Biasing Herbert w/Bias Rite

duesentrieb

Active member
Biasing of a Herbert with a Bias Rite

Take Herbert out of its Headshell. Install two adaptors of bias rite into those sockets which you want to bias:

A = 3 + 4 (inner pair)
B = 2 + 5 (inbetweens)
C = 1 + 6 (outer pair)

Put the tubes into the sockets/adaptor. Switch Herbert on, wait 30 seconds, flip standby switch. Read the mA on the Bias Rite. You should adjust each tube to get a reading on your bias rite of 35mA/tube. This is easy, if you have external bias pots (depends on model). Switch amp off. Uninstall adaptors, repeat for the other two sockets.

If you have only internal trimmers, do this:

To adjust the trimmer, locate it by flipping the amp upside down (use a set of thick books, you need to be able to have the adaptors and the tubes installed while the amp is flipped).

If the amp is safely standing, switch it on (adaptors installed into the correct sockets, also tubes in the adaptor), wait 30 sec, hit standby and read your Bias Rite.

Locate the corresponding bias trimmer (A, B, or C) inside your Herbert (pots facing you, amp flipped, leftt side, close to the midi chips). Turn the trimmer gently (!!!) until you get the desired reading.

Switch amp off and repeat procedure for the other pairs.
----------------------------------------------------------

Peter: if you could take a pic of the current bias trimmers and their labelling, it would be very cool. As you know, I don't have one here right now, so I cannot provide any pics - or anybody else . . .

Thanks.
 

shadow070

Member
Olaf,

Thanks for this.

I have the external bias on my Herbert. Is there difference in using the external capability's compared to opening the amp up and bias it that way?
 

duesentrieb

Active member
With the integrated points on your Herbert you are reading directly currents and you need to take out the fuse for the pair you are reading. With a BR you are reading volts (naturally you are also reading current, but inside the adaptors are 1 Ohm resistors, which "convert" the mA into mVs).
Also with the BR you read plate and screen currents - measuring directly on the plates just gives you - logical - plate currents.
Screen currents are few mAs, so you may set the trimmer when reading off a BR a bit (say 1-2 mA) higher.
 

duesentrieb

Active member
You're welcome.

I wish I had one of them Diezels at home, then I could make you guys another video . . . .

At least I have "ordered" the very new model, which will be launched next week :D
 

axeman81

New member
on a side note: I have external bias points and trimpots and only use a DMM, but do I have to switch the amp completely off everytime I go from one pair of tubes to another pair, or is it sufficient to switch to standby? And then take out the fuse?

uptil now I have switched to standby then off, but it takes a long time to power down/up all those times....boring to wait for minutes each time :)

- Rune
 

King Crimson

Active member
axeman81":1aon42y4 said:
on a side note: I have external bias points and trimpots and only use a DMM, but do I have to switch the amp completely off everytime I go from one pair of tubes to another pair, or is it sufficient to switch to standby? And then take out the fuse?

uptil now I have switched to standby then off, but it takes a long time to power down/up all those times....boring to wait for minutes each time :)

- Rune
Maybe I'm doing it wrong. It's always worked for me (I have external trims) to just fire up the amp in standby mode, wait 40 seconds or so, turn off standby with master volume all the way down, put the negative probe into the single black hole, positive probe in first red hole (tubes 1&6), pull the corresponding fuse and bias away. Then just continue on for the next two. Seems to work thus far, Herbs still smiling…
 

Me

Member
Is there any benefit to biasing with a bias rite and reading plate + screen individually, rather than just plate current for the pair (as is done with a herbert with external bias points like mine)?

I've heard rumours you can do more too when biasing such as using an oscilloscope and checking the crossover afterwards. How do you do that and is it worth it?

Also do you know somewhere you can get a bias-rite type device to plug into your multimeter? I have 2 multimeters so don't need to buy a full device that actually reads the current especially for biasing. Just a socket/plug with 1 ohm resistor and a couple of leads. I'm guessing I don't ever need it for my Herbie, but I have an old Marshall here too without the luxury of my Herbie's external bias points so might buy one regardless (but this is why I asked the above question).
 

Me

Member
Whilst I'm on a roll with the biasing questions...

Can changing V6 affect the bias at all (or any of the other preamp tubes). Obviously you don't need to bias preamp tubes, but I was wondering if this could effect the voltage on the power tubes grid?
 

TooL46_2

New member
Me":2a9mctuo said:
Can changing V6 affect the bias at all
Any changes of preamp tubes should not affect the biasing of the power stage -- between PI and power tubes you have coupling capacitors that separate the high voltage from PI's tails and power tubes' grids. Unless the caps have leakage the bias voltage should remain the same.

Did you encounter any problems?
 
duesentrieb":2dgdlcti said:
With the integrated points on your Herbert you are reading directly currents and you need to take out the fuse for the pair you are reading. With a BR you are reading volts (naturally you are also reading current, but inside the adaptors are 1 Ohm resistors, which "convert" the mA into mVs).
Also with the BR you read plate and screen currents - measuring directly on the plates just gives you - logical - plate currents.
Screen currents are few mAs, so you may set the trimmer when reading off a BR a bit (say 1-2 mA) higher.


what about in terms of Bias regulation correctness? are the measurement on the external points without the BR attendible? i saw a video in youtube where a tech said it was an inaccurate way of biasing tubes.
 

rabies

New member
diezel&gas":td3jb6rk said:
duesentrieb":td3jb6rk said:
With the integrated points on your Herbert you are reading directly currents and you need to take out the fuse for the pair you are reading. With a BR you are reading volts (naturally you are also reading current, but inside the adaptors are 1 Ohm resistors, which "convert" the mA into mVs).
Also with the BR you read plate and screen currents - measuring directly on the plates just gives you - logical - plate currents.
Screen currents are few mAs, so you may set the trimmer when reading off a BR a bit (say 1-2 mA) higher.


what about in terms of Bias regulation correctness? are the measurement on the external points without the BR attendible? i saw a video in youtube where a tech said it was an inaccurate way of biasing tubes.

was that eurotubes? i had seen similar but I just talked to a Diezel tech in North Hollywood and he said using the external points is the correct way. On the Diezel website it says to use bias rite.
 
i can't the source anymore, it was from a video on you tube, it was a guy saying it was inaccurate and he showed why. this on a Randall amp if i recall right.
 

gaskocher

New member
diezel&gas":19k240v6 said:
i can't the source anymore, it was from a video on you tube, it was a guy saying it was inaccurate and he showed why. this on a Randall amp if i recall right.

I think you are referring to the Peavey JSX/XXX where the external points should not be used. The external bias points on the Diezels are pretty fine.

 
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