Perhaps the hum would be coming from the table saw?
Seriously, I don’t really know enough about tube rolling to know when or if I’d be exceeding the paramerters of the self-biasing circuits. I’m sure that the prior owner did seek some advice before frankenstining a two tubes to one adapter, but I have not.
I’m willing to be educated. Yes I do know that hum is not uncommon with tubes. In building a radio and some other projects as a high-schooler in electronics class/shop we were fine with hum, it’s the flashes when we plugged things in that we wanted to avoid. But I don’t know enough about the parameters.
Per my reading and experience there is a third category between hum and explosions: Functional but with shortened tube life and/or component overheating with perhaps shortened life or degraded quality. In my experience deviating from the factory tube models is done more for the challenge or to “save money” than for quality upgrades per se. YMMV.
Maybe stick with the factory tubes (or the same model tubes) until you get bored and/or upgrade to another amp?
Ignoring bad adapters for a minute, and I do know of a case where a bad adapter killed a VERY expensive piece of equipment.
If your going to use adapters, or other types of tubes when rolling, you have to know if the amp circuit will support it.
One of the key things is how much current the heaters draw, the heater power supplies will be designed for some maximum, and in some cases they are constant current supplies and not constant voltage.
A common swap I’ve seen people do is 6SN7’s for 6SL7’s, but 6SN7’s heaters draw twice the current of a 6SL7. In most cases the supply is likely to be able to deal with the extra draw, but if it’s a DC supply everything has to be sized for the extra load. It probably won’t immediately fail is say the diodes are undersized, but they will fail more quickly.
Having said that sometimes adapters are appropriate, a 7F7 is just a 6SL7 with a different less popular base. So unpopular in fact sourcing true NOS tubes is relatively easy, so using an adapter can be considerably cheaper than trying to source NOS 6SL7’s.
Hum is pretty much always a power/ground issue in the design.
You will get the occasional noisy tube, but outside environmental factors you can make tube amps dead silent.
In cheaper tube amps it’s common to use AC heaters, because you just plug the heater into the transformer and your done. This is for the most part harmless in if the tube is indirectly heated but now you have to ensure the noisy AC heater line doesn’t run next to the signal lines, because the nice 60Hz hum will get transferred there, it’s why people twist together AC lines.
Grounding design is the other big culprit and as far as I can see that’s way more complicated that you might thing, as far as I can tell it comes down to isolate signal ground from power ground, then star ground good, everything else worse. And it’s not always practical to use a star ground.
I assume your mixing different tubes on +Ve and -Ve out?
Or is it just the way the pictures taken?
Or are the two pentodes used for something different on the Horizon?
The pentodes are used for something different.
In SE mode only the left bank are voiced, but there is a minor electrical influence on the negative right bank which is used to fine-tune the presentation.
This may be silly, but I love the look of 300B tubes, but I prefer solid state amps. Does anyone know of a desktop tube display? I’d love a few 300Bs and/or kt88 or somesuch glowing on my desk, without having to buy an amp that will sit unused. Granted it’s an expensive art piece, but some amps are that anyway.
