Bias issue

[anteroth]

Can't get the c side of the board to bias correctly.

Checked output of both capacitors and both show 220 V before trim pot adjust. After trim pot adjust I am able to get A side down to 145. However C side only ranges from 220 down to about 193. Checked both trim pots and both are showing values ranging from 1 to 2kohm.
I also show both kreg values at 6v a peice, but what I've gathered from forum searching that those can be low and it's ok.

Help?

 

[Doc B]

Swap the 6C45pi tubes and see if the voltages follows the tubes.

 

[anteroth]

Will do. To verify, the 6v on the kreg voltages is OK? And I will swap when home and see if it follows. Hope to get this up and running tonight!

 

[Paul Birkeland]

6V on Kreg is not a problem.

 

[anteroth]

Awesome will check tubes when home. If they do follow, then it's the tube. If not ill check signal path and connections for C side again.

 

[anteroth]

Swapped the tubes. issue did not follow stil un adjustable voltage at C side cap at 30.

See attached pictures

 

[anteroth]

+275vDC on power Supply: 267.9
+6.3vDC on Power Supply: 6.30
IA on A side C4S: 269.1
IA on C side C4S: 269.2
Breg(both sides: 219.3/220.6
-reg(both sides): 0/0
Kreg(both sides): 11.11/10.74

OA on center board: 3.37
OB on center board: 2.02
IA/IB centerboard: 0

A side vs C side measurements: A/C
IB: 219.3/220.6
IA: 270.1/270.1
OB: 139.9/205.9
OA: 219.2/220.6
smaal res: 216.2/217.6
big res: 264.3/264.2
also the 2 37/49 small rez match up in voltage. Only noticable difference is in OB between the A/C side

 

[anteroth]

So turns out if the trim pot is driven all the way up it behaves oddly at that end of the range. I tried turningthe working pot all the way up to see how it bhecaed(all the way up mechanically not electicaly) it behaved the same way. So i drove both pots all the way in and both went to approx 80v the low end of trim. then adjsuted both back up to 145. For anyone getting odd response try turning it until its all the way out then all the way in. if it goes down to 80 v that was your problem and just adjust up to 145. your trim pot was too far out and behaving strangely.

 

[Doc B]

Yeah, when precision pots get to the end of their adjustment they just keep spinning.  Good catch!

 

[boltd]

Hi guys,

Love the amp.  I've been running for a week now - tremendous resolution compared to what I've owned or spent any kind of significant time with. 

With respect to a bias issue, is it normal to drift with time?  For my build, I've seen approx -3V per day on A side and much less, approx +0.2V per day on C side.  I wait at least an hour after power up before measuring and re-biasing. 

Thanks much,

Guy

 

[Doc B]

I am guessing that you mean plate voltage when you say bias voltage. If it has been going down 3 volts a day since October 25th, I should think it would be quite negative by now.

 

[boltd]

Sorry, I should have started a new thread...new build for me, about a week old.

Yes, plate voltage.  I read through all the threads in the Mainline sub forums and Caucasian Blackplate responded to another saying plate voltage will drift some.  I'll check everyday and if it doesn't slow down, I'll swap out tubes.  See if the degradation follows.

 

[Doc B]

Sorry, I made the bad assumption that you were the OP. My apology. It can take quite a few hours of play for the voltage to settle. Wait a week, then check it again.

 

[boltd]

Will do and no worries, I'm very happy with the work you guys do, I'm a big fan!

Guy

 

[Paul Birkeland]

It's also worth mentioning that the fine tuning of the bias allows for absolute maximum output power from the Mainline.  Setting OA and OB on the center PC board to 1.5V will generally get you in the ballpark if the process of biasing is bothersome.

 

[boltd]

Thanks much, Paul.  The degradation rate is now at ~0.3V per day on both sides, so a periodic (weekly or so) fine tuning is ok with me.  I am curious about the cause though; I do fairly random but infrequent checks, I classify as degradation since all I ever see is lower voltages. 

Guy

 

[Paul Joppa]

This is probably the cathode being "formed" - once the process is complete, the drift will stop.

Cathode forming takes place as one of the last steps in manufacturing the tube, but it is often either incomplete (if the tube is new) or has degraded (if the tube has sat in the box for a few decades).

 

[boltd]

Thanks Paul. I was going to otherwise spend money a higher quality Vishay trim pot.

Guy