5f6a build modifications

[Mike_J]

Looking into my build from long ago I decided to modify it starting with a new board based on the Hoffman board layout for the 5f6a. The difference is I stretched the board to 15-1/4" in length. The main purpose for this change was to shorten the length of the grid wires from the PI to pin 6 of each power tube.


Now for the reason I posted this. Can I run the plate wires together in the corner of the chassis running from V3 (the PI tube) to the PI section of the board? They would need to run together for 3 or 4 inches.


As a side comment when I finished the build years ago it had an oscillation problem that was solved by the use of 1.5k resistors on the power tube sockets. I never did remove the 47pf cap in the PI that if I remember correctly was in the original design to solve an oscillation issue. Many years ago tubenit commented that the 47pf cap was detrimental to the tone of the amp so I want to remove it now. That cap is not in the Hoffman layout which probably answers the question by itself. Hopefully, the 1.5k resistors and shorter grid wires will allow this to happen without incident.

[Latole]

Hoffman board layout for the 5f6a

[sluckey]

Now for the reason I posted this. Can I run the plate wires together in the corner of the chassis running from V3 (the PI tube) to the PI section of the board? They would need to run together for 3 or 4 inches.

Yes, you can do that ***IF*** there is nothing else running with them, like heater wires or speaker wires. It would still be better to run those wires in a more direct route to the board.

[Mike_J]

The heater wires will be above the sockets so no problem there. I will need to be careful to ensure the speaker wire stays away from the plate wires and at worst cross at a 90 degree angle and as far away as possible. Believe running them on the inside corner will yield the shortest run possible. My concern was any signal crosstalk between the wires.


I know grid wires should be as short as possible and isolated from plate and cathode wires as much as possible. The grid wires should also cross the plate and cathode wires at a 90 degree angle if possible.


What is the relationship of center lugs of the volume pots that feed the 270K mixer resistors before they reach pin 2 of V2? Is this all a grid wire? The pots are being fed from the plates of V1 after they go through respective coupling capacitors. I guess what I am asking is what are the rules on the potentiometer side as it relates to wire length and how the wires should be treated in relationship to each other? I have always focused on the tube side as far as how to treat the interrelationship between wires. Not sure that is all I should be considering.

[Mike_J]

From looking at my build as it was originally constructed it looks like I braided all the wires going to each potentiometer. No clue where I got that idea from. Looked at a picture from inside the chassis of an original 59 Bassman and it looks like the wires were passed to the right of the pots and were fed to the pot wipers from above the pots. That does not scream shortest wire lengths possible. 

[Mike_J]

A reasonable solution is to duplicate the layout diagram Fender made for the 5f6a. It will work. This whole issue is caused because the Hoffman layout is less than eleven inches presumably to fit on a single sheet of paper. I am making my circuit board 15-1/4" because the wires on the tube side line up well with the tube sockets as they appear on the chassis. Also it allows me to install individual ecaps for both V1 and V2. I can install a short wire from the negative side of the ecap to the negative side of the cathode ecaps for the tube. Don't remember where I heard that was a good idea but have had good results from doing same.