Over a year ago, I was nearing completion of the transformation of my Fisher 400 receiver using the amazing modifications engineered by Dave Gillespie and I was already thinking ahead to what my next project might be. I have been holding onto a pair of the Altec-Lansing Peerless #16458 output transformers used on the Heathkit W-5M amplifier, just waiting for the right design for them. I reached out to Dave to see if he happened to have a circuit design that he recommended. His response, "Is there any reason that you don't just want to build the W-5M circuit?". I'm thinking that I imagine it's actually a fine amplifier, but was hoping that maybe Dave had something that he had fully scienced-out like his Dynaco SCA-35, Fisher 400, and several others that I've read about with amazement and great respect. Dave's next response was that he was indeed familiar with the #16458 OT's and that they were, in fact, very high quality pieces, but that his experience working with them was only in the context of the W-5M chassis.
Dave then proceeded to whet my appetite with the following statement:
"While the W-5M circuit is good enough, there are issues with it that left me feeling that it could be improved upon in a worthwhile way. What I ultimately ended up doing was converting the unit from a classic Williamson design, to a classic Mullard design, with very good results. Ultimately, the unit ended up using a 5AR4/GZ34 rectifier tube, 7027A output tubes (6L6GC or KT66 can still be used), a 6CG7 driver/inverter, and an EF86/6267 AF Amplifier stage. Besides lower distortion, greater stability, increased sensitivity, real power output is also increased somewhat as well. It does require the installation of a small -40 vdc power supply, but there's plenty of room to provide for that. If you're interested, I'll try and dig up my notes on the design.... Again, my experience with the 16458 transformer in developing the replacement circuit was entirely positive -- but that's the only experience I have."
I was hooked. And, by the way, he was remembering the details (without error) of a design that he did over 30 years ago in 1986!
Bear in mind that Dave's design is using an existing W-5M as the starting point, so it is not only using the 16458 OT, but also the 54-32 power transformer and the 46-12 7H 150ma choke. He also retained the basic design of the HV power supply, only adjusting the dropping resistors to achieve the necessary voltage changes. Since I did not have the PT or choke, I had to locate suitable replacements. The design I ultimately settled on uses a Dynaco P782 power transformer (reproduction), solid-state rectification, and a TV horizontal damper diode tube to provide the HV warm-up delay. I do expect the HV voltages to be a little lower and will update the schematic with the actual measured values once I have them.
I will end this installment by providing Dave's hand-drawn schematic (with his permission) as well as one that I drew on the computer. Please understand that Dave has given me permission to share this project because I asked to do so for the benefit of those in the AK community who might like to try it and he readily agreed. Please respect Dave's intellectual work and ensure that he is given credit for the design if shared. I expect to have my first unit operational this week and will be providing a full detailed pictorial write-up of my build and also my impression of the performance. I do have some test equipment and will attempt to provide some limited, but hopefully meaningful measurements as well.
Tom Kuhn
Dave then proceeded to whet my appetite with the following statement:
"While the W-5M circuit is good enough, there are issues with it that left me feeling that it could be improved upon in a worthwhile way. What I ultimately ended up doing was converting the unit from a classic Williamson design, to a classic Mullard design, with very good results. Ultimately, the unit ended up using a 5AR4/GZ34 rectifier tube, 7027A output tubes (6L6GC or KT66 can still be used), a 6CG7 driver/inverter, and an EF86/6267 AF Amplifier stage. Besides lower distortion, greater stability, increased sensitivity, real power output is also increased somewhat as well. It does require the installation of a small -40 vdc power supply, but there's plenty of room to provide for that. If you're interested, I'll try and dig up my notes on the design.... Again, my experience with the 16458 transformer in developing the replacement circuit was entirely positive -- but that's the only experience I have."
I was hooked. And, by the way, he was remembering the details (without error) of a design that he did over 30 years ago in 1986!
Bear in mind that Dave's design is using an existing W-5M as the starting point, so it is not only using the 16458 OT, but also the 54-32 power transformer and the 46-12 7H 150ma choke. He also retained the basic design of the HV power supply, only adjusting the dropping resistors to achieve the necessary voltage changes. Since I did not have the PT or choke, I had to locate suitable replacements. The design I ultimately settled on uses a Dynaco P782 power transformer (reproduction), solid-state rectification, and a TV horizontal damper diode tube to provide the HV warm-up delay. I do expect the HV voltages to be a little lower and will update the schematic with the actual measured values once I have them.
I will end this installment by providing Dave's hand-drawn schematic (with his permission) as well as one that I drew on the computer. Please understand that Dave has given me permission to share this project because I asked to do so for the benefit of those in the AK community who might like to try it and he readily agreed. Please respect Dave's intellectual work and ensure that he is given credit for the design if shared. I expect to have my first unit operational this week and will be providing a full detailed pictorial write-up of my build and also my impression of the performance. I do have some test equipment and will attempt to provide some limited, but hopefully meaningful measurements as well.
Tom Kuhn
