The ultimate DIY? A Stax SRM-T2!

[Craig Sawyers]

If you want the model, let me know.

Yes please.

I see you're using Linear's LTSpiceIV. I downloaded that wonderful freeby quite a while ago, but have not used it in anger yet. This sounds like the perfect excuse to do so.

Where did you get the models for the transistors?

[nattonrice]

Where did you get the models for the transistors?

I was just about to ask the same question but was worried I'd look rather silly.

[pedefede]

OK, leaving resistors aside ...

Are there any of the transistors, IC or diodes, that also can be upgraded, or exist in a better version, than on the BOM ?

[Craig Sawyers]

OK, leaving resistors aside ...

Are there any of the transistors, IC or diodes, that also can be upgraded, or exist in a better version, than on the BOM ?

Well, I guess you could go for high reliability versions, such as JANTS space grade. But at $50 for a 1N4148 diode you'd better buckle up for remortgaging the house. And they are ceramic bodied surface mount, so they wouldn't fit anyway.

Just spent well over a hundred grand (UK pounds) on the components for six circuit boards about A5 size, and only three of those have space grade components. The real killers were the ACTEL FPGA's, which were a cool $19k per chip (three off needed), and A/D convertors at $6k each (15 off needed).

So I'd leave well alone with the semiconductors. Lots of games to be played with passives to tune the sound quality.

[spritzer]

The only ones that spring to mind are the isolated versions of the regulators. I used them but only because they were the only version available. You could also swap out the 2SC3675 for one of the higher rated units but then it wouldn't really be authentic...

[Craig Sawyers]

The only ones that spring to mind are the isolated versions of the regulators. I used them but only because they were the only version available. You could also swap out the 2SC3675 for one of the higher rated units but then it wouldn't really be authentic...

I think I've been the only one with 2SC3675 troubles - and that is restricted to the one in the voltage source batteries. Formally that part is sailing very close to the wind - the spec sheet says that at 5mA collector current second breakdown happens for Vce greater than 600V. Depending on the LED in the current source Q30 the current is closer to 4mA or less. At 3mA it should just about do 750V.

Now spec sheets are always conservative, but the statistical spread of characteristics must have put my batch just about where the spec sheet says. They went short in both channels, which is classic second breakdown failure. This happened spontaneously once in my class A Krell - that generated a hell of a lot of smoke! Failure of the 3675 took out two LED's in the anode current sources, a pair of 100V zeners (they glowed briefly!), and all four 2SK216's. That it happened on both channels identically indicates that it is not a stuffing issue (I checked that first), dry joint, or probing mistake - which points the finger at the 2SC3675.

So once I get the hot stick out, I'm going non-authentic 2SC4686A (KG's suggestion). They should be good for 1200V at up to 8mA. We'll see if that cures the problem.

[luvdunhill]

Looking at Kerry's schematic... perhaps the given values for RV1 and RV2 (as well as adjusting the series resistances on either side of RV1) are suboptimal? Perhaps different values would make the adjustment a bit less tedious?

[Kerry]

...

Where did you get the models for the transistors?

The models I used where Duncan Amps for the tubes and the transistors were scavenged from the internet. I can't say that they are perfect (I'm guessing the hFEs are not corrrect), but they did simulate the volages right on with Kevin's schematic. I had to tweak an LED model to produce 1.872V which gave the correct currrents.

I've got a question on the load lines/DC operating points for the tubes on the input stage. I sat with the Mogan Jones book last night and was trying to calculate out the Vgk values for the tubes. They seem to be arranged as a Cascode with current sources/sinks. The spice model shows -2.2V for the upper tubes and just under -1V for the lower tubes.

As best as I can understand it, you subtract the anode voltage of the bottom tube (approximately 70.2V) from the HT (250V) giving about 180V. The operating point for these tubes is about 50V down from the HT so that would set it at 130V for purposes of determining the operating point. Because there is a current source above the tubes, I plotted 130V against 5mA on the Plate Characteristics graph and came up with about -3.5V for Vgk. This is pretty different from the spice model.

For the bottom tubes I took the anode voltage (approximately 70) and plotted it on the triode transfer characteristics graph at 5mA and came up with approximately -1.85. Again the model suggested just under 1V, which jives with Kevin's schematic, which it shows the grids at about 1V against a 0VDC input.

Either the models are incorrect (very possible) or I've done the math wrong (very possible). Any thoughts?

[kevin gilmore]

here are kerry's spice files for people to play with.

http://gilmore.chem.northwestern.edu/t2-ltspice.zip

and this is why i've been so busy lately

http://gilmore.chem.northwestern.edu/prospector1.jpg

http://gilmore.chem.northwestern.edu/prospector2.jpg

http://gilmore.chem.northwestern.edu/prospector3.jpg

The bad boy in the 3rd picture has the equivalent flux of 10% of the APS Argonne Labs Beam line.

i.e. about the same as 20 watt laser. Cuts thru flesh if you are stupid enough to get in the beam.

And while there is no smoke (at least there had better not be any smoke) it is in fact all

done with mirrors. Runs at 50kv and a little less than 1ma.

The one next week is a dual source. And a triumph monochrometer the end of this week.

Many thanks for funding from the State of Illinois.

Edited November 30, 2010 by kevin gilmore

[Craig Sawyers]

and this is why i've been so busy lately

The one next week is a dual source. And a triumph monochrometer the end of this week.

Nice! Here's one I prepared earlier (2002) http://www.tech-enterprise.com/tekstuff/DSCN0622.JPG

[wink]

]

and this is why i've been so busy lately

http://gilmore.chem.northwestern.edu/prospector1.jpg

http://gilmore.chem.northwestern.edu/prospector2.jpg

'>http://gilmore.chem.northwestern.edu/prospector3.jpg

Nice! Here's one I prepared earlier (2002) http://www.tech-enterprise.com/tekstuff/DSCN0622.JPG

The bad boy in the 3rd picture has the equivalent flux of 10% of the APS Argonne Labs Beam line.

i.e. about the same as 20 watt laser. Cuts thru flesh if you are stupid enough to get in the beam.

And while there is no smoke (at least there had better not be any smoke) it is in fact all

done with mirrors. Runs at 50kv and a little less than 1ma.

The one next week is a dual source. And a triumph monochrometer the end of this week.

mere bagatelles.

This is moar powahhh!!!!!

[ATTACH=CONFIG]3988[/ATTACH]

Many thanks for funding from the State of Illinois.

[Craig Sawyers]

]

This is moar powahhh!!!!!

Is that Dr Megavolt? Dr. MegaVolt: Man Meets Lightning

[wink]

That photo came from here:-

Tesla Systems Research - Tesla Coil & High-Voltage Gallery

My own little one runs off a neon transformer with a secondary of 7.5KV @ 30mA.

It goes into the Tesla primary of 5 turns, and the output is a secondary of 250 turns.

It produces about 200Kv - 250Kv and will push an arc through more than 20mm of glass.

No good for headphones - even electrostatics.........

[Craig Sawyers]

That photo came from here:-

Tesla Systems Research - Tesla Coil & High-Voltage Gallery

My own little one runs off a neon transformer with a secondary of 7.5KV @ 30mA.

It goes into the Tesla primary of 5 turns, and the output is a secondary of 250 turns.

It produces about 200Kv - 250Kv and will push an arc through more than 20mm of glass.

No good for headphones - even electrostatics.........

Neat! Megavolt has a physics doctorate from Berkeley, but found that this sort of scary activity was his true calling - watch the video here Dr. MegaVolt and his Amazing Tesla Coil . Check out him cooking a sausage in the discharge from his 8 1/2 foot high machine - it uses a 15kW generator to power it. He usually has someone standing by with a fire extinguisher because his faraday suit sometimes catches fire. The two person performance on the vid is him and his wife! Thank heavens for nutters like this.

[Craig Sawyers]

Well, that is interesting. Because I had a model already part developed for the battery, I thought I'd press on. The precise transistor type for Q16 to Q20 is not so important, but the current drawn into the base of Q23 is.

Now I do not have sensible models for the actual transistors, I simulated the base current into Q23 by using a much higher hFE part, and connecting a resistor from base to emitter to pull more current.

The behaviour of RV2 is to set the constant current through fet Q22, so I just used a constant current source there instead. Likewise I have used a 5mA constant current source from the bottom of the battery to -560V (arbitrary ground in the simulation) and a 10k resistor from the top of the battery to +810V (= 250 + 560) to simulate the load of Q5 (K216).

Decreasing the current through Q22 increases the base current into Q23 and reduces the battery voltage. It has the effect of turning Q16 off and shunting current into Q17 and hence into the base of Q23.

Likewise turning RV1 in the direction of R33 increases the current though Q17, and hence also reduces battery voltage. This seems to work fine, and gives a voltage of between 14 and 20V above -560V when the battery voltage is 740V. For any setting of RV1, the position of RV2 shifts to maintain the same operating point voltages. This works fine with RV1 throughout its travel provided that the current into the base of Q23 is not too high, ie 50 - 70uA.

But once the base current of Q23 becomes big enough, approximately more than 100uA, it becomes impossible to turn it on. That corresponds to the specified hFE of a 3675. Now Kevin measures 50uA base current, which implies that the real-world transistors used in this position are better than the data sheet implies by a factor of two.

The main point of all this calculation and modelling is because I *know* that my batch of 3675's are marginal for Q23. Swapping to the 4686 should be fine, but the hFE is lower than the 3675 by a datasheet factor of 2 - so I have to find more base current. The model suggests that decreasing R39 from 62k to 47k should have no other effect than increase the current drive to Q23.

So what is the purpose of RV1? It looks as if it does the same job as RV2. I think (?) that its purpose is to give more adjustability in the case of (a) high Idss of Q22, ( high Vgs(off) of Q22 and © low hfe of Q23.

Thoughts anyone? I'd like to examine all angles of this part substitution before firing the beast back up.

[spritzer]

Have you looked up the 2sc5563? I'm too lazy to look up the datasheets...

[Craig Sawyers]

Have you looked up the 2sc5563? I'm too lazy to look up the datasheets...

That sir looks like an interesting transistor. Do you have a source?

[spritzer]

It is a Toshiba part so Mouser should be able to source it for you.

[luvdunhill]

I believe that part is not recommended for new designs, so it might be hard to find.

[luvdunhill]

perhaps a high voltage darlington? They seem to exist, perhaps for SMPS design?

http://pdf.datasheet.su/stmicroelectronics/stp03d200.pdf

no idea if you can get them...

[Craig Sawyers]

perhaps a high voltage darlington? They seem to exist, perhaps for SMPS design?

That thought was going around in my mind, as a discrete option (but would be messy to implement). I had no idea that such high voltage single devices were available. The gotcha is that I couldn't find stock anywhere. Digikey list them at around $6 each, but have zero stock. Mouser also list them, zero stock with a 20 week lead time with MOQ of 605 pieces.

[wink]

TRANS DARL NPN 2KV TO-220 - STP03D200

[Kerry]

Some more thoughts on the battery operation.

I look at it as a high voltage op amp with inputs on the bases of Q16 / Q17 and the output feeding Q23. The resistor string R31 - R34 and RV1 forms a simple voltage divider so that on 740V RV1 will adjust the base of Q17 to the exact same voltage as the base of Q16. Q23 will then keep the output at 740V with the op amp controlling the voltage. The job of RV2 is to set a specific current against R42 which translates into a specific voltage feeding the base of Q17. So RV2 is there to adjust the current and ultimately the voltage so that it is in range of what RV1 can be adjusted to. In Kevin's KGSSHV power supply he replaced RV2, the FET, LEDs and R42 with a precise voltage reference of 10V, so the resistor string on the right is then recalculated to be able to adjust the output of RV1 to 10V against whatever voltage is required for the full resistor string. Pretty neat

So, it seems that the issue of insufficient current feeding Q23 can be solved by changing the value of R39 (increasing the current output of the op amp) and then using a higher voltage part with a lower hFE is also possible. Seems good, but I would defer to Kevin on how that could impact the operation of the amp. I wouldn't presume to know the reality associated with this change.

[Craig Sawyers]

TRANS DARL NPN 2KV TO-220 - STP03D200

Like I said - Digikey is showing zero stock.

[blubliss]

Two questions:

Would it be advisable to attach the heat sink brackets to the case before we solder the side fets to avoid any extra pressure on the fets or is this overkill?

Should we braid/twist in the umbilical?

Thanks.