Phase inverter plate resistors

[MrCoolGuy]

This might be a dumb question, but...
Why do blackface Deluxe Reverb layouts
(AB763) show two 100k PI plate load resistors, but the schematics show 100k and 82K.

I know the AA763 used two 100k resistors... I think they were left on the layout when they made AB763. They also left the mid cap value.

My main question is why would they do that? Isn't the imbalance off 100K vs 82K
Needed?

 

[Pete Farrington]

I don’t see what you mean https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf
If referring to a schematic, it’s a good idea to include a link / attachment for it.

 

[King Fan]

You mean these? Images with original markup by Rob Robinette...

 

[Paul G.]

It was a change. The different resistors balance the output of the two sides of the PI.

 

[King Fan]

It was a change. The different resistors balance the output of the two sides of the PI.

Good answer. Ever struggle with version control on your computer? Think what it was like for folks doing ink-and-ruler tech drawings they had to send to a specialized print shop -- that were then produced in large runs. Fender was penny-wise about something as simple as outdated tube charts; their schemes and layouts are fairly rich with inconsistencies, not just from one to another, but even more so over time, and as compared to what they were actually building on the shop floor.

And before you decide they were unprofessional, compare Fender to Gibson, where names, model numbers, and schematics were changing 'everything everywhere and all at once.'

 

[Lowerleftcoast]

Some people like change and some don't.

 

[MrCoolGuy]

Yes, I understand what you are all saying.
There were typo-type mistakes on layouts and schematics...
But I don't think this was a mistake... at least not at first. Didn't the AA763 actually use two 100K PI plate load resistors?
I know, @Paul G. , that the mismatch is necessary to balance the PI output. That is why I'm asking why all amps don't do this.


I know some other amps (i wish i could remember specifically) have matching PI plate load resistors... whether they are 82K, 100K, 220K... they match.


I guess my question is... do they need to be offset or not? Why do some amps have matching pairs?

(I know I'm not giving example schematics here... the people that can answer this question accurately know exactly what I'm talking about...)

Does it have something to do with the value of the Long-tail -pair tail resistor?

 

[MrCoolGuy]

You mean these? Images with original markup by Rob Robinette...

View attachment 1097400

View attachment 1097402

Thank you, @King Fan .
That's the exact layout and schematic I was talking about.

It's the same on many.

 

[2L man]

82k to 100k ratio is not "matched" but closest using standard resistors. HiFi builders forums are good places to read PI balancing.

 

[Paul G.]

...And before you decide they were unprofessional, compare Fender to Gibson, where names, model numbers, and schematics were changing 'everything everywhere and all at once.'

Even early Ampeg amplifiers will have circuits that don't match any schematic. Amps with the same model designation, similar dates of manufacture, will vary. Changes were made on the fly, I guess.

 

[__HKGuns__]

Here is a little web based calculator that allows you to change values and see the results, if you don't want to do it on paper.

LTP Calculator

 

[King Fan]

Yes, I understand what you are all saying.
There were typo-type mistakes on layouts and schematics...
But I don't think this was a mistake... at least not at first. Didn't the AA763 actually use two 100K PI plate load resistors?
I know, @Paul G. , that the mismatch is necessary to balance the PI output. That is why I'm asking why all amps don't do this.


I know some other amps (i wish i could remember specifically) have matching PI plate load resistors... whether they are 82K, 100K, 220K... they match.


I guess my question is... do they need to be offset or not? Why do some amps have matching pairs?

(I know I'm not giving example schematics here... the people that can answer this question accurately know exactly what I'm talking about...)

Does it have something to do with the value of the Long-tail -pair tail resistor?

Mistake, maybe, but equally or more often, *design or build change.*

But you’re not asking about the Fender diagrams as such, but why LTP plate loads are unbalanced, and whether it matters? See the short, clear section here about ’Balance:’

The Valve Wizard -Long Tail Pair

 

[schmee]

I think it was a simple mistake. The BFDR layout appears to maybe be a whole new layout drawing as opposed to just up dated. Also many BF and Brown face amps have the proper 82k/100k setup shown.
The non Reverb Deluxe has the 82k/100k setup with no revision.
Even the prior Brown Deluxe has that setup.

But BOTH AA and AB revisions of the Deluxe Reverb have dual 100k resistors shown.

 

[Pete Farrington]

Please see the DR AB763 schematic and layout linked to in post #2.
Drawing date codes are both C-FD.
Both show 82k and 100k LTP anode load resistors.

 

[schmee]

Please see the DR AB763 schematic and layout linked to in post #2.
Drawing date codes are both C-FD.
Both show 82k and 100k LTP anode load resistors.

That's interesting. Here's a version of the same C-FD showing 100k/100k! That version you posted has super clean legible lines etc on it, unlike nearly all the old Fender ones. I wonder if it's a more recent "re-draw"?

 

[MrCoolGuy]

That's interesting. Here's a version of the same C-FD showing 100k/100k! That version you posted has super clean legible lines etc on it, unlike nearly all the old Fender ones. I wonder if it's a more recent "re-draw"?
View attachment 1097681

Exactly. All the AA763 and AB763 layouts that show 82k on one side are obviously edited. The schematics, however, do show 82k / 100k.

I believe AA763's actually did come with 100k/100k...

Mistake, maybe, but equally or more often, *design or build change.*

But you’re not asking about the Fender diagrams as such, but why LTP plate loads are unbalanced, and whether it matters? See the short, clear section here about ’Balance:’

The Valve Wizard -Long Tail Pair

Thank you, @King Fan .
Yeah, sounds like it doesn't matter too much . I know there are a lot of other variables to consider to have balanced PI output. I think I'll use a 250k pot on one side and hear the difference.

 

[schmee]

If the originals are as far off as many of the 100K CC resistors, even if 82k + 100K new they may be equal now!

 

[Jasonpatrick]

I thought it was 82 /100 because some imbalance in the PI is actually a good thing for harmonics etc when it comes to guitar amps.

 

[2L man]

I thought it was 82 /100 because some imbalance in the PI is actually a good thing for harmonics etc when it comes to guitar amps.

Obviously out of standard resistor values 82k and 100k function better balanced than two 100k or two 82k.

With 100k a 90k make operation balanced better in "typical" LTPPI. I have sometimes installed parallel two 180k to a place of 82k.

I agree with you that push pull impalance can sound sweet because of increased 2nd and other even harmonics. Howevet there come a potential danger when impalance cause Output Ttransformer core Saturation when playing loud and long time. This increase current flow thru tubes and OT and when OT core saturate there begin to accumulate heat which eventually can destroy the OT. Occasional hard picked notes often not make any harm although OT is chosed just bit too small and saturate only briefly