ABout Fender silvers AA768 vs AA1069

[hebaton]

Hey, I have a chance to pickup a 69 SF Bandmaster reverb
THe chart is not there so I don't know if it is AA768 or the infamus AA1069 ( so I read. )

Can someone tell me how much is involveb in modding an 1069 back to 768 specs ?

thank you !

[Stevem]

The only difference I can see is the rectifier tube used which makes for about a 20 volt difference in the V+ voltage of the amp.
The 768 version used a 5AR4 low impedance indirectly heated tube, and the 1069 used a 5U4 directly heated tube.

The 768 amp in gear ( play mode) had 465 volts of V+ and the 1069 had 440 volts.
In short the 768 model will retain a bit more clean punch as it nears being driven hard enough to clip the output stage, and the 768 will have a bit more preamp stage gain, also due to this 20 volt increase.

Also both amps use the same power transformer, so the lighter heater load ( 1 full amp less! ) of the 5AR4 tube helps that transformer to run cooler!

Knowing what CBS/ Fender was look for back then this switch was done purely for cost sake, since the 5U4 was and is a cheaper tube !

Also note that any Bandmaster reverb Head is not known for having the most 60 hz hum free reverb, since the reverb pan is so close to power transformer even with the added / needed height of the cabinet!

If you like playing surf music then these might not be the amp for you unless you use a reverb stomp box to get your reverb.

Also note that these amps want a 4 ohm speaker load to produce there 40 watts of clean power, running them into a 8 ohm load which is easier on the amps output tubes will cut down that 40 watts by 33%!

[sluckey]

Put these two schematics on screen side by side and compare...

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

The 768 version used a 5AR4 low impedance indirectly heated tube, and the 1069 used a 5U4 directly heated tube.

Both use a 5U4.

The 768 amp in gear ( play mode) had 465 volts of V+ and the 1069 had 440 volts.

I see 485V on the 768.

Also both amps use the same power transformer

768 uses a 125P7D PT and the 1069 uses a 125P5D. This accounts for the different B+ voltages and is probably the main difference between the sound of the amps..

Both use a bias balance circuit but the circuits and voltages are slightly different. The 768 uses a standard 10K bias pot while the 1069 uses a center tapped 10K pot (4 lugs). I doubt that changing the 1069 bias circuit will have any effect on operation or sound.

One of the Bandmaster Reverb amps had a huge speaker cab with the 2x12 speakers mounted directly above each other. I bought this one new. I don't know which model it was. I'm sure the huge cab sounded a bit different than the smaller cab with offset speakers.

[Stevem]

The AA768 I found on EL34 world uses a 5AR4.

[sluckey]

The AA768 I found on EL34 world uses a 5AR4.

Yes it does. But there are two Bandmaster Reverb schematics in Hoffman's schematic library. I can't believe you picked the one that is almost totally unreadable. About the only things that are readable are the tube names. It would be very hard to compare circuits using that schematic. Here are both aa768 schematics. I know which one I would use.

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

[hebaton]

thanks guys. I am looking for a mainly clean amp for Jazz, from what I read, this is probably not such a good choice. So I guess this stopsher for me

[sluckey]

thanks guys. I am looking for a mainly clean amp for Jazz, from what I read, this is probably not such a good choice. So I guess this stopsher for me

The one I owned was very clean until you cranked the volume up to ear bleed levels. The Bandmaster AB763 that I have now starts sounding gritty (think CCR) at much lower volume levels.

[Stevem]

I my book and it’s need ( jazz ) for clean head room there is only two ways to go in my book.

Either you go with a amp that has about 75 watts of clean power and one very efficient high power 12” driver like a EVM, or you go with 50 watts of clean power and a lot of speaker cone area as in a super reverb.

[wpaulvogel]

I’m in possession of a Silverface drip edge Bandmaster Reverb right now that’s got solid state rectification. It’s identical to the AA768 circuit except the rectifier. The cabinet doesn’t have a tube chart but I think it’s a 1968. I’ve got it up and running great now but I don’t understand why it doesn’t fit a schematic. It has 445 volts plate voltage biased at 42ma’s.

[Stevem]

With SS rectifiers in that AA768 you should have a unloaded ( in standby ) over 500 volts on the hot end of the standby switch if its got a original power transformer, and I would think about 470 something when idling with both outputs drawing 42 ma.