biasing/balancing ???

[savy]

Refer to schematic:
http://img160.imageshack.us/img160/2...ainmodswn1.gif

OK... PP 6L6 voltage readings:


V-5 pin3 398v
pin4 399v
pin5 -36.2v
bias(pin8, 1R) -37.5mv


V-6 pin3 393v
pin4 398v
pin5 -36.4v
bias(pin8, 1R) -42.4mv

I understand the bit about static dissapation which requires multiplying the plate voltage by the bias reading from pin8 (close enough)
But in this case my output tubes are putting out 14watts and 18watts respectively given the present setting.

Even though I have a single bias supply controlled with a pot wired as a variable resistor the bias readings are different for each tube and because of the mismatch things improve a bit when tubes are swiched around. The figures above represent the better condition.

What is the most important consideration in terms of balance with what
the figures show ? One post told me to adjust the voltages at the grids until the bias readings across pin8 1ohm resistor to ground becomes even then the other voltages will fall in line.

I've not heard of this before. Wouldn't the desired result be achieved by using a balance pot in the bias circuit ?

Also, a tech told me that when biasing/balancing it doesn't matter if the applied bias voltages are the same but it is the measurement at the (grid ? plate ?) that matters.

Thanks, paul

[stokes]

Check your math.I get 14watts vs 16 watts.In my opinion this is not too bad for a guitar amp.This difference is most likely caused by the mismatch of the tubes.But it could also be caused by a slight mismatch in your 1ohm resistors or a combination of both,as well as the fact that your OT primary is not perfectly balanced.You can see this by removing your power tubes and checking the resistance between your center tap and each plate tap.Your 1ohm res.should be 1% tolerance.So if one resistor is 1ohm and the other is .99ohm you will have almost .5ma difference,when you add that to the mismatch most likely in your OT you could now be off by as much as 2ma,so your tubes may only be off by 3ma's,which for a guitar amp aint too bad.All the components in your amp are 10-20% tolerance anyway.So I wouldnt sweat the 5ma or 2watt difference in your output.This is only the idling disipation anyway,once you start putting any signal into it,and remember the signal is being passed thru all those 10-20% tolerance components things change.I think you are looking for complete and perfect balance in a circuit that is anything but,these inbalances and imperfections are what gives tubes a more desirable tone than a closer to perfectly balanced and cold solid state circuit.To achieve the balance control on the grids you describe,you need a different bias circuit,look at the bias circuit on a Silverface Fender,the Bassman 10 comes to mind,you will see this bias circuit,which doesnt actually adjust the bias,it just lets you balance a fixed amount of negative voltage on your grids.It is okay for balncing mismatched tubes,but a true bias adjustment is much better,since tubes these days are almost always sold in matched pairs and quads.For HI-Fi applications you would want closer matched tubes,but in a guitar amp 5ma is good enough.I have found no difference in the final results between 2 tubes perfectly matched and 2 that are "off" by up to 5ma's.

[Amp Kat]

Yes I agree with Stokes that this is very common in todays amps and may be somewhat desirable than a perfectly bias/balanced amp. Especially if your using it for distortion it can add even order harmonics which is a good thing. There are bias supplies that allow you to set the ma's with one pot and balance the other pot to zero acting like a wheatstone bridge but tubes drift as time goes on and can even get out of the range of the balance pot. I surely wouldn't sweat over it in any way even this being an International voltage which could be different tranny's all together which it's showing they are not so it shouldn't make a difference. BTW who the hell uses 127vac ?

[Enzo]

I agree with the above too. Check that OT winding resistance. Any difference there wil show up in readings too.

Here is the test? Put the tubes in the first way, then play the amp for 15 minutes, listening to the tone. Now turn the amp off and swap tube positions to the other way. Again, play it for 15 minutes, listening for the tone. What difference did you hear?

Bias is nowhere near the critical thing so many people think it is.

[stokes]

To a certain degree I think it is critical to find the "sweet spot" for a particular pair of tubes,but that spot is in a wider range,not a matter of 2 or 3ma's.Like if you look at the way Mesa sets their amps and then grades the tubes to fit,to me they tend to be in too cold a range most of the time,like they set it to be safe rather than to be "hot" enough to get those power tubes cooking,so I think it is critical to be able to adjust the bias setting into that range where the tubes get driven just right,but it is usually about a 10ma range before you can hear a big difference.Precise matching is not as critical as a lot of people think,but once you get outside the 5ma range on the match you will start to notice a loss in dynamics,and,although not as noticable audibly there will be a measureable difference in output power.Of course if you are driving the amp into heavy metal type distortion these things become even less critical and much less discernable.