Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: jasperok on April 11, 2008, 05:39:28 am

Title: does a bias tool exist like this:
Post by: jasperok on April 11, 2008, 05:39:28 am

gonna be  building a bias check tool but had an idea so want to see if its worth doing!

Basicaly i havea couple of spare hammonds for effect pedals here and was thinking how cheap these meters were:
http://cgi.ebay.com/3-Digital-Blue-LED-AC-0-700V-Volt-Panel-Meter_W0QQitemZ350045436811QQihZ022QQcategoryZ25411QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/3-Digital-Blue-LED-DC-0-2A-Amp-Panel-Meter_W0QQitemZ350044892061QQihZ022QQcategoryZ25411QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

now what i was thinking was having two female banana connections on the side of the box with a switch to choose which meter is measuring .  then a nother switch to AB the two sockets.?

is there a schematic anywhwere for anything like this - my main concern is that i didnt build anything that could damage myself and perhaps protect the meters some way as well,

might be a silly idea - i'm new to all this so forgive me if i have been a bit niave!
maybe these units already exist and would be cheaper and safer to buy premade?  though i am sure once the theory is right i can build it safe enough.

Title: Re: does a bias tool exist like this:
Post by: jasperok on April 11, 2008, 05:52:04 am

http://www.thomann.de/gb/tube_ampdoctor_biasmaster.htm

ah the bias master
well thats about 200 dollars by the time it is shipped.

mmm

any schematics about for that type of thing - cant be that much to em.

Title: Re: does a bias tool exist like this:
Post by: PRNDL on April 11, 2008, 09:23:20 am

I thought it would be handy to display the current in all two or four tubes at the same time, which would be nice when trying to match tubes for a Twin or 100 watt Marshall.

The second feature is to have an easy way to change sockets from octal to 9-pin.

As for the circuit, it's fairly simple:
 voltage between the anode and cathode,
 current tap between the cathode male and female pins,
 all other pins are male to female

It is important to spend some thought on designing the sockets to be rugged enough for professional use.

Title: Re: does a bias tool exist like this:
Post by: rafe on April 11, 2008, 02:05:43 pm

Here is a simple version ......make two that should be plenty to match tubes ......Get a couple MA meters or DVM's and you are in ....cheap
    http://www.el34world.com/charts/biascurrent.htm

Title: Re: does a bias tool exist like this:
Post by: jasperok on April 11, 2008, 03:18:59 pm

yeag your right -
it would be be silly to get anything more expensive.

the lost digital AM i can find is 0- +_ 2A (the one above)

however it shows to 3 decimal places so i presume it will show 0.035 A as 35ma - is that in the correct range do you think?

the 600v meter should be perfect two.

Title: Re: does a bias tool exist like this:
Post by: PRNDL on April 11, 2008, 05:41:39 pm

A better way would be to get a millivolt meter connected to a high precision 1 ohm resistor between the cathode male and female pins. This 0-20V meter might be suitable
http://cgi.ebay.com/3-Digital-Blue-LED-DC-0-20V-Volt-Panel-Meter_W0QQitemZ230241329272QQihZ013QQcategoryZ25411QQcmdZViewItemQQ_trksidZp1742.m153.l1262QQ_trksidZp1742.m153.l1262

I wonder what would be better (more accurate) - using the millivolt meter and a 1 ohm (1%) resistor, or the ammeter.

It's easier to switch with the millivolt meter, since you don't have to interrupt current flow. On the other hand, the wire length will decrease accuracy.