1st Soft Start

 


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Last Edited:
12-Sep-2004

 

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.

 

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