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Old 13th September 2011, 06:54 PM   #1
diyAudio Member
Join Date: Jul 2011
Default Power Transformer Issue

Sooooo I'm very confused. I built the AX84 P1 Tube Amp (link to schematic at bottom) and it's working, except that I should have 266V after the half-wave rectifier (380V secondaries on the power transformer) but my meter is reading around 50V less than that. I'm generally new to electronics and I'm very confused as to what could be causing this problem, because there's not much going on in the circuit before this other than a couple switches. Anyone have any ideas as to why this is happening? I would greatly appreciate any help you could offer! Thank you!
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Old 13th September 2011, 08:45 PM   #2
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Sorry, here's the schematic http://ax84.com/static/p1/AX84_P1_101004.pdf
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Old 14th September 2011, 12:55 AM   #3
! is offline !  United States
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I'm confused too. You wrote 380V secondaries, but the schematic shows 214+214 secondary (= 428V). 428V - 380V transformer you used seems pretty close to the 50V difference measured... ?
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Old 14th September 2011, 01:16 AM   #4
ChrisA is offline ChrisA  United States
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Originally Posted by Sperrino View Post
... because there's not much going on in the circuit before this other than a couple switches.

What goes on AFTER the power supplies matters too. Perhaps there is a large load pulling the voltage down. What is the bias current on the power tube? Are the resistor values correct? You measured them.

The power transformer is best described as a 190-0-190 volt secondary. Yes it is 380 from end to end but the CT is grounded and held at zero. So rule of thumb is to expect in operation, under load the DC volts to be about 1.2 times 190 or about 228 volts DC. The zero load volts would be about 1.4 times 190 or about 266 volts. Pull all the tubes out and see if you don't get closer to 266.

Also I think the newer Hammond 369X transformers have two primery taps, one for 115 and the other for 125 volts mains supply and neither might exactly match what's in your outllets.

In might also be a combination of a load and low mains. What volts are the heaters running at?

One more thing. Notice that those ren voltages exactly match my calculation. It seems those red numbers are just "ball park" figures and I'd not expect a perfect match. Certainly it will be lower.
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