Comments for page: A New Look

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Posted by Steve L. January 04, 2018 - 03:59 pm
Okay Bill, the B+ seems roughly okay. From your readings, the LM337 reference voltage is 17.52-16.30 = 1.22V, which is fine. Bias voltage on the output tube cathodes is 16.6V. You mentioned on 1/2/18 that the original spec is 13.5V. That's true but recall that the EFB mod reduced output tube bias current from 35mA to 27mA each, so a higher than original cathode voltage is to be expected. Your readings indicate that the drop across the 5R resistor is 16.60-16.30 = 0.3V. Since the nominal value is 0.27V, the bias current is, if anything, a little higher than nominal.

I just put an unused Electro-Harmonix EL84 on the Vacuum Tube Analyzer (VTA) with Va=Vg2=368V, which represents about the same conditions as in your ST-35. I adjusted grid voltage (Vg1) for the nominal 27mA plate current (Ia). The value of Vg1 was -17.14V. That suggests the cathode bias voltage you are seeing (16.6V) is about normal. From what I can see, there doesn't appear to be anything wrong. You might want to adjust the bias to make the voltage across the 5R resistors closer to 0.27V. That's about all the light I can shed on this.

Posted by Bill January 04, 2018 - 03:04 pm
360K resistor is good. I had a 560K and a 1M in parallel. I put in two 180K in series, just for the heck of it, and got the same readings. B+ the same.

Posted by Bill January 04, 2018 - 12:38 pm
At a wall voltage of 117, B+ is 385 VDC. After the 360K resistor, it is 19.06 at input pin of the 5K trimmer. Adjustment pin is 17.47, other pin before the 15K is 14.06.

Input on LM337 is 17.52, out is 16.30. After the 5R cathode resistor, it is 16.60 at the output tubes.

Posted by Bill January 03, 2018 - 07:25 pm
Thanks for your response Steve. Iíll have time tomorrow to take some readings.

Posted by Steve L. January 03, 2018 - 07:45 am
Hi Bill, Dave is away on a business trip but says he will respond when he gets back. It is strange that the output of the EFB(tm) circuit changed without adjustment. Do you have the before/after voltages of the supply voltage going to the 360K resistor (call this Vb)? You can check the LM337 reference voltage. It should be about -1.25V on ADJ (left pin) relative to the OUT (right pin). With the increase in bias, I would expect the quiescent currents of both channels to have dropped, reducing the nominal 0.27V TP voltage drops. (Is that so?) You will want to set that back to normal. Possible causes of the change: (1) Dirty 5K pot (does it seem flaky during adjustment?); (2) Bad 360K resistor (it's rated for 2W, right?)

A change in load at the wiper of the pot could cause this but since the voltage went up, it would be a decrease in load, which is the opposite of the typical faults which could be expected from the 4.7uF cap or the ADJ pin current. Anyway, after cleaning the pot and checking the 360K resistor, I suggest setting the 0.27V value correctly, noting the value of Vb and checking the bias occasionally. Perhaps Dave will have more when he gets back.

Posted by Bill January 02, 2018 - 05:20 pm
Loks like Iím the only one who posts on here anymore!

Iíve been running the ST-35 with the EFB for some time now, and red plated an output tube. My fault. Solder connection came loose. I waited a day or so and checked bias and the channel that had the abused tube, checked in at .245, while the good side had .270. I decided to put in a new quad of matched output tubes, and noticed that bias had to be turned down as it was in the .330 range. When both channels were set at .270, I took some measurements and all was good.

Before I brought it into the house a few days later, I checked measurements again and everything was pretty much the same, except for a higher cathode reading on pin 3 of the tubes. After I inserted the new tubes, I was getting 13.7 VDC. Today Iím getting 16.4 VDC. Original spec is 13.5. Wall voltage on both days was/is the same - 116 VAC. 15K resistor measures good.

Should I be alarmed?


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