Bias

[Mats A]

If i just had a tech set the bias if i want to try another set of four Power valves will i have to have the bias adjusted again?

[Anitoli]

Yes I should be checked as tubes will never be consistent in their current draw.
Learning to do this isn't all that hard depending on the amp. This is a very good read for those not familiar with doing it. Keep in mind not all the amps need to have the resistor installed. For example the DSL's have the pin/trim pot with the resistor on the board for this purpose.

http://www.duncanamps.com/technical/lvbias.html

[Mats A]

On the old DSL there is like two screes and like a contact or something in the middle of those. What is that for? Can you adjust the bias on these without taking off the chassi? If so what do you need?

[P A U L]

http://www.eurotubes.com/eurotubes-Mars ... -Video.htm

[Anitoli]

http://www.drtube.com/schematics/marsha ... ctions.gif

[Mats A]

If you adjust it will the valve then affect the meassurement you get or if you set it to a certain ma will it stay there? Don't worry i wont try this myself. What happen if i put another set of valves in without readjusting just for a while?

[Mike_LA]

Ok, so don't mean to hyjack this thread but seems like your question was almost answered.

I bought what I thought was a good bias probe set.



The info says,

you can get the reading direct from the current meter. It does not need any convert from mV to mA. It can be work with all kinds of multi meter (Digital meter or analog pointer meters) , at range of 100mA or above.

It is a safe, risk free meter, it measures the cathode current
of the power tube for bias reading (at low voltage side of the tubes ).


Steps to check the bias of your amplifier:
1. Turn off and unplug your amplifier. Make sure your multi meter works properly .
2. Remove the power tubes to be checked. If the amplifier has been used recently the output tubes will be HOT. Allow the amplifier to cool down prior to removing the tubes.

3. Insert bias meter probes into the tube sockets of the amplifier.

4. Insert the power output tubes into the bias probe sockets.

5. Turn on your power amp and let the tubes get warm (3-5 minutes) read the bias measurement. (As shown on the picture).

6. Move the switch from “A” to “B”or “C” , “D”. The reading should be no more than 2 or 3 milliamps difference for a good balance between the tubes. At this time you can adjust the current level (using the amplifiers bias adjustment control) to the appropriate level. There are many charts online that will give you the proper range of current for your tubes. Do not exceed the maximum level on these charts or you will damage your tubes. If you see a red glow in your tubes turn the amplifier off immediately. One possible reason is the bias current is too high.

7. After you have completed checking the bias on your output tubes turn your amplifier off and allow it to cool down. Again, the tubes will be very HOT at this time.

8. Remove the bias probe sockets and re-insert the output tubes.

9. You are now ready to rock and roll!

From watching this vid I think i need more parts.

He has a switch with voltage divider circuit and a 1 ohm resistor across between plate and ground?



Then lastly from Lord Valve,

WHAT YOU WILL DO:

A) Replace the ground wire on each power tube
socket with a 1-ohm resistor.
B) Read the voltage drop across this resistor
(in millivolts) with your DMM.
C) Read the plate voltage.
D) Use the above readings to calculate the
static dissipation wattage.
E) Adjust the bias to obtain the best tone,
while keeping the tubes within specifications.

<><><><><><><>
If it's ever been biased shouldn't the resistor still be there, or do you have to remove it?

[Mats A]

My brother has a multimeter so he should be able to adjust the bias om my JCM 2000 DSL 100. Isn't that all you need on the old DSL?

[dukealexan]

My brother has a multimeter so he should be able to adjust the bias om my JCM 2000 DSL 100. Isn't that all you need on the old DSL?

That's it, set it to mV and have him watch the Euro Tube vid posted above.

[Mats A]

Thanks then i wont have to wait for the holliday to end and save som money also. Must have been bad valves making my amp sparkle and crack and pop.

[Anitoli]

You can only use a probe device with a DSL if it doesn't have an internal cathode breaking one ohm resistor. If it does the readings will be wrong as there is already a one ohm resistor in there for each side. Just use the ports on the back, or take the head out, measure the plate voltage then use the ports to adjust the ma. With the voltage you can now accurately figure the dissipation.

I x V = P

For example: 35mv drop across the resistor, convert to ma = .035ma multiplied by 468v plate voltage = 16.38 watts.

Ohm's law is one mv across one ohm equals one ma.

[Mats A]

Why are this bias adjust prongs and screwes on the back of the amp if you can't use it on all? Mine is a 98 DSL. Is this the resistor between the valve and the OT? Why does he say in the video that you can do it with just a multimeter if it's not correct?
Think i mean can it be done with just a multimeter set to mv?

[Cipher]

My brother has a multimeter so he should be able to adjust the bias om my JCM 2000 DSL 100. Isn't that all you need on the old DSL?

A Digital Voltage Meter is all you for need for the old AND the new. Easier on the old because the test points and trim pots are on the back of the chassis, as you have found. You need to remove the the chassis to access the trim pots and test points on the new series. No probes needed for either.

Here's the location of the test points and trim pots in the new 100H. Red arrow indicates the test points (middle pin is ground), Yellow arrow indicates the bias trim pots.

[Mats A]

Ok thanks. It's the old DSL I'm talking about now so i'll only have to remove the back metal cover.

[Cipher]

Ok thanks. It's the old DSL I'm talking about now so i'll only have to remove the back metal cover.

Correct!