What's the buzz on hum in old tube amps?

[MrEarl]

Forget about filter caps, lets assume those are good.
What are the most common causes for excess hum, buzz, noise, etc. in the background?
Think of a 5C3 Deluxe: sounds great, and the noise level is never a problem, in fact not really something one would ever complain about. But when I take it into the studio, inevitably the engineer gives me static (pun intended) about noise from the amp.
I am aware of some tweed era and other older amps that run very quiet. Anybody out there have any thoughts???

[Tom Phillips]

...Think of a 5C3 Deluxe: sounds great, and the noise level is never a problem, in fact not really something one would ever complain about. But when I take it into the studio, inevitably the engineer gives me static (pun intended) about noise from the amp.

I believe you meant to say that the noise is never a problem for you but the studio engineer thinks the amp is noisy.
The biggest problem with the old Deluxe design that would cause noise is that the heaters are wired using the chassis as the conductor for one side of the circuit. After that you probably have deteriorated mechanical grounds, some contact corrosion and aged parts such as noisy plate resistors. The background noise was adequate for its time because the amp is relatively low gain. However, improving the ground scheme and shielding the back panel lower the hum and noise quite a bit.

Regards,
Tom

[MrEarl]

Great info. Thank you very much.
Can you suggest a method for reducing the noise caused by the heaters wiring arrangement?
Also, the mechanical grounds you speak of: do you mean things such as the pots contact with the chassis?

[Chuck H]

To reitterate Tom's spot on post...

The heater arrangement can easily be changed from how it is (hummy) to a "twisted pair" with a "false center tap" (there, now you have something to google). And because the amp is cathode biased it would be easy to bias the false center tap at an elevated DC reference. This will render hum to almost nothing and won't change the tone of your amp a bit. As for other noise, it's shielding (that amp is unshielded on one side), grounds (yes, the pots at the chassis is one place to check) and almost certainly the carbon composition resistors that amp was built with. They create more thermal noise (hiss) than any other type. If the amp is a player and not a collector I say make the changes. Or have a tech do it. If it's a collector then get a different amp to play through. If you do make the changes the amp will be so quiet that you'll think it's broken when you first turn it on. You could print this thread and take it to any good tech and they'll know just what to do. It shouldn't be too expensive.

Oh, the above advice goes along with good filters and a three prong AC cable.

[Rhodesplyr]

Can you suggest a method for reducing the noise caused by the heaters wiring arrangement?

Replace the original heater wiring with a pair of twisted wires. Provide a ground reference for the heaters via two 100-150 Ohm resistors (one from each wire) to ground--or use a hum balance pot. You can also give the heaters a positive voltage bias. There are quite a few amps that use the power tubes' cathode voltage for this positive voltage, or you can use a resistor divider from the HV supply.

Also, the mechanical grounds you speak of: do you mean things such as the pots contact with the chassis?

I don't want to speak for Tom, but I assume he does. Grounding can be tricky because it's usually NOT shown on a circuit schematic other than with the generic ground symbol. For example, on two amps I worked on recently, the main ground for the first gain stage is the 1/4" input jack's electrical connection to the chassis. Oftentimes, cleaning significant ground connections can lower noise quite a bit. These days, I often use a "Blue ESR" meter, a capacitor diagnostic tool that also functions as a low-ohms meter. It's great for tracking down ground connections that aren't as good as they could be.

If these things are over your head, you need to take your amp to a good tech. I've had to improve the signal/noise ration of numerous pieces of gear for precisely the situation you describe: what works for gigs and playing for your own personal satisfaction can become a headache for recording. It's easy to get used to a level of background noise that's immediately obvious--and irritating--to others.

[MrEarl]

Once again, excellent information. I'm really grateful to you guys.
I suppose some folks would think of this amp as collectible, but my interest is in having it perform well, especially in the studio. I want to make judicious decisions about what I do to the amp, so as not to negatively impact it's natural tone and character. I am a good enough tech to do the work, and I have quite a bit of experience doing basic repairs on old amps, but that's about it. I'm interested to learn all I can now, because I want to maintain my own gear, and my studio work is picking up and replacing live performance.

[Chuck H]

As for changing the filament supply to a "twisted pair with a false center tap on an elevated DC supply" (god that sounds kind of intimidating) I'll draw up a sketch on paint that should make it easy to understand.

Other than this, checking grounds, shielding the wood panel on the open end of the chassis and changing the preamp plate resistors from carbon composition to metal film should make the amp nice and quiet.

Oh, again, if you don't have one yet you should get a proper three prong grounded AC cable.

[Chuck H]

As it happens the 6sc7 doesn't have a hum cancelling filament option. This is still a superior way to wire the filaments for the lowest possible hum level. You will have to add the 100r resistors. You can hang them off the indicator lamp. Twisting the two unsoldered ends together and wiring to the top of the power tube cathode circuit.

Here you go:

[Rhodesplyr]

Also, try to route the heater wires so that there's as much distance between them and the signal wires as is practical. If they have to cross, try to have them cross at 90 degrees. The worst thing to do is to bundle them all together, creating long parallel runs of heater & signal wire. You do see this because people think it looks neat and tidy.

[MrEarl]

Many thanks to all those offering helpful tips, and taking time to explain things. They're all well within my grasp and abilities, and will hopefully turn out some results with this amp. I'll post back here after working with the amp to let you all know how it works out.

[tubeswell]

Setting up the 6SC7 with DC heater supply should also reduce the (filament) hum.

[Steve Conner]

One important point is that the acceptable level of hum has decreased a lot over the years. When vinyl (with its hum-susceptible magnetic cartridge) went out, and solid-state hi-fi amps with their heavy hum-cancelling feedback came in, the world became a less hummy place.

Even if a vintage amp is working perfectly, and literally as good as new, people can complain that it hums too much.

[Enzo]

And hum you wouldn;t normally notice, becomes quite apparent to a microphone perched a couple inches from its speaker. And every quiet moment on your guitar will reveal that small hum. Recording engineers will hate that.

[Steve Conner]

Bah, recording engineers.

One of my home-built amps has the PT quite near the speaker. It doesn't hum when checked by ear, but if you mic it with a SM57, the little transformer in the SM57 picks up hum off the PT. Another thing for recording engineers to get worked up about.

I have laid down bass tracks with my amp hooked up outside the room by long wires, because the engineer didn't like the cooling fan.

[Enzo]

And we do get that occasional question, "how do I hook up a switch to turn off the fan?"


Local guitar legend "Frog" used to get his tone by cranking the bejesus out of his amp. AT gigs, he had a cab out in his van, which he ran a cable to from his amp head, and a mic with a line running back to the PA. Hum wasn;t the issue, but, well, long wires...