S.E.X. C4S upgrade

[timbro52]

I've just completed installation of the C4S upgrade on my SEX amp. Resistance values check out fine but in doing the voltage checks I'm getting 330 VAC on A5 B5 & A6 B6 and they should be 72 & 2.5 VDC respectively. Can anyone help me out. I've double checked my soldering and diodes are lighting up fine.
Thanks,

Tim

[JC]

You know, I've been looking over the schematic trying to figure out how that is possible.

How is it that you decided to measure AC Voltage on those particular terminals?

[Grainger49]

Ok, let's think about this for a minute.  The C4S shouldn't affect the power supply rectifiers.  You built stock and the diodes and capacitors shouldn't have changed.  So the voltage feeding the C4S should be DC already, not AC.  It could be that your meter swaps between AC and DC automatically and some noise is confusing it into thinking it is AC.

Having high voltage on tube pins 5 and 6 indicates that there is no conduction in the cathode leg of the first triode.  As I am not familiar with how the C4S is installed in the SEX amp I can't be too specific.  I would guess that there is on C4S for the first plate and one for the first cathode.  I would look at A6 and B6 to ground.  See what is there and inspect it.  If it is a C4S then are all the LEDs lighted?

[JC]

FYI, Grainger, there is only a single C4S on each side on the SEX.

I suspect that we are thinking along similar lines:  There is some sort of issue with the reading which needs to be sorted out first.  Hence, my question.  I have been unable to come up with a scenario which would account for raw AC getting to those points.

[Grainger49]

JC,

Thanks for the information. I don't have any information on the SEX other than a manual.  

I will try to remember it but I can't remember yesterday so it is doubtful.

Does that mean it has one of the "universal" boards?

[JC]

It uses the original C4S board, one per side.  Recently, I see that Doc upgraded that to a new "green" version PCB, with mounting hardware.

The original version (like mine), you'll recall, was basically held in place by its leads.  Which worked fine in that it is a small, light board.

[timbro52]

Sorry guys, that should be 330 VDC not AC. Yes all the diodes are lighting up.

Thanks ,

Tim

[JC]

OK, so on each tube socket, Pins 5 and 6 measure 330 Vdc with respect to ground?  And, the two LEDs on each C4S are lit?  And, the only tube pin you worked on during the installation was Pin 5?

Have I got all of that straight?  If so, I would double check that each lead on the C4S is going where it is supposed to, and that each transistor is correctly oriented.

I must admit, though, I am still stumped about how that Voltage can be getting to Pin 6 on each tube.

[Paul Joppa]

Ok, now if pin 6 is at 330 volts, and is also 1270 ohms to ground, then the current through that 1270 ohm cathode resistor must be 260mA and it is dissipating 86 watts. It's rated 1/4 watt, so that seems unlikely  :^)  I conclude at least one of the premises is not true. The measurements may have seemed correct - probably because there is an intermittent connection somewhere, which changes with the mechanical forces of the measurement probe.

[timbro52]

Guys I think I found the problem and I'm more than a little embarrassed. I had removed the wrong resistors and soldered the jumpers to the wrong pins. (Oh, the learning curve of a newbie)
I was on A6 and B6 instead of A5 and B5. In partial defense, the instructions I believe are incorrect. It says, "Remove the 150K ohm plate load resistors from terminals 17 to A5 and 37 to B5.
After looking at the original S.E.X. amp instructions those resistors go from terminal 16 to A5 and 36 to B5. In hindsight I should have measured the damn things before I removed them.
So I'll wait patiently for Eileen to send me 2-1.27K 1/4 watt carbon resistors and hopefully that will do the trick.
Thanks for all the help!
Humbly Yours,

Tim

[Dr. Toobz]

Guys I think I found the problem and I'm more than a little embarrassed. I had removed the wrong resistors and soldered the jumpers to the wrong pins. (Oh, the learning curve of a newbie)
I was on A6 and B6 instead of A5 and B5. In partial defense, the instructions I believe are incorrect. It says, "Remove the 150K ohm plate load resistors from terminals 17 to A5 and 37 to B5.
After looking at the original S.E.X. amp instructions those resistors go from terminal 16 to A5 and 36 to B5. In hindsight I should have measured the damn things before I removed them.
So I'll wait patiently for Eileen to send me 2-1.27K 1/4 watt carbon resistors and hopefully that will do the trick.
Thanks for all the help!
Humbly Yours,

Tim

Nothing to be embarrassed about - for those of us who aren't electrical engineers (I myself am a psychologist, for heaven's sake), making mistakes seems to be the best way to really learn about how things work. Looks like you removed the cathode resistors for the small triode halves of the tubes vs. the plate loads. If you have any HLMP-6000 LEDs laying around, you can use one in series with a 500 ohm resistor, or two in series, in place of the 1.27k resistors. This may actually give a cleaner sound than the 1.27k resistors, though I myself seem to prefer the resistors over the LEDs in that particular instance, having tried both ways. If not, Eileen is pretty quick with shipping!

[JC]

No need for embarrassment whatsoever.  One of the trickier things in electronics is to keep your place when the pins are in a circle, IMO.  Every time I turn a chassis over to look at a tube socket, I take several minutes to re-orient myself; that's how many times I've counted the pins from the wrong end of the circle.

Be sure to post back when you take your readings after the re-work; I always look forward to hearing how these things turn out!

[Grainger49]

I built my first kit in 1964.  I number as many pins as it takes to keep my place.  Numbering is not intuitive.

[timbro52]

Well I'm back needing assistance again. I replaced the the 2-1.27K resistors in the right locations this time and when I went to fire it up the fuse blew. I've been over the manual a zillion times and for the life of me I can't find my mistake. Resistance values are all OK, checked and rechecked my solder joints but still when I power up the fuse blows. Can anyone point me in the right direction.

Thanks,
Tim

[JC]

I wonder if the tubes themselves may have been damaged.  I would suggest trying again with the tubes removed, to see if the fuse will hold.

If my guess is incorrect, of course, you're going to blow another fuse, I'm afraid.  Sorry about that.