DIY TUBE MATCH

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Andy Le Blanc
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DIY TUBE MATCH

Post by Andy Le Blanc »

So...

I'm working a rig an goin to her for hum an buzz.

Its a 50 w P-P with 6l6 and one of my goals was to explore
the early designs that have the screen grids tied together.
No screen grid resistors.

I was swaping power tubes going thru the bench spares.
I noticed that one pair 6l6 had alot of hum where another did not.
Push pull cancels hum and reduces noise so it was a quandary.

With screens tied... And the plates running 10 to 12 volts positive...

The measured difference anode to anode was where the answer showed up.

The quiet pair had a measured difference of less than 0.5 v.
The noisey pair was 8v difference plate to plate.

It struck me as a good way to match power tubes under practical conditions
on the bench with not but a shite meter.
lazymaryamps
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pompeiisneaks
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Re: DIY TUBE MATCH

Post by pompeiisneaks »

Makes perfect sense, the voltage drop is related to how much current the tube is dropping, and that means one is doing much better than the other, I think, though, that, imbalance in the circuit could attribute some general fault with that idea. I.e. you could get 'closer' to balanced tubes that way, but not as precise as measuring them on a tube tester. A good spitball approach for sure. Very cool, never thought about that.

~Phil
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RJ Guitars
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Re: DIY TUBE MATCH

Post by RJ Guitars »

I know some folks that use the hum factor as their form of tube matching... that is they like the flavor of a little mismatch and if it doesn't hum then that is as close as they need to be.
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Phil_S
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Re: DIY TUBE MATCH

Post by Phil_S »

Andy,
I've done this sort of thing. It's not fool proof as the tubes interact with the circuit, so that has to be considered. I'm not sure I'm right, but have theorized that it's a good idea to find one strong tube and leave it in one socket. Then swap all the other tubes through the other socket. This introduces (more or less) a constant that is helpful. We all know that scientific method says to only change one variable at a time. The additional problem is that there are several parts inside the tube, so as a unit, two that appear to be the same might turn out different anyway.

Once you find what you think is a good match for a pair, try those together and see what you find when you measure voltage.

One real advantage over a tester is that you are running the tubes at intended voltage rather than some lower test voltage.

Another thing that might be of value in a fixed bias amp is to test the tubes at 3 different bias voltages. This allows for a (crude?) calculation of transconductance, or maybe it just allows you to draw a curve that suggests how the tube might behave over a range, which is what it does when you run the instrument through it. This is yet another consideration in matching.

Or maybe I'm just full of it and don't really understand this stuff. It was fun to share the thought, in any case.
Phil
Andy Le Blanc
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Location: central Maine

Re: DIY TUBE MATCH

Post by Andy Le Blanc »

Your not wrong of course. This is most certainly a ballpark method that can not really give you
a valid trans-conductance figure.

But it can give you a practical circuit measurement under static conditions, a quick double check with a measurable observation.

I think it works in this case because of how I've chosen to wire the rig, both the cathodes and screens of both valves
are held at the same potentials.

The only variables left are plate resistance and the dc resistance of the OPT primary.

But as stated, its the ability to make a practical measurement on the bench that makes all the difference.

I have a measurement check list for the power side of any amp that I'm working on.

Plate voltage
Bias
screen voltage
grid voltage

Push pull makes the assumption that the tubes and voltages on both legs of circuit are identical or balanced.
This is becomes interesting in practical dynamic circuits.

I've learned to observe and measure the "difference" or the balance in a practical circuit where ever possible.
The plate voltage is the measured difference between cathode and plate.
The screen voltage is the measured difference between the screens and plate.
The grid voltages are the differences AC and DC between the grids.
bias is the current over a 1ohm resistor in the cathode circuit no matter which bias method.

And now I can add to the list the difference between the plates .

You see where this goes, the long and short of it is that you decide and choose which design factors to utilize.
But you need to get practical measurements an observe the voltages of your circuit under static conditions.

I had happy moment this morning... I found a pair of 6l6 that where closely matched in the practical circuit
with a measured difference of 50 mv ( 0.05v), I was surprised, they came out of a pile of old bench spares.
I was hoping for a couple volts or so... and you can hear it in the response of the amp... 5v / 0.5v / 0.05v
surprised the hell out of me.
lazymaryamps
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