BIAS on SC200+

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BIAS on SC200+

Postby Vucinic on Sat Oct 07, 2017 5:10 am

Hi to all,
Last night i check Bias on my SC200+. It's was set at -44V, like in the book :). I had a little hum so i started tuning. I realize that at beginning and at the end of bias pot i have most hum, and at the middle of the pot is not hum at all. So i set up my bias according the hum.
When i check the voltage i have difference of 7V between pairs of KT88's, but i dont have hum and sounds better.

Any thoughts?
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Re: BIAS on SC200+

Postby Vortexion on Sat Oct 07, 2017 4:06 pm

This is quite normal, and is down to the fact that no set of tubes (regardless of what the tube sellers would like us to think :wink: ) can ever be perfectly matched - and, with use over time, each individual tube will drift off-spec as they all age. This behaviour is more pronounced with modern-production tubes: they simply aren't made to the same exacting tolerances as back in the day (sigh).

The -44V recommended by the manufacturer is just a rule-of-thumb figure (one that used to work just fine with the rugged tubes of yesteryear, which could withstand a fair bit of abuse).

Adjusting your bias pots for minimum hum is a good thing to do, provided you keep an eye on how different from each other those bias supply voltages are. If you have to adjust them way out of whack to reduce the hum, then it means that your output tubes have become hopelessly unbalanced.
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Re: BIAS on SC200+

Postby pdf64 on Sat Oct 07, 2017 6:45 pm

http://soundcitysite.com/soundcity200plus.pdf
I think it would be a very good idea to check the idle plate or cathode current of each power tube; 30-35mA should be fine. Using the method described in post #1, the power tubes may be idling unnecessarily hot.

I don't think that there ever has been a golden age when any tube that passed GEC's QC could be swapped for any other of the same type and manufacturer without adjustment being necessary. eg see p3 http://www.mif.pg.gda.pl/homepages/fran ... k/KT88.pdf
"It is essential to provide 2 separately adjustable bias voltage sources, each having a voltage adjustment range of +/-25%"
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Re: BIAS on SC200+

Postby Vucinic on Sat Oct 07, 2017 8:43 pm

Thanks people for your replays.
On the plates, I have 703V, on schematics from soundcity.com is 680V.
Now, one side bias is -44, second side is -51. I think is not too much.
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Re: BIAS on SC200+

Postby Vucinic on Mon Oct 09, 2017 5:38 pm

pdf64 wrote:http://soundcitysite.com/soundcity200plus.pdf
I think it would be a very good idea to check the idle plate or cathode current of each power tube; 30-35mA should be fine. Using the method described in post #1, the power tubes may be idling unnecessarily hot.

I don't think that there ever has been a golden age when any tube that passed GEC's QC could be swapped for any other of the same type and manufacturer without adjustment being necessary. eg see p3 http://www.mif.pg.gda.pl/homepages/fran ... k/KT88.pdf
"It is essential to provide 2 separately adjustable bias voltage sources, each having a voltage adjustment range of +/-25%"


I have old GEC, but 3 of them. So i buy new quadmatched JJ's. I must agree with Vortexion about matching. I never checked GEC's
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Re: BIAS on SC200+

Postby Papa Dog on Mon Oct 09, 2017 9:48 pm

if the voltages in the amp do not match the voltages called for on the schematic, then the issue is that either the selector on the back is not set to the correct voltage or that the wall voltage does not batch any of the selections on the amp, or both.
You need to know what wall voltage is, and either get a variac, bucking transformer, or some other means of setting the voltage for what is called for on the amp. Of course, that matters more if the HT voltage is above spec, but it still matters because the heater voltages with not be 6.3, and that matters far more than you might think.
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Re: BIAS on SC200+

Postby Vucinic on Tue Oct 10, 2017 4:17 am

Thanks Papa, i will check the voltage in the wall, selector and HT.
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Re: BIAS on SC200+

Postby pdf64 on Tue Oct 10, 2017 9:17 am

Vucinic wrote:Thanks people for your replays.
On the plates, I have 703V, on schematics from soundcity.com is 680V.
Now, one side bias is -44, second side is -51. I think is not too much.

What is the basis for you thinking that?
The purpose of bias voltage is to control plate current; to know that the bias voltage is suitable, the plate current must be assessed.
The GEC tube info I provided previously shows that if the nominal bias voltage (given the nominal g2 and plate voltages) is -44V, the actual bias voltage required by any real tube may be +/-25% of that, ie -33 to -55V.
The plate current resulting from that bias voltage IN THAT PARTICULAR TUBE is what matters.
Note that if the actual HT differs to the nominal, then the magnitude of the actual bias voltage must be scaled proportionately.
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