• 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.

Kofi Annan in: "Kofi's Baby Huey"

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Kofi Annan

Kofi Annan

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Joined 2004
homepage.mac.com/jasonandpatti/PhotoAlbum1.html
By the way, in gingertube's description of the resistors he wrote the following:

2) If you don't have a separate 5V winding to generate the -12 to -13V supply for the diffamp CCS then use what ever low voltage winding you have. The voltage isn't critical - anywhere between about -7 and -30 would be OK. You may have to adjust the 1K6 and 1K values in the current source to keep LED current at around 2 to 5 mA.

Just offering this as evidence that the intent seems to be a 1K and a 1K6...

Kofi
 
Kofi Annan

Kofi Annan

Member
Joined 2004
homepage.mac.com/jasonandpatti/PhotoAlbum1.html
I'm getting about -13V at the - terminal of the cap feeding the CCS.

Also, I checked the voltage drop across the 470R in the CCS and I came up with -3.13VDC, which I believe is .006A-- WAY too little... This should mean something is wrong with the CCS, correct? Remember that the B+ is around 390VDC, which is WAY too high...

Any correlation possible?

Thanks for all the help. I know I'm close here.

Kofi
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
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Everything looks in order if you have wired correctly and you have no dead BJTs. A 1.7V Vf Red LED will use 3.5mA and the CCS will be biased and will pull 1mA. I loaded a single 12AT7 tube to check it as a whole. That would pull 4.55mA via the 470R but if your LED's Vf is a bit stronger circa 2V it can easily set near 1.4mA CCS. Now tell me that your LED is IR and you can't see it alight.:D Nah, just joking.;)
 

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Salas

Salas

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Something can be wrong either in connections or parts. See that the 1k6 and 1k resistors show drop across each that can be showing towards 3-5mA through them for instance...find circa 1V-1.4V across the 1K opposite to the LED and so on. If you can't find those clues, inspect connections against schematic thoroughly, if nothing's still in error change LED and BJTs. A dried out joint, a wrong value for a resistor even...
 
Kofi Annan

Kofi Annan

Member
Joined 2004
homepage.mac.com/jasonandpatti/PhotoAlbum1.html
OK-- so here are the voltages I'm seeing across the resistors / LED.

1K6: 2.9V
1K from supply: 2.5V
LED: 7.3V
1K between BJTs and LED: 6.6V (Right); .11V (Left)

Looks like a problem across the LEDs and an inconsistency with the 1K resistor above.

I'm still poking around, but is anyone enlightened by this?

Kofi
 
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Salas

Salas

diyAudio Chief Moderator
Joined 2002
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It must be forcing too much Ip. So the 1K to the right (from bottom BJT to -V) has 6.6V across? That is 6.6mA when it should do 1mA. Pulls down hard on the 220K plate resistors if so. See if the BC547s show Vbe 0.6-0.7V. Maybe they got destroyed when you had the PSU problems and need changing.
 
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