Ramblin Ray
Tele-Afflicted
I have a line on a twin with the AA270 circuit, although it has AA763 stamped on the chart. What are the differences and if I get it, should I bother having it changed over? Thanks!
'70 Twin Reverb AB763 vs AA270
- Thread starter Ramblin Ray
- Start date
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SoK66
Tele-Afflicted
The chart circuit number is meaningless on a SF amp. The AA270s were the "revised" CBS circuits thet eliminated the atrocious mess they created in mid 68 through 69. If it's a clean example, unmolested, etc., it can make a great amp. Those are easily blackfaced, which just means removing the power tube suppression caps, changing a few resistors in the phase inverter circuit and either converting it to bias adjust, or, adding a bias adjust pot to the existing bias balance circuit. Piece 'o cake. I've never done one and had the amp's owner ask me to convert it back to SF specs.
I once had to contact fender to find a schematic for a non-pull boost master volume twin reverb. I could not find this configuration anywhere. I didn't even know this configuration existed.
Most of the schematic updates are minimal, ie Princeton reverb aa1174 and b1270, one shows a 70uf bias cap the other a 25uf. One has a 22k bias resistor, the other a 27k. And one has a small value cap across a resistor, I believe, in the reverb circuit.
Most of the schematic updates are minimal, ie Princeton reverb aa1174 and b1270, one shows a 70uf bias cap the other a 25uf. One has a 22k bias resistor, the other a 27k. And one has a small value cap across a resistor, I believe, in the reverb circuit.
bigben55
Friend of Leo's
My '70 was already blackfaced when I got ut, with verb and vib on both channels. All I can say with certainty is that it sounds glorious.
I have a question. The SFTR are supposedly 100 watts instead of the blackface 85 watts. If you blackface a silverface, is it now 100 or 85 watts?
I have a question. The SFTR are supposedly 100 watts instead of the blackface 85 watts. If you blackface a silverface, is it now 100 or 85 watts?
umasstele
Tele-Afflicted
My '70 has AB763 stamped on the tube chart. They were using up old charts, the REAL circuit mine had in it was an AA769
In this particular case, what the chart says is incorrect.
In this particular case, what the chart says is incorrect.
SoK66
Tele-Afflicted
Not a surprise, they sound fantastic. No, it will still be 85w. Believe me, that's PLENTY of amp.
fauxsuper
Tele-Afflicted
Wattage ratings on guitar amps are nebulous anyway as there's no official way to do it. Power ratings on stereo and consumer audio equipment use a distortion figure as a qualifier. You'll never see that on a guitar amp. I have a hunch that the 100 watt figure was more marketing department than engineering. More important anyway, is how the amp responds to dynamic inputs, since you're not going to play steady state test tones. And at that, the difference between 85 and 100 watts would be close to inaudible.
Raybob
Tele-Meister
Not real familiar with an AA763. The difference between AB763 Twin and the AA270 Twin, besides bias balance instead of bias adjust, is the resistor values around the phase inverter long tail pair and at PI plates. Supposedly, the AB763 is 80-85 watts, AA270 is 100 watts. I believe that's CBS hype on inflated RMS values.
FYI: The Dual-Showman Reverb is same circuit as Twin AA270.
T Prior
Poster Extraordinaire
100 watts vs 85 watts.
It is known that there are no POWER AMP changes to any of the non MV circuits, tone stacks yes...
Here is one theory, the one I prescribe to..
Way back at the factory in 62 or 63 whatever, who actually knows what AC power they had and at what AC line level they used for the equation of output power.
The theory is that after the SF amps came to be, the new procedure or MATH was to calculate the power using the transformers highest available input rating which is probably in the 125vac area. Fender magically found 15 watts ( just shy of 20% ) and can now advertise that they are the same as Marshall, Vox and Mesa.
It is fascinating how they found 15 watts without any power amp circuit changes. But if you re-do the math with different criteria you can find as much as you want !
Still, the amps are great, 68,69,70 and 71 ( non MV / SF era) are flat out amazing amps.
t
It is known that there are no POWER AMP changes to any of the non MV circuits, tone stacks yes...
Here is one theory, the one I prescribe to..
Way back at the factory in 62 or 63 whatever, who actually knows what AC power they had and at what AC line level they used for the equation of output power.
The theory is that after the SF amps came to be, the new procedure or MATH was to calculate the power using the transformers highest available input rating which is probably in the 125vac area. Fender magically found 15 watts ( just shy of 20% ) and can now advertise that they are the same as Marshall, Vox and Mesa.
It is fascinating how they found 15 watts without any power amp circuit changes. But if you re-do the math with different criteria you can find as much as you want !
Still, the amps are great, 68,69,70 and 71 ( non MV / SF era) are flat out amazing amps.
t
bigben55
Friend of Leo's
SoK66 said:
Yeah, it don't matter to me at all. I was just curious.
