Amp tube advice please!

Pedro

Hello folks. I've just bought a new 2 x 10 cab to sit my 1970-something fender bassman on (having been without speakers for 4 years or something) and the old thing HUMS like hell. The tone is lovely, I mean really really nice, but I'm having to crank it up to mask the hum (the hum is constant volume however the amp is set). This is clearly going to drive a wedge between me and the rest of the neighbourhood if I can't play at lower volumes 😆 , so what is it? I mean, should I replace all the tubes at once, or what? Please bear in mind that I'm a novice with these things and don't know a power tube from an output tube! All I know is there's 4 big fat ones, and 4 little thin ones with shielding over them, and that they all glow a warm orange but aren't mega bright.

If I buy a full set do I need to get it re-biased, and is that something I should get done by a tech?

Sorry if I'm being a dumb-ass, I have trawled through old threads looking for info and there's clearly a few people on here that know their stuff so I thought I'd ask!

Thanks

Doog

The fact the hum doesn't change volume with the amp suggest it's an earthing problem rather than tube problem. Does the hum eminate from the speakers or from inside the amp itself? If it's the later, it might be a problem with the amp's power transformer.

For your reference:

The four thin tubes with metal shields are the preamp tube, this is where your tone is shaped

The four bit fat ones are the powertubes, the section of the amp that basically "makes the preamp loud" 😁

I'd get a tech to look at it before retubing.

Pedro

Right, thanks Doog. It's coming from the speaker, and i tried a different cab and it still hums, but if I take a line out of the line/recording socket into a mixer there is no hum. I don't know if that suggest that it's a powertube problem, I'll try and find somewhere to get it checked out if I can't find any obvious loose earth connections

Doog

Hmm.. well the recording line out will be coming straight from the preamp, so it's a problem with some facet of the powerstage as I thought..

I take it you're using a proper speaker cable to connect the head to the cab and not a guitar lead?

Pedro

umm.. 😡 ... well, no I'm using a guitar patch lead. What's the difference?

Doog

Put bluntly, guitar leads are not beefy enough to carry the current a tube amp puts out. Worse case scenario, you could kill your amp's mains transformer which ain't cheap to replace. Guitar leads are generally shielded, something you don't want in a speaker leads for a few different reasons.

Grab yourself a nice beefy speaker lead, for the safety of your amp and your toooooone. May even solve your humming problem too.

Also, make sure the speaker is equal to or above the amps wattage output and impedance (ohms) rating.

SawaotheBuster

someone said: Guitar leads are generally shielded, something you don't want in a speaker leads for a few different reasons.

just outta curiousity, what are they Doog?

Doog

"Speaker connections should not be shielded, because the power transmitted in speaker lines is much greater than any induced signal, and there are side effects of shielding that can change the frequency response."

SawaotheBuster

I coulda googled that couldn't I have.....

thanks, I woulda thought shielding would be good