Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on March 10, 2021, 11:19:01 am
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I modified the Hoffman 5f6a layout to better fit my chassis which I may have purchased from Doug 22 years ago. Looks just like the original '59 Bassman chassis. Idea was to place the components in front of the tube sockets just like they are shown on Doug's schematic. Allowed for more spacing between components and for the ability to use larger coupling and tone section capacitors, for example the 6ps caps in the LTP on the board.
That posed a small problem for me because the trimmer pot that I moved was now too close to the power tube sockets. I modified the bias section by moving it more towards the middle of the board. Also wasn't sure which resistor I would need to change to adjust bias range even though there is a huge sign pointing to a different resistor than the 1K/3W resistor that says bias range. Anyway, I gave the 15K resistor its own pair of turrets so if I ever needed to change it I could do it relatively easily.
Undoubtedly the largest change was the decision to use radial caps instead of the axial capacitors in the dog house. Was referred to Aiken and Merlin for advice on where to ground components. With the supervision of pdf64 I learned the appropriate ecap spots to ground components to.
Then I read a few comments by Merlin. One was the ecaps ground point should be as close as possible to the circuit section it is being filtered by. Well my dog house seemed like a long distance to go so a solution had to be found. That solution was going to radial caps from axials. Still have the axials. They are in their dog house hoping to be needed again.
Second comment by Merlin he called bus grounding. In his case it involves using a 14awg bare copper wire from the reservoir cap to the input jacks. I compromised and used 18awg except for the pot buss ground which I believe is 10awg. Anyway, Merlin figures this is a good way to make a quiet amp and the Hoffman board layout is perfectly suited, with minor modification in the power and LTP areas, for use as a bus grounding vehicle.
Attached is a picture of the populated board as it exists today. There are no radial ecaps installed because I ordered them many moons ago and they still haven't arrived. This is because their warehouse is in Texas near where I was when the power went out in much of Texas during a recent Arctic blast. Since I was in Houston at the time and got to experience it first hand their delay is somewhat understandable. If this works out the way I hope it does hopefully Doug will stock radial ecaps in the 100uF @ 350VDC, 47uF @ 350VDC and 22uF @ 500VDC values. I would be very happy to purchase from him and if all goes well I will be using them in all my rebuilds.
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Took three more pictures. One is the grounding point near the input jacks. It will be the only chassis ground for the whole amp. Will put a regular nut on top of the keps nut and tighten it until it screams for mercy. Second picture is where the PT CT will be connected to the negative side of the first stage filter capacitors without being grounded. Third picture is the vertical three terminal strip the heater wires from the PT, to the tube sockets and the lamp housing. The 100 ohm balancing resistors go from each sides wires to the third terminal strip. It was going to be grounded near the PT with all the other power amp grounds before the change to the bus bar idea. However, Merlin said it can be difficult finding a good place to ground the heater wires. Well I didn't need to hear that. Decided to hook up the DC supply circuitry and run the wire that would normally run to the wiper of a humdinger pot, which is still backordered, to the juncture of the 100 ohm resistors and hope the heater supply is balanced. This means the heater supply will only be grounded to the chassis with everything else near the input jack.