Tubes Asylum

Posted by Jimmy (A) on January 21, 2004 at 20:04:17

In Reply to: Help on bring up a Scott 299-B that has not seen use since 1965 ! posted by HiFiBuff on January 21, 2004 at 14:16:30:

You can use the light bulb 'trick'. I have perform this on & off for 30 years! I just did this on a Fisher SA-100 that was stored since the late 1960s. The variac medthod is better as you have excellent control. The 40 to 60 watt light bulb does add more risk.

Before starting this proceedure, make sure the equipment is at room temperature & condensation is not present. A room temperature soak for 24 hours is good if no condensation was present. If condensation was present I would let the equipment soak for 4-5 days at room temperature. Chech the amp for any smoked parts. A slight dull color on the parts is not necessarily a bad part. Black or dark grey is suspicious.

First, if your rectifier in good condition it will conduct at about 2.5 to 3.0 volts filament. It just takes a minute or so. I used a 60 watt light bulb on my Fisher SA-100. I shorted the speaker output terminals with a jumper wire and kept all tubes installed. After something less than a minute you will measure B+ voltage. Start pulling the small preamp tubes one at a time until approximately 150-175 volts is present. Let this amp sit for a few hours at this voltage. Next, remove tubes to get an increase to 200 volts. Take the amp up in approximately 50 volt steps and let sit at that increased voltage for an hour each. Once all tubes are removed, you should have approximately 325 volts. That voltage depends upon your bleeder resistors and other resistive circuitry.

If you are not capable of measuring this high voltage, you will have to perform this in a slower process. Assuming the 60 watt bulb is in series with the 120vac line voltage input. Turn on the amp & let the amp sit for 3-4 hours then pull one tube at a time every 30 minutes starting with the smaller (less tall) preamp tubes after the first 3-4 hour soak. After all tubes are removed, shut off the amp & reinstall all the tubes. Remove 60 watt bulb and power up at full line voltage. If any time during the process the lamp goes and stays bright, you have a short. Then, the amp must go to a technican.

My 2 cents.