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Old 12th December 2010, 02:06 AM   #1
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Default Vintage power transformers

Hey All,

I have pair of vintage power transformers from circa 1948 Magnavox consoles. The transformers powered the amp as well as the preamp and tuner. The high voltage supply is 800 volts. The bias supply is 35. The heater supply is 5.5 volts.

My question is, are they safe to use in an amp today? The insulation on the leads is in surprisingly good shape. But can a 60+ year old power transformer be trusted? And is there any way to insure they won't cause damage or let the smoke out?

Kevin
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Old 12th December 2010, 02:11 AM   #2
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Should be okay.
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Old 12th December 2010, 02:33 AM   #3
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or may not...

Best bet - take them to a motor rewinder or transformer shop and have them tested at the very lest for shorts and leakage to ground and interwinding shorts and leakage.

Smoke can be expensive...

BTW, its probably a centre tapped supply, so that's normally described by each phases voltage to the centre ie 400-0-400
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Old 12th December 2010, 02:59 AM   #4
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If you only have a 5v heater supply, you will probably need an additional heater transformer (e.g. 6.3v)
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Old 15th December 2010, 11:19 PM   #5
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Actually I have a 5.4 volt tap and a 6.7. Probably were intended to be 5.0 and 6.3. But my mains voltage runs high. The output tubes were originally 6V6's.
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Old 16th December 2010, 12:05 AM   #6
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Quote:
Originally Posted by mr2racer View Post
Actually I have a 5.4 volt tap and a 6.7. Probably were intended to be 5.0 and 6.3. But my mains voltage runs high. The output tubes were originally 6V6's.

Those measured voltages will come down, when current is drawn.

People forget that the long time North American rule for AC mains is 105 to 125 V. What's currently different is a higher average mains voltage.

If the old power trafos are electrically sound, powering a 6V6 or EL84 monoblock should be fine. My guess is that the rectifier was a 5Y3 and the 5 VAC winding is rated for 2 A. You can safely bring the B+ rail voltage up by using either a 5V4 or 5AR4, as those types draw 2 A. of heater current. Forward drop in a 5Y3 is large, while it's modest in a 5V4 and small in a 5AR4.
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Old 16th December 2010, 12:24 AM   #7
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cellulose breaks down over time.
if the windings are in good condition, i suspect a short to case is less likely than a shorted turn.
i'd use them, but you might be surprised at how much power they draw at idle.
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Old 16th December 2010, 03:14 AM   #8
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Thanks Again. The 1650n's are on eBay. If they sell I'm planning to buy a pair of Dynaclones. And Eli you are right. They were 5Y3's

And in general do you guys like tube rectification? From my earlier research I found it was considered a bad idea?

Kevin
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Old 16th December 2010, 04:10 AM   #9
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it was not a bad idea before the advent of silicon rectifiers....nowadays you have better options....
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Old 18th December 2010, 06:18 PM   #10
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Hey All,

Just for yucks I put the two 5Y3's back into the chassis and powered it up. I am amazed and humbled. The 5Y3's had 520 VDC output. And when I checked beyond the two 60 year old filter caps there was 1.26 volts ripple and, 0.00 after the second!
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