6sn7 grounded cathode amplifier problem

[tubenut33]

Hello i'm rebusistorilding a KT88 amplifier with a1 parts.But I have a problem .The voltage on the 33k ohms plate resistor is 380 v dc on the power side then on the tube [plate] side 20 dc volts !?And the 2w resistor gets hot quick.Tried different 6sn7's and they work fine on the Cornet phono pre.So ,looks to a short circuit of sorts...Had it working fine before I started rebuilding with a1 prts i.e. Takmam resistors ,Auricaps Why would a tube behave like this ? It is not a child , it does not have tantrums!Someone have a trail for me to follow? Al

 

[ruffrecords]

The probability is there is nothing wrong with the tube, especially as it works fine elsewhere. Most likely you have made a mistake. Triple check your wiring and quadruple check the component values - easy to get a factor of 10 wrong.

If you post a schematic with your description it is much easier for use to help.

Cheers

Ian

 

[abbey road d enfer]

What do you mean by "grounded cathode"? (in the title)
For correct bias of a tube, there are 3 possibilities:

• Fixed bias, which involes a source of negative voltage applied to the grid resistor
• cathode bias, which means a resistor between cathode and 0V
• grid leak bias, which takes a very large value for teh grid resistor, several megohm

 

[tubenut33]

Thank you for replying ,it is cathode bias The voltage on the cathode is 10 volts so current is flowing. But why 380 volts on p.s. side and 20 volts on the plate with 33k ohms. How can this happen? Where is that 360 volts go?

 

[abbey road d enfer]

What is the value of the cathode resistor? Apparently, you have 11 mA circulating through the tube, which has to go through the cathode resistor also.
You say there's 10V across it. That suggests a 90 ohm resistor. What tube is it?
Obviously there is something wrong in the calculation of the bias or a wrong resistor value.

 

[rock soderstrom]

I guess it is a 6sn7, see the thread title. Cathode resistor seems to be ~920R.

Is there a cathode capacitor? If so, is the polarity right? Negative feedback loop?

Can you sketch your circuit?

Can you measure cathode, plate and grid resistor?

 

[abbey road d enfer]

I think there is a wrong value for the cath resistor. 910 seems OK, 91 very wrong.