[runeight]

I believe this one is right.

 

[FrankCooter]

I'll sign up for a set of the "sand" boards. Normally, I'm a member of the "use a transistor go to jail" school of audio, but I'm looking forward to making an exception here! Really impressed with your work and commitment, Alex!

 

[digger945]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif Alright, back from Xmas shopping, how about this then? Gate is now positive to source by a variable - and measurable - amount.

Whatever your Idss measurement is(from your first test), then that is going to be about the lowest current measurement your going to get(with the gate and source shorted together). For example, if you have one with a Idss of say 11mA, then as you lower the Vgs from a point where both gate and source are the same to something where the gate is seeing less potential than the source, then the current is only going to increase.
For the 271 jfet, the max current will be at 0v Vgs, and the lowest current that will be flowing from drain to source at about 3v Vgs or higher(The 3v number is from the datasheet, it will probably vary from device to device).

 

[Beefy]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif Whatever your Idss measurement is(from your first test), then that is going to be about the lowest current measurement your going to get(with the gate and source shorted together). For example, if you have one with a Idss of say 11mA, then as you lower the Vgs from a point where both gate and source are the same to something where the gate is seeing less potential than the source, then the current is only going to increase.

Isn't that highest? And current is only going to decrease?

My extensive reading of, ahem, Wikipedia suggests that for a p-channel transistor, as the gate becomes more positive, current progressively decreases.

*

In any case, my little rig is having a problem. I can derive 12V just fine from the two 9V batteries, but the Idss measurement doesn't work when I do so. Nothing measures higher than about 5mA.

I can get measurements just fine with 9V from one battery connected directly to the JFET, and fine with 18V from two batteries, but not with the trimmer in there. I think that the 10k value is restricting the current way too much...... to get the 12V out of the 'middle' of the trimmer, the 'top' resistance is ~3500 ohms. 18V at 3500ohms is - drum roll please - approx 5mA maximum current through the whole circuit.

I may have to have stern words with somebody over this......

 

[runeight]

You got a 12V zener?

 

[digger945]

The 271 is not a transistor, it is a depletion mode fet.
For the P-channel, as the gate voltage increases with respect to the source, the flow of current decreases.
Perhaps this will help.

This is for an N-channel device, so to test the P-channel j271 hook up the positive on one 9v to "-10" on the schematic, and ground to the point where the two 9v's connect, and the negative 9v to where it has "+10" on the schematic. Take the Vgs as noted on the diagram, take the current across the 10k or inline, whichever you prefer. I just built this and used 4x1M for the 5M on the diagram and it works fine. Matter of fact, you can play around with the numbers and do some calculations based on the exStata schematic and probably get this to look pretty close to actual "in amp" conditions.
I haven't done the math.

 

[digger945]

Sorry, I posted at the same time Alex did.
Carry on.

 

[sumo-kun]

Just a quick question about power transformers.

What is the recommended secondary voltage spec for the hybrid? On site, it mentions 240VCT on the scematic but the BOM says to get a 250VCT. I remember a load of discussion a while back in the thread about the Hammond 270xxx and 269xxx and which one works/doesn't work but I can't remember. I seem to recall the 270 was supposedly too low in the current rating but it worked fine in practice and the 269 produced overheat problems, which resulted in the need for a 300V zener string instead of the 291?

I will probably be getting a transformer locally because the voltage over here is 100V/50Hz. I found a candidate but I just want to make sure its OK. Its a 240VCT 100mA. Over specced in the current department but is the 240V OK? It also has 3 6.3V/2A taps. Is the 2A rating enough to run both heaters off of it?

 

[Beefy]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif You got a 12V zener?

Unfortunately not, and I just don't trust the 12V supply I have here. I do have 1k and 2k resistors though, to create a fixed voltage divider that can supply enough current at ~12V.

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif The 271 is not a transistor, it is a depletion mode fet.

Brainfart

Quote:

For the P-channel, as the gate voltage increases with respect to the source, the flow of current decreases.

I'm pretty sure that's what I said!

Quote:

Perhaps this will help.

Ah, where were you six hours ago! In a round-about way, that seems to be exactly what I have done.

Quote:

I haven't done the math.

Pretty sure Alex said earlier that 1mA current was the appropriate operating point. That is what I was aiming for anyway......

 

[digger945]

Well to be honest, I was thinking of another project(jfet amp

) so I am thinking along the lines of the current with a 10-ohm resistor in series and 12v. 1mA may very well be on target for your application.
Sure enough, with a 271 in the circuit I posted and 387mV Vgs, I get 1.17mA.
I'm using the Placid with +-12v.

Touche'. Flatulence flows from this direction as well.

 

[BoilermakerFan]

Quote:

Originally Posted by sumo-kun /img/forum/go_quote.gif Just a quick question about power transformers. What is the recommended secondary voltage spec for the hybrid? On site, it mentions 240VCT on the scematic but the BOM says to get a 250VCT. I remember a load of discussion a while back in the thread about the Hammond 270xxx and 269xxx and which one works/doesn't work but I can't remember. I seem to recall the 270 was supposedly too low in the current rating but it worked fine in practice and the 269 produced overheat problems, which resulted in the need for a 300V zener string instead of the 291? I will probably be getting a transformer locally because the voltage over here is 100V/50Hz. I found a candidate but I just want to make sure its OK. Its a 240VCT 100mA. Over specced in the current department but is the 240V OK? It also has 3 6.3V/2A taps. Is the 2A rating enough to run both heaters off of it?

You want a 480VCT for 240V from each leg to the center tap. Parallel two 6.3V heater windings and your fine.

 

[Beefy]

Well, I did all of my JFET testing for Idss and Vgs@1mA using the final schematic I posted above...... the little rig worked nicely. My procedure was to put the JFET into a screw terminal, connect the batteries, measure Idss after 30 seconds at Vgs=0, then increase Vgs until Id dropped to 1mA. Repeat measures on an individual JFET were within 1%.

Of the 25 I bought from Mouser, 16 were Idss <10mA....... not great. Four were between 10 and 12, but not making the 10% criteria. The five remainders were:
13.65mA, 2.26V
13.9mA, 2.26V
14.15mA, 2.42V
14.4mA, 2.55V
14.4, 2.58V
So pretty good for Idss, but Vgs? I really don't know.

Thankfully, a magical fairy dropped some of their spare J271 in a package for me, and I got an absolutely perfect quad from three of those and one of mine - both Idss and Vgs are within 2%.

 

[ChrisX]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif Well, as it turns out, splitting the panel will only add cost. At this point I will just get both amps and assume that they all will go out.

Although I already have some tubes I would be in with a solid state amp in this case.

Alex please add me to the list. I will be on a vacation during X-Mas, so I hope it's not too late for payment next week.

Chris

 

[runeight]

Latest list.

 

[Beefy]

In a stroke of luck, my custom SumR transformer arrived today - I walked into my building just as the postwoman was walking out. It is sitting on a CD for size reference. A touch scuffed on the encapsulation, but who the hell cares?

I now have absolutely everything I need to have my amp fully built and cased, except for Stax Jacks (in the mail) and FPE panels (99% designed, pending jacks), so assuming no failures or part oversights, its going to be a productive holiday season!