Comments for page: A New Look

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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?

Posted by Bill November 03, 2017 - 08:25 am
Thatís good to know since I may be using a PT that is less ďrobustĒ than the usual 330-0-330. If I were to use a 300-0-300 PT, my B+ figures will be lower - all things being equal - and I guess I would have slightly less power. But itís reassuring to know that I can still dial in a 0.27VDC bias. I donít have anything sophisticated enough to measure distortion.

Posted by Dave November 02, 2017 - 08:23 pm
Hi Bill -- Assuming that you are maintaining the same output stage mode of operation (UL) with the same screen tap location (25%), the deviation could be relatively large, as large goes. For the discussion at hand, any deviation would almost certainly be in a downward movement, since the 380 volt B+ level is already at the upper limits of what the tubes can safely handle.

With that in mind, the EFB circuit already compensates for about a 35 volt drop in B+ voltage as both amplifiers are driven from quiescent to full power output simultaneously. Dropping it another 35 volts (bringing the static B+ to 310 vdc would not be hard for the EFB circuit to respond to at all, but it would certainly be best to confirm that the .27 vdc level specified in the Dynaco environment would still hold at a 310 volt level as the low distortion operating point. The point is that the EFB circuit will compensate very proportionately over a very wide range of B+ levels, but that doesn't mean that the tubes themselves do, as so a recheck would be recommended. Ultimately, as long as the new operating point remains on the linear portion of the operating curve for the output tubes, then the EFB circuit will compensate very well for any changes made in the B+ operating levels.


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