C4S voltage check

[RW]

Hey everyone,

I may have a small issue with my C4S voltages but I'm not sure. I finished the main build and all of resistance, and voltage checks were within range. After I installed the c4s, I had 350v 0n 10/20 (falls within range), OB 69.8v (within range), OA 58.6v (which is out of the 10%-15% range). is it bad that OA is that far out of range? is there anything I should check, or a bad component? Or, is it not bad and I can move onto the impedance switches? I appreciate any input, thanks

[Paul Birkeland]

How does the amp sound?

58.6V on OA could just indicate a very strong tube.

-PB

[RW]

Hey PB, thanks for the reply.
Unfortunately, I haven't had a chance to spend any quality time with it. I just finished last night, and I didn't want to run it until I found out it was ok. From what I've in the forum, it seems that the valves should cook for 50 hours before any critical listening is done. I will be able to spend a few hours with  it tomorrow or this weekend, should I analyze before or after that 50 hour mark? Do you know if the stock tubes are matched? Would getting another pair or two and swapping them until the voltages are closer? I don't mind grabbing them because they will be used eventually. Is there another component bring the voltages closer? Should I hold off on installing the impedance switch until I'm done analyzing

[Paul Birkeland]

Stock pairs of SEX tubes are construction matched (almost always). 

The 10V plate difference is not going to be audible, and electrically matched tubes are not at all required for the SEX amp.

-PB

[RW]

will this value change with different tubes or will it remain the same no matter which ones go in? what is this plate voltage difference a bi-product of? And how would it be remedied? Even tho its not audible, I'm curious why this is out of the recommended range yet still acceptable? thanks for your help PB

[Paul Birkeland]

If you switch the tubes side-to-side, that voltage will follow the tube itself.  It's not really anything to be remedied, just part of electronics in general.  If the voltage didn't follow the tube, then that would be something to look into.

Another indicator would be pin 6 on each tube, which is the cathode of the driver stage.  Since we have a constant current load, those voltages should be very close to identical.

The actual voltage variation can take on several forms.  If you had one tube that was 75V, and one tube that was 110V, we might speculate that the 110V tube was worn or needed some time to cook and emit properly.  The high plate voltage indicates low emission, which is not a good thing. 

The 75V value might be what we see on the tube curves themselves, which are drawn from a "bogey" tube.  There is some production variation in tube manufacturing, and it's not that unusual to see some variation from that bogey set of curves.  Your tube that's providing a lower plate voltage is just emitting a bit more than usual on the small triode. 

-PB

[Paul Joppa]

... I'm curious why this is out of the recommended range yet still acceptable? thanks for your help PB

That voltage is the plate voltage. In the stock amp it is stabilized by the cathode resistor, which provides negative feedback at DC - it's sometimes called "automatic bias" for that reason. With the current source plate load (C4S) the bias voltage is fixed, leaving the variation between tubes unattenuated. So once the C4S is installed, there is more likely to be a +/-30% variation or thereabouts.

[RW]

Awesome. Thank you all for your replies, and giving me some knowledge on whats going on

[stormbringer69]

Hello guys! I got the same issue when I hooked the C4s up to my S.E.X. Videlicet, I have the 10 volts plate difference now. And the statement that this difference is not going audibly is an incorrect one. The cause of it is that the tubes are not matched unfortunately. I will certainly find and buy the matched tubes. But till I do so ,could I equalize the plates voltages by selecting nominal 499 ohm resistor on PCB of C4s without any trouble? Maybe it's not worth doing that, because it may not improve sound etc? I will be very grateful for your advice.

[Paul Joppa]

The operating point is defined by both voltage and current. The design target is a balance among current drive capability, distortion, headroom, and treble response - it's NOT just the voltage!

Changing the current to match voltages causes a larger deviation from the design target than leaving it alone. Changing the cathode bias resistor also causes a large deviation, this time mostly affecting the treble response through the effective plate impedance. The best balance and symmetry is achieve by leaving the current and bias the same.

[Grainger49]

   .  .  .   The 10V plate difference is not going to be audible   .  .  .  .   

If you hear a difference between channels with mono source material it is most likely a tube difference, not the voltage and current. 

Swapping the tubes from side to side might show you it is a tube difference.  It might also change the voltages you have. 

Give it a try, it is free and might give you more information.

[stormbringer69]

Paul and Grainger 49 , thank you very much for your reply and advice. Paul, I leave it the same, you are right: it will be better.))  Grainger 49, I already tried to swap the tubes from side to side and I have drawn a conclusion as you correctly surmised : the tubes are not matched,so the best path to find matched ones, not playing with bias and current.

[Grainger49]

Often tubes need to burn in for 50 hours.  The voltage and sonic problem might disappear.

If it measures bad and sounds bad after the 50 hours it is probably a bad tube.  Bottlehead usually replaces those for new kit builders.

[stormbringer69]

Grainger 49, thank you very much for the hint, at the same time, I also want to try matched pair of 6SN7 GTB tubes, condition NOS, made by Sylvania in 1962. I would like to sense a  difference.))

[i luvmusic 2]

 I have been using  6SN7 GTB for a few months now,I don't think if using a match 6SN7's for the SEX is important i tried using match paired and none matching pairs and i can't hear any difference.