New SEX build, high voltages [resolved]

[teamrushpntball]

So I just finished wiring my Bottlehead SEx I've had for at least half a year.  All of my resistances were good, with terminals 3, 11, 17 and 25 all reading out of limit on my cheap meter.  Double checked all their connections and joints just to be sure it was my meter and not a bad joint.

When doing my voltage checks everything is on the high end of the ranges.

Terminals 3 and 17 I'm getting: 41v
Terminals 6 and 20 I'm getting: 450v
Terminals 7 and 21 I'm getting: 475v
Finally terminals 10 and 24 I'm getting 470v

These are all slightly higher than the 10% tolerance (or way high on 3 and 17).  Is this too far out of range?  Any suggestions on the issue?

 

[Paul Birkeland]

Can you tell us what your measured AC line voltage is, where you live, and which primary winding voltage range you used?

What voltage do you get at terminal 4?
How about terminal 18?

 

[teamrushpntball]

Live in Ohio, USA.  AC was 115V, and used the 115V to 130V wiring.

 

[Paul Birkeland]

If you use your meter to measure a fresh 9V battery, what voltage do you see? 

 

[teamrushpntball]

Reading 9.51v on a new battery. 

 

[Paul Joppa]

You said "All of my resistances were good, with terminals 3, 11, 17 and 25 all reading out of limit on my cheap meter" - this worries me. Those should read 880, 680, 880, and 680 ohms.

There should be a 680 ohm, 3 watt resistor from 5L to 11L, and another from 19L to 25L. The measurements suggest these are missing or not connected or connected to the wrong place. That would explain the voltages.

If I am correct, then the 1000uF/35v capacitors may have been damaged.

 

[teamrushpntball]

Yep, that will do it.  That resistor is is on 5 to 12 and 19 to 25.  When the resistance check failed I just assumed it was meter not reading on those 4 points.

I'll move it in the morning, check again, then see if I need to replace those caps.

Thanks for your insight.

 

[Paul Joppa]

The caps were seeing 41v and rated for 35v. The current will have been very low though, so it's possible they are not damaged. Look for signs of swelling - it's the heat and gas pressure that does the damage. Personally, I'd replace them anyhow.

Incidentally, that voltage is the bias voltage for the power section of the tube. The high voltage there indicates the tube is very nearly cut off with almost no current flowing. That low current draw is why the other voltages have drifted so high.

 

[Paul Birkeland]

We have indeed seen high cathode voltages like these on really old SEX amps with blown cathode resistors. 

 

[teamrushpntball]

Corrected the 3w resistors to the right locations.  Resistances came out on target.  Voltages are all within 2-3% on everything except terminals 24 and 17 which aren't reading anything (resistance for 17 is on target, 24 is reading O.L). 

I assume I destroyed a 630v capacitor there? 

 

[Paul Birkeland]

Your meter should give you a voltage number.  Without knowing what voltage is actually there, it's tough to say what might be going on.

Nothing about any of this indicates a damaged 630V capacitor.

If you have the correct voltages at terminal 3 and 10, at least you're half way there.

If you have 0V at terminal 17, then you may have destroyed the 1000uF/35V cap.  To check that, you can remove it from the circuit and see if the voltage at terminal 17 pops up.

 

[teamrushpntball]

Re did my joints on those two terminals.  Everything is in spec now.  Amp is up and running!

Little static but sounding great.  Might wait a few weeks to do the C4S upgrade.

Thanks so much for the help to both of you!