Mainline BIAS Issue

[Mike V.]

Hi guys, long time no chat, Finally I´ve dicided to build my Mainline kit that I bought in Dec'14. I have completed the kit, all messurements were correct, the 275V is 300V in my Reading. the rest of ohms and volts are perfect.

The issue I got is adjusting the BIAS of the A side. The B side I set it at 145V, but the A side is stucked at 80V if I rotate the switch I can get up to 81 or down to 74V until I start listening a click every turn I do. I switched the tubes and I was able to set BIAS at the B side at 145V, but The up I could get on the A side with the other tuve was 97V.

I have inspected all the soldering joints, and look fine, all leds are lighting. the only issue I guess I have is one Capacitor ZU5 that I solded it to close to its body.

Please help me!

 

[Paul Birkeland]

OB on the center board is the adjustable bias voltage for the A side.  What voltage do you have on OB at each end of the trimmer pot rotation? 

 

[Mike V.]

Sorry the side that I don't get the correct bias is the one of the C socket.

The OA measurements are the following: 3.7v - 0.69v

 

[Mike V.]

I already re heat and solder all the C side, I get the same 97v

 

[Mike V.]

I have checked and compare all voltages from both C4S boards, and all are equal except the one related to the bias 145v - 96v. I guess that will take me to the tube socket and after I have resolder everything, maybe the problems is with a ZU5 or a 1/8w resistor that I have blown  or is defective?

 

[Mike V.]

OB on the center board is the adjustable bias voltage for the A side.  What voltage do you have on OB at each end of the trimmer pot rotation?

Paul,
any thoughts on where might be the issue?

 

[Doc B]

Can you verify that the resistance across the trim pot is actually changing when you turn the trim screw?

 

[Mike V.]

Can you verify that the resistance across the trim pot is actually changing when you turn the trim screw?

Doc, How can I do that, do I messure the ohms from one leg of the trim to an other? the amp needs to be turn on?

 

[Doc B]

Yes, from one end pin to the other end pin on the pot. Power off and amp unplugged as you are measuring resistance, not voltage.

 

[Paul Birkeland]

Photos would be helpful at this point.  Can you confirm that you get the same adjustable range of voltages (approximately) from OB and OA on the center PC board?

If so, but your plate voltage won't move, this suggests a miswire or shorted connections, most likely on the bottom of the high current C4S board on the offending side.

 

[Mike V.]

Yes, from one end pin to the other end pin on the pot. Power off and amp unplugged as you are measuring resistance, not voltage.

Doc, The resistance values are from 1 ohm to 2kohm

 

[Mike V.]

These are the 3 boards

 

[Doc B]

Could you send a pic of the underside of the middle board?

 

[Mike V.]

Could you send a pic of the underside of the middle board?

 

[Mike V.]

Photos would be helpful at this point.  Can you confirm that you get the same adjustable range of voltages (approximately) from OB and OA on the center PC board?

If so, but your plate voltage won't move, this suggests a miswire or shorted connections, most likely on the bottom of the high current C4S board on the offending side.

Yes I get the same. Tonight I will check the C4S board and check if everything is soldered as the manual.

 

[Mike V.]

I just notice that 2 lends on the C side board takes a little to turn on

 

[swich401]

On C side, you have resistors from the center lug going to 3 and 8 of the tube socket. This is supposed to be 2 and 8.

Cheers,
Mischa

 

[Mike V.]

On C side, you have resistors from the center lug going to 3 and 8 of the tube socket. This is supposed to be 2 and 8.

Cheers,
Mischa

I knew the issue was down there! But couldn't find where, That's the problem of building an amp at late night hours.

That for sure resolved the Bias issue. Thanks!

The final result a clean pure delightful sound!

 

[Mike V.]

Doc, Paul, Mischa,

Thanks for your help! A tired eye is a dangerous thing lol hopefully all my soldering was clean and well done. Got a clean sound.

Delightful sound!

 

[Paul Birkeland]

I would also resolder the middle leg of the MJE5731A on the "A" side, as it looks like the flux hasn't flowed out of the joint completely.

-PB