You’ve just dug an old Tube amp out of Grandpa’s attic; or maybe got a deal
on eBay or Craigslist. So, how do you
safely turn it on to assess its health?
Well, there’s really NO way that’s guaranteed to be safe, but here is
the best advice I can give ya.
Don’t Just Plug it in, turn it on and hope for the best.
Believe me, this is a proven recipe for smoking resistors and
capacitors. And oh how terrible the
smoke smells! It’s the reverse of the
old Nike line: JUST DON’T DO IT!
Never turn a tube amp on without a speaker load.
On eBay I’ll often see amps listed as something like “turned on and all the
glass bottles lit up”, and the accompanying pic will show the amp with the
tubes glowing and NO SPEAKER connected. The
output transformer of a tube amp expects to see a certain impedance on the tube
side of the tranny, and a reflected impedance on the speaker side. Now, most old amps are forgiving of loads up
to +/- 100% of the expected load (in other words an amp expecting an 8-ohm
speaker impedance is generally safe at 4-16 ohms. But, no speaker load at all, nope, that ain’t
safe for even a few seconds! Figure any
tube amp ran without a speaker load has a pretty good chance of having bad
output tubes, output transformer, or both.
Again, JUST DON’T DO IT!
This pic was taken directly from an eBay auction. What's wrong? Yep, the tubes are all aglow ... with no speaker load! Don't do that!!!
what SHOULD you do?
First visually inspect all of the innards.
Carefully look for fried resistors or blown out electrolytic capacitors. Then, check all the filter caps and other
big-value caps with an ESR meter and replace any bad ones prior to powering up. Also check the important grid and coupling
caps and resistors and replace ones that are way out of spec before powering
Next check all the tubes on a tube tester.
They don’t have to test “as new” … but if they are just flat shorted,
gassing, or WAY below minimum spec, replace them before powering up. In a push-pull amp with 2 or 4 power tubes
(or more) you will also want to make
sure the output tube pairs are at least FAIRLY closely balanced.
Then remove all the tubes except the Rectifier tube.
And fire the old girl up on a variac starting at about 25 volts AC. Note that you do NOT need to have her hooked
up to a speaker load at this point because you have REMOVED the output
tubes! Increase the voltage about 20
volts every 20 minutes until you have reached full voltage. If nothing is smoking and you haven’t blown a
fuse, you are ready to continue.
Now load her up with tubes.
At this point, make sure she’s hooked up to an appropriate speaker load, and
you might want to run her through a visual over-current protection device
(light bulb); the bulb should be an old fashioned incandescent of about 150-300
watts (or multiples that add up to this).
For info on the “light bulb” current limiting device, click HERE.
With amps that have been dormant for a long time, I like to start with it on
the variac at about 90 volts for a few minutes before slowly bringing the power
up to full line voltage (provided nothing snaps, crackles, pops, or
smokes). Now listen for unusual noise as
you turn the controls through their ranges.
A little “warm heater” smell is normal at this point, but keep your nose
and eyes open, and if you smell ugly burning smells or see smoke, cut the
If all is still well, give her some input signal.
I like to start with music from my mp3 player, etc. Just turn her up and see what she sounds like. If she sounds entirely happy, then it’s time
to plug in a guitar and see how she sounds.
I won’t get into any mods you might want to make to an old amp as that
is out of the scope of this blog. I’ll
say this: if the amps happy, and you are
happy, then you’re done!
Here's a peek inside my amp shop, where several old tube amps are curently being loved back to life!