I just got, on eBay, a non-working silverface AA769 without speakers. Has a wonderful Mather aftermarket cabinet. It doesn't appear from the pictures of the guts to be either an AB763 or AC568. It really looks like an AA769 like the Dual Showman Reverb.
I need some speakers.
The AA769 schematic says 8 - 16 ohms for the speaker load, with two jacks in parallel, one for internal speakers and the other for an extension cabinet. So I assume the internal speakers would be 16 ohms. Does anybody know whether that was two 8 ohm speakers in series of two 32 ohm speakers in parallel? Or some other combination?
I need some speakers.
The AA769 schematic says 8 - 16 ohms for the speaker load, with two jacks in parallel, one for internal speakers and the other for an extension cabinet. So I assume the internal speakers would be 16 ohms. Does anybody know whether that was two 8 ohm speakers in series of two 32 ohm speakers in parallel? Or some other combination?
You sure the jacks are just in parallel and nothing else?
Some of those amps had an unusual jack connection which also switched transformer taps, please post the schematic to check.
Some of those amps had an unusual jack connection which also switched transformer taps, please post the schematic to check.
Here's the AA 769 schemo and it shows the internals wired in parallel and output total at 4 ohms, so the internals would be two 16 ohms in parallel for 8 ohms and an extension 8 ohm cab in parallel for total of 4 ohms. The OT only has two secondary wires, so no switching. Everything is in parallel, internals and ext cab.
View attachment Fender_twin_reverb_aa769_schem.pdf
View attachment Fender_twin_reverb_aa769_schem.pdf
Last edited:
Twin
Yes, I checked several more places, and indeed it's generally regarded as a 4 ohm output transformer, which really surprised me. But...I already bid on a pair of very nice low-hours 8-ohm Italian-made Jensen P12Q. So the question is whether they will handle the power in parallel? Jensen calls it a 40 watt rating and 80 watts musical power. So...is that like 30 watts with a non-musical distorted square wave? So what's an AA769 capable of putting out full-out and distorted? Maybe 90 watts? Is that asking for trouble?
Yes, I checked several more places, and indeed it's generally regarded as a 4 ohm output transformer, which really surprised me. But...I already bid on a pair of very nice low-hours 8-ohm Italian-made Jensen P12Q. So the question is whether they will handle the power in parallel? Jensen calls it a 40 watt rating and 80 watts musical power. So...is that like 30 watts with a non-musical distorted square wave? So what's an AA769 capable of putting out full-out and distorted? Maybe 90 watts? Is that asking for trouble?
It does look as if you will be pushing those Jensens to the max under full distorted load. Maybe the answer is to wire them in series for 16 ohms and add an ext cab at 16 for total of 8 ohms. The amp is made for 8 ohms since without an ext cab, it originally had an 8 ohm load with just the internal speakers. The ext cab will absorb half the power so relieve the Jensens from being pushed to the max. If you don't intend on maxing out the amp, re-wire the internals in parallel for 4 ohms. The beauty of this OT is that they designed it for 4 or 8 ohms without adding a second tap. From what I have read, generally Fenders tolerate a one degree mismatch well, as in 4 ohm speaker on 8 ohm tranny. That is GENERALLY, not written in stone.
Those Twin output transformer are 2K Plate to Plate impedance.... made by Schumacher in Chicago in those days.,,,,
The Fender Twin of that era never used 16 Ohm speakers....
It used 2 x 8 Ohm speaker to make 4 Ohm load ... 12" Jensen Black frames or Utah speakers..
Ceramic magnets... Occasionally JBL D-120F speakers as option...
The extension cab jack is a marketing dept add on that... With the 4 Ohm extension cab plugged in , the transformer plate load will dip to 1K and still work , since the wire gauge in the OT was hefty at 1000 CM/A .... Tubes are not linear current sources, and the current will limit under normal playing conditions at 1K plate load..
In Reality, no one really plugs in extension cabs into the twins....so not to worry ...
The Italian Jensens are not made with the same materials as the original speakers...so don't expect to get a vintage tone or vibe from that amp...
The Fender Twin of that era never used 16 Ohm speakers....
It used 2 x 8 Ohm speaker to make 4 Ohm load ... 12" Jensen Black frames or Utah speakers..
Ceramic magnets... Occasionally JBL D-120F speakers as option...
The extension cab jack is a marketing dept add on that... With the 4 Ohm extension cab plugged in , the transformer plate load will dip to 1K and still work , since the wire gauge in the OT was hefty at 1000 CM/A .... Tubes are not linear current sources, and the current will limit under normal playing conditions at 1K plate load..
In Reality, no one really plugs in extension cabs into the twins....so not to worry ...
The Italian Jensens are not made with the same materials as the original speakers...so don't expect to get a vintage tone or vibe from that amp...
- Status
- Not open for further replies.