[Vintage-Audio] Tube amps with no load

Sun Mar 2 21:35:01 2003

Bob,

When you bring the AC voltage up slowly with a Variac, you will reach a
point where the rectifier just starts to conduct. While you are doing this,
you want to be measuring the B+ voltage with a VOM. Since the AC voltage is
still relatively low, the B+ will also be low. It will increase slowly as
the capacitor charges. You must wait until the filter capacitor stops
charging as indicated by no further increase in B+. Once you are satisfied
the B+ is not increasing anymore, you increase the AC slightly and watch the
B+ increase. You repeat this procedure until you reach full line voltage,
and therefore full B+. If the capacitor becomes warm, or the B+ does not
increase, you need a new filter capacitor. You must be patient, as it may
take hours for a capacitor to reform.

Bob, I know I answered your question in the first sentence. However, I
thought there might be someone out there that had not reformed capacitors
while they were in the power supply. There is certainly no reason why you
can not take them out of the amp and reform them with a variable DC supply.
The old Sprague Capacitor Checkers had the ability to do this and measure
the current drawn during the reforming process. You would run the voltage up
until about 10 ma. of current draw and wait for the current to drop to zero.
Then increase the DC voltage and repeat the process.

By the way, one advantage of reforming the capacitor in the amp is that you
are also slowly warming up the power transformer and other components which
may contain moisture.

Steve W9QLF
-----Original Message-----
From: Robert J. McKee <[email protected]>
To: [email protected] <[email protected]>
Date: Sunday, March 02, 2003 8:07 PM
Subject: Re: [Vintage-Audio] Tube amps with no load

>Can you explain what good it does to bring it up slowly
>when you have a tube rectifier?  I can understand it if
>the rectification is solid state so you are truly bringing
>it up slowly... reforming the power supply capacitors.
>It would seem that the way to reform them would be to
>use a variable DC supply to do the reforming.
>Looking for the pros and cons of each.
>Bob McKee
>
>
>----- Original Message -----
>From: "Duane Fischer, W8DBF" <[email protected]>
>To: <[email protected]>
>Sent: Sunday, March 02, 2003 5:05 PM
>Subject: Re: [Vintage-Audio] Tube amps with no load
>
>
>> The real issue, may be in powering up an old amplifier that
>has sat idle for
>> several or more years. Unless it is brought up slowly on a
>Variac to allow the
>> capacitors to reform, Kapow! "Sold as is, a non-smoker, so
>not sure where that
>> slight smell of smoke is coming from."
>>
>> DBF
>>
>>
>> ----------
>> From: [email protected]
>> To: [email protected]
>> Subject: Re: [Vintage-Audio] Tube amps with no load
>> Date: Sunday, March 02, 2003 11:55 AM
>>
>> No problem at all....
>>
>> It's only when there's an audio signal going through the amp
>that there's
>> a problem....
>>
>> Don't forget a transformer is an AC device.  The load is
>only reflected
>> back to the tube during an AC excursion.  There's no problem
>or
>> refelction with no AC, when the amplifier is at idle.
>>
>> Ed K1ZOK
>>
>> On Sun, 2 Mar 2003 10:28:03 -0600 "Steven K. Wenger"
><[email protected]>
>> writes:
>> > Is it just me, or does anyone else cringe when someone on
>eBay writes
>> > the amp's tubes light up but I have no way of testing it?
>There is a
>> > Fisher X200 amp on eBay right now. You can actually see
>the amp's
>> > line cord plugged into an extension cord, the tube's
>filaments are
>> > on, and no speakers are connected.
>> >
>> >
>>
>>
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