PI long pair differences between SR and Bansmaster Reverb

[Helmholtz]

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

Just like early 100W Marshalls.

[Roe]

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

Just like early 100W Marshalls.

Yes, from 1966-68 they were like this, presumably because they followed the schematic without much understanding (as was the case earlier when they copied the 5f6 bassman, keeping the 27k NFB resistor despite changing tap from 2ohms to 16ohms)

[pdf64]

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

That cap seems to have been typically used with 12AX7 paraphase then LTP phase splitter designs, generally without grid stoppers on the output valve control grids.

With cathodynes, the cap wasn't appropriate, and if stability improvement was used, grid stoppers to the output valve control grids seems to have been used.

When they swapped to a 12AT7 in the LTP circuit in the 1963 and onwards designs, that cap was dropped, and typically the grid stoppers which had been used with many cathodynes were brought back.

Cathodyne's have a pretty low output impedance, and I guess that the 12AT7 LTP does too.

So perhaps the rationale used for the stability improvement arrangement used was 'if low output impedance at phase splitter, used grid stoppers but if higher output impedance, use phase lag cap across anodes'?

Yes the 5F6 used both, but belt and braces makes doubly sure. And the design only seems to have lasted a few months before it got superseded (5F6 docs date coded F-EG, 5F6A I-EG) and the grid stoppers were removed.

I can't recall any vintage designs after 1963 using the anode to anode phase lag cap? Though the 2nF caps fitted between the output valve lug 5 and chassis were an alternative implementation of achieving the same effect.

[Helmholtz]

Yes the 5F6 used both, but belt and braces makes doubly sure.

Like Marshall has been doing since the beginnings.

I can't recall any vintage designs after 1963 using the anode to anode phase lag cap? Though the 2nF caps fitted between the output valve lug 5 and chassis were an alternative implementation of achieving the same effect.

Both of my BF SRs ('65 and '66) have the PI cap, though not shown on the AB763 schematic.
I don't think Leo relied on simple design rules but rather did stability testing with new designs.
The cap was then added if found necessary.

Those 2nF power tube grid caps added by the CBS engineers are overkill and have a negative effect on sound.
They are equivalent to a 1nF cap between the PI plates.
I removed them from my 1970 Vibrolux Reverb but had to use a 270p PI cap to get the amp stable with speakers.

[pdf64]

... Both of my BF SRs ('65 and '66) have the PI cap, though not shown on the AB763 schematic. ...

How interesting! What cap value?

And surprising too, as the SR has a seemingly higher spec interleaved OT than the Vibrolux (so less phase shift), and a lower degree of NFB.

[Helmholtz]

How interesting! What cap value?

100pF like my '63 Vibroverb RI.
I don't think stability can be read from the schematic.

[pdf64]

How interesting! What cap value?

100pF like my '63 Vibroverb RI.
I don't think stability can be read from the schematic.

Indeed, OT interleaving isn't shown
The VVRRI uses a 12AX7 LTP,whereas the SRs use 12AT7.
I'd expect a somewhat lower corner frequency with a 100pF lag cap on the former, compared to the latter.
Due to the 12AT7's lower anode resistance.
What are your findings / thoughts?

[Helmholtz]

[
Indeed, OT interleaving isn't shown

Interleaving is used to lower leakage inductance.
But winding capacitance is just as important.
As well as parasitic wiring capacitances in the amp's layout.

I mentioned the VVRI because the original (I think 6G16) schematic didn't show the PI cap either.
BTW, the OTs of the VVRI and the SF VLR measure almost identical.
The VVRI sounds much better to me when using the same speakers.

The VVRRI uses a 12AX7 LTP,whereas the SRs use 12AT7.
I'd expect a somewhat lower corner frequency with a 100pF lag cap on the former, compared to the latter.

Yes, but even with the 12AX7 the corner frequency should not be below 20kHz.