Plate load resistors on LTP PI

[brewdude]

Many amps use 100k/82k for the PI anode resistors. Some use 100k/100k in order to imbalance the PI. What is the expected effect or tonal response of either reducing them to 82k/82k? Or, raising them both to 120k/120k? How does the PA tube effect these schemes? Would one be more effective for a 6V6 and the other better suited for a 6L6?

[JamesHealey]

An amp I made has dual 82k just because it's all I had.. sounds fine to me.. experiment draw your own conclusions.

I know Randall Aiken's Phase Inverter design on his website uses dual 82k..

They balance the signal for sure but having a large tail resistor like 56k vs 10k means dual 100k is near enough balanced anyway.

[brewdude]

Actually, the reason I asked was because I recently changed the plate resistors on the PI of my 6V6 equipped D'Lite to 120k/120k and, so far I think I like it. I was just wondering if there is some rule of thumb, or something that would create an issue if it was outside the normal spec's.

[VacuumVoodoo]

PI balance by itself means nothing. You need to think of the whole chain of imbalances besides PI: power tubes matching idle plate current and transconductance at operating point as well as DC and ac balance of output transformer. Each imbalance contributes to the tone and it's not wholly unimportant where in this chain you balance or intentionally unbalance the 2 phases.

[brewdude]

Honestly, I'm not particularly concerned with whether the PI plates are "balanced" or not. I was more interested in the other potentially different characteristics that using an equal pair of lower value resistors vs. an equal pair of higher value resistors.

[ampdoc1]

Try a 220k or 240k pot where you can "inject" imbalance if you want. I've tried this on a couple of amps, and I like the harmonic content when the PI is unbalanced. But this gives you the choice.

a'doc

[Andy Le Blanc]

Put a pot in the plate circuit and balance the DC on each side of the inverter.
give it a listen.

Then try balancing the AC output, give it a listen.
the larger plate resistor value will give you a little more gain at the stage.
Circuit design tries to keep the operating point of the tube around the most
linear area , when you change the plate load in relation to the bias you will
eventually see more distortion, thats the color your hearing, a slight
rounding of one side of the wave form.

[brewdude]

Thanks for the replies. I guess there is nothing dangerous about using the 120k pair then. I think it sounds pretty good... So far, anyway. I intend to keep it this way for the time being.

Have any of you guys tried the higher plate resistors in the PI?
If so, what are your observations and/or opinions?

[Structo]

I run the 110K / 120K plates on my D'lite (6L6)
I also have a 5K trimmer between them.
I set that trimmer for the best sustain and feedback.


Randall says:

Higher values will result in less high frequency response. When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal.

In other words, the two plate resistors are made unequal in an attempt to balance the output to the power tubes.
But since nothing is perfect, there will always be enough of a difference for those lovely 2nd order and higher harmonics we love.

[brewdude]

For what ever reason, the 82k/100k sounded kind of flat or sterile to me and I had a hard time finding a tube that balanced any where near the expected plate voltages. I don't have a trim pot to balance the two triodes. I then tried them at the "tweaked" schematics values, but that wasn't much better. Then, I tried the 100k/100k combo and it sounded much livelier and felt more dynamic. Over the weekend I tried the 120k/120k and I like it so far. However, I haven't really had chance to turn it up or play with other instruments yet.

[Tone Lover]

Structo how does that 5k trimmer look between the 110 and 120k plate resisters in other words how do you hook it up. Dont mind me just a newbie trying to learn something. Bill

[Sonny ReVerb]

You can see the 5k pot setup in the #124 schem and layout:

https://tubeamparchive.com/viewtopic.php?t=5719

[Structo]

Yes like Sonny eluded to, #124 shows how this is done quite well.

You run your PI B+ to the wiper of the trimmer, then the plate load resistors along with the coupling caps connect to the two outside lugs of the trimmer.

The way Tony of VVT amps described it to me when adjusting it was, to turn the amp up pretty load in clean mode, and using a single coil guitar....have the neck pickup on and lightly brush a chord and let it ring.
This is where a buddy could come in handy.
A strat works well for this.

Have him brush the strings and you adjust the trimmer.
I've shocked myself doing this before trying to do both so, it's a safety thing so have someone help you.

As you brush the strings, listen for increased sustain and the note wanting to bloom into harmonic feedback. The sweet kind not the bad howl. If you get that you probably have the amp up too loud.
Anyway, make very small adjustments with your screwdriver. There is a lag between the adjustment and effect due to the capacitors there, so turn the trimmer a smidge and brush the strings and wait and brush again.

You may have to back an forth in the whole range of the trimmer to get a feel for it.
You may have to try a different tube if the first one doesn't get you there, some just aren't balanced right and don't give up the goods.
Once you find the right combination you will know it.
It can really sweeten up the whole amp when you get this right.
It's worth the effort for sure.

Good luck!

[David Root]

I've done #124 w/5K trimmer. I found it difficult to tell the optimum off-balance setting with a LP.

I put a 5K trimmer in my 50W Princeton '70s ODS with the same result. Probably my just desserts for such a heretical mod!

I think 10K trimmer is probably better, about to try that with a 4x6L6GA Bluesmaster HRM circuit in a Bassman chassis/head with 91K/110K plates (which I am not sure will balance perfectly anyway with a 10K trimmer) and a GE 12AX7 long plate PI (recommended by Brandon, not specifically for a Bluesmaster PI, I think that was for a low plate PI). We shall see.

[andrew]

Ampdoc1, Would an Alfa 220kL pot work? Thank you.