PI long pair differences between SR and Bansmaster Reverb

[hebaton]

I just got this AA768 Bandmaster Reverb ( thanks you Slucky )
Looking to see if I can somehow "black face" it even if I know thwre never was a BF Bandmaster Reverb.
Looking at the PI I find those two 47K resistors where the BF circuit comes with 100K and a 82K but the voltage to the plates of the 12AT7 is 230V for the AB763 and 340V ont the AA768
WHat is the effect of those differences both are then going to 6L6GCs trough .1 uf caps...

[GlideOn]

It's not just the 47k/47k plates, it's the 330k/470r/330k Long Tail config too. Further, the Power tubes are wired in Ultralinear config. This is definitely the later Silverface spec and more "clean."

If it's a more project type of amp there's plenty of worthwhile mods you can perform without screwing up the wonderful high headroom clean tone.

I worked on an AA768 Twin Reverb which is the same preamp circuit except with more power tubes.

I "blackfaced" the PI section to 82k/100k and the Long Tail to 1M/470r/1M spec and didn't make a huge difference...until I put a 12AX7 in place.

A simple 12AX7 made the biggest difference in gain structure and creaminess, even with the Ultralinear power section intact.

The Vibrato/Reverb channel was left alone and still had crazy headroom (as expected for a 130w amp), but it definitely got a nicer albeit dirtier tone when really cranked at venue volumes.

I also swapped the Pre-Phase Master Volume for a LarMar + a higher gain Normal channel and it worked exceedingly well. Post-Phase works great in an AA763 circuit, just had to source a lower profile dual Ganged 100k/100k pot to fit inside the slanted panel. Hilarious concept, but the end result was that you could actually get really great cranked bedroom tones out of a 130w Twin.

Short version: Try a 12AX7 first. Then Blackface the PI later if it's not enough. IMHO, it's not as dramatic as people make it out to be without first considering a tube swap.

[Roe]

some of us like the SF 12at7 driver.

[GlideOn]

some of us like the SF 12at7 driver.

If I'm being honest I prefer it in a clean Fender too. More chime and bite.

I gave example of using a 12AX7 PI as it was for a friend who picked up a beatup UL Twin and was looking to make something else out of it. It had ridiculous headroom and wattage to spare so it could get away with being not so pristine clean at all times. It ended up being more of a Dumble thing without the Dumble pricetag.

I have no idea what OP wants to get out of his, but with my experience "Blackfacing" the PI values didn't have as dramatic effect as changing the tube type. Honestly change maybe the PI entry cap to a .022 or .0068 and call it a day. Far more effective way to clean up the muddy bass of a SF circuit.

We read things online thinking they are somehow better and have mystical properties, this is instance of "Blackfacing" is one such example. It was underwhelming and didn't really come into its' own until far more was done and by that point it was something else.

[pdf64]

I just got this AA768 Bandmaster Reverb ( thanks you Slucky )
Looking to see if I can somehow "black face" it even if I know thwre never was a BF Bandmaster Reverb.
Looking at the PI I find those two 47K resistors where the BF circuit comes with 100K and a 82K but the voltage to the plates of the 12AT7 is 230V for the AB763 and 340V ont the AA768

It seems normal for 100k anode load resistor will drop more voltage than a 47k?

WHat is the effect of those differences both are then going to 6L6GCs trough .1 uf caps...

Slightly lower LTP stage gain, but more significantly, the maximum signal output voltage of the LTP will be about halved.
That will reduce the overdrive recovery time, hence somewhat higher levels of power amp overdrive can be tolerated before farty blocking distortion becomes apparent.

The 100k grid leaks that load the LTP outputs should act further improve both the above.

https://www.ampbooks.com/mobile/amplifi ... alculator/

I suggest to beware of reverting to a typical AB763 power amp. The 125A6A / 022848 OT may be crude low spec, with too much high frequency phase shift to maintain stability with the feedback loop.
The low pass filters at the 6L6 pin 5 control grids may have been a necessary counter measure to improve stabilty.

See the nasty https://www.thegearpage.net/board/index ... t-38846765

https://el34world.com/charts/Schematics ... 8_sch.jpeg

[pdf64]

It's not just the 47k/47k plates, it's the 330k/470r/330k Long Tail config too. Further, the Power tubes are wired in Ultralinear config. This is definitely the later Silverface spec and more "clean."

If it's a more project type of amp there's plenty of worthwhile mods you can perform without screwing up the wonderful high headroom clean tone.

I worked on an AA768 Twin Reverb which is the same preamp circuit except with more power tubes.

Have you got a schematic for that TR?
The closest I can find is the AA769. That has the 330k LTP grid leaks you describe, whereas the AA768 BR uses 1M.

https://el34world.com/charts/Schematics ... 8_sch.jpeg

https://el34world.com/charts/Schematics ... _schem.pdf

[GlideOn]

CBS 70 UL.jpg

Referring to this.

Appears "Ultralinear" were 1978 and beyond.

Entirely depends what year model# OP has.

There were a transition period as with the AA763 schematic where 47k were being used for Plate R in the PI. This too was adopted by other Fenders while simultaneously moving to cleaner Long Tail PIs that really take advantage of the lower gain, higher current triodes. One of the few things Silverface era did well on improving upon, actually.

If it is the earlier and already has, adds more to the point that Blackface PI doesn't do a whole lot.

It's easy enough to try, though.

There are so many other things that can be done to this circuit to "warm it up" though. We really need to know what the OP wants out of his amp.

[pdf64]

CBS 70 UL.jpg

Referring to this.

Appears "Ultralinear" were 1978 and beyond.

Entirely depends what year model# OP has. ...

Do you mean the SR reference? There seems no mention of it other than the thread title.
The Bandmaster Reverb is AA768

I just got this AA768 Bandmaster Reverb ...

I think the revisions with OT primary taps for the screen grids were first introduced in 1976.

There were a transition period as with the AA763 schematic where 47k were being used for Plate R in the PI

Yikes I'm finding this discussion confusing!
Do you mean the Bandmaster AA763 or the Super Reverb AA763?

https://el34world.com/charts/Schematics ... ematic.pdf

https://el34world.com/charts/Schematics ... _schem.pdf

Whichever, I can't see the 47k LTP anode load you're referring to in any early 60s model?

Rather the 47k LTP anode loads don't seem to have been introduced until the May 1968 revisions of various models, eg https://el34world.com/charts/Schematics ... _ab568.pdf

[pdf64]

Oops

[GlideOn]

CBS 70 UL.jpg

Referring to this.

Appears "Ultralinear" were 1978 and beyond.

Entirely depends what year model# OP has. ...

Do you mean the SR reference? There seems no mention of it other than the thread title.
The Bandmaster Reverb is AA768

I just got this AA768 Bandmaster Reverb ...

I think the revisions with OT primary taps for the screen grids were first introduced in 1976.

There were a transition period as with the AA763 schematic where 47k were being used for Plate R in the PI

Yikes I'm finding this discussion confusing!
Do you mean the Bandmaster AA763 or the Super Reverb AA763?

https://el34world.com/charts/Schematics ... ematic.pdf

https://el34world.com/charts/Schematics ... _schem.pdf

Whichever, I can't see the 47k LTP anode load you're referring to in any early 60s model?

Rather the 47k LTP anode loads don't seem to have been introduced until the May 1968 revisions of various models, eg https://el34world.com/charts/Schematics ... _ab568.pdf

...All of the above?

The circuits are so similar that I think you'd be hard pressed to hear a difference. Aside from the cripplingly loud 130w UL Twin Reverb I doubt you'd be able to hear much between the Super Reverb and the Bandmaster of that era.

If we're talking differences in tone between Fender circuits, I think the Silverface Bassman would be far better example of a contrast as it made use of multiple (heavily attenuated) gain with their triodes stages vs using them for Vibrato and Reverb.

In any event, what are we debating? My answer was trying to deduce what the OP was after, which was to warm up the preamp (?) a bit.

Raise the plate resistors, sure.

But why stop there?

[Roe]

why did fender leave out the 47pf fizz cap across the anode pins here?

[Helmholtz]

why did fender leave out the 47pf fizz cap across the anode pins here?

A low value cap between the plates of an LTPI is often used to prevent ultrasonic HF oscillation at certain conditions.
Its value (and if it's necessary) depends on the circuit, amount of NFB, layout and particularly on the OT used.
The method is called lag compensation because is causes a HF phase lag, preventing the NFB turning into positive feedback.
Amp instability shows best with square wave testing.
In case of doubt I'd use the cap.

[pdf64]

why did fender leave out the 47pf fizz cap across the anode pins here?

I think Fender moved to using grid stoppers to mitigate ultrasonic oscillation.

[Helmholtz]

I think Fender moved to using grid stoppers to mitigate ultrasonic oscillation.

As the input capacitance of power pentodes/tetrodes is typically below 20pF, a 1.5k grid stopper has little to no effect below the MHz range.
It would take huge value grid stoppers to get the same effect as a 47p cap between the PI grids.
Already the 5F6 BM used both, PI cap and grid stoppers.

[Roe]

why did fender leave out the 47pf fizz cap across the anode pins here?

I think Fender moved to using grid stoppers to mitigate ultrasonic oscillation.

but the grid stoppers were introduced much earlier than the silverface PI. the 1k5 grid stopper can be seen on tweeds, blonds and the ab763 blackface

See here for a strange case with grid stoppers on two of four 6l6s: https://www.prowessamplifiers.com/schem ... _schem.pdf