OT mod for Princeton Reverb issue???

[Govmnt_Lacky]

Building a Princeton Reverb and ran into an issue. I am using an "updated" Output transformer that has 4, 8, and 16 ohm taps. The original PR only had an 8 ohm output. Looking at the circuit (link below) you can see that the output (both the internal and external jacks) run back through as the negative feedback to V3B.
If I am using a multi-tap output, how would I wire the NFB? I really do not want to use an output impedance selector switch (want to have a jack for each output impedance) so... how can I get around this? Or... can I???

*** using the AA1164 version ***

[Chuck H]

If your OT secondaries are actual taps then you would just hard wire the NFB from the 8 ohm jack/tap. If your OT has any complicated parallel/series requirements for secondary impedance (like the standard Hammond 16xx units) it would be more complicated.

[Govmnt_Lacky]

If your OT secondaries are actual taps then you would just hard wire the NFB from the 8 ohm jack/tap. If your OT has any complicated parallel/series requirements for secondary impedance (like the standard Hammond 16xx units) it would be more complicated.

The OT for the project is the Hammond PT-1760E. It has individual outputs of 4, 8, and 16 ohms. I want to have each output impedance going to it's own output jack HOWEVER, I do not know how this plays into the NFB.

For example... if I plug my cabinet into the 16 ohm output jack BUT, the 8 ohm is the only one wired into the NFB loop. How would that work???

Sorry if I am sounding dumb on this....

[hylaphone]

The OT for the project is the Hammond PT-1760E. It has individual outputs of 4, 8, and 16 ohms. I want to have each output impedance going to it's own output jack HOWEVER, I do not know how this plays into the NFB.

For example... if I plug my cabinet into the 16 ohm output jack BUT, the 8 ohm is the only one wired into the NFB loop. How would that work???

Sorry if I am sounding dumb on this....

Keep the NFB connected to the 8ohm tap, it will function no matter which jack you use

[Chuck H]

Keep the NFB connected to the 8ohm tap, it will function no matter which jack you use

This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[mozz]

Loading down/ no load that tap will not change the feedback voltage? Just curious.

[Helmholtz]

For example... if I plug my cabinet into the 16 ohm output jack BUT, the 8 ohm is the only one wired into the NFB loop. How would that work???

Consistent NFB requires a fixed fraction of the OT secondary voltage to be fed back to an earlier stage.
This fraction depends on the ratio of primary turns to the secondary turns where the NFB connects.
In a transformer all turns/windings are coupled, so the feedback voltage at the 8 Ohm winding will stay the same regardless if an 8 Ohm speaker is connected to the 8 Ohm tap or a 16 Ohm speaker is connected to the 16 Ohm tap.

[Govmnt_Lacky]

So.... even if I have 3 separate Output jacks (one for each of the 4, 8, and 16 ohm outputs) all I have to do is connect the NFB connection to the 8 ohm output jack and I am good no matter which output jack I use??

[Helmholtz]

Loading down/ no load that tap will not change the feedback voltage? Just curious.

The difference is insignificant as secondary DCR is very low and reflected primary impedance will be the same in both cases.

[Chuck H]

So.... even if I have 3 separate Output jacks (one for each of the 4, 8, and 16 ohm outputs) all I have to do is connect the NFB connection to the 8 ohm output jack and I am good no matter which output jack I use??

Yes.

[Helmholtz]

So.... even if I have 3 separate Output jacks (one for each of the 4, 8, and 16 ohm outputs) all I have to do is connect the NFB connection to the 8 ohm output jack and I am good no matter which output jack I use??

Yes.
But you should use only one output at a time connected to rated load impedance.

[Chuck H]

Yes.
But you should use only one output at a time connected to rated load impedance.

THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can't, for example, plug an 8 ohm cabinet into the 8 ohm jack AND a 16 ohm cabinet into the 16 ohm jack and present a proper load to the power tubes. For this reason I don't like the idea of a jack for each impedance because the potential for an error goes up. Also, there's no way to plug in two cabinets at the amp. With a switch you can use an extension speaker jack and just choose the correct impedance tap closest to the averaged load of your cabinets.

[Govmnt_Lacky]

Thanks all! I fully understand that you cannot connect more than one impedance load at a time.

[Greg Robinson]

THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can't, for example, plug an 8 ohm cabinet into the 8 ohm jack AND a 16 ohm cabinet into the 16 ohm jack and present a proper load to the power tubes. For this reason I don't like the idea of a jack for each impedance because the potential for an error goes up. Also, there's no way to plug in two cabinets at the amp. With a switch you can use an extension speaker jack and just choose the correct impedance tap closest to the averaged load of your cabinets.

Does open up the option of connecting mismatched cabs though - an 8ohm cab on the 4ohm out and a 16ohm cab on the 8ohm out provides nominal primary load. My preference is for 2x4ohm jacks, 2x8ohhm jacks, and a single 16ohm. Using jacks with a ring shunt you can still wire this arrangement to short the secondary with no plug inserted like typical Fender wiring for no-load protection. Clear labelling is important of course, but that's always true.

[g1]

My preference is for 2x4ohm jacks, 2x8ohhm jacks, and a single 16ohm. Using jacks with a ring shunt you can still wire this arrangement to short the secondary with no plug inserted like typical Fender wiring for no-load protection.

I don't understand how this works? For the same reason that the Fender doesn't have the shunt on the 'ext.spkr' jack (and Marshall's don't have 'no-load' protection).