Fender 5F6A oscillates after new OT

[lindope]

I have a homebuilt amp in which I copied the 5F6A circuit mostly. Originally I had in it a cheapo single output tap output transformer with a measured input impedance of about 3176 ohms. This is being driven by 5881 valves. I also had added a switch to wire the 5881's in tetrode configuration--as well as a switch to eliminate the negative feedback resistor. It all worked fine for years, but now I decided to upgrade the output transformer to a better multi-tapped Hammond with an input impedance of 4000 ohms. After installing the new transformer, the amp works fine in tetrode mode, with both no NFB and with a 39K NFB resistor--but when I switch it to pentode mode (standard 5F6A circuit) it works with no NFB but when I switch the NFB resistor in it oscillates with a horrendous low frequency.

I tried lowering the value of NFB resistor to 10K and no difference.

Any ideas?

[Enzo]

Reverse the plate wires. On a standard transformer those would be the blue and brown.

[Helmholtz]

. I also had added a switch to wire the 5881's in tetrode configuration.

I'm with Enzo. Wrong OT phase will turn NFB into positive feedback and can turn the amp into an oscillator.

But you probably mean triode (not tetrode) mode, as the 5881 is a tetrode (not a real pentode).
So tetrode mode would mean standard wiring.

[pdf64]

I think it’s fine to refer to eg 5881 as a pentode, albeit a beam pentode. As the beam forming plates are a 5th electrode.
The beam output valve types that we use don’t seem to have been referred to as tetrodes by their manufacturers through the 50s and 60s. In fact almost all manufacturers used the term ‘beam pentode’, Tung Sol included https://frank.pocnet.net/sheets/127/5/5881.pdf
Only RCA didn’t, preferring the ‘beam power’ term, maybe due to earlier bad blood between them and Phillips over the ‘pentode’ term / suppressor grid technology.
https://frank.pocnet.net/sheets/049/5/5881.pdf
For reasons unknown (perhaps marketing to differentiate them from their EL34 pentode competitor?), GEC revived the ‘beam tetrode’ term in the mid 60s, despite having previously referred to eg the KT66 as a ‘beam pentode’.
https://frank.pocnet.net/sheets/126/k/KT66.pdf
https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf

Sorry for the diversion, I’ve got a bee in my bonnet about ‘beam tetrode’!

[Helmholtz]

The beam output valve types that we use don’t seem to have been referred to as tetrodes by their manufacturers through the 50s and 60s. In fact almost all manufacturers..

You're right about the confusion regarding terms in older datasheets and one might argue that the beam reflectors are not only a constructional detail but something like a fifth electrode.
A beam tetrode is certainly an improvement over a simple tetrode, which has problems with kinks in the plate curves caused by secondary electron emission. (IIRC, simple tetrodes had a tendency to self-oscillate, because the negative slope at the kink means negative resistance.)

Nevertheless there are characteristic differences between (comparable) real pentodes (having a suppressor grid) and beam tetrodes regarding screen current, plate curves, gm and Rp.
And I think they sound different.

[pdf64]

…Nevertheless there are characteristic differences between (comparable) real pentodes (having a suppressor grid) and beam tetrodes regarding screen current, plate curves, gm and Rp.
And I think they sound different.

Absolutely
But as they’re pretty much functionally equivalent, I think of there being output pentodes, with subsets of suppressor grid pentodes and beam pentodes.
Hence it seems reasonable to refer to a 5881 or any beam output valve being pentode connected, ie as opposed to triode or UL / DL?
I suppose that eg an EL34 really could be tetrode connected, as it has a dedicated g3 pin, as opposed to being internally connected to the cathode, as with most other pentodes

[Helmholtz]

But as they’re pretty much functionally equivalent, I think of there being output pentodes, with subsets of suppressor grid pentodes and beam pentodes.
Hence it seems reasonable to refer to a 5881 or any beam output valve being pentode connected, ie as opposed to triode or UL / DL?
I suppose that eg an EL34 really could be tetrode connected, as it has a dedicated g3 pin, as opposed to being internally connected to the cathode, as with most other pentodes

All is fine as long it's clear which wiring is meant.
(I personally prefer to call it pentode wiring/operation with pentodes and tetrode wiring/operation with (beam-) tetrodes.)

It's much more important to agree on triode wiring.

[Enzo]

We can get semantic over pentode/triode, but the two options in the amp are triode and pentode/tetrode and not tetrode vs pentode. The tube is a tetrode or a pentode by its nature, not how it is wired.



(Yes, I am skipping over the separate pin for suppressor grid in some tubes)