haven't heard of this before - any thoughts?

[gary]

John Chambers of Champ Electronics ( google chambonino ) makes the following comments about a mod to Deluxe Reverb


".....indulged in letting me do a little mod that I always recommend on any amp. This is to fit a 200k pre-set pot in place of the two plate resistors of the phase splitter, usually an 82k & 100k. This small but effective addition balances not only the splitter tube & associated components, but the output tubes (no matter how many) and any balance deficiencies within the output transformer too. It works wonders!"

Taking John's vast experience into account, what do you think of this mod? Is it more about output performance than tone? Could someone without a scope test it safely?

[selloutrr]

it makes sense. Fender used a ball park blanket approach that worked but wasn't the best possible.

the actual values used will depend on the rest of the circuit's component values, voltages, etc.

You could start with the pots at the value of the resistors they are replacing and work down by ear. but a scope is highly recommended so you don't cause any damage.

[es345]

I used this method in a little modified way by exchanging the 82K with 47K and a 100K pre-set pot in series and let the 100K like it is. That works quite well.

Some guys want to run the powerstage unbalanced, it creates a specific sound in powerstage overdrive. If you have a pot with >500V isolation you mount it at the backside and adjust the balance to the sound you want.

[Andy Le Blanc]

Push-pull assumes two identical signals 180 degrees out of phase.
Its what you see with AC and DC balance in hifi amps.
closely matched resistors in a cathodyne inverter can really help.
Any inverter type can really benefit from improved balance.
You can use it to compensate for tube age drift too.

Or you can go for a specific difference to reduce push pull cancellation
of even order harmonic distortion at clip, more tone color...

I think there is a Peavey patented tone thing that put a user control on the
inverter balance, can't remember at the moment....

just run an 1k tone and use a DDM set to AC and find zero +\- within a few hundredths or thousandths ma..

[JamesHealey]

I've spoken t john on the phone about this mod before. He's a clever chap I've used this mod on a hifi build before

[Reeltarded]

Its what you see with AC and DC balance in hifi amps.
..

Cool! Is that how they get that sound? I love Back in Black! One of my favortie records.

[Zippy]

Very similar to the Dumble setup - except that the Dumble trimmer is much smaller and more idiot proof to someone taking the adjustment to the far end of its travel.

P.S. Don't EVEN start asking how best to set such a trimmer.

Oscilloscope or ears???

Balance output voltages, balance PI voltages, or tweak 'til it sounds good???

[Reeltarded]

Turn the amp up and it's just south of stops humming.

[ampdoc1]

I've used a 250K off the plates to balance/unbalance the PI. I used a scope and my ears. For my taste I found I liked the harmonics present with a little imbalance. It's a simple thing to add or replace to almost any amp.

a'doc1

[Zippy]

Turn the amp up and it's just south of stops humming.

This is very different from the Fender "hum balance" control - potential of bloom and enhanced harmonics in some amps.

[tubeswell]

And while you are busy twiddling with the plate resistance values, don't forget that the size of the tail resistor in a LTP also affects how 'balanced' the output signal strength is on both sides (albeit at the expense of gain as the tail resistor size increases).