more original s.e.x

[sl-15]

i recently bought a new unassembled original s.e.x. monoblock amp kit. it is the revision 2.0 and was originally bought in 11/97 (not by me but jennifer crock of jena labs whom i have to thank for such a great deal). this kit definitely took me on a journey back in time to the beginning of the bottlehead era back when they were still electronic tonalities. it also made me read through a lot of the old valve magazines which i never paid much attention to. (big mistake) this kit was just as much fun to build as the other stuff that i put together (seduction and foreplay3). today i was finally ready, resistance checked out, voltages too. i was just measuring the last terminal on the voltage check list when the first filter capacitor started to smoke and bubble. i immediately turned of the power to prevent any further damage. the cap was very hot and lost some fluid. to me this is pretty clear, the caps are old and failing and i will go ahead and change all 4 filter caps. i went ahead and measured the resistances again on both amps to check if the other filter caps were failing as well. they all measured lower then the first time and i am ot sure if that means they are all failing? and it makes me wonder if there is something else going on. please bare with me while i try to explain the layout a bit where the filter caps are. after the power supply is a soft start rectifier, then a filter cap, than a choke and then a filter cap again. the caps are 10mF 450V. all resistance and voltages readings were within tolerance before the cap smoked up, except the resistance on the rectifier terminals 1 and 2, instead of 275 ohms i got around 200ohms. the terminals on the filter caps 3 and 6 should have read 110ohms and i got 100ohms. because all the numbers were consistent on both amps i went head and fire them up to check voltages which were all spot on.  after the cap failed i checked the resistances again and now terminal 3 and 6 on both amps read much lower 78 ohms and 60ohms on the second amp. i am not sure why these readings are so consistently lower. has it something to do with the lower reading of 200ohms instead of 275ohms? i can see how one cap might fail due to age but all 4 of them at the same time? i know this kit is 16 years old and most people moved on to the current stuff which i hope to do after this little experiment eventually. hopefully doc or paul can chime in to help with this problem . i will try to source new caps and fire them up again. thanks for reading and i hope somebody has some ideas.

 

[sl-15]

i am also curious if these 4 filter caps can be substituted with a different value in case my local electronics store does not carry the exact value. from the voltage reading there is about 352 AC on the caps, so my guess is that i should not go below a 450 volt rating to be on the safe side. the manual calls for 10mF and i am wondering how much lower or higher i can go. thanks

 

[Paul Joppa]

As I posted elsewhere today, I am working on a "History of S.E.X." document which will include a chapter on "Better SEX" i.e. improvements that have proven themselves over the years since these kits were produced. So just a couple comments from working on the draft...

I am puzzled at the T1, T2 resistances. 210 ohms is in the Rev 2.0 manual that I have, and it is correct for the deYoung power transformer (which I think is the most suitable one). 275 was the original power transformer, which proved to be unreliable. 155 ohms is the value for the PGP8.1, a good transformer but with a bit too much voltage for the SEX design. If you have a Rev 2.0 ,manual with the old resistance measurements, then it's a manual version I didn't know existed!

Exploding caps can be because they are old - electrolytics don't store well, and modern ones are much better than old ones in other ways. In any case, I recommend replacing the 10uF/450v caps with 47uF/450v caps for better hum reduction. But there is another possible cause - if the caps are in backwards (or if the diodes are in backwards...) they will be reverse biased and will quickly self-destruct. So make sure you check that first. One version of the manual referred to the "+ stripe" on the cap, but the stripe is usually the - side - it has a big black rectangle that is actually a minus sign! It's so big that most of us miss the fact that it's a minus sign.

The writing exercise got me thinking of making a "Commemorative Edition Original SEX Amp" in time for its 20th anniversary, parafeed using the Paramount iron and the PT-7 power transformer, and all the best mods with today's knowledge. I haven't even told Doc B about this idea yet ...  :^)

 

[sl-15]

paul, thanks for your reply. while i am writing this i am listening to my now working original s.e.x. amps! i got some replacement caps today and put them in after i got home from work. finally, after a second voltage check i connected them. first, one amp was not working but it turned out to be a intermittent connection on one of the speaker binding posts. but now everything is working and i am impressed. i only have been using SS amps to this point and the first two things that stick out are: who ever said that tubes do not produce good bass? i feel these amps play a bit lower than my hafler that i was using before, quite surprised by that. and i am impressed that with my 90db efficient speakers i have plenty of volume to fill the room. so if these already sound so good how will a new sex version sound...? so tempting, but i have plenty of tweaking to do before i will upgrade. anyways, these will need some time to break in and open up but so far i am already very happy with what i hear. paul i checked the polarity of the caps and diodes over numerous times and they were right. so i guess the caps were failing. so you are saying that even caps that have never been used but are older degrade? i definitely have the rev 2.0 version of the power transformer it is 1.5 inches stack height so i think it is the de young. but my manual says 275ohms  instead of 210ohms so i guess my specs are mixed up. it would be interesting to get the right numbers. is there a way to copy them for me? i replaced the caps with the same value but got some 22mF 450V just in case. so you are saying 47mF 450V is better? i am sure i could exchange them if i need to. at the moment i have the 10mF in and the hum is very low, impressive how silent these are. this has been another very exciting build project. i will report back with further impressions and am looking out for your better sex post. thanks

 

[Paul Joppa]

As long as you retain the original circuit, the feedback will control the hum, so you are good to go.

Most of the upgrades remove the feedback (you need a more "audiophile" output transformer to get away with that), at which point larger caps are needed.

I've just been reading about electrolytic capacitors and how they fail. The electrolyte inside the cap has a small portion of water. The caps use a thin layer of aluminum oxide on the surface of the foil as the insulator, and are self-repairing. When the oxide fails, an electrolytic reaction combines oxygen from the water with aluminum from the foil - liberating the hydrogen as a gas which leaks out around or through the rubber seal.

If too many repairs happen too fast, you get too much hydrogen pressure and the cap blows out, literally. And eventually the rubber seal will oxidize, letting the water evaporate. When the water is gone, the cap can't repair itself and shorts out.

Sometimes old caps, which still have a good seal but have accumulated a lot of repair work, can be brought back by slowly raising the applied voltage so the repairs happen slowly. This is commonly called "re-forming" the cap. It's a slow process but works.

 

[2wo]

I am working on a "History of S.E.X." document which will include a chapter on "Better SEX"

Paul, I didn't know you were an Ampthropologist. 8)...John 

 

[Paul Joppa]

Chapter 1 - the Versions
Chapter 2 - the Perversions

Couldn't help it! Chapter 2 is the documented mods, of course. Chapter 3 is the current (2013) recommended mods, Better SEX.

 

[Grainger49]

I was told in school that a Psychologist was the only guy who could get federal funds to go to a town and study the whore houses.  I was studying Psychology at the time.

 

[najo49]

Stephan I am happy to see you got your amps and fixed your electrolytics.Paul and everyone here on the forum are so free with their expertise. It is a seldom seen thing in our society. I have some things I want to send you that paul J sent me that will help when you upgrade your version. You should I think convert your amps to parafeed with a c4s upgrade. please see c4sing the sex amps In valve. there is a schematic for this in valve. You can use your original sex transformer for plate choke and I can give you some suggestions for a opt with out spending big buck on magnaquest stuff. but eventually getting magnaquest iron is the way to go.I just bought some things from jennifer crock she is another wonderful person.

 

[sl-15]

thanks jan. i got my amps from jennifer crock as well and i couldn't agree more that she is wonderful. one of the nicest people i ever dealt with. and i would also like to thank paul for giving such great tech support on such an old product. i am still amazed at my new amps and am trying to listen to them as much as i can. i have been having them on almost around the clock and hope that that speeds up break in even if they are just sitting idle. my first upgrade will be to swap out the wires for jena labs wire that jennifer was so kind to provide. once that is done i will continue with more drastic modifications. so yes i am very curious about your suggestions jan and please send me the info you got from paul as well as what opt you are suggesting. just send me a PM. thanks, best, stefan

 

[sl-15]

after about a week of the amps working they suddenly developed both a nasty distorted buzzing tone after being turned on. this sound happens with or without an input connected. it takes about 15 seconds before i get a reading on my multimeter on the output with a loading resistor connected and the input shortened. it reads around 1 to 2 VAC and is often fluctuating rapidly. you can also hear an oscillating shrieking sound without speakers connected either comming from the tubes or the loading resistor on the output terminals.
i have rev 2.0 of the amps but my manual seems to have the resistance and voltage numbers for the original version.  i am not sure what the differences are. i checked voltages and resistances again and the numbers match my initial readings that i got after finishing the amps with a few exceptions:

A1 and B1 read still very low 0.04 VDC instead of 0.2 VDC and are heavily fluctuating on the meter on both amps (grid, sec.2)

A2 on one of the amps is heavily fluctuating (plate, sec. 2)

B5 is fluctuating on both amps (plate, sec. 1)

B7 and B8 are fluctuating on both amps but

A7 and A8 read normal at 97 VDC on both amps (heater)

i am surprised that both amps are failing pretty much at the same time. my guess is that some of the tubes are going bad. i tried swapping them around  but it does not make things better. one of the tubes makes a rattling sound when you shake it as if something is loose inside. my guess is that it only takes two bad tubes, one per amp to make the amp fail?
i was also wondering if there is something wrong in the circuit e.g.. a cap or resistor  that wears out the tubes faster. any help troubleshooting this further would be great and i would appreciate any kind of input. thanks, stefan

 

[Paul Joppa]

The voltages in the manual should be pretty accurate, it's only the transformer winding resistance (210 vs. 275 ohms) that is much different. So the first thing is to check the voltages. A good starting point is to report the DC voltage at all tube pins.

If I guess correctly, you have replaced all of the 10uF 450v caps with the same value but new? How about the 220uF 10v and the 100uF 16v caps? If they are originals, inspect them for damage (leakage); if you suspect them you can disconnect one lead temporarily - the amp will work without them, the only loss will be poor bass.

It's not likely to be the tubes, they were probably manufactured 50 years ago and the age has not damaged them yet.  Many 80-year-old tubes are in use today.

It is not unusual to hear some sound from the output transformer, as the fluctuating signal current magnetically excites the laminations. The fluctuating voltages probably just indicate a fluctuating signal, when it is present.

If the voltages all look good, then we'll have to look for other things. But I want to see the voltages anyhow! It is extremely odd to have the same problem with both amps at the same time, leading me to wonder if there is an external cause. You don't have a diathermy machine in your living room, do you? (That was a funny, not serious!) It's possible one of the amps is oscillating and it's picked up by the other. Try each amp one at a time with the other one off to check this.

It's also possible that there is a cell phone nearby, in use or checking for messages or something.

 

[sl-15]

thanks paul, i replaced all the electrolytes just in case. i tried the amps one at a time. not sure what suddenly would have changed. i am suspecting the feedback loop? i will report voltages again tonight when i am back home.

 

[sl-15]

okay here are my voltage readings from tonight. (resistances are spot on.) numbers in blue are the values from the manual. the first reading is for amp A and at that point the line voltage was 122AC, the second is amp B and the line voltage was 117AC. (pretty big fluctuation)

1  355AC     355AC    340AC
2  355AC    355AC    340AC 
3  430DC     443DC    420DC
6  412DC    422DC    406DC
9    78DC       86DC      70DC
10  170DC    179DC    116DC
11  .475DC    .574DC      .465DC 
14  338DC    362DC    350DC
15  89DC      96DC      92DC
16  170DC    175DC    115DC

A1  0.2DC     0.06DC    0.05DC
A2  310DC     334DC    319DC
A3  15.5DC   11DC      12.7DC
A4  0DC      -0.007DC    -0.003DC
A5  106DC    89DC    57DC
A6  3.2DC     3.87DC      3.2DC
A7  89DC    93DC    93DC
A8  89DC     93DC    93DC

B1  0.2DC    -3.1DC    display switching too fast to get any reading
B2  310DC    320DC    327DC
B3  15.5DC    17DC    13.2DC
B4  78DC     69DC    84DC
B5  280DC    282DC    280DC
B6  175DC    172DC    145DC
B7  89DC    93DC      93DC
B8  89DC    93DC    93DC 

A1,  B1, B2 and B3 are fluctuating rapidly, never getting a steady reading which did not happen as much in my initial measurements a week ago.
on terminal A3 my meter (Fluke87) starts beeping and i have no idea what that means. it never did that before. it does that on both amps.

as soon as i put a loading resistor on the outputs ( i use a 10ohm 10watt wire wound for that) the output transformer starts chirping or better almost screaming, high pitched with a high switching frequency sound to it i do not think that that is normal as i have not heard the transformer make that much noise before. it does that continuously until i take the resistor off again.
i visually inspected the caps and can not see any damage to them. all the electrolytes are switched for new ones with the exact same value.
i have not tried to swap tubes to see if the readings follow the tubes. not sure what to think of this. hope people can help.
thanks again,
stefan

 

[sl-15]

i was trying to study and understand the circuit of the amps and stumbled over a lot of things i do not quite get.
i was comparing the schematic in the manual with the build diagram and there is one thing in particular that doesn't make sense to me:

the schematic says the 'heater' output coming from the power transformer is 6.3 VAC but the actual heater voltage is around 90VDC. there are two 100 ohm resistors going from the heater of one tube (terminal B7, B8) to terminal 15. these two resistors do not show up in the schematic at all or i am not seeing them. the voltage reading on terminal 15 should be 89VDC which is the heater voltage. so my guess is the heater voltage is part of the B+ voltage that gets filtered with cap, choke, cap, resistor. but what is the 6.3 VAC, coming from the secondary winding of the transformer, for?

this has nothing to do with my current problem it's just one of the things that might help me to understand the whole circuit better. thanks

 

[Paul Joppa]

The 6.3v winding of the transformer has two wires, with 6.3vAC between them. There are connected to tube socket terminals 7 and 8 respectively, so that the heater current (0.9 amps for each 6DN7) flows through the heater wire inside the tube - producing that dull orange glow.

There is a DC voltage difference between the heater wire and the cathodes. Only the tiniest current flows from heater to cathode. But the voltage difference can't be too large; most tubes are rated to withstand +/-100 volts maximum. Since one of the first-stage cathodes (B6) is at a high voltage (175v in the manual), the heater is biased halfway between the lowest cathode (A6 at 3v) and the highest (B6), or 90 volts.

There is other reason for this biasing; it reduces the possibility of hum being coupled from the AC-powered heater to the cathode. Traditionally the optimum way to do this is to place the bias at the midpoint of the 6.3v heater power, using either the 6.3v winding's center tap or a voltage divider (the two 100-ohm resistors that failed to show up on the schematic). The Rev 2.0 amp follows this traditional form of biasing the heater power even though it is only optimal for some 6DN7s, not all of them.

 

[Paul Joppa]

OK, thanks for the numbers. The voltages look basically good - some are further off that I'd like to see, but nothing indicates a wiring error - besides, it worked for a while!

So I'm going on guesswork here - I have an Original S.E.X. of my own, but I was not part of the design team back then, and more important not involved in tech support either.

However, based on the available data I suspect some instability of the tubes, specifically the power section which has an especially high transconductance - that makes tubes prone to unstable behavior throough tiny stray capacitances between wiring and the extremely tiny inductance of wire itself. The standard cure is what is called a "grid stopper" resistor, which damps the resonance before it can become an oscillation. So I dug out my own SEX amps and looked at the insides - sure enough, I had installed grid stoppers on mine at some point. I have no memory of doing this or of why I might have done it, but there they are. So I suggest you do the same.

Get four small resistors; any value from 100 ohms to 1000 ohms is suitable. Mine appear to be 100 ohms if these old eyes are reading the color code correctly, today we use 220 ohm stoppers in our products. Carbon composition is traditionally the best, but carbon film or metal film should work fine. On each tube socket, pin 1 is the offending grid. Unsolder whatever is connected there, and solder one end of the resistor as close to the terminal as possible and with the resistor vertical so there are no current flow kinks between the tube and the resistor. Kinks are inductive, and you want to separate the grid from all the parasitic inductances and capacitances with the resistor. Now re-connect whatever had been connected to that grid pin, to the other end of the resistor. Repeat on the remaining sockets.

 

[sl-15]

okay that makes sense, the only thing that was changed after swapping out the electrolytes on the tube sockets were the wires that are going from one tube to the other. i moved them around to get to the terminals and did not bend them back the way they were.

tonight i bent them back the way i had them originally and did not get any crazy sounds from the output transformer. then i was playing around with bending the wires with a wooden stick while the amp was on until the OT started to make high pitched sounds again. so the wires  definitely pick up something and i will get some resistors tomorrow and put them in.

is the high transconductance something that is a characteristic of this particular tube type (6DN7) or does one have it more than another like microphonics? is it something that gets worse the longer you use the tube?

anyways, thank you so much paul for taking the time to dig out your old amp and trouble shoot this. your help is very much appreciated. i will report back once i put the resistors in.

 

[Paul Joppa]

Thanks for doing the experiments, it pretty much confirms my guess about the problem. I'll add this experience to my notes on the Original SEX amp.

Transconductance is a property of the tube design - essentially, it's the current gain. It's a good thing, it just has some side issues. The power section of the 6DN7 has 7.7mA per volt transconductance, while the driver section is 2.5mA per volt. A 300B runs 5.5mA per volt, which was considered very high when it came out in the late thirties.

 

[Doc B]

Guys, you are over-thinking this. That squeal that comes and goes when you move tube socket pins means you have a dirty tube socket or one with loose connections to the tube pins. Just crimp the socket sleeves a tiny bit to get a tighter fit on the pins. Might want to reheat the solder joints too.