Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on January 24, 2016, 01:16:33 pm
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Since Sluckey was nice enough to post really good gut shots of a 1964 Deluxe Reverb, I finally noticed something in the AB763 circuit that is curious. The first gain stage of each channel gets its own 1K5 resistor and 25uf cap (although 2 caps in one package - that's what got my attention at first). But the second gain stages of each channel share an 820 resistor in parallel with a 25uf cap. Same is true for every AB763 circuit I looked at. Appears to begin with 6G16 Vibroverb circuit but I may have missed something earlier.
So what happens when two gain stages share a common cathode RC network that Leo Fender wanted to avoid in the first gain stages of his AB763 circuits?
In the Tweed era 5F6-A and 5E3 circuits, the first stages of each channel share a common cathode RC network. In the Blackface Bassman, Leo sprung for separate RC networks for all 4 cathodes.
I'm probably trying to pick fly poop out of pepper here, but Fender didn't do anything by accident as far as I can tell.
Chip
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So what happens when two gain stages share a common cathode RC network that Leo Fender wanted to avoid in the first gain stages of his AB763 circuits?
Leo put another quarter in his pocket. Notice that the reverb recovery and VIB channel third amp also share a cathode RC. So, put a quarter in each pocket! That way, he doesn't lean to one side when he walks to the bank. :icon_biggrin:
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So what happens when two gain stages share a common cathode RC network that Leo Fender wanted to avoid in the first gain stages of his AB763 circuits?
Leo put another quarter in his pocket. Notice that the reverb recovery and VIB channel third amp also share a cathode RC. So, put a quarter in each pocket! That way, he doesn't lean to one side when he walks to the bank. :icon_biggrin:
But there must be a reason why Leo didn't share cathode RC networks every time he had a chance to. Sometimes he saved those pennies, but other times he didn't.
Respectfully,
Chip
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But there must be a reason why Leo didn't share cathode RC networks every time he had a chance to. Sometimes he saved those pennies, but other times he didn't.
I can only guess.
It looks like every place where the parts in question are next to each other, he shared cathode resistors. Something not obvious from the layout is that although 2x 25uF caps are shown bypassing 2 separate cathode resistors, the part used in the amp is a single 2x 25uF cap (what a tech I once worked with called "3-legged critters" with 1 ground lead and 2 positive leads).
I'm guessing there might have been an issue with cross-talk from the non-reverb channel into the reverb channel and vice-versa when input stages shared a cathode resistor. That would be the only sensible explanation to me, since those parts are next to each other & he might have saved another quarter or 2 by cutting down to 1 resistor & 1 cap for the input stages.
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I find the history of amp development to be fascinating, but am drawing a blank. Leo began tinkering with amps in the 1940's, but I don't know how much direct involvement he personally had later on. My understanding has been that a lot of Fender designs were ad hoc, or even mistakes made while copying from general audio amp designs.
Also, regardless of the design specs, the builder is faced with certain pragmatic realities that may call for makeshift solutions. This occurs all over, such building a home, etc. If the solution works it might become "standardized" and carried forward, even though no one later remembers how or why it started. Some worker may have felt too lazy to go get another batch of 1500 Ohm resistors, and his boards got standardized by accident.