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Old 3rd May 2012, 04:28 PM   #1
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Default Troubleshooting help needed

I am building a slightly modified version of this:

Click the image to open in full size.

Everything is pretty much the same, except:

- my supply voltage is 80V (instead of 60V)
- my plate resistor is 9k ohms (instead of 18k ohms)
- my cathode resistor is 100 ohms (instead of 200 ohms).

I am supposed to get at least 19V voltage drop between the power supply and the plate.

On one channel, I got about 27V of voltage drop, which is fine.

On another channel, I got about 1V. And of course that channel isn't working.

I swapped the tubes around, same thing.

What is wrong here? Is any transistor toast? Or is there a bad joint somewhere? (although there isn't as far as I could tell)

Thanks.

Last edited by Navyblue; 3rd May 2012 at 04:34 PM.
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Old 3rd May 2012, 04:48 PM   #2
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Is the voltage from Kathode to Grid equal in both channels?
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Old 3rd May 2012, 04:51 PM   #3
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Quote:
Originally Posted by Navyblue View Post
I am building a slightly modified version of this:

Click the image to open in full size.

Everything is pretty much the same, except:

- my supply voltage is 80V (instead of 60V)
- my plate resistor is 9k ohms (instead of 18k ohms)
- my cathode resistor is 100 ohms (instead of 200 ohms).

I am supposed to get at least 19V voltage drop between the power supply and the plate.

On one channel, I got about 27V of voltage drop, which is fine.

On another channel, I got about 1V. And of course that channel isn't working.

I swapped the tubes around, same thing.

What is wrong here? Is any transistor toast? Or is there a bad joint somewhere? (although there isn't as far as I could tell)

Thanks.
It's all fine.

Although it doesn't seem, the circuit is symmetric and on the upper right transistor collector there must be the same voltage the is on the upper left transistor collector ( ~ 1V between the power supply and the collector).

Please do the right math and you will find that the circuit "must" work as measured.

Last edited by diy_audio_fo; 3rd May 2012 at 05:05 PM.
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Old 3rd May 2012, 04:56 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Assuming the heater is on, on both triodes..

What voltages do you have on the plate a/b on each channel so we compare and what cathode voltage do you have measured to Gnd?

Regards
M. Gregg
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Old 3rd May 2012, 05:03 PM   #5
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What does the 18K in first anode do? Appears doing nothing.
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Old 3rd May 2012, 05:07 PM   #6
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Is the voltage from Kathode to Grid equal in both channels?
Working channel
grid: 0V
cathode: 1V

Non working channel
grid: 0V
cathode: 1.5V

Yeah, something is wrong.
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Old 3rd May 2012, 05:10 PM   #7
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Quote:
Originally Posted by diy_audio_fo View Post
It's all fine.

Although it doesn't seem, the circuit is symmetric and on the upper right transistor collector there must be the same voltage the is on the upper left transistor collector ( ~ 1V between the power supply and the collector).

Please do the right math and you will find that the circuit "must" work as measured.
I did not come out with that circuit, I am merely following instruction here:

The SOHA II Headphone Amplifier

The modification is merely to move the tube operating point and apparently there are a few people who successfully performed the modification.
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Old 3rd May 2012, 05:13 PM   #8
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OK. Then, are tubes equal? May be a fake triode and or dissimilar used so not equal current in them, if used before. I have lots of 12AU7 rescued from TVīs, and are differences between plate currents.
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Old 3rd May 2012, 05:15 PM   #9
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Quote:
Originally Posted by M Gregg View Post
Assuming the heater is on, on both triodes..

What voltages do you have on the plate a/b on each channel so we compare and what cathode voltage do you have measured to Gnd?

Regards
M. Gregg
Heater is apparently working, both tube are glowing and hot.

Working channel
plate 1: 52V
plate 2: 53V

Non working channel
plate 1: 78V
plate 2: 57V

The current mirror on top isn't working? And the left BC560C is toast?
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Old 3rd May 2012, 05:18 PM   #10
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Quote:
Originally Posted by Osvaldo de Banfield View Post
OK. Then, are tubes equal? May be a fake triode and or dissimilar used so not equal current in them, if used before. I have lots of 12AU7 rescued from TVīs, and are differences between plate currents.
Please pardon me, as you can tell, I know quite little of these stuffs, and I don't understand what you mean. But I did swap the 2 tubes around and the same channel isn't working.
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