The following is an excerpt from Bill
Bittle, an avid collector of vintage tube hi-fi and frequent
contributor to the Vintage H.H. Scott ListServer. Be sure and checkout Bill's
complete Vintage Tube
Audio web site. Follow along, as Bill takes us through a "soft
start" procedure of his vintage H.H. Scott 299C using a Variac.
"... 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. I remove and inspect the fuse and
often replace it with one of lower known value.
Assuming everything looks good, I next will install all the tubes and
again connect it to the Variac. I will start off at about 30-40 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. I also monitor the amps' current or wattage draw.
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 (filter) 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! But by using the Variac, I had the time to safely ascertain
that I did have a leaky (filter) capacitor.
Hot Hint: If you do not have access to a Variac, you can make a substitute
dim-bulb tester (a.k.a. poor-boy's safe soft-start rig) by carefully wiring a light socket in series with the amp's power cord and the
AC wall outlet. Here's how it works:
You use progressively higher wattage bulbs to gradually raise the voltage seen at the amp's
power source. Always start out with lower-wattage bulbs, i.e., 15-watt, 30-watt,
and gradually work up to 60-watt, 75-watt, 100-watt. If the lower wattage
bulbs, get bright STOP IMMEDIATELY! This indicates a high current fault
within the amp. For an excellent, illustrated, construction guide, see: Phil's
Old Radio's, Dim-Bulb Tester.
But if you are really into collecting and restoring antique radios and
vintage tube audio gear, 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 - to over $200, depending on
their size. They are reliable too, so don't be afraid of buying an old one.
Now, after the amp has successfully operated at 60 - 80
volts for a while, it is now time to increase the Variac to 100 volts and plug
in a signal source, such as a turntable and tuner.
Switch the amp's input selector 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.
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. (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.)....."
-- B.B.
