if you left V6 hooked up at the cathode and anode, it also needs a ground reference for the grids or the bias voltage can fluctuate widely. doesn't really seem likely that's creating the hum as it's disconnected from the audio circuit, but i'd put 1M's from the grids to ground regardless, or just pull the tube until you replace the channel components. pulling the tube might be instructive as to whether or not it's contributing to the hum.
guessing your earlier issues with poor tone were just related to too cool of a bias on the power tubes. but it is a bit concerning that it went from no output and showing signs of the preamp not really being loaded, then jumped right to more or less proper voltages. i'd look for bad solder joints in the power supply, and probably give the amp a good 'chopsticking', take a solid piece of wood like a chopstick and gently tap components and nudge wires while monitoring preamp voltage nodes, to identify intermittent connections. just make sure the piece you're using is all wood (don't use a pencil), keep one hand in the pocket, and i'd put it back on the dim bulb limiter for that test.
my money's on there being oxidized and intermittent contacts within the power switch, as it is also a DC Standby switch for everything besides the power tubes. your results sound exactly like the primary AC voltage connection was getting made, but not the Standby connection.
many old switches are able to be (carefully) opened up so the contacts can be cleaned, or have a hole where cleaner can be sprayed, i'd highly recommend doing so. but definitely DON'T use deoxit here. if you can't get it open but there's a hole for spray, i'd recommend using CRC QD Electronics Cleaner, then rotate the switch end to end a few dozen times.
if you can get it open in a way you're confident you can reassemble it, the same cleaner works great, but it may be simpler to use 91% or 99% isopropyl alcohol and q tips. if the contacts are really oxidized or pitted, mineral spirits/lighter fluid/naptha may be more helpful, but test a bit on any plastics they come into contact with first to make sure there's no damage. i find a brass brush, especially on the end of a rotary tool, to be amazing at burnishing electrical contacts - tough enough to blast any crap off, but not tough enough to actually score the metal.
bucking transformer is great for reducing primary voltage, but if cost or space is an issue, i find a high resistance NTC thermistor to often be pretty effective at dropping a few volts off the primary. it can be installed anywhere in series with the live or neutral wire, which is handy, but it gets quite hot, so you need to make sure it's at least an inch or two away from wires and other components. to bring the amp back to the 117VAC spec it was designed for, you'll want to drop anywhere from 5-8VAC on the primary... with a 2A fuse, we can assume primary current is about 1-1
5A... which means you'd need the on-resistance of the thermistor to be anywhere from 3-8 ohms, so 5-6 ohms is a good target. none will be designed to have that much resistance at rated current, but you can get that resistance by choosing one rated for much more current than the amp can draw, at the same time giving it a great safety factor. the Vishay MS2212104 fits the bill very well for only $5:
https://www.digikey.com/en/products/detail/vishay-ametherm/MS2212104/749846 looking at the datasheet, it's rated for 4A max and would be 0.75 ohms at 4A, or about 2 ohms at 2A, so for your 1-1.5A, that should land around 4-5 ohms. you could also get multiple lower resistance ones and put them in series.
be still my beating heart... is that really a bias circuit with only 145K total grid leak resistance per 6L6?! i can't remember the last time i've seen an amp that even gets that close to the datasheet max rating of 100K! them's some happy control grids, there.
Topic: HIGH Voltage right off the PT (Read 1359 times)