OB voltage low

[atul.mehta6]

Hello All..

I had a C2A which was not getting used for some months (ty Mainline!)... Today I put it back in the rack with a new production 12AU7 tube. I measured all the voltages just now and everything checks out EXCEPT OB on the high current C4S boards. Both read somewhere around 75-78V range. Everything else is in spec and all the LED's also light up.

I am assuming that this is just a matter of the new 12AU7 tube needing to cook a little bit.. Is that correct? Can I run the amp still?

Thanks!

 

[Paul Birkeland]

What are OA and OB on the center board?

 

[atul.mehta6]

What are OA and OB on the center board?

both are around 80 V DC..

 

[Paul Birkeland]

How well is the 6080 glowing?  Those voltages indicate that the 6080 is really struggling to draw appropriate current.

 

[atul.mehta6]

I dont have a 6080... instead I am using a TungSol 5998.. the 12AU7 is the new Apos Ray 12Au7..

 

[atul.mehta6]

Hello PB,

Any update on this? Is the new 12AU7 tube not a good idea?

Rgds

 

[Paul Birkeland]

Can you try a 6080?

 

[atul.mehta6]

I tihnk I might've sc***ed up something.. Now the voltages esp. on A side are all bad..

So this are the voltages with the TungSol 5998

Low Current C4S
IA        100                                  IB        147
OA      64.6                                  OB        68.4
KRegA  0.219                              KregB    6.04
bRegA    100                                bregB    147.3


High Current C4S

IA      193/192                                              IB    0/0
OA    100/147                                              OB  67.1/78.9
bA        0/0                                                  bB    100/147.3

and these are the voltages with a stock 6080

Low Current C4S
IA      99.9                                    IB        147.3
OA      65.5                                    OB        68.8
KRegA  0.22                                    KregB    6.06
bRegA    100.2                                bregB    147.3


High Current C4S

IA    193/193                                              IB  0/0
OA    100.2/147                                          OB  71/91
bA    0/0                                                      bB  100/147.3

So all in all, not looking good..

I noticed one curious thing. When I measure the voltage at IA on the low current board, it first shows 147V but in a flash drops to 100. Might be a clue perhaps on what the real issue is....

Thanks

 

[Paul Birkeland]

Yeah that isn't a working amp.  What voltages do you get with no 6AQ5s plugged in? 

The 100V regulated voltage and 0.22V on the corresponding Kreg voltage would have me suspect that the A side of the high current C4S feeding that side of the circuit has a short, solder bridge, or misconnected wire.

 

[atul.mehta6]

Hello PB,

I took out the 6AQ5 and these are the voltages across the low current C4S board (with a 6080 plugged in)

IA                    205            IB            205
OA                  65              OB          68.9
kRegA              0.006        kRegB      1.308
bRegA              205          bRegB      205

Rgds

 

[Paul Birkeland]

The 431 regulator on the A side is likely shorted out.

 

[atul.mehta6]

CRAP okie.. let me try and take a look at it tomorrow.. I will update this thread with my findings later on..

thank you PB!

 

[atul.mehta6]

The 431 regulator on the A side is likely shorted out.

So I tried reflowing the solder joint around the LM431A and still no joy! Do you think it is fried? Should I try and procure another LM431A and give it a shot..

Also I understand that the C2A is retired. But looking at some of the soldering I did on that C4S board, I am seriously tempted to get another low current C4S board from Queen and stuff it all over again. Do you know if the board and parts are still available?

 

[Paul Birkeland]

You can definitely ask the Queen for some parts.  That 431 definitely needs to be replaced, but it may not be the only issue.

 

[atul.mehta6]

thanks PB! qq.. is it also worth upgrading the 0.1uF 400V caps on that board ?  will it yield significant sonic benefits?

 

[Paul Birkeland]

is it also worth upgrading the 0.1uF 400V caps on that board ?  will it yield significant sonic benefits?

When the amp is back in functional shape, that is something you could explore.  I wouldn't do modifications to an amp that isn't working, as sometimes that can make extra problems.

 

[atul.mehta6]

When the amp is back in functional shape, that is something you could explore. 

yup! wise words.. I intend to follow this plan too.. waiting to hear back from Queen about the spare parts....

*once* the amp is back up running, do you think caps like these are worth exploring?
https://ebay.com/itm/112795427528?hash=item1a4321a2c8:gI0AAOSwLjxadzOc

 

[Paul Birkeland]

I do not recommend putting caps with metal bodies into the Crack-a-two-a.  That is just an extra way for shorts to develop.

 

[atul.mehta6]

I do not recommend putting caps with metal bodies into the Crack-a-two-a.  That is just an extra way for shorts to develop.

I was reading earlier posts on this and someone had wrapped the caps in Teflon and then stuck it in one of the kits.. would that be risky too?

 

[2wo]

I like the K40's. I have used heat shrink tubing over the bare body... John