Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Boots Deville on March 30, 2012, 02:13:49 pm
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I would like some help understanding the power supply design choices between the AB763 Super Reverb and the AB763 Deluxe Reverb.
They both have the same preamp but since the 6L6's in the Super operate at higher voltages than the 6V6's in the Deluxe, I would expect larger dropping resisters in the power supply rail of the Super than what is found in the Deluxe so that the preamps in two amps would have similar supply voltages.
In reality the opposite is true. The Deluxe has a choke followed by a 10K dropping resistor, following by another 10K dropping resistor. The Super has a choke followed by a 1K followed by a 4.7K.
I recently built these two amps side-by-side which is how I happened to notice it. It left me scratching my head.
Can someone shed some light on why the two power supplies were designed the way they were?
-John
Deluxe: http://www.el34world.com/charts/Schematics/files/fender/DELUXE_REVERB_AB763.pdf (http://www.el34world.com/charts/Schematics/files/fender/DELUXE_REVERB_AB763.pdf)
Super: http://www.el34world.com/charts/Schematics/files/fender/super_reverb_ab763_schem.pdf (http://www.el34world.com/charts/Schematics/files/fender/super_reverb_ab763_schem.pdf)
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One thing is 6L6's need more to drive them than 6V6's. AB763 SR has 230v on PI plates, DR has 180v.
Could just be they wanted to give the SR a little more head room on the PI and pre amp tubes, for a cleaner sound and a little more juice?
I giged with a BF SR for years, and it was a real good amp, for me anyway. Never had anything go wrong with it. I did change the tubes a few times. Never played it above 7, and 90% of the time on 5 or 6.
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> so that the preamps in two amps would have similar supply voltages.
Why?
Theory 1 - low hum _with_ low capacitor cost is good business
Theory 2 - a couple-hundred volts is plenty for any player
Theory 3 - pay more for a Super, should get more-of-something
Factor 4 - driver for Super needs to deliver 52V, DeLuxe only 35V
AB763 Super Reverb
final B+ 465V
screens 460V
driver B+ 450V
preamp B+ 410V
AB763 Deluxe Reverb
final B+ 415V
screens 415V
driver B+ 325V
preamp B+ 250-267V
We see that the driver supply to output ratios are similar: 52/450 vs 35/325 or 0.11 vs 0.107. The smaller flavorful amp's preamp runs on 2/3rd the supply of the big clean amp's preamp. The 1K vs 10K is deliberate matching of driver overload to final stage overload.
MAYBE someone sat a Super and a DeLuxe side-by-side with a resistor box and selected the other dropper "to taste" (for market differentiation). Just as likely it just happened.
You may change these to suit your own tastes, of course. Value under 2K feeding the single-ended stages may want a larger last capacitor than 16/20uFd; 40uFd is cheep enuff.
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Theory 1 - low hum _with_ low capacitor cost is good business
I take that to mean, it's the reactance of the R/C pi filter that matters ? The Bigger the R, that let's you use a smaller C and = the same low hum? AT less cost?
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Thanks for the info guys, I learned something, but this raises another question:
driver for Super needs to deliver 52V, DeLuxe only 35V
I looked over the 6V6 and 6L6 datasheets and it wasn't immediately obvious to me how these 52V and 35V figures were derived.
Could someone explain how to calculate this?
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Thanks for the info guys, I learned something, but this raises another question:
driver for Super needs to deliver 52V, DeLuxe only 35V
I looked over the 6V6 and 6L6 datasheets and it wasn't immediately obvious to me how these 52V and 35V figures were derived.
Could someone explain how to calculate this?
Not from the data sheets. Well, maybe Fender originally figured based on data and curves, but...
Look at the schematics for both amps. See what the design bias voltage is, as indicated on the schematic? That's also the peak output the phase inverter must deliver to push the output tubes to full output.