#26 pre amp

[stajo]

Ok good. Show some kind of drawing or description and I can help you find your test voltage.

 

[merlin el mago]

Stajo do you want that I do like the attached psud?

 

[stajo]

Start with that and measure voltage over 20k and 30k.

 

[merlin el mago]

OK

 

[merlin el mago]

Voltage across 20K 109V, across 30K 165V

N.B. measurements done only with one rail 20K & 30K connected.

 

[stajo]

ok, im in a meeting, brb in 30

 

[merlin el mago]

ok don't worry I will wait for you.

 

[merlin el mago]

Can't be used CCS+Gyrator with OPTs? If can't be used I have to buy a new box and use SSHV2.

 

[stajo]

Lets do the math first.

So what do we know now? You have 165 volts over a 30 k resistor that ties B+ and ground together. Thats the simulated audio circuit.

Ohms law says U=R*I or the other way I=U/R

165 Volts divided by 30 000 ohms is 5,5 mA

We can check this over the 20 k filter resistor also since all current passes that one also.

109 volts divided by 20 000 ohms is 5,45 mA.

If you should now plug in a tube, biased at 5,5 mA (bias is controlled by the resistor between cathode and ground in the relation to B+) you should now have 165 Volts B+.

Are you with me on that? Shall we try?

 

[merlin el mago]

OK I will try with one tube.

 

[merlin el mago]

87,5V

 

[stajo]

Ok, measure voltage over 10 ohm cathode resistor.

 

[merlin el mago]

10,95V

 

[stajo]

Hmm, peculiar. Voltage over 20 k filter resistor?

 

[merlin el mago]

170V

 

[stajo]

Ok, can you parallell the 20 k with something like a 100 k, or change it to something like a 15 k? And measure drop over "the new" 20 k and B+ on tube? Measure the new resistorvalue in ohms to plz.

 

[merlin el mago]

I have 15K, please wait I do the change.

 

[merlin el mago]

Across 15K 151,8V, plate 98V

 

[stajo]

Ok, tube starts to bias up. Im at work so I have no PSUD. Try same with 10 k

 

[merlin el mago]

ok