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Is this the right place to find solutions to the above problems?

I'm trying to make this amp usable, but after some rebuilding and reconfiguring, I still have a hum problem and what seems to be odd voltages.

The current configuration is 2 inputs (one with a treble peaking tweak added,) 4x6AT6 preamp tubes, volume and treble/bass controls after V2, variable NFB added on cathode of V3, V4 cathodyne phase inverter, EL844s V5 and V6 cathode biased, 5Y3GT rect.

I have changed all capacitors with close to same values, added 510 ohm/5W screen resistors.

B+ is 370V on power plates (recommended 250), 320 on screens; and preamp plates V1-4 measure 123,87, 112, 174 (recommended 100V).

How can I tame the voltages and clean up the output?

The heaters are 5.05V and 6.54. There is a filament CT but no secondary HV CT on the transformer.

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  • \$\begingroup\$ As a fellow guitarist I feel your pain. As an electronics engineer (and that is what this site is), you need to post schematics. \$\endgroup\$
    – Andy aka
    Feb 20, 2021 at 23:38
  • \$\begingroup\$ Thanks Andy. I'll try to cobble something together. Meanwhile, it is not a lot different from the Fender 5F10 circuit (early Harvard) <is.gd/MZiWoa>, apart from the main differences I mentioned. \$\endgroup\$ Feb 21, 2021 at 2:53
  • \$\begingroup\$ The devil will hide in the details. \$\endgroup\$
    – Andy aka
    Feb 21, 2021 at 8:01
  • \$\begingroup\$ This is the best schematic I can produce for now. Might be errors. My alterations: Fender style input, treble peak on guitar channel, tremolo converted to adjustable NFB with on/off switch on pot, tube converted to preamp stage, line out jack. Note that preamp tubes are triodes, speakers stand in for jacks, bridge rect symbol instead of 5Y3 tube. link \$\endgroup\$ Feb 22, 2021 at 2:42
  • \$\begingroup\$ Note that some odd resistor values are as measured, not according to color code. \$\endgroup\$ Feb 22, 2021 at 2:51

1 Answer 1

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If your hum problem is 50 or 100 Hz, my first step should be to remove V4 (of course no power) and check. If no hum, check removing V3 and so on V2 and V1 to find where this noise comes from. Check heater wires are twisted and as far away as possible from signals, specially in pre-amp.

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  • \$\begingroup\$ Many thanks for the response. I'm in Canada, so its 60 or 120 Hz. Trying to whip up a schematic so you have a better idea. I have used shielded cable on the long preamp signal runs, and kept the wires up in the air as far from filaments as possible. I will give your suggestions a try once the schematic is more or less presentable. Can Kicad files be uploaded? By the way, I have just started using a USB oscilloscope, so I could step through the connections with that. \$\endgroup\$ Feb 21, 2021 at 17:52
  • \$\begingroup\$ I should mention that the amp was discovered in a crawl space under a house where it may have lain for 30 years or more. I was surprised that the transformers hadn't shorted. It is made by Lifco, a Montreal firm that produced various cheap electronics devices, from radios to turntables and guitar amps in the 1950s and 1960s. The latter were mainly copies of other brands. (e.g., they did a copy of the Traynor YBA-1) Gut shots are available if anyone is interested. \$\endgroup\$ Feb 21, 2021 at 17:52
  • \$\begingroup\$ For the caps I used what similar value items I had available, and bought a few extras from a local electronics store that has been in business for 61 years. Some of his caps may be about that old too. The owner, in his 80s, remembers doing a few subcontract jobs for Lifco in the 1960s. \$\endgroup\$ Feb 21, 2021 at 17:53
  • \$\begingroup\$ Here's the hum report. Hum not present when volume at zero. Rises in volume as volume increased, 60Hz plus a shrill metallic whistle. Attenuated as more NFB added. Same when V4 removed. Disappears when V3 removed. Could it be the NFB insertion on cathode of V3? \$\endgroup\$ Feb 22, 2021 at 2:46
  • \$\begingroup\$ I think there is a missing groung connection in output transformer secondary winding (opposite to D point). \$\endgroup\$
    – Bravale
    Feb 22, 2021 at 20:11

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