Power Tube load sharing

[Helmholtz]

Well a bit of 'chicken or egg first' scenario then. The instructions were to adjust for minimum hum. Maybe that was actually the goal, or maybe they thought it was the simplest way to get to symmetrical clipping.

What I meant say is, that I think it's both "chicken and egg"".

Adjusting for least hum allows any untrained user to get decent balancing without using a meter.

[g1]

Well a bit of 'chicken or egg first' scenario then. The instructions were to adjust for minimum hum. Maybe that was actually the goal, or maybe they thought it was the simplest way to get to symmetrical clipping.

[Helmholtz]

Equal gain? Or equal idle current?

Equal gain (at idle) I think, but might be wrong.
Hum cancelling requires AC currents through both primary halves to be identical (and in-phase).
I'm not sure if equal DC currents can ensure that with somewhat different tubes.

With similar tubes, equal gain at idle typically (though not always exactly) goes along with equal idle currents.

Neither equal gain nor equal idle currents ensures symmetrical clipping, but chances are good, if tube characteristics
- especially saturation voltages - don't differ much.

[Enzo]

Equal gain? Or equal idle current?

[Helmholtz]

The later SF Twins etc. did have the 'output tubes matching' trim pot, but it was all about nulling audible hum, not balancing power output (or symmetrical clipping).

Not so sure about the last part, as minimum power stage hum is a consequence of equal gain on both sides of the primary, which typically means balancing.

[g1]

He didn't think matching tubes was a thing, or even from side to side on a Twin as far as I know.

The later SF Twins etc. did have the 'output tubes matching' trim pot, but it was all about nulling audible hum, not balancing power output (or symmetrical clipping).
Modern Fender's no longer have this, but they come equipped with matched power tubes, so excess hum due to mis-match is not an issue.

[Helmholtz]

The '65 Twin RI isn't even a 100W amp.
It's rated at 85W like a BF Twin.

Actual full power output before clipping depends on:

1) B+ (squared) at full output, and B+ depends on actual mains voltage. A 10% mains voltage variation can change power output by up to 20%.

2) The residual plate voltage (aka "saturation voltage") of the power tubes, which lowers available plate voltage swing.
Saturation voltage can vary considerably (say between 50V and 100V) between individual new tubes and tends to increase with tube age.

3) Symmetry. With unbalanced tubes, one side will clip earlier than the other, thus lowering available "clean" power.


Regarding THD:

Each preamp triode might add something like 1% THD.
The Vibrato channel can be expected to have a little more THD than the Normal channel.
Fender specifies the 5% THD with the normal channel.

[Enzo]

A 100 watt amp, 15 watts either way won't be a noticeable difference, seems to me anyway.

[DrGonz78]

I think tube matching and the sort leads to further OCD. I am guilty of it when I finish up an amp where everything is perfect but the tubes are not perfectly matched. Then after the compulsion lets go I just play a guitar through the amp and the amp sounds great! job Done!!

edit: There is another process when dealing with SVT amp and other behemoths with 6 output tubes. In those cases it is very important to balance the load.

[Randall]

The way I look at it is, what would Leo do? He didn't think matching tubes was a thing, or even from side to side on a Twin as far as I know. And who complains about the sound of those amps? I just match them like a see saw. I put the heaviest and the lightest kid on one side, and the other two on the other side. Then I get on with my day. If Leo Fender and Jess Oliver didn't worry about if one tube was taking more of the load than the next, than neither should I, I should think.

[Leo_Gnardo]

Another way of assessing whether load is evenly distributed in a group of output tubes, laser infrared thermometer. I'm sure I've mentioned it before. Very affordable and handy as a shop tool say $20-40. Also reveals whether any output tube is hogging current or running cold. I let a set of tubes warm up say 15 minutes or so, handling a nominal signal, whether it's 100% or a fraction, it's easy enough to tell who's running hot & who's not. Only hassle is if tubes are set up in an uneven heating/cooling scheme, such as Mesa's power amps that have a fan blowing at one end of a row of tubes.

[Helmholtz]

I plugged in my 4 ohm 200W dummy load, with AC Voltmeter across it,

Sorry, wasn't clear from your statement above.

[nevetslab]

If you care about accurate output power measurements, you should measure RMS voltage at the output jack lugs, not at the dummy load.
Reason is that each phone plug contact involved can have a contact resistance of maybe 0.1R.
There are 4 contact points involved, which in total might steal around 6W at a current of 4A.

The RMS meter WAS across the output jacks, not across the cable end of the load.

[Helmholtz]

If you care about accurate output power measurements, you should measure RMS voltage at the output jack lugs, not at the dummy load.
Reason is that each phone plug contact involved can have a contact resistance of maybe 0.1R.
There are 4 contact points involved, which in total might steal around 6W at a current of 4A.

[nevetslab]

Well, a bias balance pot like in a SF Twin would be nice here...
What model of Twin is it?

65 Reissue Twin Reverb