Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on January 17, 2021, 11:09:26 am
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Can I run the grid wires going from the PI to the power tubes together? I mean right on top of each other. Also one of the grid wires needs to get pretty close to the bias trim pot. Should I come away from the board before I get to the trim pot or does it matter that much? For reference I am still referring to the 5f6a Hoffman layout.
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Can I run the grid wires going from the PI to the power tubes together? I mean right on top of each other. Also one of the grid wires needs to get pretty close to the bias trim pot. Should I come away from the board before I get to the trim pot or does it matter that much? For reference I am still referring to the 5f6a Hoffman layout.
Also curious, hopefully someone can chime in
My slightly educated analysis is that this would not be a problem. Running wires very close to one another can lead to capacitive coupling, which in this case would be the same as the PI plate-to-plate "fizz" cap that is common on Marshall that use the same overall config. Probably no impact, possibly a few pFs leading to a low-pass filter that only dogs can ear the impact of.
Both wires conduct signals of opposing polarity, which cannot induce positive feedback into one another
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Can I run the grid wires going from the PI to the power tubes together? I mean right on top of each other. Also one of the grid wires needs to get pretty close to the bias trim pot. Should I come away from the board before I get to the trim pot or does it matter that much? For reference I am still referring to the 5f6a Hoffman layout.
Also curious, hopefully someone can chime in
My slightly educated analysis is that this would not be a problem. Running wires very close to one another can lead to capacitive coupling, which in this case would be the same as the PI plate-to-plate "fizz" cap that is common on Marshall that use the same overall config. Probably no impact, possibly a few pFs leading to a low-pass filter that only dogs can ear the impact of.
Both wires conduct signals of opposing polarity, which cannot induce positive feedback into one another
If all the dogs in the neighborhood start howling I will know why. I was leaning towards the not a problem side as well, especially because the Hoffman layout has the wires running pretty close to each other. Nice to get sluckey's blessing on things like this though when I am not positive. If you do three things that are screwed up while you are building an amp it can be very hard to troubleshoot. That is my reason for all the questions.
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I had no problem running the phase inverter outputs right next to each other in an amp I built where the phase inverter was on the Preamp Chassis, and a cable routed those outputs to the separate Power Chassis.
The length of the wire run probably matters more, in which case instability is best handled with grid-stoppers right at the output tube sockets.
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I had no problem running the phase inverter outputs right next to each other in an amp I built where the phase inverter was on the Preamp Chassis, and a cable routed those outputs to the separate Power Chassis.
The length of the wire run probably matters more, in which case instability is best handled with grid-stoppers right at the output tube sockets.
The original Fender 5f6a Bassman didn't have grid-stopper resistors on the output tube socket, or anywhere else feeding the output tube grids for that matter. My amp suffered from a high frequency parasitic oscillation that I cured with 1.5K grid-stopper resistors on the output tube sockets. Have no idea where I got that idea from but was glad it worked. The Hoffman layout has them on the tube sockets. I am limited to what I can do about the length of the wire because of the length of the chassis. The one I am using is a reproduction of the chassis used for the original '59 Bassman. How Fender got away with wiring the output grid wires without grid-stopper resistors I will probably never know.
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Kind of mixing what SnickSound and HotBluesPlates said I am wondering if Fender found out that by running the grid wires together it created enough capacitance to eliminate parasitic oscillation. Could it be that was their fix but they never adjusted their layout diagram to reflect it?