High voltage checks - help please!

[Paul Birkeland]

I would double check that the -reg pads on the center C4S board are all reading 0V.  Typically a failed 431 will show 1.8-1.9V at the X terminal and Kreg, so what you're observing is more like a 431 that's partially disconnected (hence my initial inquiry about that jumper that's easily left off).

[OutFishing2]

Thanks Paul.

The -reg pads on the centre C4S board are at 0v effectively, <4mV for both A & B.

I resoldered all the pins on the board for good measure and replaced the black and red wires under the board. Also fitted new wire bridges across +reg. Voltage readings are unchanged.

[Paul Birkeland]

I'm still leaning towards a solder problem.  The common problems we find that prevent solid joints are insufficient heat and insufficient lead in the solder.  You want 900F/500C or so of iron temp, and 60/40 leaded solder is also extremely helpful.  Missing one of those elements will make soldering a lot more difficult than it should be.  Along with the sufficient heat, you also want enough time for the solder to flow through the PC board holes to really grab onto the component leads.  Looking at your board, you can see how this has happened with the LEDs (they have low thermal mass) but not the 431 regulators (slightly higher thermal mass). 

More info can be found here:


I've been building this stuff for 25 years and I still have problematic joints here and there.

[OutFishing2]

Yes, I appreciate that. My soldering skills have certainly got better whilst building the kit. The troubleshooting phase has given me chance to go back over the wiring around the transformer and power and it's all looking neater now.

I followed your advice and reflowed all the points on the board in the last hour except the LEDs. My iron only goes to 480'C but fairly close to what you recommend.

I had done most of the kit with the iron set at 380'C and I can see now how the extra heat creates much shinier joints. Solder is 60.40 0.7mm.

I'm still 12v on KregA, 3.xV on the X terminal.

[Paul Birkeland]

It's also worth mentioning that having the wires that go to each Kreg terminal swapped can give funny consequences like this.  Same with the red wires connected to IA/IB on the center C4S board.  If one pair of those wires is swapped, the regulators will not function properly.

[OutFishing2]

Thanks Paul, really appreciate your patience working this through with me.

The wires are definitely in the right places. It's a funny one. Is there any chance it could be the vregs, or the 6aq5s or dudes at Q1 and Q2?

I'm sure it's not the 6aq5s since switching them over makes no difference. Would switching the Q's from A to B reveal anything?

[Paul Birkeland]

You have higher regulated voltages than we would expect with adequate Kreg voltages at each 431 regulator to allow them to do the regulating they would normally do, but yet nothing is happening with them.  Having this happen on both sides would suggest a build issue rather than a component issue.  Were any parts substituted by chance?  You could certainly try replacing the 431 regulators, but they aren't behaving like failed 431s.

[OutFishing2]

Alternative 431s arrive on Wednesday so I'll have a go with them.

6AQ5s arrived today and made no difference to voltages but that was expected. Fascination anticipated with future tube rolling.

I used the BH parts except in two areas.

The 0.1uF caps on the low current board are mundorf. I substituted the stock dayton's in/out during this troubleshooting. It made no difference to voltages.

The four tube sockets are from a local firm. Ceramic gold plated. Described as B9A, B7G and octal sockets. I've since read a thread here where you advise against substituting sockets as they can give trouble so I'm readying to switch those over to stock.

[Paul Birkeland]

If you swapped in ceramic sockets that are about the same as what we provided, just leave them in there.  The teflon ones and some of the other exotic types can cause some real problems though. 

If you do completely remove and replace all the sockets, I would expect more problems to pop up, so definitely don't do that.

[OutFishing2]

Great news. The new 431s are in and the voltages immediately went to spec. 4.6v on both kreg and 149v on the IA and IBs.

Quite how you narrowed the fault down to the two vregs from the other side of the Atlantic is something I will take a long time to understand. Thank you!

[Paul Birkeland]

something I will take a long time to understand.

You and me both!  I'm glad you got it nailed down. 

[OutFishing2]

Thank you. The C2A really is a superb amp. I am extremely impressed at how well it sounds as a preamp as well as with headphones. Rock solid performance. The stepped attenuator went in today and it's all working beautifully. Thanks again.