to what extent that may effect the amp. I am primarily concerned about the cathode bias and heater voltages. Anyone do this before or had the same concerns?

[Chuck H]

I am primarily concerned about the cathode bias and heater voltages.

Well it's precisely the cathode bias that is your friend WRT higher B+ voltage because it's somewhat self adjusting. And it's easy to check. As to the heater voltages... Those were 115V (117V?) amps. So even if your AC is on the high side at 125V you'll only have 6.8V on the filaments. No great shakes. Changing to a grounded chassis line cord and checking/replacing electrolytic caps would be my first priority. If the amp hasn't been used regularly and the electrolytic caps are original they will almost surely be hinky. So get a grounded AC cord and crack it open so you can see the caps and replace that cord. If the caps aren't original and none display leakage or bulging you may wish to turn the amp on with one of those light bulb current limiter gadgets (instructions on build and use available on line everywhere). If it checks out you can test the bias. If the bias is good you and play it.

[bob p]

As Chuck says, the filament voltages on a 115 VAC amp running at 125 VAC should still be within the 10% tolerance band for 6.3 V heaters, but why push the limits? If it were me I'd just plug the amp into a bucking transformer to bring the mains voltage down to a safer area for the amp. If necessary, google for RG Keen's "Vintage Voltage Adapter."

Chuck, I don't have incandescent light bulbs to use in the current limiter any more. We have CFL lamps and LED lights. When I'm dealing with old caps I reform them with a variac. Rather than recommending light bulb limiters, I think we need to get with the times and start recommending current limiting resistors. At least they're still available.

[Enzo]

Common bulbs are no longer available, but specialty bulbs are. For example a heat lamp.

A current limiting resistor stays static.

[g1]

For regular incandescent type bulbs, search "rough service bulb"

Articles - Rough Service Light Bulbs

[Enzo]

Rough service bulbs, appliance bulbs, decorative bulbs.

[Chuck H]

I actually bought a dozen when it first seemed future stock could be an issue. Still have a couple left. I should have ten left, but my wife likes them for the good light they offer compared to energy efficient bulbs.

But, yes, there are still special purpose and industrial sources for hundies. They may have a silicone coating or something on them. Or not. NBD.

[bob p]

A current limiting resistor stays static.

and that's all you need to reform caps.

we're not talking about dealing with a circuit that's got a dead short.

[Chuck H]

and that's all you need to reform caps.

we're not talking about dealing with a circuit that's got a dead short.

We might be. The visual aspect of the light bulb current limiter can be a fast reference/useful tool. And usually fast enough that in the event of a short the bulb can offer some small circuit protection and be an alert to power down. Since we don't always know what's wrong going in, the light bulb limiter is a fast reference and reasonably safe trouble isolation tool. In fact I wouldn't recommend it for reforming caps. Because I wouldn't recommend reforming caps. Once a cap has crystallized I just don't trust it. I've tried reforming caps with a current limiter with less than peachy results. Somebody said you can heat the caps in a shoe box that has a hole and a vent cut into it. Once the electrolyte is heated you can begin the reform with current limited voltage and more heat. Gradually backing off the heat and the current limiting... REALLY!?! Why the hell would I do that? Does it work? Does it work with EVERY electrolyte product across the possible brands? And who would trust a reformed cap leaving their bench when a new cap can be had for a few bucks!?! Frankly, anyone trying to reform OEM NOS caps for their "mojo" is acting like a chowder head and so is anyone troubling with a reform as outlined on any old caps just to save the cost of a new one. JM2C on that.

[Enzo]

Though it may be useful for it, we almost never find ourselves recommending a light bulb limiter for cap forming, it is almost always for use in servicing circuits that are blowing fuses.

The OP said he was going to recap the amp first anyway, THEN he would concern himself with any other matters the amp might have. That didn't sound like cap forming to me.

Not everyone has or can afford a variac, but the bulb is simple and easy and cheap to make.

[bob p]

I reform new production caps. you don't? you just solder them in and flip the switch?

[Enzo]

Generally yes, but that misses the point. The OP was concerned with problems with the amp, not cap forming. How do I protect the amp from circuit failures during service is not best answered by here is how we reform caps.

[stokes]

Just an FYI,you can get exact cap replacements for Ampegs at Fliptops.net

[frankeg]

Thanks everyone! I certainly didn't mean to start an arguement but for what it's worth, I have built a light bulb limiter for troubleshooting, I don't reform new caps although I may now since it can't hurt. I will look into the suggestion by Bob on the Vintage Voltage Adapter (good to know about this option) and I did find "Fliptops". I am going to change the bias resistor from 250r/10w to 300r/10w mainly because they are GE 6L6's and I'd cry if I blew em. On a lighter note and please don't get offended but I'm thinking we're all true geeks spending time here on Christmas. I'm perfectly ok with that too!

[Enzo]

You didn't start an argument, we are just an argument waiting to happen. Merry Christmas.