Biasing a Herbie

[tlingen]

I need help biasing my new (used) Herbie. It has the external bias pots but do I just turn the gain and volume down then bias it? I read in posts about pulling the fuses when having to do internal bias'ing. With the gain and volume all the way down and the fuses in, I'm getting 15mv 15mv and 16mv.

 

[MARK2C]

Your multi meter needs to have the red lead in mA or it will not give you a correct reading.

You can pull all three fuses and bias them all at once.Don't forget that Herbert needs to have each pair balanced. When you bias one pair of tubes
the bias set on the other may go up or down. You have to slowly balance them all out to your target MPD. Adjust, check, adjust, check, adjust and keep checking all three duets
until they are at your calculated MPD or 60-70mA per pair.

 

[tlingen]

These are all older tubes, 20-30 hours on them I'm told. When I switch to ma, I then get readings like 7.85, 6.71 and 8.02
The range wont change but I guess these are then 78, 67 and 80. Assuming that's right, I tried to adjust to balance them all out. But couldn't feel any pots inside with a couple different screwdrivers. I stopped when I was alarmed at how far the drivers went inside. Maybe it would be easier with the head out of the head shell?

 

[slyvren]

I have an Einey, but the screwdriver goes a good ways down. Use a skinny long flat tipped guy. Push it down till it hits bottom. You might have to apply a little force to get it past the rubber grommets. Be gentle though. Then turn the screwdriver while applying a little down force and you'll fill it catch. Keep an eye on the volt meter as you turn it. The pot is pretty sensitive. You're right about the readings on mA your 8.07 is 80 mA.

I use one of these screwdrivers (flat tipped side).

Good luck! I hope that helps. Let us know if you get it sorted.

 

[tlingen]

Thanks for your help

I'll give it another go tonight!