audio amp. versus guitar amp
- Thread starter Marcelinho
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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.
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.
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.
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.
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.
d1
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.
d1
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.
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.
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
Cheers!
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
Cheers!
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
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
It's the guitar amp that is humming. It is not that old (2 years) and always done so. Nevertheless i consider it a "normal" behaviour for a guitar amp.
The hum that bothers me is already apparent without guitar attached.
To keep the neighbours happy, i want to play at lower volumes and keep the hum at a minimum. At higher volumes the hum wouldnt't be such of an issue.
The hum that bothers me is already apparent without guitar attached.
To keep the neighbours happy, i want to play at lower volumes and keep the hum at a minimum. At higher volumes the hum wouldnt't be such of an issue.
Go to the forum at el34world.com and post your problem. That is where all the professional tube guitar repair techs hang out. I am sure they will be able to help you out quickly.
Make sure you tell them the correct frequency of the hum and indicate that the hum occurs with the guitar unplugged.
Make sure you tell them the correct frequency of the hum and indicate that the hum occurs with the guitar unplugged.
A quick look at the AC-30 schematic shows the culprit - minimal power supply filtering. The 1960 AC-30 has a choke and a single 8µF cap.
In your amp, that cap most likely needs to be replaced.
Most guitar amps have three or more filtering caps, usually with larger values.
You could add some more caps before and after the choke to improve filtering, although the amp will stiffer and less reactive. That's the trade-off and most likely the reason for the AC-30 famous tone.
In your amp, that cap most likely needs to be replaced.
Most guitar amps have three or more filtering caps, usually with larger values.
You could add some more caps before and after the choke to improve filtering, although the amp will stiffer and less reactive. That's the trade-off and most likely the reason for the AC-30 famous tone.
All true, but design and implementation play a large part in it too.m6tt said:
All true, but almost all designs used are based upon 'classic' ccts which had little filtering and often poor design and layout. There are often 'better', 'alternate' ways of doing it, but then you lose a big part of the market; those for whom classic is god.1400WATTSRMS+ said:
Simple PS, grounding, layout and other mods can clean up an old amp well, but most of my customers won't do it because it'll 'spoil the mojo'. Bollocks to that.
Have a load across the secondary at all times whilst it's on for safety's sake (OPT protection).Marcelinho said:
Hum doesn't come from speakers.
DC supply can pretty much gaurantee quiet, but it isn't neccessary.
+1 Geek.Geek said: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
Cheers!
The his has to do with the top boost channel. I found some mods to improve that.
Have a load across the secondary at all times whilst it's on for safety's sake (OPT protection)..
For instance adding a bleeder resistor (like in borderpatrol psu)?? http://www.borderpatrol.net/psuind.htm
Wat does OPT protection stand for?
Have a load across the secondary at all times whilst it's on for safety's sake (OPT protection)..
For instance adding a bleeder resistor (like in borderpatrol psu)?? http://www.borderpatrol.net/psuind.htm
Wat does OPT protection stand for?
High speed high voltage diodes in parallel with output tubes save output transformers.
Another option is, to use metal oxide varistors as a last resort: they may die saving your transformer and killing a plate fuse.
Speaking of hum and hiss, it was already said that guitar amps need more gain to add a character to electric guitar sound when tubes add harmonics, the more the louder you play, so hum and hiss is more amplified than in Hi-fi amps. And it is true that guitar amps had lack of B+ filtering and sometimes bad layout; for example I modified several SVT amps resoldering only few wires to silence them completely (both hum and RF oscillations).
Another option is, to use metal oxide varistors as a last resort: they may die saving your transformer and killing a plate fuse.
Speaking of hum and hiss, it was already said that guitar amps need more gain to add a character to electric guitar sound when tubes add harmonics, the more the louder you play, so hum and hiss is more amplified than in Hi-fi amps. And it is true that guitar amps had lack of B+ filtering and sometimes bad layout; for example I modified several SVT amps resoldering only few wires to silence them completely (both hum and RF oscillations).
Essentially, when there is no speaker attached, all the power put out by the tubes has nothing to do except break things. I have never tried using the diode/varistor method..I have an old lamp cord connected to a gloriously crappy speaker. When I'm working on an amp, I just connect the lamp cord (with a speaker plug on it when necessary) to the amp and toss the speaker into a closet or something. I've turned on amps without speakers before, and played through them, but I seem to have been able to figure that out quickly enough and yank power cords to prevent a problem.
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