Bringing a preamp up on a variac

[fini]

I have been asked by a buyer to use a variac to power up and test a Fisher console preamp (ca. 1962). I've never plugged it in, in the year I've owned it, and don't know how long it has been since it was in use. The only potentially useful test equipment I own is a digital VOM (in addition to the variac). The goal is to do a listening test (and not harm the preamp). How would this procedure be different from doing it with a power amp? What are the (pre-smoke ;^) things to look for as I do it? If testing is needed (using the VOM), where and how do I test it?

You'd think by now I'd have learned how to do this...

[JL Sargent]

I don't there there is one "holy grail" way to do it. The general idea is to have the variac turned all the way down when you plug in the amp and then slowly turn up the voltage to that specified on the amps nameplate. Now the term "slowly" can mean anything you deem necessary I guess.

My personal experience with 1/2 doz. or so console tube amps from various makers is to simply plug it in and see if it works. I haven't killed one yet and I do have a nice variac sitting there on the workbench, WTH.

[fini]

JL, I'm being particularly cautious with this one. It's a 400CX-2 (very collectible). I've read several threads on variac use for power amps, but none for preamps. I don't mind if it's a waste of time. []

[JL Sargent]

Oh no, not a waste of time by any means. I'm just a try it and see kinda guy. I found this write up a guy did about how he does an initial power up. Looks pretty darn intensive to me.

Initial Power up

this can be the most stressful
time in
the restoration process. What the heck is going to happen when I put
the juice to this baby? Well, to
help avoid catastrophe, I have a few suggestions. First, what I always
do is connect an
ohm meter across the amps power cord, turn the power switch on and see
how many
ohms I get. If it is above 2 ohms, things are cool and I will assume
that the
power transformer is not shorted.Next I remove the tubes and plug the
unit into my Variac
and bring the power up while monitoring the AC voltages coming out of
the power transformer. I check the filament,
bias and high voltage values to make sure everything is there and
within specification.If the amp passes this test I will install
all the tubes and again connect it to the Variac.

I will start off at
about
40 volts on the variac and just let the amp run till things start to
glow, and
I begin to get high voltage in the power supply. At this point, I look
for any
unusual voltage drops in the power supply circuit. Such voltage drops
could
indicate a shorted capacitor or an open resistor. Next I will crank
the variac
to around 55 volts. At this point, I will usually be able to hear a
little hiss
and hum in the speaker, and the amp will also, if everything is
working, pass
a signal. It may not sound too great, but it should be there. It was
at this
stage that my first Scott 299C exhibited the leaky capacitor
phenomena. When
I pushed the variac to 80 volts, the B+ did not increase and the first
filter
capacitor began to get noticeably hot. This amp actually worked as
long as I
kept it's line voltage below 60 volts! Anyway, by using the variac, I
had the
time to safely ascertain that I did have a leaky capacitor. You can
make a makeshift
variac of sorts by wiring a light socket in series with the amp's
power cord
and the wall socket. You can use different wattage bulbs to change the
voltage
seen at the amp's power cord. If you are really into collecting and
restoring
antique radios and vintage audio you owe it to yourself to purchase a
variac.
Used, I have seen them as cheap as $20.00 and new they range from
$80.00 - over
$200. depending on their size. They are reliable too, so don't be
afraid of
buying an old one. My variac is from the 40's! Yea, it may look really
retro
a la Frankenstein, but it works great!Now, after the amp has either
successfully
run at 60 - 80 volts for a while, or you have replaced whatever
components were causing it not to work at 60 - 80
volts it is time to crank the variac up to 100 volts and plug in a
turntable
and tuner.

Now, switch the amp to tuner and see if both channels work
and sound
clear using the tuner as a source. Let her rip for a while paying
special attention
to any abnormal distortions of the sound, or any unusual noises like
squeals,
pops and hisses. Now turn the volume control down and see if there is
any hum.
Sometimes an amp will be quiet for the first several minutes of
operation, then
it will start to hum. This can be indicative of a power supply
capacitor problem,
weak or gassy tubes, and improperly balanced and biased tubes. Some
amps have
a hum balance control that can cause the problem as well. Next, give
the phono
section a check out by selecting the turntable and playing an LP. If
everything
is all right, you should hear a nice clean crisp signal coming from
both speakers.
I don't care what anyone says, tube amps from the 'golden era' of
hi-fi (1955-1965)
have the best sounding phono preamp sections. So, if you have been
listening
to those old LP's on a solid state amp (that is if you hadn't become
so disgusted
with the sound you tossed them and the turntable!) you won't believe
how good
they will sound through a tube amp.When I finally got my first 299C
going,
I plugged a Techniques SL3300 turntable into it, plugged it into a
pair of Mirage
190is speakers and felt my jaw drop at the sound. I couldn't believe
it! This
thing sounded better playing an LP then new solid state gear does
playing CD's.
You must bear in mind that my LP's are, for the most part, in quite
good shape.
If you have LP's that have been played extensively on inexpensive
'close-n-play'
quality turntables then they are probably trashed and nothing is going
to make
them sound great.Well, at this point, you are ready to
just plug the amp into the wall socket. It is fully functional, but
not guaranteed
reliable.

Any amp that is 40+ years old can and
will have certain components that are all but guaranteed to fail. In
the sections
that follow I outline what components I have seen fail frequently. I
also make
suggestions for component replacement so as to make the vintage hi-fi
amp or
receiver a reliable and enjoyable piece of equipment to own.There is
just something satisfying about
restoring and being able to use an elegant piece of true hi-fi
history. Vintage hi-fi is not for everyone, but
you owe it to yourself to at least give it a listen. Many people
become converts
after listening to a friends system. Everyone that hears one of my
Scott systems,
or my Dyna and Fisher systems is amazed. They just cannot believe that
such
great sound can come out of equipment that is so old. Yep, like the
guitarist
trying to play rock 'n roll on a solid state combo amp. He just
doesn't get
it. The solid state amp just doesn't have the 'feel', the 'touch'.

[ZAKO]

COULD YOU PRINT THAT IN BLACK AND WHITE..

[ZAKO]

SOME OF US ARE COLOR BLIND..

[mungkiman]

Initial Power up

This can be the most stressful time in the restoration process. What the heck is going to happen when I put the juice to this baby? Well, to help avoid catastrophe, I have a few suggestions. First, what I always do is connect an ohm meter across the amps power cord, turn the power switch on and see how many ohms I get. If it is above 2 ohms, things are cool and I will assume that the power transformer is not shorted.Next I remove the tubes and plug the unit into my Variac and bring the power up while monitoring the AC voltages coming out of the power transformer. I check the filament, bias and high voltage values to make sure everything is there and within specification.If the amp passes this test I will install all the tubes and again connect it to the Variac.

I will start off at about 40 volts on the variac and just let the amp run till things start to glow, and I begin to get high voltage in the power supply. At this point, I look for any unusual voltage drops in the power supply circuit. Such voltage drops could indicate a shorted capacitor or an open resistor. Next I will crank the variac to around 55 volts. At this point, I will usually be able to hear a little hiss and hum in the speaker, and the amp will also, if everything is working, pass a signal. It may not sound too great, but it should be there. It was at this stage that my first Scott 299C exhibited the leaky capacitor phenomena. When I pushed the variac to 80 volts, the B+ did not increase and the first filter capacitor began to get noticeably hot. This amp actually worked as long as I kept it's line voltage below 60 volts! Anyway, by using the variac, I had the time to safely ascertain that I did have a leaky capacitor. You can make a makeshift variac of sorts by wiring a light socket in series with the amp's power cord and the wall socket. You can use different wattage bulbs to change the voltage seen at the amp's power cord. If you are really into collecting and restoring antique radios and vintage audio you owe it to yourself to purchase a variac. Used, I have seen them as cheap as $20.00 and new they range from $80.00 - over $200. depending on their size. They are reliable too, so don't be afraid of buying an old one. My variac is from the 40's! Yea, it may look really retro a la Frankenstein, but it works great!Now, after the amp has either successfully run at 60 - 80 volts for a while, or you have replaced whatever components were causing it not to work at 60 - 80 volts it is time to crank the variac up to 100 volts and plug in a turntable and tuner.

Now, switch the amp to tuner and see if both channels work and sound clear using the tuner as a source. Let her rip for a while paying special attention to any abnormal distortions of the sound, or any unusual noises like squeals, pops and hisses. Now turn the volume control down and see if there is any hum. Sometimes an amp will be quiet for the first several minutes of operation, then it will start to hum. This can be indicative of a power supply capacitor problem, weak or gassy tubes, and improperly balanced and biased tubes. Some amps have a hum balance control that can cause the problem as well. Next, give the phono section a check out by selecting the turntable and playing an LP. If everything is all right, you should hear a nice clean crisp signal coming from both speakers. I don't care what anyone says, tube amps from the 'golden era' of hi-fi (1955-1965) have the best sounding phono preamp sections. So, if you have been listening to those old LP's on a solid state amp (that is if you hadn't become so disgusted with the sound you tossed them and the turntable!) you won't believe how good they will sound through a tube amp.When I finally got my first 299C going, I plugged a Techniques SL3300 turntable into it, plugged it into a pair of Mirage 190is speakers and felt my jaw drop at the sound. I couldn't believe it! This thing sounded better playing an LP then new solid state gear does playing CD's. You must bear in mind that my LP's are, for the most part, in quite good shape. If you have LP's that have been played extensively on inexpensive 'close-n-play' quality turntables then they are probably trashed and nothing is going to make them sound great.Well, at this point, you are ready to just plug the amp into the wall socket. It is fully functional, but not guaranteed reliable.

Any amp that is 40+ years old can and will have certain components that are all but guaranteed to fail. In the sections that follow I outline what components I have seen fail frequently. I also make suggestions for component replacement so as to make the vintage hi-fi amp or receiver a reliable and enjoyable piece of equipment to own.There is just something satisfying about restoring and being able to use an elegant piece of true hi-fi history. Vintage hi-fi is not for everyone, but you owe it to yourself to at least give it a listen. Many people become converts after listening to a friends system. Everyone that hears one of my Scott systems, or my Dyna and Fisher systems is amazed. They just cannot believe that such great sound can come out of equipment that is so old. Yep, like the guitarist trying to play rock 'n roll on a solid state combo amp. He just doesn't get it. The solid state amp just doesn't have the 'feel', the 'touch'.

[ZAKO]

THANK YOU,,!!!!

[seti]

Fini did you find that one under your couch?

[fini]

Fini did you find that one under your couch?

Yeah, I decided to get rid of it after I stubbed my toe too many times.

[artto]

I have been asked by a buyer to use a variac to power up and test a Fisher console preamp (ca. 1962). I've never plugged it in, in the year I've owned it, and don't know how long it has been since it was in use. The only potentially useful test equipment I own is a digital VOM (in addition to the variac). The goal is to do a listening test (and not harm the preamp). How would this procedure be different from doing it with a power amp? What are the (pre-smoke ;^) things to look for as I do it? If testing is needed (using the VOM), where and how do I test it?

You'd think by now I'd have learned how to do this...

I've been told by Audio Research that it doesn't really need to be done on preamps, but there's no harm in it.

I do it with any power or preamp just to be on the safe side and as far as I know it's more of a tube amp type of thing after the amplifier has been out of use for months, or especially years. It's mainly done to reform the capacitors. Some amplifier designs may be more or less susceptible to damage without a slow voltage start up after being out of use for long periods of time.

I use both a variac and a little, easy to make home-made device which is simply a low wattage light bulb (15-25w) in series with the AC power from the wall. When turning up the voltage on the variac, if anything blows in the amp, the low watt light bulb will act as a current overload protector and burn out, most likely before any damage is done to the amp. This kind of device can, and should also be used, in the event you don't have a variac. When the light bulb begins to glow dimly the capacitors are reformed.

I usually just take my time turning up the voltage on the variac, taking as much as 24 hours to reach 100%. I also remove the tubes from the amp during this process. However if the amplifier has a vacuum tube rectifier be sure to leave that in.

I once turned my Luxman MB3045 triodes on after they had been in storage for a few years, and I had just replaced the 8045G power output tubes. Five of the output tubes (3 amplifiers) blew within 30 minutes ~ a very expensive mistake. So now I'd rather be safe than sorry.

[fini]

So, tomorrow I'll have time to do this. I have a few questions specific to preamps.

1.) With power amps, a dummy speaker load is needed. With preamps, obviously there is no speaker output, but should I provide some "line level" load for the initial start up?

2.) Where and how (specifically) do I check voltage output at various stages of "bringing it up?" Seriously, I need someone to tell me where to stick my probes (insert joke here).

[fini]

Well, nothin' blowed up.