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quote:

[By runeight] This says that the tube is drawing 161mA. This is not impossible, but it is likely that the tube will draw even more current at 12.6V.

I think that you can wire everything up and put the tube in.

But first, dial your trimpots so that they are in the maximum resistance position. This will put the maximum bias on the tube.

If nothing has fried then measure the voltage at one of the plates and adjust the trimpot until the voltage comes down to 40V. Do this with the other plate.

Then measure the voltage across the 1k2 resistors in each CCS. I would like to know what these are, but they should be around 1.2V.

After all of this adjustment, measure the B+ again. It should still be between 55-60V.

At some point you might need to replace the tube to get good results.

Have I mentioned that your board looks terrific.

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Thanks for the comment

Ok, I just wired up a filament of a 12ax7 to test measure the voltage and i get 13.6!!!!

I'll wire everything up now with the 12au7 and see what happens.

[edit] just tried another 12au7 I had lieing around, it's some chinese one , and i get a measurement of 11.8

[Edited by pho_boi on 11-19-2005 at 05:38 PM.]

[Edited by Blooze on 11-19-2005 at 06:10 PM.]

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[By Blooze] Is this just a case of when measuring voltage across the tube heater the internal resistence of each tube is a little bit different? As long as you're getting 12.6V going to the tube heater it should be fine right?

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quote:

[By blooze] Is this just a case of when measuring voltage across the tube heater the internal resistence of each tube is a little bit different? As long as you're getting 12.6V going to the tube heater it should be fine right?

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Yes. Also, the range of heater voltages for a 12AU7A should be about +/- 1V (maybe a little more). So, anywhere from 11.6 to 13.6 is within range. The 10.8 is a little low.

[Edited by runeight on 11-19-2005 at 06:20 PM.]

I cant get the voltage down to +40v, it sits around +50v no matter how much I adjust the trimpots. I then put some music through and it was very loud and distorted, I turned the volume knob all the way down and nothing changed. I seem to be running into heaps of problems aren't I.

Also, am I meant to ground pin 9 of the tube? Because I have and not sure if it should be.

[Edited by pho_boi on 11-19-2005 at 07:23 PM.]

What is the B+?

What are the voltages across the 1k2 CCS resistors?

Are the opamp voltages stil at +/- 12V.

Also, unground pin 9. It should not be grounded.

Thanks

[Edited by runeight on 11-19-2005 at 07:41 PM.]

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[By runeight] I hate to ask, but if you could make a few measurements.

What is the B+?

What are the voltages across the 1k2 CCS resistors?

Are the opamp voltages stil at +/- 12V.

Thanks

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B+ is 56.2v

Voltage drop across CCS resistor is 1.5v and 1.7v

And opamp is still +/- 12v

I didn't have anything plugged into input this time, and the voltages at pin1 ranges from 37 to 39 and pin6 ranges 44.1 to 46, depending what position the trimpot is at.

Can you dial up both plate voltages to 44V? This is a good place to start and if everything else is working properly, the amp should work correctly.

If you do this, and dial down the volume do you get decent sound?

And even when no music is playing and volume is all the way down, I get this really loud hum/noise coming out of the headphones. And adjusting the trimpots only quitens it down a bit but its still very loud.

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quote:

[By pho_boi] Pin1 can't reach 44v.

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Right. I knew that.

I have a question about the bipolar supply. Are the 470u caps before the regs and the 47u after? It's hard to tell from the photo.

The very narrow range on the trimpots is puzzling to me. You should be able to swing the voltage much more than 2V. Are your cathode bypass caps across both the 1k5 and the 2k pot? Are the cathode resistors actually 1k5 and 2k trimpot and are they in series?

Do you have a scope?

If not, ground the inputs to the opamp and see if the hum goes away?

Also, is everything grounded properly?

Sorry for the trouble. But, things in the amp don't seem far from normal and we know that the amp does work under these conditions.

[Edited by runeight on 11-19-2005 at 08:54 PM.]

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[By runeight] Right. I knew that. :-)

I have a question about the bipolar supply. Are the 470u caps before the regs and the 47u after? It's hard to tell from the photo.

Do you have a scope?

If not, ground the inputs to the opamp and see if the hum goes away?

Also, is everything grounded properly?

Sorry for the trouble. But, things in the amp don't seem far from normal and we know that the amp does work under these conditions.

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Yup the caps are where there meant to be.

I rechecked grounding and I missed one

Well music is playing fine now, But I can't bias pin6 to under 50v. I've also noticed that when I put the aligator clip onto pin6 to measure the voltage it sounds better

[edit] The amp is also very microphone? You can hear it through the headphones when you move the wires or tap the board

[edit2] I've also noticed that it distorts real easily. Like I only turn it maybe 30degress? and its really loud and distorted. I am using 100k Linear pot and 300ohm resistor like the schems.

[Edited by pho_boi on 11-19-2005 at 09:29 PM.]

If you wouldn't mind a little change, the CCS resistors appear to be too small. Can you easily replace the 1k2 resistor in the CCS with a 1k5 resistor? If so, this should help.

Also, you should get much more variation in plate voltage when you adjust the trimpot. You are certain of the wiring for the cathode resistor/trimpot and bypass capacitor?

You are certain that the jfet SGD are connected properly? Batman and I are also wondering if, somehow, the jfets on the 50V plate are toasted.

Hate to ask, but it's my job.

The high plate voltage (50V) is hampering the CCS. We have to get it lower.

It is also possible that this is a bad tube.

The amp is very sensitve. I am not surprised, for the moment, at how your volume control is behaving.

[Edited by runeight on 11-19-2005 at 09:53 PM.]

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[By runeight] pho_boi, we can work on the plate voltage now.

If you wouldn't mind a little change, the CCS resistors appear to be too small. Can you easily replace the 1k2 resistor in the CCS with a 1k5 resistor? If so, this should help.

Also, you should get much more variation in plate voltage when you adjust the trimpot. You are certain of the wiring for the cathode resistor/trimpot and bypass capacitor?

You are certain that the jfet SGD are connected properly? Batman and I are also wondering if, somehow, the jfets on the 50V plate are toasted.

Hate to ask, but it's my job.

The high plate voltage (50V) is hampering the CCS. We have to get it lower.

It is also possible that this is a bad tube.

The amp is very sensitve. I am not surprised, for the moment, at how your volume control is behaving.

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I replaced it with a 1k5 and same results.

Does this mean I have to put in new jfets?

And I'm kinda not sure about my trimpot wiring now. My trimpot has 3 pins, is it correct that I've only used 2 of th 3pins?

http://users.tpg...nlo/layout1.jpg

The trimpot is the square box with an "X" inside the circle, you can see where I've only used 2 of the 3 pins.

When you rotate the screw, does the resistance of the trimpots change between the two pins that are wired? If not, you'll need to determine which terminals are the ends and which is the wiper. You can use just two of the terminals if one of them is the wiper.

Could you kindly measure the voltage across the 1k5 CCS resistors now.

Don't replace the jfets yet until we make sure that the trimpots are connect properly.

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[By runeight] The trimpots might be the problem, depending on which connections you used.

When you rotate the screw, does the resistance of the trimpots change between the two pins that are wired? If not, you'll need to determine which terminals are the ends and which is the wiper. You can use just two of the terminals if one of them is the wiper.

Could you kindly measure the voltage across the 1k5 CCS resistors now.

Don't replace the jfets yet until we make sure that the trimpots are connect properly.

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The voltages across CCS resistors are 1.2v and 1.3v

Also, as you change the trimpots the voltages across the CCS resistors should stay constant. If the do, the jfets are working.

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[By runeight] Well, the CCS voltages are now pretty good. If the CCSs are working properly, you should be able to adjust the trimpots and get more than 2V change at the plate.

Also, as you change the trimpots the voltages across the CCS resistors should stay constant. If the do, the jfets are working.

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Voltage across CCS resistor decreases as I adjust the trimpot for more voltage at the plates!!

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

[Edited by runeight on 11-20-2005 at 12:43 AM.]

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[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.]