Herbert biasing guide questions

[Sam Laming]

This guide is from Diezel's site, I just wanted to ask some question about it so I know what's going on when I get my amp retubed/so I can learn to do it myself

1) Remove the backpanel. You'll see three (four) jacks above the fuses.
No prob.

2) Set your multimeter to mA and plug your black probe into the black jack and the red to one of the red jacks.
I assume it's to the red jack that's in line with the 'what tubes are blown' light above? e.g on my Herbert 1&6 are lit up so that would mean I plug the red socket into the far left socket, correct?

3) Remove the jack-corresponding fuse from the fuse holder which is located on the backplate, turn volume controls to 0 and make shure a load or speaker is connected to the amp.
No prob.

4) Now switch it on and read the bias current from your multimeter. Adjust it to the proper value by turning the bias pot which is accessible with a little flathead screwdriver through the hole in front of the red jack.
Are you biasing both tubes at the same time? + does 1&6 = far left and far right tubes? or far left and far right tubes?

5) When finished, turn off the amp, install the fuse and repeat the same
steps for the other pair(s) by using the other red jack(s) and removing the other fuse(s).
Install the fuse? Is that under the kettle lead socket?

 

[hairychris444]

One thing to note: the tubes should be paired far left w/ far right, 2nd left with 2nd from right, and finally the middle pair. This is the order in which the red sockets are when looking at them left to right (eg tubes 1&6, 2&5, 3&4). The fuses that you remove are also ordered in this way (the 0.5 Amp slo-blow ones pictured above).

When you remove the fuse, the circuit is made via the multimeter instead. So, removing the left hand fuse of the 3 with the red probe in the left hand socket will allow you to bias the pair of tubes on the far ends of the row (positions 1 & 6 counted left to right).

Makes sense?

 

[Peter Diezel]

Thanks Chris !

 

[Sam Laming]

Awesome, thanks tons Chris

So you remove the fuse (already done) replace it, take out 1&6/far right and left, put new tubes in, then get your multimeter out fire up and get biasingin that order?

One last silly question... how do you take the tubes out? Do you just have to undo those 2 screws that are either side of each tube and then they'll pull out?

 

[hairychris444]

... and I don't even own a Herbert...!

Thanks Peter!!



OK, Sam...

1) Yup, that's what you do, then power down and replace the fuse after biasing.

2) No, don't unscrew anything. Ease the tubes out using gentle side to side movement. You don't want to stress the tube base or the socket as both can crack and you really don't want that... You then slide the replacement tube straight in. It may be worth spraying contact cleaner onto a paper towel and ***lightly*** coating the new tube's pins first.

Cool.

 

[Sam Laming]

... and I don't even own a Herbert...!

Thanks Peter!!



OK, Sam...

1) Yup, that's what you do, then power down and replace the fuse after biasing.

2) No, don't unscrew anything. Ease the tubes out using gentle side to side movement. You don't want to stress the tube base or the socket as both can crack and you really don't want that... You then slide the replacement tube straight in. It may be worth spraying contact cleaner onto a paper towel and ***lightly*** coating the new tube's pins first.

Cool.

Awesome, thanks again Chris, your really helping me here, though the fuse doesn't matter until AFTER you've done everything? So I can bias my amp without a fuse? (As obviously it's blown at the moment and though I've taken it out I haven't replaced it).

+ Is there anyway of telling which of my 2 tubes it was that blown so I can keep the other for a replacment for say if my amp blew at a gig? Or does it not work like that and I need to just grab some spares?

Thanks tons.

 

[hairychris444]

You put the fuse back in when you want to connect everything back together and use the amp, so you can bias up now but don't use the amp until the fuse has been replaced*.

As for blown tubes, you'll often be able to tell in a similar way to blown light bulbs. Gently tap the tube (with something hard like a pencil, or you can use your finger nail) - if the tube rings cleanly you're probably OK, if the tube rings & rattles then you've definitely got a break. This won't always work but it'll show any blatant problems...

Spare tubes are a good thing to keep. If you replace a set (and know that the ones you pull out are good even if old) then keep them together in their pairs and you can swap them back in in emergencies. A re-bias is not necessary in this situation but you will need to do that when new tubes are put in. If you're not sure about a tube, I'd say throw it out as you don't want to keep blowing fuses.

* Well, I suppose that you can but you'll only be driving 2 pairs of tubes rather then all 3. Not sure what this does to the output impedence, etc..!!

 

[Sam Laming]

Fucking awesome, if you ever go a Nile gig drinks are on me Thanks man

 

[hairychris444]

Haha, no worries.

 

[Joeytpg]

ok i have a question.........if i don't know jack shit about biasing or electronics for that matter, and there comes a time to retube the amp.......do i have send the amp to a distributor to get it retubed? isn't this a pain?

or is there a way to get the amp with tubes don't don't need biasing?

 

[Sam Laming]

ok i have a question.........if i don't know jack shit about biasing or electronics for that matter, and there comes a time to retube the amp.......do i have send the amp to a distributor to get it retubed? isn't this a pain?

or is there a way to get the amp with tubes don't don't need biasing?

Oooooo I can answer this one! ok, I knew jack shit about biasing and electronics but well... read this thread! phone your distributor/wherever your from and tell them your a tube dummie and ask them to explain, I knew nought but in this thread and from a conversation with Doug of Diezel UK I'm very confident I can do it myself now. Though it's only easy because my amp has external bias pots...

and second question = yes, don't buy a tube amp! Only way out I believe...

 

[Joeytpg]

well actually I have a Mesa Boogie F-50 and i CAN order tubes that are withing the Mesa Boogie's bias range. Is there a way i can order tubes that are within Diezel Bias Range and NOT have them biased?

 

[hairychris444]

Strictly speaking, all tubes of a specific type fall within a certain range. Mesa amps tend to be fixed bias (relatively cold fwiw), so you select the tubes to fit the bias. The Mesa tube sets (and others supplied for Mesa amps) are graded with this in mind.

Most other amps are different. As matched tube sets will have biases that are the same, but probably different to other matched sets, you bias the amp to meet the new set using a multimeter. If, however, you order a set of tubes with the same rating as your current ones you will be able to drop them straight in.

Swapping like for like tubes without rebiasing will not break the amp. It can affect tone (esp if the tubes want more current then you give them) and tube life (if you are giving the tubes too much current).

For an Einstein or a Herbert you'll be able to bias yourself with a multimeter and a screwdriver. Other amps will usually need opening up and that can be dangerous!

 

[Me]

I was just reading through some older posts and thought I'd add a bit of extra help for those of you who don't use multimeters much (or at all)...

Often when you change a multimeter to current measurement (Amps or mA) on the front dial, you also need to change where the lead comes out the multimeter. Usually it has 3 or 4 connectors. One is common - for the black ground/- lead, then the red/+ usually goes to a Voltage (and other non-current measurement) connection on it. To measure current you'll probably need to pull the red lead out and plug it in to a different connector. This will usually be labelled I, A, mA, or some particular amperage (e.g. 500mA, 2A, etc.). This is because when you measure voltage with a multimeter the meter is a high impedance so barely any current flows through the meter, it just measures voltage difference between 2 points of the circuit by being parallel to that part of the circuit. To measure current with it however, all the current must flow through the meter. This is why you have to remove the fuse; usually the current flows through the fuse, the multimeter test points are in parallel with the fuse, so you have to remove the fuse to make the multimeter be the only path through the circuit get the current to flow throught the MM so you can measure it!

Because the current has to flow through the MM (unlike the V measurements terminal), this input is fused. Therefore any rating on this input (e.g. 500mA, 2A, etc.) is the maximum current allowed through the MM before the fuse goes. Therefore it should be rated to higher current than the current you're going to be measuring. If you have more than one current input, use the lowest current rated one that's big enough for your needs as it'll give a more accurate reading.

Hope that helps, and if in doubt get someone to do it for you...