OB voltage low on one board

[atul.mehta6]

Hello,

I was doing a voltage check on my C2A after a long time. Everything is in spec except OB voltage on one side. It was reading 78.2 V while on the other side  it is closer to 88V. Is the 78V something to be worried about? The manual says it should be between 90 - 110V…

Thanks!

 

[Paul Birkeland]

If this is on the center PC board, that is OK. 

 

[atul.mehta6]

Hello PB..

This was on the high current board.. Not the center one unfortunately..
All the other voltage measurements are spot on.. FWIW I am trying out a new (NOS) 12AU7 .. Could that have any impact?

Thanks for all your help!

 

[Paul Birkeland]

What are the OA and OB voltages on the center C4S board?

This voltage that you've measured indicates an issue, but without knowing all the voltages, it's hard to say what it may or may not mean.

 

[atul.mehta6]

Oh ok.. I’ll post the voltage readings later today.. Ty!

 

[atul.mehta6]

What are the OA and OB voltages on the center C4S board?

These are the voltages I measured:

Low Current C4S:

IA              149.6           
OA              69.3
KregA        6.3
bregA          149.6

IB                  150.2       
OB                72.1
KregB          5.39
bRegB        150.2


High Current C4S boards.

IA                    191.3 /  191.3   
OA                  149.6 / 150.2
bA                  0 / 0

IB                    0 / 0
OB                  79.1 / 82.5
bB                  149.6 / 150.1

Thanks!

 

[Paul Birkeland]

I think I'd be trying a different 6080.

 

[atul.mehta6]

I think I'd be trying a different 6080.

So I went and replaced the TS 5998 with a Tung-Sol 7236 and the readings are pretty much the same.. Should I try a proper 6080 instead?

 

[Paul Birkeland]

Yes, a 5998 and a 6080 will have different voltages on the OB terminals of the high current C4S board.

 

[atul.mehta6]

Yes, a 5998 and a 6080 will have different voltages on the OB terminals of the high current C4S board.

That was it.. I popped in a Tung-Sol 6080 and the OB voltages are back to the normal values.. 91V or so.. Rest are more or less unchanged..

Thank you for your help.. I assume i can still go ahead with using the 5998 tube even though the OB voltage is a bit on the low side?

 

[Paul Birkeland]

Yes, when you have a family of tubes that have different values for me but are otherwise relatively similar, the higher mu tubes will require less bias voltage to reach the same current as their lower mu counterparts.  If you had seen that set of voltages you posted with a 6080 in the socket, that would generally indicate a very dead 6080, as the C2A is adjusting the bias very low to get the desired current out of the cathode follower section, but this is normal for a 5998.