OG S.E.X. take two: which schematic should I follow?

[tpdorsey]

Hi folks, hoping you can point me in the right direction for completing a project I botched 20+ years ago. It's a pair of '99 vintage S.E.X. kits that I bought and started to build, but never got working properly. They went in a box, we moved across the country a couple times, raised a family, now I'm looking at this again. The stubborn part of me wants to try to make them work.

Here's what we've got:

The invoice says ParaSEX, but my recollection is that's because S.E.X. kits were technically not sold anymore, but the powers that be hooked me up with an S.E.X. kit setup with:

PGP 8.1 power transformer
TFA-204 output transformer

What I believe to be the correct filter choke is labeled:

EIA-17
831
0005-1A
105-6

The paperwork has both S.E.X. construction booklet and a ParaSEX upgrade guide.

First thing I did was go back through the booklet and "check my work" and while I mostly got it right, there are some discrpancies between the guide and what I built. Close but not right, and I'm not sure what I was thinking. I carefully checked voltages and outside of the power supply they're significantly off in a few places to the point I have no confidence in my build.

Output is hum and very faint hint of the input signal.

So rather than try troubleshooting whatever I built, which doesn't match any of the provided instructions, I'm inclined to take most of this back apart and start over.

(Also, it's a pretty ugly build. I have higher standards today.)

I have over the years restored or built a number of other projects successfully (a working Foreplay, ss preamps and amps, other vintage hifi gear, guitar effects, speakers), so replacement wire, resistors, and caps are either on hand or I know where to get appropriate bits.

The question is: what's the definitive guide to building this thing _correctly_?

Open to other suggestions, but I think what I need is a tested recipe. Thanks in advance for your advice.

[Paul Joppa]

The S.E.X. Manual Rev 2.0 dated 1997 is probably the best reference; it has some instructions for connecting the TFA-204s. There was also a short 4-page PGP 8.1 installation and wiring manual. Let us know if you don't have it.

However, there have been some improvements in reliability, performance, and safety in the last 23 years(!), and I would urge you to make a few changes. The circuit was optimized for a different power transformer and a different output transformer, and should be re-tuned for your components.  I wrote up some notes a decade ago - I'll review them and see what still makes sense.

If you could post a picture of the transformers and chokes, that would clarify what you are starting with. The early history of the S.E.X. amp is pretty chaotic!

[Paul Birkeland]

You are also welcome to send this to me and I can make something conceptually similar to what the original SEX amps were, but with some modern updates.

[tpdorsey]

Thanks for your words of encouragement. Some photos of what I have attached.

[Paul Joppa]

Excellent - you have the parts I hoped you had.

The main issue is that the PGP8.1 power transformer makes more voltage than the previous power transformers, leading to excessive plate dissipation in the output tubes and reducing the lifetime of the filter capacitors. Here are the essential changes, relative to the Rev 2.0 manual:

1) The 10uF/450v electrolytic filter caps should be replaced with some rated 500v or more. They are harder to find but do exist. I would recommend higher capacitance values, such as 22 or 47uF, as long as they fit. Get 6 of them (see note 3).

2) There is a 1000 ohm 5 watt resistor between power supply and the output transformer (terminal 6 to terminal 14; p.23 in the manual). Replace it with a 1000 ohm 10 watt resistor - the original is under-rated. This resistor gets hot, so don't let it touch any adjacent electrolytics!

3) Install the third 500v electrolytic filter capacitor with the negative to terminal 13 (a ground) and positive to terminal 14.

4) There is a 270-ohm 1-watt resistor between B3 and the ground bus (p. 23 bottom). Replace with 330-ohm 5 watts - the original is badly underrated.

5) The resistor in 4) is bypassed with a 100uF/16velectrolytic  capacitor. Replace this with 100uF/35volts.

6) Be sure to skip the two instructions (p.24 6th instruction and p.25 3rd instruction) as noted for the TFA-204. This removes the feedback/treble boost circuit that is only needed with the old stock output transformer.

These changes will make the amps more reliable and better matched to the TFA-204, while making as few changes as possible from the manual. It should sound quite good!

Both PB and I have a fondness for these old designs, and we'll be available to help if you have questions. (If you should change your mind and just want someone to make them work for you, PB does excellent work, and would probably add a few more good tweaks!)

[Paul Birkeland]

Yes, if you sent me those parts,  I would substitute new power transformers (with covers) that make a little less B+ than a PGP 8.1!

[tpdorsey]

Paul Joppa: Thank you. I'll go through the circuit again with these tips in mind. It may take some time as I need to travel on business next week. Stay tuned.

Paul Birkeland: Thank you as well. I'm going to try sorting this on my own first, but will be in touch if that doesn't work out.

Cheers.