• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Troubleshooting help needed

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Navyblue

Member
Joined 2012
I am building a slightly modified version of this:

An externally hosted image should be here but it was not working when we last tested it.


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. :)
 
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D

diy_audio_fo

Member
Joined 2004
Navyblue said:
I am building a slightly modified version of this:

An externally hosted image should be here but it was not working when we last tested it.


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. :)
Click to expand...
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.
 
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N

Navyblue

Member
Joined 2012
diy_audio_fo said:
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.
Click to expand...

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.
 
N

Navyblue

Member
Joined 2012
M Gregg said:
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
Click to expand...

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?
 
N

Navyblue

Member
Joined 2012
Osvaldo de Banfield said:
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 TV´s, and are differences between plate currents.
Click to expand...

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.
 
Osvaldo de Banfield

Osvaldo de Banfield

Member
Joined 2011
Navyblue said:
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.
Click to expand...
What I´m trying to say is that although both triodes are in same envelope, but the may have great differences in plate currents in case that they had working in another device and having different roles in it. In example, if the tube was horizontal (line) oscillator in a TV set, one triode is more time conducting than the other, more time at cut off, so, they are used unevenly.
 
N

Navyblue

Member
Joined 2012
Osvaldo de Banfield said:
What I´m trying to say is that although both triodes are in same envelope, but the may have great differences in plate currents in case that they had working in another device and having different roles in it. In example, if the tube was horizontal (line) oscillator in a TV set, one triode is more time conducting than the other, more time at cut off, so, they are used unevenly.
Click to expand...

Thanks for the explanation. :)

I swapped the tubes on both channels, whatever imbalance is there, the tube still works for the other channel. I have another pair of 5963 that I plugged into this amp, same thing happens. So I guess we can safely say it is not the tube here.
 
M Gregg

M Gregg

Disabled Account
Joined 2010
I would change both BC560c...
Looking at your measurements..Then post what voltages you get..

You could measure the voltage drop across the 18K<<on diagram and work out the current through the first tube compare this with the transistor max current..
It will give you more information about whats going on compare to the good channel...

One other thing to check...the continuity between the grid and the Gnd on the input pot....I'm not a fan of just having the wiper of the pot to Gnd..I would put a 1M from wiper to Gnd.

Regards
M. Gregg
 
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Navyblue

Member
Joined 2012
M Gregg said:
You could measure the voltage drop across the 18K<<on diagram and work out the current through the first tube compare this with the transistor max current..
It will give you more information about whats going on compare to the good channel...
Click to expand...

This prompted me to poke around to find out the voltage drop of all the resistors in the schematic. I found that the 200 ohm resistor on the top right has no voltage. So I thought try my luck and reflow the solder joints on both the resistor and the transistor, it actually worked!

Thanks guys. :)
 
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Navyblue

Member
Joined 2012
Thanks guys. :)

I have been trying to solve this for days, but it was pretty much poking in the dark. Once you guys started to approach this methodically, it really pointed me in the right direction. I can read all I want but actual trouble shooting experience like this is priceless.
 
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