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quote:

[By runeight] How much does it change?

Please check the SGD connections for the jfets. They are correct in your drawing as it it labeled.

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Drops to around 0.75 to 0.8

[Edited by pho_boi on 11-20-2005 at 12:44 AM.]

I might have to get some sleep soon.

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quote:

[By runeight] That's not good. Perhaps you could measure the voltages across the S-D of each fet?

I might have to get some sleep soon.

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Sorrry

I get 0.04v, 0.1v, 0.8v and 0.14v

[edit]I'm in no way educated in transistors, but I was reading the datasheet for the J113 and it states that maximum Drain-gate voltage is +35v. Does this mean that the J113 will not work in this circuit because the B+ is 55-60v?

[Edited by pho_boi on 11-20-2005 at 05:47 AM.]

The max DS voltage for these is 35V. But, they only have to see the voltage difference between the B+ (60V) and the plate voltage (40V).

When the tube is cold it is an infinite resistance. And then as it warms up it starts to conduct. So the jfets should see no voltage when the amp is turned on and then this voltage should slowly go to 20V.

It's hard to know what happened to them, but they do appear to not be working at all. In the schematic, the top jfet should have some significant volts across it. The bottom jfet should have one volt.

I'm sorry to suggest this, but I think you got to replace them. For the purposes of testing you can put a zener in to keep the voltage across the jfets to below 35V as in this drawing.

The sourcesa and drains were not reversed accidentally when they were wired in?

Also, have you verified that your trimpots are wired properly?

[Edited by runeight on 11-20-2005 at 08:46 AM.]

Please confirm the cathode resistor/bypass cap is wired correctly so we know the bottom end of the valve is under control.

Also, please make it clear how you have wired the trimpots.
Agreed, you only need two out of three pins wired (although three is best).

You could have experienced "over voltage" but I don't think so.
The FETs shouldn't see more than 20V(ish) worst case.

But we'll get to the bottom of the problem

[Edited by mains_hum on 11-20-2005 at 11:34 AM.]

http://i31.photo...ho_boi/draw.gif

I've checked the pin layout of the trimpot and the resistance does change from 0 to 2.2 when I use those 2 pins.

The only thing I wired incorrectly was the -/+12 regulators. Could that have damaged them?

I have also check the CCS section and thats wired correctly also.

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quote:

[By pho_boi] I've checked the pin layout of the trimpot and the resistance does change from 0 to 2.2 when I use those 2 pins.

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Do you mean 2.2 or 2k? The trimpots should go from 0 to 2k ohms.

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quote:

[By runeight] [quote=pho_boi]I've checked the pin layout of the trimpot and the resistance does change from 0 to 2.2 when I use those 2 pins.

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Do you mean 2.2 or 2k? The trimpots should go from 0 to 2k ohms.
[/quote]

Oh yeh thats right 2k, silly typo.

If this is so, then I'm afraid you'll have to replace the jfets. If you must order more of them, grab a few 33V 1/2W zeners to protect them while we troubleshoot.

Or, if you want a solution that is more expensive but impossible to break, find some 1N5297 constant current diodes and use them as shown in the schematic. Make sure you install them the right direction. You can also use J505s. They just have a smaller forward breakdown voltage (50V instead of 100V)

Basic Stoopid Hybrid Amplifier 2

However, if you've got some jfets on hand then you could try replacing them. Turn the trimpots to maximum resistance and power up. You might have a voltmeter attached to the one of the plates. Just to see what happens when starting up.

The main reason why we're not suggesting the diodes for the primary build is that they are much more expensive than the jfets and we're trying to keep the cost as low as possible.

[Edited by runeight on 11-20-2005 at 04:32 PM.]

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quote:

[By runeight] Well then, I'm not sure why your jfets seem to be cooked. But, if you're measuring less than 1V from D-S on all four jfets, it would seem that they are. This means that the plate voltage on both plates is about the same as the B+. Correct?

If this is so, then I'm afraid you'll have to replace the jfets. If you must order more of them, grab a few 33V 1/2W zeners to protect them while we troubleshoot.

Or, if you want a solution that is more expensive but impossible to break, find some 1N5297 constant current diodes and use them as shown in the schematic. Make sure you install them the right direction. You can also use J505s. They just have a smaller forward breakdown voltage (50V instead of 100V)

Basic Stoopid Hybrid Amplifier 2

However, if you've got some jfets on hand then you could try replacing them. Turn the trimpots to maximum resistance and power up. You might have a voltmeter attached to the one of the plates. Just to see what happens when starting up.

The main reason why we're not suggesting the diodes for the primary build is that they are much more expensive than the jfets and we're trying to keep the cost as low as possible.

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I have another 4 spare J113, and I just replaced the old ones out and NOW the voltages are both 45v when i turn the trimpots all the way down.

It still distors very quickly, like when i turn up the volume.

BUT at medium volume this amp sounds VERY VERY nice

Also, the voltages between the D and S terminals are 1v, 1,v, 8v and 8v.

[Edited by pho_boi on 11-20-2005 at 05:26 PM.]

May I request a few more voltage readings and suggest one more change.

First, could measure the voltage across the 1k5 CCS resistor again please. This will help me in setting this value for others who may build the amp.

After I know these values I can suggest one more change.

What headphones are you using and what is the nominal impedance?

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quote:

[By runeight] Great. Now that makes more sense. Very glad to hear this.

May I request a few more voltage readings and suggest one more change.

First, could measure the voltage across the 1k5 CCS resistor again please. This will help me in setting this value for others who may build the amp.

After I know these values I can suggest one more change.

What headphones are you using and what is the nominal impedance?

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They are 1.3v and 1.1v.

My headphones are Seinheiser HD201 with 24ohm impedence

Now, you can try replacing the 1k5 cathode resistors with 1k. Turn the trimpots to maximum value again and then fire it up. With this change you should be able to get 40V on the plates. Anything close to that like 41 or 42 is good enough.

Is the B+ still 55-60V?

The amp will be very sensitive when it has 24R headphones. With 24R you don't really need any voltage gain in the amp, just current gain. This amp has a voltage gain of about 11 or 12. Thus, you won't be able to turn the volume up much. The opamp limits at 40mA, but you should be able to get clean audio up until the opamp limit. I assume that you haven't wired in any of the bass boost stuff so that the tube is driving the 1M resistor. Yes?

One of the tricks to use, as you know, would be to put a resistor in series with the output. 120R is often used for this.

[Edited by runeight on 11-20-2005 at 06:06 PM.]

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quote:

[By runeight] Those numbers are not bad. The real resistance that we want is somewhere between 1k2 and 1k5. What you want is 1mA through the CCS. This means that the voltage should be the resistance divided by 1000. Yours are both a little low, but not bad at all and if the amp sounds good, well . . .

Now, you can try replacing the 1k5 cathode resistors with 1k. Turn the trimpots to maximum value again and then fire it up. With this change you should be able to get 40V on the plates. Anything close to that like 41 or 42 is good enough.

Is the B+ still 55-60V?

The amp will be very sensitive when it has 24R headphones. With 24R you don't really need any voltage gain in the amp, just current gain. This amp has a voltage gain of about 11 or 12. Thus, you won't be able to turn the volume up much. The opamp limits at 40mA, but you should be able to get clean audio up until the opamp limit. I assume that you haven't wired in any of the bass boost stuff so that the tube is driving the 1M resistor. Yes?

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B+ is still 56.6v and I'm using bass boost and it is only driving the 1M resistor.

I'll go and replace the 1k5 with a 1k now.

So, you have the bass boost circuit wired but not switched in?

[Edited by runeight on 11-20-2005 at 06:23 PM.]

PLates are at 40v now. voltage drop across D and S are 1.4, 15.4, 1.2 and 15.4. B+ is 56.

Oh no, the bass boost isn't even wired up at all.

[Edited by pho_boi on 11-20-2005 at 06:31 PM.]

Now you can listen for a while and let it break in. You can experiment with resistors in series with the output. And you can, after a while, start thinking about the version with the diamond buffer output so you can get more power.

Let us know how it sounds after some listening time. Mains and Batman are very impressed with their Stoopids. We'll all be building new ones soon.

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quote:

[By runeight] I am truly sorry for the work it took to get here, but this is exactly where the amp should be.

Now you can listen for a while and let it break in. You can experiment with resistors in series with the output. And you can, after a while, start thinking about the version with the diamond buffer output so you can get more power.

Let us know how it sounds after some listening time. Mains and Batman are very impressed with their Stoopids. We'll all be building new ones soon.

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No need for sorries, I found this whole build as a learning experience .

By putting resistors in series with the output, will it let me use the full sweep of the volume pot without running into distortion? The resistors would decreases there sensitivity?

[edit] antoher question, does the 300ohm resistor go before or after the volume pot?

[Edited by pho_boi on 11-20-2005 at 06:48 PM.]

This partly depends on your source. What are you using to drive the amp and what is its output voltage?

The 300R goes between the wiper of the volume pot and the grid of the tube. It's called a grid stopper resistor and is there to prevent parasitic oscillation from the tube. It's probably not necessary in this amp, but most of the time we put them there anyway.

[Edited by runeight on 11-20-2005 at 06:50 PM.]

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quote:

[By runeight] It depends on the size of the resistor. If you use 120R, you'll be able to use much more of your volume control. But, probably not quite all of it.

This partly depends on your source. What are you using to drive the amp and what is its output voltage?

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My source is a DIY USB DAC:

http://www.mell...ler/usbdac.html

http://www6.hea...ead.php?t=51870

http://www.ecp.cc/gud.html

I'm really not sure what it's output voltage is though.

[Edited by pho_boi on 11-20-2005 at 06:52 PM.]

The SOHA needs only about 0.05Vp to reach full output into 24R. This why you don't have much volume control.

120R on the output will make the SOHA require about 0.3V to reach full output. This will be more of your volume control. I think you might have to try this to tell us how much it helps. Given that the volume control is logarithmic, it might help more than we think.

[Edited by runeight on 11-21-2005 at 09:04 AM.]