Hello.

Here are two pictures of my amp.


I want to measure the bias volt and I have read a thread about how to do it in this forum, but if you look at the picture I have tried to measure between the red and the black wire on the tube, don't get any values. Where is the correct point to measure?

My last question is, where can I find the OD pot to adjust? Here is another picutre of my amp:
It would be nice if someone could tell me where in the amp I can find that one.

Best Regards.

No disrespect to mr fabulous, but I don't think the bias is used to balance the tubes. I am a newbie also. but I believe the function of biasing is to apply enough negative voltage to the grids to limit the current from the plates. Without this neagative current, the tubes would overheat and self destruct. Think of this negative current as the blinds of a window to limit the amount of light coming in. Since electrons are negative and like charges repel each other, the negative bias limits the number of electrons passing through the grid.  If you are running 6l6 tubes, you need to bias to not exceed 70% of the 30 watt operation capacity. ODS's seem to like a cold bias. Usaually in the -34mv to-38mv range. Adjust to taste as long as you stay under the 70% max output of the tube.  If this seems too much to take in as it did for me innitially, you can probably set your bias at -34 to -38 range and be good to go if this was a prebuilt Ceriatone unit.  Let me restate that I am no expert and I hope other more experienced people can correct me if I have mislead you.
Good luck and enjoy.

Dennis

Dear dennis
i am an electronics engineer, and know my stuff believe me. the current on the valves is more or less set to the nominal value. the bias pot is there to compensate for any differences in transdconductance between the two tubes, allowing them to be symmentrical.

i suggest you look at the schematic and see how its done

OK here goes. I'm not an electronic engineer and until 3 years ago knew absolutely nothing about valve electronics but thanks to the people here, the internet in general and some good books I have learnt enough to know that the bias control is not just to compensate for unmatched valves.

The bias control on the OTS sets the current flow in the valve at idle (so all controls should be set to 0 and nothing plugged in) by varying the bias voltage on the grid. It changes the bias voltage of both valves together thats why you need matched sets. You could mod it to have individual bias supplies but thats not what we're talking about here.

The bias controls how the valve behaves under load and it is generally recommended that the bias be set such that at idle the current flow is not greater than 70% of maximum plate dissipation. Some people do go higher but this will shorten the life of the tubes, but if this is the tone you want and are prepared to pay, then so be it. However if you set it so high that the valves start to 'redplate' then you will do serious damage and not just to the tubes.

70% plate dissipation equates to around 35-38mA depending on the valve make which because of the bias test system equates to 35-38mV across the test points. A lot of people prefer the tone at around 28-32mV and you should experiement within that range.

You should let the amp warm up fully before doing the measurements. Please make sure you have a load connected to the output 

+1 Erwin and Hywelg

http://www.webervst.com/tubes1/calcbias.htm

Put in your plate voltage - mine's around 470v, class is AB and tube type 6L6GC = 44.6mA (or mV, as measured at the bias points), that's 70% of plate dissipation and so is deemed a "hot" bias. As has been said, try from  32mV upwards to get the tone that you like.

I really can't believe you're a Electronic engineer; attached is the schematic where I draw a line in the section we're talking about.
BTW this isn't a cathode biased amp. Maybe there is where you go wrong.

Even I, the valve electronics newbie know this is utter pants. The bias pot does not control the current directly, it controls the negative voltage (read the schematic and look how the bias supply is rectified) on the grid of BOTH output tubes. If you have the same B+ the same cathode voltages (which you do) and the same arrangements for grid and screen voltages then the bias voltage does affect BOTH tubes. And its purpose is to get the valve operating in the correct manner. Nik did not do anything different to any other amp designer, this is how variable, fixed bias octal tubes are setup. And yes you can set it to too high a quiescent current AND risk redplating if you so desired.

Mr Fabulous, you might well be a highly qualified electronics engineer, but you are obviously not familiar with the way valves operate. Please do some research before handing out misleading information that might harm a users amplifier.