Username: Password:

Author Topic: Biasing Amps  (Read 4108 times)

TomW

  • Featherweight
  • ***
  • Posts: 330
    • http://www.myspace.com/wolfiosg
Biasing Amps
« on: August 10, 2009, 09:09:35 AM »
Does anyone here know of any decent books on biasing amps, as I have decided I want to learn how to bias my amps myself to save me from having to take them to an amp tech every time the valves need changing?

Also what equipment am I likely to need to bias my amps apart from a multimeter and a bias probe? My amps are a marshall jcm800 4104 and a dsl100 if that is any help.

Cheers
Riff Raff/Stormy Monday, Holydiver and Mississippi Queen!!!
www.myspace.com/therealamorist

hunter

  • Middleweight
  • *****
  • Posts: 5262
    • http://www.myspace.com/christophjaeger
Re: Biasing Amps
« Reply #1 on: August 10, 2009, 09:39:52 AM »

There's not much to know if you use a Bias Rite.

https://taweber.powweb.com/biasrite/br_page.htm

Just plug it in, fire up the amp and let it warm up, then bias with the internal pot according to some bias table depending on your plate voltage (see attachment).

Maybe it's a bit amateurish to do like this, but it works fine for me.
Tweaker's Paradise - Player's nightmare.

jpfamps

  • Lightweight
  • ***
  • Posts: 767
    • http://www.jpfamps.com
Re: Biasing Amps
« Reply #2 on: August 10, 2009, 11:44:16 AM »
Best info on the theory behind biasing is on Randall Aiken's web-site in the Tech Info section.

www.aikenamps.com

Also make sure you check out his safety tips.

Essentially you need to measure the current passing through each valve and calculate the "resting" power dissipation in each valve (power = plate voltage x current). This is normally adjusted to between 50 and 70% of the maximum permissible for each valve (25 W for an EL34) for the usual 50/100W Marshall amps using El34s or 6L6s.

The DSL has test points and bias adjustment pots on the outside of the chassis so you can adjust the bias without opening the amp up. However, depending on how old your amp is, I've found it quite a hard amp to adjust the bias on due to interaction between the adjustment pots.

The JCM800 has a bias adjust pot on the inside so you will need to remove the chassis to adjust the bias, thus potentially exposing yourself to high voltages. You will also need some form of bias probe to measure the current draw.

To measure the plate voltage in either amp you will need a multimeter and have the amp open on the bench and on i.e. with the high voltages exposed.

Alternatively, you can assume that the B+ is around 470-490 for each amp and use this figure for your calculations, which is plenty good enough for rock and roll.

The Weber bias right is a good tool because it allows you to measure both plate voltage and current draw without poking around inside the amp when it is on. Although it might seem expensive, it's probably less then a couple or 3 trips to a tech. It will also let you check that your filter caps have been discharged as you can measure the B+.

Some extra practical tips:

I always leave amps for at least 20-30 minutes (longer is better) when biasing them, checking the bias every few minutes or so. This is especially important in Marshalls which can have a very large time constant in their bias supply, and can take several minutes for the bias supply to reach its final voltage. It also allows the valves to get up to a typical operating temperature. Also generally if anything bad is going to happen to new valves, then it will happen in the first few minutes of operation.

Always have a load attached to the amp (this can be disconnected when removing the amp chassis). I like to plug the amp into a speaker/s so I can hear if there are any problems with the new valves. I also turn down all the controls.

If you are going to poke around inside a live amp (and I can only recommend this if you are very sure you know what you are doing), make sure that the amp chassis is very stable and don't use both hands inside the chassis ("one hand in your pocket").




« Last Edit: August 10, 2009, 11:46:47 AM by jpfamps »

jpfamps

  • Lightweight
  • ***
  • Posts: 767
    • http://www.jpfamps.com
Re: Biasing Amps
« Reply #3 on: August 10, 2009, 11:45:59 AM »
Sorry about extra post!

TomW

  • Featherweight
  • ***
  • Posts: 330
    • http://www.myspace.com/wolfiosg
Re: Biasing Amps
« Reply #4 on: August 10, 2009, 12:21:16 PM »
Thanks for the help guys,
It seems alot easier than I thought looking at that weber bias rite website, so I'll get one of them ordered I think. I'll make sure I have a good read of that aiken amps website aswell. Hopefully this should help save me alot of money in the long run.
Riff Raff/Stormy Monday, Holydiver and Mississippi Queen!!!
www.myspace.com/therealamorist

TomW

  • Featherweight
  • ***
  • Posts: 330
    • http://www.myspace.com/wolfiosg
Re: Biasing Amps
« Reply #5 on: August 10, 2009, 01:13:14 PM »
Does anyone know of a UK website that sells the weber bias rite, as I can't seem to find any?
Riff Raff/Stormy Monday, Holydiver and Mississippi Queen!!!
www.myspace.com/therealamorist

HTH AMPS

  • Middleweight
  • *****
  • Posts: 5649
    • HTH AMPS
Re: Biasing Amps
« Reply #6 on: August 11, 2009, 11:22:43 PM »
Does anyone know of a UK website that sells the weber bias rite, as I can't seem to find any?

I think Hotrox sell them

FELINEGUITARS

  • Middleweight
  • *****
  • Posts: 6609
  • London & Southeast's Number 1 BKP stockist
    • http://www.felineguitars.com
Re: Biasing Amps
« Reply #7 on: August 11, 2009, 11:45:38 PM »
I have one of these
http://www.compu-bias.com/

Dont know if they are as good as the Bias Rite

Any clues HTH or JPF ?
www.felineguitars.com - repairs & custom built
Great fretwork!
Buy your BKPs & Earvana from ME!

HTH AMPS

  • Middleweight
  • *****
  • Posts: 5649
    • HTH AMPS
Re: Biasing Amps
« Reply #8 on: August 11, 2009, 11:52:43 PM »
That Compu-bias looks great - instant readouts of plate voltage, current draw and static dissipation.  It's quicker than using a DMM - might have to get one of those myself.


hunter

  • Middleweight
  • *****
  • Posts: 5262
    • http://www.myspace.com/christophjaeger
Re: Biasing Amps
« Reply #9 on: August 12, 2009, 07:11:31 AM »
That one seems to be better than the Weber, has some more fancy stuff to it. So with the CompuBias, you set bias based on Wattage?

The probes look also shorter than on the Weber, which is a bit annoying as they hardly fit in small housings, such as the VHT (where I first removed the fan to be able to plug it in, but then got some advise from Steve Fryette about how to do it with a multimeter on the OT connections).
Tweaker's Paradise - Player's nightmare.

jpfamps

  • Lightweight
  • ***
  • Posts: 767
    • http://www.jpfamps.com
Re: Biasing Amps
« Reply #10 on: August 12, 2009, 10:43:15 AM »
That one seems to be better than the Weber, has some more fancy stuff to it. So with the CompuBias, you set bias based on Wattage?

The probes look also shorter than on the Weber, which is a bit annoying as they hardly fit in small housings, such as the VHT (where I first removed the fan to be able to plug it in, but then got some advise from Steve Fryette about how to do it with a multimeter on the OT connections).

Well you effectively always set the bias on the basis of power (wattage), it's just that the Compubias calculates it for you, which is a nice feature, although I'm not sure how much extra I would pay for this......

We use a probe that we built that measures both current and plate voltage in two valves using a multimeter. This cost us probably 15 in parts, however as it involves wiring to the B+ it's not a rpoject I would recommend to someone not used to working with high voltages.

Fitting bias probes to some amps can be a pain and often require removal of the chassis (which you may have to do anyway if the bias adjust is internal).

The other method mentioned above is the transformer shunt method. This can work OK, however it is also potentially very dangerous, not just to you but also your amp, so I would not recommend this method for general use.

hunter

  • Middleweight
  • *****
  • Posts: 5262
    • http://www.myspace.com/christophjaeger
Re: Biasing Amps
« Reply #11 on: August 12, 2009, 10:59:42 AM »

This is what Steve Fryette suggested for biasing the VHT:

You don't have to install 1 ohm resistors to use a DVM. There already is a resistor in each plate circuit in the form of the output transformer primary resistance.

To read the bias for each tube, first read the resistance of each half of the OT primary - Red to Blue and Red to Brown leads (power off for a while) and then the voltage drop with BOTH standby switches on after warm-up - Red to Blue and Red to Brown.

The resistance is typically around 38-40 ohms. Normally the voltage reading will be in the range of 1.35 - 1.5VDC. So for example: 1.5/39=0.038 or 38mA.

Observe all of the usual precautions...!


I've done it a few times without any issues, it seems to work better than any other method (unless I wanna buy 90 converters for the bias rite, which cost a fortune)
Tweaker's Paradise - Player's nightmare.

jpfamps

  • Lightweight
  • ***
  • Posts: 767
    • http://www.jpfamps.com
Re: Biasing Amps
« Reply #12 on: August 12, 2009, 01:32:44 PM »
OK, that's different from the transformer shunt method.

I've actually used this method for some amps in which I can't fit a bias probe.

It can work very well, and is obviously cheap (assuming you already have a multimeter!).

However, I would only recommend this method for someone with a more advanced understanding of amps, as not only do you have to correctly identify the OT primary taps, there is a bit more maths involved.

You must ensure your filter caps are discharged fully before you take the resistance reading otherwise you will get a duff reading (and you can destroy your multimeter). You can also get some strange resistance reading on some OTs, which I assume is due to their primary inductance.

It involves having the amp on with high voltages exposed though, with corresponding risks, and doesn't allow you to measure the current through individual valves, which is often very useful (think SVT).

A bias probe is safer for the inexperienced user, however they certainly aren't cheap (which is why I made one!!).