biasing

[blues man]

Here is a dumb or not so dumb question. Where do you set the volume and tone controls when biasing? I typically set everything on 0 but then I started experimenting and noticed when I started turning the volume and tone controls as well as the presence control the bias starts going down like from 35milli amps to 25 and 20. I am not playing guitar but have it plugged in with the volume off. I searched the world over and found nothing on this question.

[eleventeen]

That's incredibly weird. I have never head of such a thing, which does not mean it's impossible.


Is this an otherwise functioning amp, commercial or home-built? And/or, did you replace the OT or seriously mess with the NFB loop? 


The only thing that would cause this, to my way of thinking, is there is a high-frequency oscillation (past your hearing range) going on ALL the time which is dampened or defeated by the position(s) of the vol/tone controls. That I find very hard to believe; on a presence control it's far more believable because *some* presence controls sit in the feedback loop.


Are you sure of the polarity of your OT?


Have you tried your measurement(s) while pulling the first or first & second preamp tubes? 

[blues man]

It is a 5f6 that I built with 2nd input not used. I have a toggle switch to switch that in for more gain. The output tr is a bogen that was for 7868 tubes but I am using 6L6 tubes. I am going to replace that tr with the right one as soon as I buy one. The presence control is a little scratchy when you turn it. If the OT tr polarity was wrong it would be howling with a loud hum wouldn't it? I have all the tubes in and haven't tried pulling any of the pre amp tubes.The amp does seem to be working fine and I cranked it way up.

[eleventeen]

If the OT is mis-phased, yes, in 98% of cases it would produce "motorboating" which is a gross, unmistakable low-frequency feedback; but in some odd circumstances it can produce positive feedback of a different nature above normal audio range, so...I just threw out the possibility even if remote.   


Judging as best I can from a distance, I am still suspecting some kind of high-freq oscillation. You could be getting some kind of interstage feedback caused by sharing a filter cap (or sharing a cathode bypass structure---the paralleled R & C) between successive preamp stages.
 
I would also suggest you measure G1 volts on your 6L6 tubes while the amp is undergoing such a big move in output tube current. 

[HotBluePlates]

Original Question:

Where do you set the volume and tone controls when biasing?

Tone controls don't matter, but set the volume at 0.

... when I started turning the volume and tone controls as well as the presence control the bias starts going down like from 35milli amps to 25 and 20.

When you're measuring, you're checking direct current (or voltage across a resistor due to direct current).

When you add a signal (even background hiss due to turning up volume with not signal input) you no longer have direct current at the output tubes but an alternating current bobbling around some d.c. midpoint.

When you measure d.c. when there's really a.c. present (and possibly a very random a.c. due to noise), there's no telling what your meter will display. It depends on if you're using a digital or analog meter & the details of exactly how the meter does the actual measurement in d.c. mode. The latter indicates whether the meter can measure d.c. while rejecting a.c. also present at the measurement point.

Digital meters sample the voltage so many times per second. For d.c., maybe the meter displays each sampled voltage (not likely, because the display would be switching very fast between each measured value). Maybe the meter averages the measures for a certain time period. Even if it averages, there are also discrete samples used for the average, and those could land more on the negative end of the waveform and drive the average downward. Some meters can reject a.c. present very well. Some can only reject a.c. if it is a repetitive waveform (like that from a signal generator, not random noise). Some claim to reject a.c., but only reject well at 50/60Hz.

The best answer is don't set up the conditions for the test equipment to lie to you. Don't have a.c. present for a d.c. measurement. Turn the volume down. If background noise is really an issue, pull the phase inverter tube from its socket.

[blues man]

I thank you for the replies. The amp actually sounds good and I am still working on it. I will order the right  OT tr from Doug and get rid of the bogen tr. I original had this amp set up with stock deluxe transformers and 6v6 tubes but I decided I wanted more headroom and power so I swapped out the transformers with others I had and beefed up the filter caps. The distortion is a little rough but I do think it could be the mismatched bogen I have in there. Trying to save a little money, My bad. I am using a fluke meter. I also didn't have the scratchy sounding presence control pot until I put these tr's in the amp. Thank you all for your time.

[blues man]

I purchased a new OT and when I went to change it out I discovered I never soldered the ground wire of the OT. No wonder the bias was acting weird. After installing the new OT the presence control doesn't sound scratchy and doesn't change the bias anymore. My bad again.