Integration Operating Points and Bias Current

[OJneg]

Hello everyone,

So I have built up my Reduction kit with Integration boards. So far most of the operating points seem to be nominal. I get sound out and all that. But it does seem to me that my build with the Integration upgrade has changed the bias current for the worse  ???

I completed the build without the upgrade first and all the operating points seemed perfect. I was getting 3.8mA to the anode (measured via 220ohm plate resistor). With the Integration kit in place, the anode current has dropped to 1.5mA (0.35V measured on all the 220ohm plate resistors). On the Integration's schematic it still specifies that the 3.8mA should be flowing through the tube, although I suppose this could just be a holdover misprint from the non-Integration schematic.

So my question is, is this reduction ( ) in bias current normal for the Integration upgrade?

I can't preclude the possibility that I've somehow borked the upgrade. I should mention that on each CCS board, I only get 2 of the LED's to light up. The pair that do light up have about 1.5V across while the pair that don't have 1.2V across. So both seem to be conducting at least so I assumed this is normal.

 

[Doc B]

Has the plate voltage changed? That may be a more reliable measurement. Can you post which voltages are off?

 

[OJneg]

All 4 plates are ~65V. Raw B+ is 150V (instead of manual's 130 V, makes sense since less current draw). Regulated on the shunt is 105V.

 

[Doc B]

Raw B+ is also influenced by the AC mains voltage, and as long as the regulated is 105VDC you are fine there. 65VDC is within the acceptable range for plate voltage, and it can vary with different tubes. You might try carefully measuring the voltage across each R1 (237R) on each front board to see what the voltage drop is there.  These resistors set the current though the tube. Should be in the ballpark of 0.85VDC or maybe a bit less.

Re the LEDs - is it both LEDs one one side that are not lighting up, or is it one LED on each side that is not lighting up? They don't flow a lot of current and don't get very bright. Have you looked at them in a darkened room?

 

[OJneg]

I mean the Raw B+ after the CRCRC regulation, which should be influenced by the current draw of the system.

Voltage over R1's tends to be a ~0.58V, which seems to correspond to a little bit higher current (2.4mA) than what the plate is getting. Not sure how this is explained since there seems to be only one path to ground.

As far as I my eye can tell, the LEDs with 1.2V (the LED that is "above" the other) are basically not emitting any light.

It seems to me that if both of these LED's had a ~1.6V drop, that would correspond to ~1.0V (subtracting the transistor's Vbe) across R1 which would make for the nominal ~4mA of plate current. Replacing the LED's bias resistor to ground with a lower value should allow both LEDs to get that, but I don't have the part. Perhaps I could replace the LED bias resistor with the plate resistors I just removed to allow a great voltage drop on the LEDs.

 

[Doc B]

I just checked the manual build Reduction/Integration. Voltage at the top of the 120K LED bias resistors (R2) is 114-115VDC. Both LEDs are lit. So I think it's a matter of production tolerance with respect to the two 51V rated zeners diodes in series that regulate the supply voltage to the C4S. In other words your Zeners are closer to spec than the ones in our prototype. I'm going to guess that something on the order of 100K to 105K would be an appropriate substitution for R2 if you want to tweak it. Or just put a 1meg in parallel across each 120K.

 

[OJneg]

Ok so I tried something a bit different.

Adding a 1Meg to the LED resistor didn't make much of a difference to the operating point. I then removed the shunt regulator board and hooked up the CCS boards directly to the B+. Instead of a 1Meg, I paralleled with the 18k plate resistors that were removed. I now have all LEDs shining bright with a healthy 1.6V on each. 1V drop across R1 which should equal about 4mA. Raw B+ is sitting around 115V with ~40mA being drawn from the supply. Voltage on the plates is sitting around 74V now. If my math is right, this is not particularly efficient as each LED path is drawing more current than the triode! I would need to use just the right value to bias the LEDs to a lower current draw but still maintain a 1.6V drop on both.

Apologies if I seem like a nuisance customer here. I work in electronics myself and I realize how much of a nuisance it can be when you have customers that know just enough to be dangerous, and how much time you might spend to satisfy their whims ;D  My original curiosity with regard to the bias current needed to be satisfied though and I don't expect you guys to indulge me if you'd rather not deal with tweaks that aren't in the manual.

But I do wonder if there's more benefit (measurably or sonically or otherwise) to optimizing for more bias current vs using the shunt regulator. Obviously the transformer can only deliver so much current (53mA according to the label) and I know a shunt regulator will burn a fair bit of juice itself so both features might not be possible for this design. My understanding of triodes is that more current = more linearity, while I know that a shunt power supply can offer the benefit of lower noise and lower impedance to ground. So I could try to listen both ways and decide for myself, but I figured I'd consult the Doc first and see if you guys maybe explored this route during the R&D process and verify that I'm not just treading old territory that you guys have already passed by.

 

[Paul Birkeland]

The C4S is a constant current source load, and if you have a massive change in plate current when installing them, something isn't quite right with the installation. 

Are all of the outputs of the pair of forward C4S boards behaving in the same fashion?  Have you verified that all R1 resistors are the correct values?

Having not all the LED's lit would tend to indicate issues with the installation, as the LED's themselves only need ~1mA of current to adequately do their jobs on the forward C4S boards.