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Old 9th January 2013, 08:58 PM   #1
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Default the old light bulb trick

Hey guys and gals...at least I hope there are bottle head girls out there.

I recently scored a pair of Altec 1570B mono tube amps. Everything looks OK with them (no leaky caps, burned wires or resistors etc.). As much as I would just love to plug these in and "light em up" I know better. These monstrosities have sat forgotten for 25 years or so. I don't have a variac so I was going to use the old light bulb in series trick. I figure I would start out with a low watt bulb (25 watts) and let it run about 12 hours then start stepping up the wattage of the bulbs (40, 50, 60, 75, 100 etc.) And let it run 12 hours at each wattage to very slowly ramp up and allow caps to reform.

Am I on track with my reasoning?...just a sanity check.

Btw yes I am aware of the dangers of top capped 811A's and nearly 1kv on the B+!

I use the 4 step method:

1. Test it.
2. Ground it.
3. Test it again.
4. Only after 1-3 shall you touch it!
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Old 9th January 2013, 09:10 PM   #2
wicked1 is offline wicked1  United States
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Quote:
Originally Posted by oldschooltube View Post
wattage of the bulbs (40, 50, 60, 75, 100 etc.)
Are we allowed to talk about illegal activities and banned items here?

(sry, soap box moment about lightbulbs)
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Old 9th January 2013, 09:12 PM   #3
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They're not banned. If you own them, you can use them.
It's only the manufacture that is stopping.
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Old 9th January 2013, 09:19 PM   #4
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"just a sanity check"

Don't mess around: buy a variac. Those amps are worth considerable money; a good variac will be some small fraction of that amount and you will use it for years. You need the basic tools if you are going to work on your own equipment. Set the amp up with your DVOM connected to B+, then start to ramp up the voltage. When the voltage stabilizes, turn up the AC until the voltage stabilizes again. Go slowly and watch your meter and the amp for any signs of distress.
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Old 9th January 2013, 09:32 PM   #5
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Quote:
Originally Posted by Frank Berry View Post
They're not banned. If you own them, you can use them.
It's only the manufacture that is stopping.
They are still allowed for non-domestic use here in UK, so manufacture is continuing, and everyone pretends they are contractors or something.
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Old 9th January 2013, 09:53 PM   #6
DF96 is offline DF96  England
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Light bulbs are a useful precaution if big caps need reforming, but they are not a suitable reforming method as their resistance is likely to be too low.
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Old 9th January 2013, 10:57 PM   #7
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Yeah. What he said. Light bulbs are excellent current limiters / self resetting fuses, but not for capacitor reforming. Use a variac.
- Unplug rectifier tube.
- Apply power. Ramp up to normal while checking that tubes light up and the power transformer remains cool. (Shorted turns are not uncommon, especially in countries with 230 volt mains - thinner wire, more turns.)
- Reinstall rectifier. Monitor the B+ while winding the voltage up slowly. Wait for the B+ to stabilise before each increase. Also check the plate voltage on the output tubes, it's not uncommon for the OPT to be open circuit and this will damage the tubes if full B+ is applied.

You will, of course, have already gone over the chassis, resocketing tubes and cleaning contacts, and checking all non-electrolytic capacitors for leakage.

Last edited by Don Hills; 9th January 2013 at 11:01 PM.
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Old 9th January 2013, 11:53 PM   #8
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There's nothing to reform. Oil caps in the main B+.

There are some smaller electrolytics on board, but I'd not be terribly concerned about them, I've yet to find a bad one thus far on these amps. I've had a bunch over the years. If you have a bad one, the voltage will just not come up or have a ton of ripple... easy to spot, then just put a replacement cap for the bad section(s) under the chassis and preserve the look

The variac is a good idea, and you don't need a super big one...

But to test, I'd pull the B+ off the 811s and assuming the tubes are tested good, fire it up and see where the voltages come up to... the bulb in series is ok, then the voltages will be slightly lower... probably good enough in lieu of the variac.

The variac won't buy you that much because the amp uses 4 rectifier tubes, so you have to get their filaments up first before they will conduct... unless you strap in solid state rectifiers for testing - which is an ok idea.

What one wants to test is the driver choke.
It is known to fry.

It should have approximately, not exactly equal DCR between legs. I seem to think I recall about 80 something ohms. But I'd suggest taking the time to pull the three wires, and remove the choke from the chassis. Then remove the end bells and look for heating and smell for burning.

IF they are suspect it is a good idea to replace *both* channels, since the part is unavailable. PM or email me if you find this to be the case and need guidance on a replacement.

The other thing I like to do is to clip the cap off the feedback network. This makes the amp respond better under square wave test, less ringing, like none. Also it sounds better.

Be cautious about increasing the main capacitance (6 or 8 ufd oil caps) over the stock, since this is a tuned filter, and adding capacitance seems to make the ripple greater, unless you figure out how to re-tune it.

Adding an RCA input to where the barrier strip is, and/or taking it out is something that I have done.

Also, skip the path through the level pot.

Definitely replace the coupling caps on the board, the ones that go to the driver tubes as a precautionary measure.

Eyeball under bright light and a magnifying glass all the resistors - look for heating, discoloration or cracking. Measure them too. Carbon comps tend to go south when old.

As it turns out I am refurbishing a pair pretty much right now...
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Last edited by bear; 9th January 2013 at 11:59 PM.
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