Bench test AC transformer from stereo tube equipment

  • Thread starter JohnM24018
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  • #1
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Main Question or Discussion Point

I would like to disconnect the secondary windings from a stereo preamp to test the voltages of the secondary windings. The transformer has a 330 - 0 - 330 volt center tapped secondary and a 10.9 volt secondary. It is to be used with house current - 110 or so in Virginia. I have measured the current from the plug. It is OK.

My intent is to use a digital multi meter to measure the voltages of the secondary windings. I intended to measure the 330 leg to zero then zero to the other 330. I was going to test the other windings in the same way.

Is there a better way to do that? If so what is it? Should it be loaded - what ever that is.

The reason I feel this needs to be done is that after some work to the unit all 4 of the 12AX7 tube's pins (#1 and 6) (4 tubes, pins 1 and 6 on each) are currently measuring over 450 volts. They normally measure 115 and 135 volts DC. I tried two meters and got the same results. I see no burns and saw no smoke or sparks. The unit is around 60 years old but has been working recently. I have rechecked my work and am satisfied.

I possess the sheet from the distributor and could upload it if requested. Some sites prefer me not.
 

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  • #2
tech99
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I think it is unlikely to be a transformer fault. It could be measurement error. Just trace the plate voltage back towards the filter caps and rectifier and transformer and see where it comes from.
Actually, 330-0-330 sounds ridiculously high for a pre amp.
 
  • #3
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I think it is unlikely to be a transformer fault. It could be measurement error. Just trace the plate voltage back towards the filter caps and rectifier and transformer and see where it comes from.
Actually, 330-0-330 sounds ridiculously high for a pre amp.
The transformer is sold as a PA211 Power Transformer. You seem to be more familiar with these than I. It would be helpful to me if you would search for one of these to check the accuracy of my statements. I could certainly use your experience. I will see if I can upload a photograph of the interior of the preamp. Thanks John McIntosh
 
  • #4
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Here is a photograph of the inside of the Dynaco PAS-3 preamp without the tubes. I was afraid to install them with 500 volts showing. The two red wires are the 300 volt secondaries. The centertap to these is grounded to the chassis underneath and the two blue wires are what was represented as the 10.9 volt secondary.
Preamp.jpg
 

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  • #5
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The PC-5 board is the "line" board as opposed to PC-6 which is the phono board. PC-5 is powered by eyelets 16, 18 and 19. They all come from the Capacitor board and eyelet 16 is the high voltage and currently measures 500 volts. 18 & 19 are the low voltage lines. The schematic shows the bottom of a 4 capacitor "can" capacitor. Also a rectifier tube is shown. Both have been replaced by the updated solid state capacitor board which sits (pictured) next to the transformer. As far as tracing them back, there are only three wires: 16, 18 & 19. They go straight to the cap board. The other eyelets on this board are to tone and volume controls and a scratch filter switch.
 

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  • #6
Tom.G
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What is the voltage rating of the capacitors on the Capacitor Board? This will give us a clue for the designed B+ voltage (high voltage).

From what I've been able to find on the Internet, about 470V without the tubes installed is normal. When the tubes are installed they will draw about 2 milliamps each thru a plate load resistor of 150K Ohms, this will drop the Plate voltage to around 150V. The B+ on eyelet 16 will go up and down a few volts as the line voltage varies. It will also drop when the tubes are in, probably by 20 to 40 Volts.

Let us know what you find.

Cheers,
Tom

ADDENDUM:
The specs look excellent regarding noise, distortion, pulse recovery, etc.
User Manual and Schematic are available for download at: https://www.hifiengine.com/manual_library/dynaco/pas-2.shtml
They require a valid e-mail address and what country you are in.

The PAS-2 was introduced in 1960 for $60 as a kit, $80 assembled. I found one in original condition offered for sale (reverb.com) at $575 + shipping. Sounds high but that works out to only 3.5% inflation per year!
 
  • #7
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This information is extremely helpful! I bought this pre amp and sister power amp in 1972. I can't remember whether it was $50 each or $50 for the pair. They got me through college and beyond but were replaced . . . I recently started listening to them and just replaced the selector switch because I was losing the left channel. When I got the huge voltage I stopped and sought advice.

To answer your question I have uploaded the blurb from the replacement cap board. I elected to replace the rectifier tube (12X4) as suggested by Mr. Stokes, the designer of the board with a pair of diodes. The large capacitors are rated at 450 volts 47 mu F, the 1800 mu F are 25 volts. The orange things are 104K 630V. That seems to be .1 mu F film Capacitor rated for 630 volts.

You need to understand that at the time when something went wrong I took all the tubes down to the drugstore, hardware store or Radio Shack if there was one and replaced the ones that failed. No one ever told me how to measure the voltage at the tube pins. I just assumed that they would measure the same both ways.

I intend to wait until later tomorrow (Saturday) to test with the tubes installed. That would give time for any last minute emergency instructions.

Best

John
 

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  • #8
Tom.G
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The large capacitors are rated at 450 volts 47 mu F
OK, good. That fits well with voltages estimated in my previous post. Unless someone comes up with a panic alert, it looks like you are good to go with tube replacement and enjoy the sound!

Tom
 
  • #9
Bandit127
Gold Member
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I had a similar experience with a guitar amp build recently and I can confirm that the voltage does drop to "normal" with the tubes installed and working. Can you post back and let us know how it went?
 
  • #10
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Thanks to everyone. With the tubes installed the voltage measurements were within specifications. I hooked up the unit and now it works fine.
John
 

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