Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Crack-a-two-a => Topic started by: Happy Ghost on August 27, 2024, 05:22:43 PM

Title: OB voltage low on one board
Post by: Happy Ghost on August 27, 2024, 05:22:43 PM: 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!
Title: Re: OB voltage low on one board
Post by: Paul Birkeland on August 28, 2024, 05:05:52 AM: If this is on the center PC board, that is OK.
Title: Re: OB voltage low on one board
Post by: Happy Ghost on August 28, 2024, 05:18:44 AM: 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!
Title: Re: OB voltage low on one board
Post by: Paul Birkeland on August 28, 2024, 05:20:06 AM: 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.
Title: Re: OB voltage low on one board
Post by: Happy Ghost on August 28, 2024, 06:01:21 AM: Oh ok.. I’ll post the voltage readings later today.. Ty!
Title: Re: OB voltage low on one board
Post by: Happy Ghost on August 29, 2024, 06:17:11 AM: Quote from: Paul Birkeland on August 28, 2024, 05:20:06 AM
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!
Title: Re: OB voltage low on one board
Post by: Paul Birkeland on August 29, 2024, 07:20:40 AM: I think I'd be trying a different 6080.
Title: Re: OB voltage low on one board
Post by: Happy Ghost on August 29, 2024, 08:07:41 AM: Quote from: Paul Birkeland on August 29, 2024, 07:20:40 AM
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?
Title: Re: OB voltage low on one board
Post by: Paul Birkeland on August 29, 2024, 08:15:33 AM: Yes, a 5998 and a 6080 will have different voltages on the OB terminals of the high current C4S board.
Title: Re: OB voltage low on one board
Post by: Happy Ghost on August 29, 2024, 08:59:50 AM: Quote from: Paul Birkeland on August 29, 2024, 08:15:33 AM
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?
Title: Re: OB voltage low on one board
Post by: Paul Birkeland on August 29, 2024, 04:21:37 PM: 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.