The ultimate DIY? A Stax SRM-T2!

[justin]

It is possible that current production 2sk216 parts don't have the protection zener built in...

That could explain it.

Those parts would be excellent for counterfeiting

[Craig Sawyers]

Those parts would be excellent for counterfeiting

That is a dark and worrying though, Justin. I've got a couple of spares (they have the Hitachi logo, but that counts for nothing) that I'll throw at the curvetracer and see what they look like.

[kevin gilmore]

That sure scares me too. I have a batch that i know really came from hitachi with original hitachi packaging.

Even when an input tube went nuts at CJ, all i did is pop a new tube in, all the bias's were fine, and kept going.

I'm using NOS sylvania i think... But 50 year old tubes are a pain in the neck. I've never had a problem with

the el-cheapo chinese output tubes either. Don't sound great, but don't die either.

[Craig Sawyers]

That sure scares me too. I have a batch that i know really came from hitachi with original hitachi packaging.

Well, the two K216 left over from the build produce traces identical to those in the Hitachi data sheet - to the extent you could pretty much substitute the curves. My Tek 177 curve tracer won't generate enough voltage to trigger the gate protection zeners. But the drain-source diode seems to be present just using the diode function on the DVM.

The thing I don't understand is the 2N3675 in one of the active batteries (the one connected to Q4) has suffered second breakdown - it is essentially short circuit between all three terminals. Q27 and Q28 (2SK316) are dead, as are D21 and D22 (100V zeners). Three dead LED's in the current source triples (D10, 11 and 7). So the sequence of events is a little puzzling, and I cannot think of a mechanism that would lead to dead LED's - other than infant mortality. Everything connected to them in the current sources is A-OK.

[Inu]

Inu, what adjustment protocol did you use? ie where to preset RV1 and RV2, and which order in which to adjust, whether they need to be adjusted simultaneously or not, etc.

.

Following is adjusting procedure that I am using.

Presetting: Set RV2 to Center, RV1 to up (= Wiper is R32 side) position.

Confirm voltages between +/- Output and Output to GND are not too High (< 80V)

If voltage between Out to GND is too high (300 -400V), prepare to replace the 2SK216s. if around 160V, check 100V zener diodes.

Set both batteries to 740V with RV2.

Adjust RV2 alternately (2-3V step each and upward) to Output

[Inu]

But I also read that the 6DJ8 was originally introduced as a TV RF stage, and that with its high mu if it can burst into self oscillation it will. Most folks seem to add a 1k grid stopper to quite the things down.

QUOTE]

I added 1Kohm to the Grid (with cutting the trace) while I am playing with old Ampelex, but nothing changed. It will works for the point to point constructions.

[Inu]

Those parts would be excellent for counterfeiting

I have never seen the counterfeit MOS FET yet but many of Japanese small signal transistors. 2SC2240, 2SA970 etc… purchased from MC… M.

[luvdunhill]

sorry to bother the thread with more trivial things... but, are there only 14 contacts needed in each umbilical? I get:

| A | Grey | Bias |

| M | Orange | +500 |

| P | Black | Circuit Ground |

| F | Blue | +250 |

| M | Yellow | -560V |

| N | Purple | -500V |

| R | White | -260V |

| K | G/Y | Chassis Ground |

| G | Green | -12V |

| E | Red | +12V |

| C | Black | 6.3V 1 |

| J | Black | 6.3V 1 |

| D | Black | 6.3V 2 |

| H | Black | 6.3V 2 |

[Inu]

sorry to bother the thread with more trivial things... but, are there only 14 contacts needed in each umbilical? I get:

| A | Grey | Bias |

| M | Orange | +500 |

| P | Black | Circuit Ground |

| F | Blue | +250 |

| M | Yellow | -560V | ----------------------> | L |

| N | Purple | -500V |

| R | White | -260V |

| K | G/Y | Chassis Ground |

| G | Green | -12V |

| E | Red | +12V |

| C | Black | 6.3V 1 |

| J | Black | 6.3V 1 |

| D | Black | 6.3V 2 |

| H | Black | 6.3V 2 |

Correct. I assigned |B| for Low bias line.

[Voltron]

I never did the parts kits. Never will either, not enough time.

All current chassis are spoken for. There is actually one extra chassis, but no board set to go with it.

I just don't see me doing another chassis run.

Some other fool would have to take that on.

I could sell somebody my spare board set for whatever I paid Kevin so as to not let the chassis go unused.

[mrarroyo]

That sure scares me too. I have a batch that i know really came from hitachi with original hitachi packaging.

Even when an input tube went nuts at CJ, all i did is pop a new tube in, all the bias's were fine, and kept going.

I'm using NOS sylvania i think... But 50 year old tubes are a pain in the neck. I've never had a problem with

the el-cheapo chinese output tubes either. Don't sound great, but don't die either.

Hi Kevin, instead of an NOS 6922 have you tried the Russian 6H23EB?

[kevin gilmore]

Hi Kevin, instead of an NOS 6922 have you tried the Russian 6H23EB?

See this is exactly the problem Mikhail created. Lets just throw any tube in there and watch what happens.

I like mine the way it is. Tube rolling on this thing is probably a real bad idea.

[Craig Sawyers]

See this is exactly the problem Mikhail created. Lets just throw any tube in there and watch what happens.

I like mine the way it is. Tube rolling on this thing is probably a real bad idea.

I'd agree. Given Inu's comments on the fact that using old stock Amperex 6DJ8's killing the 2SK216's no matter what he did, I think that that one messes with this design with real caution.

[spritzer]

Yeah, I wouldn't even try tuberolling this amp. For instance the ES-1/2 should only be used with ECC81's but Mikhail probably thought they looked odd driving the EL34's so used octals instead...

[mrarroyo]

See this is exactly the problem Mikhail created. Lets just throw any tube in there and watch what happens.

I like mine the way it is. Tube rolling on this thing is probably a real bad idea.

Well, I mentioned the Russian 6H23EB because I have obtained excellent results in other gear in place of the 6922/ECC88. Of course it may not be applicable in this design thus so be it.

[luvdunhill]

See this is exactly the problem Mikhail created. Lets just throw any tube in there and watch what happens.

I like mine the way it is. Tube rolling on this thing is probably a real bad idea.

as much as I'd like to blame Mikhail, if the original used 6DJ8, anything labeled 6DJ8 should work. The issue is that *some* 6DJ8's don't work, that's a totally different issue than trying to cram a 2c51 into a 12au7 socket.

[spritzer]

To be clear, the stock amp didn't use ECC88's but rather E188CC

[luvdunhill]

To be clear, the stock amp didn't use ECC88's but rather E188CC

point taken, but still, an amp that can take a 7308 but not a 6dj8? where do you draw the line, e188cc but not a 7308? something isn't right, IMHO.

That being said, Inu are you using the same tube / manufacturer as Kevin? If so, that should be stated somewhere probably.

oh, and of course I'll still try another tube, of course. heh. what MOSFETs typically blow?

[kevin gilmore]

I've seen pictures of the original with 7308's and 6dj8's. I'm using 6dj8's.

My experience with the russian tubes that say they are the same as something else

usually is that they are not even close to the curves that are published for their tube

let alone compared to the original. For ac coupled standard tube circuits, you would

never tell the difference. For a fully dc coupled circuit with the top tube as a cathode

i'm willing to bet that the russian tubes would pull positive grid current, and all sorts

of bad would happen.

[luvdunhill]

Inu:

what about swapping C12 and C13? You mentioned that one of your pots you cannot adjust at all, or you get the squeeling.. i was just thinking about that when I noticed this asymmetry in the schematic.

[Craig Sawyers]

Hokay - here's where I've got to.

No progress on zapped channel. Waiting semiconductors.

But some progress on the one with stuck battery voltage. Symptom was two LED's in one of the current source triples would blow. They would flash very brightly and then blow. One of the 3675's in that triple was open circuit too.

So I spent a cheerless hour or two pulling semiconductors. Everything checked out fine - all MOSFET's absolutely fine, all junction transistors likewise fine (apart from that one 3675).

Then I spotted it - it was either Q32 or Q33 - I had missed soldering a pin! Boy did I feel a dope - I must have been interrupted half way through soldering and skipped a joint.

Anyway, whereas previously both batteries were stuck, but at significantly different voltages and LED's died, they are still stuck, but within a volt of each other at 570V. LED's are all OK (checked with a 9V battery in series with 6k8 resistor) so LED death is cured. Current triples and bottom current source are not turned on, but the battery LED chains are lit. So no anode current in the EL34's.

About the only semiconductor I haven't checked are the JFETS in the batteries. If they were phut, that would explain lack of adjustablilty using RV2. Sounds like the next check. All other semis in the batteries are fine - checked in circuit just using diode test on the DVM.

[spritzer]

I've had something like this happen before so now I triple check every solder joint.

[Craig Sawyers]

Bored and twiddling my thumbs waiting for semiconductors to arrive to fix the zapped T2, I thought I'd try to get a grip on what the actual signal level circuit was doing.

I've replaced all the current sources and 740V battery with ideal symbols. I've also left out compensation components, and some of the details of one or two current sources which I think are error handling, to prevent the thing blowing up if a tube goes down badly. I think the result shows the rather neat symmetry of the design, and the clever way in which DC voltage shifts are generated using the FETs and batteries to move the ground referenced input signal to -500V.

File is here http://www.tech-enterprise.com/tekstuff/T2deconstruct.pdf

[kevin gilmore]

yep that is exactly how it works. Such a simple circuit

[spritzer]

...and we have people complaining about it being too complex...