ron_swanson

Checking vintage Triad transformers
« on: August 08, 2020, 10:54:59 PM »
Hello,

I have three vintage Triad R-7a transformers, two of which have been used to build two P2P Pultec MEQ-5 units and the third still unused in it's original box.  Both of the built MEQ-5 units are working for the most part, but one seems to have excessive DCv around the 12AX7 tube.  I've rolled the tubes between each unit hoping that the problem follows the tubes ( didn't ) and have gone over the wiring & components multiple times trying to understand why one unit has excessive voltage around the 12AX7 tube - something like 80-100V higher that what is stated on the schematic.  The other unit more less falls in line with the measurements indicated on the schematic I'm using.   Both units are on the higher side in the same area, but the second as noted above seems beyond expectations.   So, I've gone back to the beginning trying to confirm that all is well starting with the power transformer. 

So, my question here... Is it considered normal for a transformer to read over 100VAC higher ( unloaded ) than stated on the datasheet on the secondary understanding that an unloaded transformer will read somewhat high?  The Triad datasheet indicates a 115ACV input, but in this case my wall voltage is a steady 120VAC.  As such I would expect readings to be slightly higher than the 115ACV datasheet specifies, but 100VAC unloaded and 60-70VAC loaded?    The difference between 115 and 120 should result in something like 625V if I undertand the math correctly?

Trying to understand what I'm measuring here.

Transformer specs attached.

Cheers!


ron_swanson

Re: Checking vintage Triad transformers
« Reply #1 on: August 08, 2020, 10:57:51 PM »
Schematic...

ron_swanson

Re: Checking vintage Triad transformers
« Reply #2 on: August 08, 2020, 10:59:37 PM »
Measurements...

scott2000

Re: Checking vintage Triad transformers
« Reply #3 on: August 09, 2020, 10:26:11 AM »


So, my question here... Is it considered normal for a transformer to read over 100VAC higher ( unloaded ) than stated on the datasheet on the secondary understanding that an unloaded transformer will read somewhat high?


It's normal.... 12% more  ish unloaded

And it will also obviously be higher voltage than specced in circuit if current the transformer is rated at isn't being used..... The circuit  takes this into consideration...
« Last Edit: August 09, 2020, 10:36:58 AM by scott2000 »

ron_swanson

Re: Checking vintage Triad transformers
« Reply #4 on: August 10, 2020, 08:47:59 PM »
It's normal.... 12% more  ish unloaded

And it will also obviously be higher voltage than specced in circuit if current the transformer is rated at isn't being used..... The circuit  takes this into consideration...

Thanks, scott2000. I knew the readings would be higher unloaded, but was surprised by how much.  I guess I never paid that much attention previously.

Be that as it may be, I also figured out my high voltage issue.   Incorrect sized resistor on pin 6 of the 12AX7. A replacement resolved the issue.

Thanks again for your response.


 

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