BHK Preamp Bias Jumper

When changing the bias jumpers to 5 ma do you insert both pins into the jumper or only the 1 pin?


Hi twperil,

Here is the basic tube replacement tutorial:

You would need to remove power from the BHK Pre.

Then, remove the top by unscrewing the two threaded caps on the top between the vent screens.

Allow the tubes to cool completely.

Remove the tubes while wearing gloves to avoid fingerprints.

There is a three pin header located behind each tube.

There is a screen print on the PCB and a 2 pin shunt.

Connecting the center pin with the right pin allows the use of 12 volt tubes.

Connecting the center pin to the left pin allows the use of 6 – 7 volt tubes.

Move the 2 pin jumper to the appropriate location for the tube you intend to use.

When changing to an alternative 9 pin vacuum tube, it is essential that the heater current does not exceed the values shown below:

6-7V Tubes : < 300mA

12.6V Tubes : < 150mA

We also recommend a medium Mu value.

Typically data sheets will say low, medium and high.

Let me know if you have any issues replacing the tubes in the BHK Pre.

Best wishes,

  • Jeremy

For the bias jumpers, the default is 4 ma with the jumpers hanging on one pin so in effect not in use. As the manual specifies, for 5 ma the jumpers should cover pins JP7 and JP8 for each tube.

Thanks for the reply. I have been out of town.

I dont know if im being dense. JP7 and JP8 have two pins each. When I received my preamp each of the jumpers were on the pin closest to the tude, with the rear pin on each side uncovered. As I understand it with my new PCC887DJ8 tubes I need to change the bias to 5Ma. Does this mean the I move the jumper to cover the rear pin for JP7 or JP8 or does the jumper need to cover both pins for JP7 and JP8.

I had called the tech line and was told to cover the rear pin only but maybe he wasnt understanding what I was asking. If I understand SteveM I would insert both pins for JP7 into the jumper and the same for JP8.

Can someone please clarify.


For 5 ma the jumper should cover both pins so it connects those pins, which in this case changes the bias from 4 ma to 5 ma.