Early Herbert Biasing Questions

M

mrb1946

New member
Hey guys,

Just got my first Deizel amp last month and I wanted to put some fresh tubes in it. It's one of the earliest Herberts that does not have the external biasing options. I found this guide online: http://www.diezel.co.uk/Einstein & Herbert Biasing.pdf

My version of the Herb is the one on the second page. Just want to ask you guys a few questions for clarification.

1) I am ok to put all of the new tubes in before I start biasing, right? I have all new Pre amp and Power amp tubes.

2) Am I supposed to connect the alligator clips on my DMM to where the fuses go? Is there a polarity issue to worry about there?

3) What is the best way to remove the fuses?

4) I need to have a cab plugged in, right?

Thanks a lot guys.

Also, I am putting the stock recommendation tubes in (Ruby Pre amp tubes and JJ KT77 power amp tubes)
 
I'm not familiar with that amp but if it doesn't have 1 ohm cathode resistors on the power tubes, or some other easy way to measure a VOLTAGE to correlate to idle current, then imo you are in a bit over your head. Adjusting bias using a transformer shunt method can be very dangerous to you, your amp, and your meter.

Measuring across a fuse won't tell you anything except whether the fuse is blown or not. If the fuse is good they are essentially the same point in the circuit. But just the fact that you asked this question leads me to think that this is, imo, too risky.

You can swap all tubes before adjusting bias but I always turn the bias voltage to a more negative voltage before switching from standby just in case the new tubes need more bias voltage.

Polarity won't really be an issue except the measurement would be negative if you had the leads reversed. Pretty much all voltages in most tube guitar amps are positive, the obvious exception being bias voltage for the power tubes.

Not trying to dissuade you from learning and at some point you have to jump in. But I just think that, again unless the amp is very easily and safely biased, that the money saved here just isn't worth it.

FWIW, I got into amp building/modding because I wanted to learn to bias my own amps. Little did I know the rabbit hole was so deep (and fun).
 
SpiderWars":1bgarbtw said:
I'm not familiar with that amp but if it doesn't have 1 ohm cathode resistors on the power tubes, or some other easy way to measure a VOLTAGE to correlate to idle current, then imo you are in a bit over your head. Adjusting bias using a transformer shunt method can be very dangerous to you, your amp, and your meter.

Measuring across a fuse won't tell you anything except whether the fuse is blown or not. If the fuse is good they are essentially the same point in the circuit. But just the fact that you asked this question leads me to think that this is, imo, too risky.

You can swap all tubes before adjusting bias but I always turn the bias voltage to a more negative voltage before switching from standby just in case the new tubes need more bias voltage.

Polarity won't really be an issue except the measurement would be negative if you had the leads reversed. Pretty much all voltages in most tube guitar amps are positive, the obvious exception being bias voltage for the power tubes.

Not trying to dissuade you from learning and at some point you have to jump in. But I just think that, again unless the amp is very easily and safely biased, that the money saved here just isn't worth it.

FWIW, I got into amp building/modding because I wanted to learn to bias my own amps. Little did I know the rabbit hole was so deep (and fun).
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The only reason I asked about connecting the clips to the fuse area is because that's what it looks like is happening in the picture on that pdf. I do have some electronics experience, but not much. I've built a couple pedals from kits and whatnot, but I definitely do not want to get shocked while doing this. I'm not sure I want to take it to a guitar shop and have it done either because I'm sure they would be confused as well.

I do also want to learn this, since it is my amp after all. Thank you for your help
 
The big hazard is that in order to do a transformer shunt method (which I'm not sure you need to do) is that you have to set up your meter to read current (change the dial setting but also change your positive lead to the current-measuring jack). When set up this way, the leads on your meter are common. So, touch a high voltage point with one lead and you now have that voltage on the other lead. Ready to shock you or any piece of grounded metal (like your amp). There will either be pain or fireworks or both when that happens. There is usually a 10 amp fuse (that is HUGE!) inside your meter just for this.

Running around in circles yelling "fuck, fuck, fuck..." because you got shocked isn't fun. And that's prob best case.
 
Ok. Definitely does not sound fun. So what do I need to do to check if I would have to use that method or not?

Edit: did a little research, and it looks like I have to check for a 1 ohm resistor on the power tubes as you were referring to. I think that if I can find the resistor, I will attempt to bias the tubes myself. But if it looks like the transformer shunt method will have to be used, I will definitely have someone else do it.
 
mrb1946":qz4s8aj7 said:
Hey guys,

Just got my first Deizel amp last month and I wanted to put some fresh tubes in it. It's one of the earliest Herberts that does not have the external biasing options. I found this guide online: http://www.diezel.co.uk/Einstein & Herbert Biasing.pdf

My version of the Herb is the one on the second page. Just want to ask you guys a few questions for clarification.

1) I am ok to put all of the new tubes in before I start biasing, right? I have all new Pre amp and Power amp tubes.

2) Am I supposed to connect the alligator clips on my DMM to where the fuses go? Is there a polarity issue to worry about there?

3) What is the best way to remove the fuses?

4) I need to have a cab plugged in, right?

Thanks a lot guys.

Also, I am putting the stock recommendation tubes in (Ruby Pre amp tubes and JJ KT77 power amp tubes)
Click to expand...

1) - Yes you can but I'd drop the bias a little with the old tubes in and bring it up again with the new tubes

2) - Yes you connect them to either side of the fuse housing, if you get it round the wrong way you will get a negative number on the meter

3) - You should be able to simply pull them out or at most pop them (gently) out with a flat blade screw driver

4) - Yes you need a load on the amp, and to set the volume to zero too

Hope that helps - my Herbert is a little different but its the same basic operation - if you have any questions I'd email Peter his email is in a lot of the posts on this subforum!
 
Alright. Thanks a lot. Still trying to decide if I should do this myself or not. I seem to have the instructions pretty clear, but it still worries me that I might slip up.
 
OK. I have tried following the directions in the pdf attached in my first post, but when I turn on the amp, the DMM reads 0 and does not show anything else. I have verified that it is on 200 mA DC and I have the leads plugged into the COM and mA ports. The leads are clipped onto individual alligator clip wires and the other ends of those are on the fuse housing.

Oddly, when I shut the amp off, the DMM begins showing readings steadily declining from about 15 mA and stops after a few seconds.
 
You don't want to measure current directly. That's what those 1 ohm resistors are for. Current flowing through a resistance creates a voltage drop across that resistance. Using a 1 ohm resistor (as opposed to some other really low value) just makes the math super easy. If you read 40mV across that 1 ohm resistor, that equates to 40mA of current.

Usually the cathodes of fixed-bias power tubes are connected directly to ground. The current you are trying to measure is all flowing to the cathode and then straight to ground. Those 1 ohm resistors are connected to the cathodes of the power tubes and then go straight to ground. So now all the current is flowing through those 1 ohm resistors. You simply need to measure the voltage drop across those 1 ohm resistors (after you set your meter back to voltage measurement).

The cathodes of your power tubes should be pin 8. If you can't find the 1 ohm resistors but can get a lead on pin 8 of the power tubes, then just measure from pin 8 to chassis (which is of course ground). But you need to know if it's only one tube connected to each 1 ohm resistor or more than one. Often with 4 power tubes, each pair will get a 1 ohm resistor so there you'd actually be measuring the idle current of two tubes.
 
SpiderWars":27qgzh8q said:
You don't want to measure current directly. That's what those 1 ohm resistors are for. Current flowing through a resistance creates a voltage drop across that resistance. Using a 1 ohm resistor (as opposed to some other really low value) just makes the math super easy. If you read 40mV across that 1 ohm resistor, that equates to 40mA of current.

Usually the cathodes of fixed-bias power tubes are connected directly to ground. The current you are trying to measure is all flowing to the cathode and then straight to ground. Those 1 ohm resistors are connected to the cathodes of the power tubes and then go straight to ground. So now all the current is flowing through those 1 ohm resistors. You simply need to measure the voltage drop across those 1 ohm resistors (after you set your meter back to voltage measurement).

The cathodes of your power tubes should be pin 8. If you can't find the 1 ohm resistors but can get a lead on pin 8 of the power tubes, then just measure from pin 8 to chassis (which is of course ground). But you need to know if it's only one tube connected to each 1 ohm resistor or more than one. Often with 4 power tubes, each pair will get a 1 ohm resistor so there you'd actually be measuring the idle current of two tubes.
Click to expand...

Thanks for all the help. I went ahead and dropped it off at a shop I found in the area. The guy has a lot of experience with Diezel amps so I feel much better now.

Thanks again!
 
So I got my Hebert back today. Turns out, there was a capacitor on the PCB that had a lead broken off of it. That's what was causing the clicking and cutting in and out. So that's all fixed now! He also replaced a few other capacitors that had begun leaking. And he biased the KT77's to around 65-67 mA's.

Sounds awesome, and I'm glad to be a Diezel user now!


Thanks
 
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