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In Reply to: RE: Dynaco A470 Output Transformers posted by mark.korda@myfairpoint.net on August 17, 2014 at 10:58:45
Mark;
First, thanks for responding. I get consistent DC resistance measurements from CT to either screen grid on both transformers - about 35 ohms to either screen grid. From CT to plates, one measures about 100 ohms to either plate while the other transformer from CT to either plate measures about 200 ohms. Clearly the plate circuits of the two transformers are widely divergent from one another. I could understand them being perhaps 10% or maybe 20% different from each other, but not one transformer having plate to plate or CT to plate DC resistance twice the value of the other.
What I would like to know is how these compare to some known good working transformers. One of them has to be bad. The question is, which one? Both are OEM transformers never having been changed.
Could you measure DC resistance plate to plate on your unit and let me know the approximate measurement?
Thanks in advance.
Joe
Follow Ups:
I thought I would go back and provide some more detail.
Output Transformer Measurements
Test Conditions:
Center tap disconnected from power supply electrolytic
Allied KG-625 VTVM
Output tubes removed from sockets
Left Channel:
Center Tap to front plate lead = 92 ohms
Center Tap to rear plate lead = 103 ohms
Center Tap to front screen lead = 32 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 192 ohms
Right channel:
Center Tap to front plate lead = 180 ohms
Center Tap to rear plate lead = 178 ohms
Center Tap to front screen lead = 58 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 390 ohms
I have a Heathkit IB-2A Impedance Bridge and decided to take measurements of the two transformers being discussed. The test frequency used is the bridge internal 1kHz oscillator.
My measurements of the two transformers with their center taps disconnected from the power supply electrolytic were:
Following the Heathkit instructions for the bridge the setup of the bridge that gave the lowest left hand 100uA reading of the AC level when the range switch was set to the 10H range. Then the other dials are adjusted for best null reading as the generator level is gradually increased to maximum. The controls are basically adjusted for the best deflection back toward the left end of the scale on the meter.
DQ dial adjusted to read 1.0
Final readings of the CRL Dial were:
Left channel transformer 1.57 X Range dial reading of 10, so 15.7 Henries
Right channel transformer 1.42 X Range dial reading of 10, so 14.2 Henries.
Those measurements do not look all that different, but I am still troubled by the difference in the plate to plate DC resistance.
Thoughts?
Joe
"Clearly the plate circuits of the two transformers are widely divergent from one another. "
I'm not sure that is the language I would use.
I think the output tubes in the dyna 70 draw about 50ma. each,
50ma across 200 ohms will drop 10 volts. 50ma across 100 ohms will drop 5 volts.
The voltage at the plate of the tube is 410 vdc.
A 5 volts difference amounts to a difference of about 1.25%
I wouldn't call that "widely divergent from one another".
If you want a matched pair of output transformers for the sake of having a matched pair (and I don't blame you) that's fine but I doubt you would hear it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Mark and Uncelestu;Thanks to both of you for your help. I found one comment on DIY Tubes forum that mentioned when measuring from plate to plate it was normal to see about 200 to 235 ohms, so measuring from CT that would be about 100 to 117 ohms. So it appears that the transformer that is problematic is the one measuring about 200 ohms from CT to either end.
The same DIY Tubes forum post had a follow-up post that said it was common to see ohm readings CT to either plates or screens vary by 15% to 20% and the unit would be OK. Reason is the winding diameter per turn increases as more layers are built up during construction.
This amp may well be one that was subjected to tubes going into run-away conduction (loss of grid bias) on the right channel. It may also have been in a high humidity environment. There is considerable small pitting of the chassis.
Looks like I will be ordering all new tube sockets throughout, new electrolytics, film capacitors and bias rectifier. A new transformer for one channel will be ordered and later a new stainless steel chassis so it will look decent. The power transformer top bell and the one good OEM transformer's bells will get a rust removal treatment, sanding, priming and repainting to look good again. The PC board is in need of a complete cleaning if not outright replacement. However I may try reusing the OEM PC board first to see how it does.
I now have some used 6CA7 JJ outputs and 7199s to use for a trial that seem to check relatively balanced output although a couple of them are slow to warm up. After I get things operational with a new set of tubes I will decide whether to do any modifications.
Cheers!!
Joe
Edits: 08/17/14
I wouldn't rush out to buy new iron. If you have thermal runaway in the output stage, it will cause the output transformer to get incredibly hot. This heat will eventually cook the insulation on the wire in the transformer, and you'll eventually develop shorts inside the transformer. This will appear as a much lower than expected impedance across the primary, and potentially can give you shorts between primary and secondary. (As already indicated)
I'm not aware of a failure mode that would give you 2x what you would expect.
It's also important to remember that Dynaco was a budget product. Your two output transformers may be wound differently (the one with higher DCR may have more turns of slightly thinner wire).
I have about 6 St-70's and IIRC the readings could vary widely.
So I checked another pair I have out of chassis. Plate to B+ was 145 /175 on one transformer and 179/199 on other transformer: both were working when removed for rebuild.
I would run your transformer and then order another only if they are actually bad.
I am powder coating my transformer covers and chassis. Picture enclosed. If you get the Stainless chassis , remember that Dyna uses the chassis as a ground plane, articularly for the cathode resistor for the power tubes. Stainless is a lousy conductor, so run a wire to tie the chassis ground points together. Everything will sound better: faster and quicker. more open.
Uncelestu;
Thanks for that additional information. Your measurements agree with one other statement I found on DIY Tubes forum today. That gives me a good reading to compare to. I will be ordering a new transformer and it should come reasonably close to the best OEM transformer. I will not be throwing the old one away, but may save it to have it rewound later on. Heyboer is one company several have said can rewind these transformers and do a good job at a reasonable price. I sent them a request for a quote.
Best wishes!
Joe
On a pair working when removed out of chassis for rebuild: 31 to 33. Ohm screen to center tap. One transfomer plate to center. tap measures 100 to110 ohm. Other tranny measures 110/176 ohms.
Hi Joe, The transformers I have that need to be tested are from a pair of Dyna Mark 3's. Not recalling the transformers exact numbers or letters, Bob Latino said ST-70's output transformer would have about the same specs. I'm in the process of moving and every thing is kind of packed up now. I do have a real good working ST-70,packed too. I would really like to thank Unclestu for getting the specs to you. He kind of got me off the hook. Your way ahead of the game. I just wrote down from B. Latino what I could do to test my transformers before I attempted a rebuild project.....good luck..Mark K.
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