I just acquired a NOS Variac from a shop that is liquidating it's stock.

Now that I have this useful piece of equipment my next
question is on how to use this to reform caps and bring-up old tube amps
slowly.

Can anyone give me a Variac-for-dummies short do's and dont's list?

Does one use the Variac differently on Tube rectified and Diode rectified
equipment?

I would appreciate some hints.

Thanks,

Rich Sherman

"Rich Sherman" wrote in message
...
I just acquired a NOS Variac from a shop that is liquidating it's
stock.

Now that I have this useful piece of equipment my next
question is on how to use this to reform caps and bring-up old tube
amps
slowly.

Can anyone give me a Variac-for-dummies short do's and dont's list?

Does one use the Variac differently on Tube rectified and Diode
rectified
equipment?

I would appreciate some hints.

Thanks,

Rich Sherman


I have two "variacs" (not this brand) in my workshop and have wired
both so as to never exceed the line voltage, i.e. no "140 VAC"
maximum. We tend to be on the high side anyway, particularly at
night, say 125 VAC, saw 127 once. One "variac" has an AC voltmeter on
it and I tend to leave it at 115 VAC. The other has just a knob so
I've marked that at about 115 VAC when the line is 120 VAC (it's in an
large inverted coffee can with a PC power cord/skt in, and a two skt.
receptacle out.)

To re-form caps I put a meter on the reservoir cap and just turn up
the variac until it reads a steady 25 VDC or so - them leave it "for a
while" (subjective!) Then I advance it in steps of 10 to 15 VDC to
full B+ with careful sniffing and listening all the way .

Cheers,

Roger

It would of been nice if you told us the capacity (V/A) of your variac.
Since there are many RATs that can answer application usage with tube
equipment, I'll offer a troubleshooting technique using solid state
equipment.
Significant amount of intermittent problems can be cause by a microscopic
fracture on a pcb's trace. They often are most difficult to troubleshoot. If
you lower the input AC voltage as much as you can an still have an
operational circuit, the fracture should manifest itself. Conversely if the
PCB is dead because of the microscopic fracture, raising the AC to let's say
135 volts momentarily, can cause the current to jump the fracture. Try it,
you like it.
Cordially,
west

"Rich Sherman" wrote in message
...
I just acquired a NOS Variac from a shop that is liquidating it's stock.

Now that I have this useful piece of equipment my next
question is on how to use this to reform caps and bring-up old tube amps
slowly.

Can anyone give me a Variac-for-dummies short do's and dont's list?

Does one use the Variac differently on Tube rectified and Diode rectified
equipment?

I would appreciate some hints.

Thanks,

Rich Sherman

I put an ammeter in series with the primary of my viarac. This way I can tell how much current
is being drawn when the power is just nudged. Helps when testing to see if that direct coupled
transistor amp is planning to burn up again, and limiting the current the "needs reforming"
caps want to draw.

Phil

"Rich Sherman" wrote in message
...
: I just acquired a NOS Variac from a shop that is liquidating it's stock.
:
: Now that I have this useful piece of equipment my next
: question is on how to use this to reform caps and bring-up old tube amps
: slowly.
:
: Can anyone give me a Variac-for-dummies short do's and dont's list?
:
: Does one use the Variac differently on Tube rectified and Diode rectified
: equipment?
:
: I would appreciate some hints.
:
: Thanks,
:
: Rich Sherman
:
:

It is useful to be able to monitor both the output voltage and current
drawn when using a variac. But don't connect an ammeter in series.
Instead, use a large 1 Ohm resistor which appears on two terminals on
the case of the variac. Then you simply measure the voltage across
the resistor. By Ohms law, if you get 1V then 1A is being drawn.
I find an analogue volt meter better in this application - a swinging needle
is easier to following than a fluttering digital readout.

When cranking the variac up slowly, watch the meter all the time
for a current rush.

On valve amps, I usually use a connect a separate dedicated heater supply
so that the heater voltage is not dependent upon the variac.

It's a great piece of equipment. Enjoy.

Iain

"anybody-but-bush" Anybody But wrote in message
hlink.net...
I put an ammeter in series with the primary of my viarac. This way I can
tell how much current
is being drawn when the power is just nudged. Helps when testing to see if
that direct coupled
transistor amp is planning to burn up again, and limiting the current the
"needs reforming"
caps want to draw.

Phil

"Rich Sherman" wrote in message
...
: I just acquired a NOS Variac from a shop that is liquidating it's stock.
:
: Now that I have this useful piece of equipment my next
: question is on how to use this to reform caps and bring-up old tube amps
: slowly.
:
: Can anyone give me a Variac-for-dummies short do's and dont's list?
:
: Does one use the Variac differently on Tube rectified and Diode
rectified
: equipment?
:
: I would appreciate some hints.
:
: Thanks,
:
: Rich Sherman
:
:

Before doing anything else I would suggest wiring a fuse in series with
the output of the variac; you could put one in the input also but the
output fuse is critical. If the variac is rated for 5a, you could
possibly draw 25a out of it with it set to 20+ output volts without
blowing a 5a fuse in the primary - but you would fry the variac.

________
)
)----o put fuse here
120 in )
)
)
________)_____

We fried a lot of variacs at work before I added output fuses to them.
better to loose a $0.25 fuse than a $50 variac!

Bob

Rich Sherman wrote:

I just acquired a NOS Variac from a shop that is liquidating it's stock.

Now that I have this useful piece of equipment my next
question is on how to use this to reform caps and bring-up old tube amps
slowly.

Can anyone give me a Variac-for-dummies short do's and dont's list?

Does one use the Variac differently on Tube rectified and Diode rectified
equipment?

I would appreciate some hints.

Thanks,

Rich Sherman

"west" wrote in message
...
It would of been nice if you told us the capacity (V/A) of your
variac.

Sorry. It's about 100 VA.

Cheers,

Roger

"Bob Cleary" wrote in message
news:6Lqvc.289$%F2.143@attbi_s04...
Before doing anything else I would suggest wiring a fuse in series
with
the output of the variac; you could put one in the input also but
the
output fuse is critical. If the variac is rated for 5a, you could
possibly draw 25a out of it with it set to 20+ output volts without
blowing a 5a fuse in the primary - but you would fry the variac.

________
)
)----o put fuse here
120 in )
)
)
________)_____

We fried a lot of variacs at work before I added output fuses to
them.
better to loose a $0.25 fuse than a $50 variac!

Bob

Two very good ideas above - as a fuse and an ammeter!

Unfortunately, my AC ammeter is an old, classic GE brand 0 - 10A, 0 -
20A, a bit course for delicate work. However, I have a spare Micronta
DMM that I might rig up with the one ohm resistor idea and leave in
permanetly in this service.

Cheers,

Roger

"Rich Sherman" wrote in message . ..
I just acquired a NOS Variac from a shop that is liquidating it's stock.

Now that I have this useful piece of equipment my next
question is on how to use this to reform caps and bring-up old tube amps
slowly.

Can anyone give me a Variac-for-dummies short do's and dont's list?

One thing you may do, is to trash the notion at the outset that one
can "reform" caps in-circuit with an autotransformer connected to the
equipment, because it is not possible in any meaningful sense of
"reforming." All you can do to caps this way, is to baby them up into
operation in case they haven't been in service for a long time, with a
little less chance of popping one or ruining what may be left of its
film & anodizing/healing ability. This is not "reforming." It is, at
best, a precautionary measure when first evaluating old gear, or at
worst a lazy attempt at avoiding doing things right by not checking &
chucking poor parts.

To reform an electrolytic in any meaningful way, you need to connect
it by itself it to a variable DC supply which is capable of exceeding
the rated working voltage of the cap, and in series with an accurate
means of measuring its current leakage continuously. When the cap may
be brought up to its full rated voltage with only very small leakage -
say, 50 microamps or less - it may be considered reformed, and it is a
good idea to run it up a little further toward its design voltage
rather than stopping at its working voltage, with the same criteria.
A cap will not form up completely at lower voltages often encountered
in its particular circuit service. Once you do a few dozen, which
takes considedrable time and individual judgement along the way, you
will come to realize that new caps, which have zero leakage, are a
very good deal, and that most (not all) reforming is a waste of time
that is ultimately more expensive. And after it is reformed, it must
be tested for capacitance (and ESR if you are able), which will often
be disappointing.

The issue of forming up to full capaciatance at full rated voltage is
also the reason why it is poor practice to use even a new electrolytic
that is rated a great deal higher in voltage than the circuit calls
for. It will often not form up to full value, or may have reduced
service life.

An autotransformer is very handy for initial checkout of old or
suspect equipment by limiting voltage & accompanying currents so that
bad parts don't toast or take something else out with them before they
may be more easily located by observation & measurement. While having
provision for an ammeter is very handy, and a fuse is just common
sense, it is even much more practical to have 1 or 2 lightbulbs
switchable in series with it as current limiters, and this is how they
have been used on repair benches for 60+ years. There is no advantage
to wiring it up not to exceed line voltage, because increased voltage
has other uses, and with current-limiting bulbs switched in & a mark
for its knob you will not get in trouble. A bulb with wattage
approximately equal to line voltage (100w in USA) will limit current
to the neighborhood of 1 amp, and a second switchable in parallel to
it will give around twice that. I have a nice ammeter on my Variac
panel & I look at it about twice a year, and at least one of those
times is out of boredom. A voltemeter is a must-have unless you want
to tie up a good DMM that's more useful elsewhere.

*Always* power a bench Variac with an isolation transformer of
sufficient VA rating to match it. If you can't afford one (they are
much more expensive than scrounged Variacs these days), build one from
2 salvaged microwave oven transformers (free, requires metalworking
skills & an enclosure). If you don't have one, don't use it at all,
period. If you build one remember to have an IEC equipment ground
uninterruptible from the tested equipment to the mains outlet, and
make sure the mains outlet ground is good.

You can also break in new speakers (if you're into that) *directly*
from the Variac through a bulb, and can crank it up considerably
without exceeding the voicecoil rating if its a 25-50w job (monitoring
it of course). If you don't know how to monitor it, don't do it.

Higher-than-line voltage is good for burn-in routines on demanding
gear, and other things.

Most of the better autotransformers were not designed for electronics
servicing, but for control of industrial equipment, often polyphase
motor circuits & the like, and are commonly available as pulls
generously marked up. They may be wired up a number of ways, and
often a 240v unit may be used for 120v and vice-versa (while rerating
current accordingly) for our non-demanding testing purposes. Do not
spray contact cleaner or other canned **** on it, or you may pay later
accordingly. If its winding contact surface is unhappy, clean it with
fine sandpaper. Many pull Variacs have a worn but serviceable brush.
Be nice to it, they are not always cheap & easy to replace. The
smaller sizes of Superior Powerstats are better for light bench work
than common 5 amp Variacs.

wrote in message . com...

Higher-than-line voltage is good for burn-in routines on demanding
gear, and other things.

Ooops! I forgot that having it wired up for higher-than-line voltage
is also a *necessity* for practical bench use with current-limiting
bulbs, or full line voltage will not be available at the equipment due
to the drop across the bulb(s). So even if you never do overvoltage
burn-ins, it is useless wired for only line voltage.

You can build all this but wouldn't a proper HV lab supply do as
nicely? They come up pretty reasonably fairly often or you can build
Bruce Rozenblit's nice one.

For anybody in London I have a large variac FS. Heavy so must be picked up -
open to any offers, even silly ones. Andy

=== Andy Evans ===
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