Digital Multi Tester Question for External Herbert Bias

[ttosh]

I picked this up at a local store today for a good price. Will it work for biasing the Herbert or am I asking for trouble??

 

[duesentrieb]

If you can read 700-1000V DC (DCV) and 200mA DC A (DCA) you're fine, the pic is a blurry, I can't read the stuff on it . . .

Please take a pic when its is out of the plastic shell.

 

[ttosh]

here are two better pics sorry about the orientation of them.

 

[Ventura]

Got your mA values fine... 500VDC max on the top right, should be okay...might be a bit slim. But correct me if I am wrong (anyone??) the key here is the mA reading for biasing a Herbert (or a VH4 or the like).

V.

 

[King Crimson]

You're fine with that DMM, ttosh.

When setting the bias with your DMM on Herbert, make sure you connect the positive (red) lead to 'mA' and the negative (black) led to 'COM.'

 

[ttosh]

500 DCV correct

 

[King Crimson]

200mA on your DMM for measurement. I set my EL34 pairs to 65mA.

 

[ttosh]

200mA on your DMM for measurement. I set my EL34 pairs to 65mA.

There is a 200m on DCV and DCA, I think DCV is correct but I want to get clarification on this. I am going to read through the DMM manual and get a better knowledge of it before I do this. I was going to have my friend who does this type of stuff a lot do this for me however he is unavailable at the moment and I figure it will be good to learn how to do this myself, one of the reasons I like the Herbert with external bias, I would not want to touch moving it out of the headshell and shocking myself across the room....

 

[King Crimson]

DCA - 200m

 

[ttosh]

DCA - 200m

Thanks!

 

[ttosh]

Okay I did a bias test today and here are my steps and results. I want some input before I proceed to make any changes.

I put the black in COM and Red in MA from the Multimeter, I took out the fuse that aligns to the tubes I am testing. I put the black end in the far right black slot, the red end in the slot of the tubes I am testing. I noticed this is not a snug fit, they just lay in there. Is that correct?

I turned the Multimeter to DCA 200M and then power the amp on and below are the readings I got for the 3 sets of tubes. Note: I powered the amp off between each reading.

1&6 biased out at 37.5

2&5 biased out at 45.3

3&4 biased out at 35.7

I am thinking these numbers should be closer to 65-70ma not this low as long as I am understanding the above properly. If the above is correct and I am doing this right is there a certain screw driver I need in regards to length of the screwdriver to reach the trim pots. Will I feel a slot for a flat head that is turning? What should I expect.

After confirmation of these items I will move forward to slowly bias each set 3 times as Peter suggests and see how they align at that time.

Thanks for all the help. Once I have this down and know all the ins and outs changing tubes when needed and experimenting with tubes will be a breeze!!!

 

[duesentrieb]

Yes, 60-65mA would be correct for each pair.

 

[King Crimson]

I turned the Multimeter to DCA 200M and then power the amp on and below are the readings I got for the 3 sets of tubes. Note: I powered the amp off between each reading.

1&6 biased out at 37.5
2&5 biased out at 45.3
3&4 biased out at 35.7

ttosh, if these readings are correct, then your Herbert is biased way too cold – over-biased. This means that you are in crossover distortion and can’t produce a clean, warm tone and are sluggish in response. Remember that Herbert's bias setting is measured in pairs. So for example a single EL34 biased at 30mA x 2 = 60mA and that is approximately what you are looking for to get out of crossover distortion. You do not need to power the amp off between readings. Just set it to standby if you want to.

...is there a certain screw driver I need in regards to length of the screwdriver to reach the trim pots. Will I feel a slot for a flat head that is turning?

I use a typical flat head jeweler screw driver. The length of the business end is exactly 1" and it works perfectly. You will feel it when you insert it into the test point. Also, the test point bias adjustment is very sensitive and you need only turn it ever so slightly.

 

[ttosh]

I turned the Multimeter to DCA 200M and then power the amp on and below are the readings I got for the 3 sets of tubes. Note: I powered the amp off between each reading.

1&6 biased out at 37.5
2&5 biased out at 45.3
3&4 biased out at 35.7

ttosh, if these readings are correct, then your Herbert is biased way too cold – over-biased. This means that you are in crossover distortion and can’t produce a clean, warm tone and are sluggish in response. Remember that Herbert's bias setting is measured in pairs. So for example a single EL34 biased at 30mA x 2 = 60mA and that is approximately what you are looking for to get out of crossover distortion. You do not need to power the amp off between readings. Just set it to standby if you want to.

...is there a certain screw driver I need in regards to length of the screwdriver to reach the trim pots. Will I feel a slot for a flat head that is turning?

I use a typical flat head jeweler screw driver. The length of the business end is exactly 1" and it works perfectly. You will feel it when you insert it into the test point. Also, the test point bias adjustment is very sensitive and you need only turn it ever so slightly.

Yes those readings were the correct readings I was getting and I upped it to around 60mA on each set of tubes. I did this 3 times as instructed by Peter. I will get to crank up and play tonight. I think this will make a difference to the lack of depth I was hearing and the hair on the notes or displeasing sound I was hearing as well. My other Herbert never sounded like that and I bet this one comes to life tonight.

Thanks to everyone who posted, it is really easy to Bias the Herbert with the external Bias points. I wish all amps came this way, and wish the Govt had not made them stop doing this for future Diezels.

Thanks again!!

 

[King Crimson]

Jolly!

Just a suggestion, ttosh - since you are beginning to understand the concept of the bias settings with Herbert, when dialing in the tone that you want to achieve you might want to set all controls (except channel volume & MV) to 12:00 and tweak it from there.

I've never heard a Herbert sound (to say the least) 'bad' in the past 5 years set neutral like this.

Attached is a Herbert sample settings portable document format chart that you may find interesting.

Enjoy...