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Marcelinho 23rd February 2008 11:14 PM

audio amp. versus guitar amp
Why do guitar amps in general hum and somtime hiss like hell and audio amps in general do not.

(I have both a vox ac30 and an audio innovation 500)

m6tt 23rd February 2008 11:18 PM

Old guitar amps were commodity items, whereas old hifi amps were expensive. Thus, more time was spent in the old hifi amps to quiet them. Another factor is that most hifi poweramps have at most two gainstages before the power tube (excluding a phono preamp/RIAA)...many guitar amps have more gainstages, and also higher-gain gainstages. this tends to amplify a lot of hum unless careful precautions are taken.

JonnySwitchblade 23rd February 2008 11:21 PM

Well, for starters when you boost your signal to overdrive it so it can be clipped and generate distortion, you're also boosting the noise that's present in the input signal. Hi-Fi amps would in fact do the same thing only you'd never be boosting the signal into clipping like we do for instruments.

However, the more you refine your "distorting" set up, the less noise you'll have aside from the pesky grounding buzz which is really determined by the quality of your power system's ground.

I play with a lot of gain but, unless I'm hitting the strings it's pretty much dead silence and no, I don't use a noise gate of any kind. Just keep experimenting, working with different circuits and toying with the gain levels. You'll find you can get a roaring metal distortion that is pretty darn quite until you hit your strings.

Here's a hint, you don't want your amps gain cranked, a heavily clipped signal sounds pretty lousy whether it's a solid state or tube.

Marcelinho 23rd February 2008 11:30 PM

With my audio amp a little hum comes out of the speaker and it changes when i change tubes.

With the guitar amp it doesn't seem to come from the speakers (difficult to be shure though) So what is humming (even without input signal)

m6tt 23rd February 2008 11:40 PM

I would imagine it is coming from the speakers, but a way to check would be to attach lamp cord to the speaker output on the amp (disconnect the original speaker), and use another 8ohm speaker quite a distance away. Then the speaker and the amp will be somewhat isolated. Sometimes if there is a power transformer problem, the transformers will hum if they are under too much load, but I'd imagine your hum is coming from the speaker. Different tubes hum differently, even within a different type. If your input tube is a 12ax7, you could try a 12ay7 to reduce gain at the input and input tube hum (12ay7 is a low-noise low-hum type). If it's a pentode like ef86, try using different ef86 tubes, some are better than others. Long guitar cables hum way more than short ones, and long extension cords can introduce hum into the power supply. Try turning the volume off on the instrument, if hum persists, it's probably the cable or the amp--guitar strings are essentially antennae.

Marcelinho 24th February 2008 12:49 AM

detaching the speakers is a good idea.

What about changing heater supply from ac to dc for the three 12ax7's

d1camero 24th February 2008 05:43 AM

How old is the amp? Which one hums? The AC30? It may be a grounding issue or an elec cap NFG. Do you know if the hum is 60Hz or 120Hz?

As for hiss - not unusual for a high gain amp. Guitar amps are design quite different than hi fi amps, and consequently hvae these kinds of idiosyncrasies.


SHiFTY 24th February 2008 07:09 AM

some 12AX7s i've used in guitar amps have been dead quiet in the phase splitter but unusable in the front end- hissssss!

You need a box of old 12AX7s and then you can sift through for quiet ones.

As for hum, that can be reduced a lot by using regulated DC filaments on the preamp and a decent grounding strategy.

Geek 24th February 2008 07:54 AM

Hmmm, I've never designed a noisy guitar amp and also never needed DC filaments. All of them have headphone outputs and are quieter than any SS guitar amp I've tried.

Just decent layouts, proper tube selection (keep 12AX7's out of the V1 position) and bias the heaters +40V or so above ground and instant quiet :)


Marcelinho 24th February 2008 10:48 AM

if the hum is 60Hz or 120Hz?

In europe it's 50-100hz. i have to dubbelcheck because i am already disassembling parts, but i think it's a 50hz i am hearing.
(ok i checked my hearing-memory with a wave generator, it must be a 50hz i am hearing)

I swapped tubes and used my dead quiet ones from my audio-amp.
I even removed a tube which is used for a kind of boost channel with only little effect.

The 50hz hum doesn't seem to be increasing very much, if i turn up the volume.

So what is the next suspect? A cheapy transformer or choke.
What about the tube rectifier

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