Clean or Clean sound?

[Tdale]

I've asked about this before, but can't seem to get an answer I understand...

Why is it that most guitar amps tend to distort once you turn it up a bit. Most 50W tube amps I've heard, tend to break up/distort quite a bit, a long time before they reach their maximum output volume, while tube Hi-Fi amps can be about 12W, and still play Hi-Fi, and almost not distort at all, even at quite loud volumes?

Basicially what I'm asking: What is the difference in construction, between a guitar amp and Hi-Fi amp....if there is any?

Tommy

[Bob-I]

I don't know if we can make that blanket statement. I had a Dynaco stereo that I built in the 60s, dual 70 power amp. I could make it break up pretty bad, same as a guitar amp. I'd bet the lower power 12 watt stereo's you're talking about simply have lower gain preamps.

[Tdale]

So lowering th gain of my preamp (higher cathode resistor value for instance) will give me a cleaner sound, even at high volume?

When I'm at it.. There is something I have had trouble understanding about the placement of the volume pot.

If the pot is placed before a tube, you can attenuate the signal before it's amplified, and achieve a clean amplified signal.

But very often, the volume pot comes after the preamp tube, attenuating the already amplified, and possibly distorted, signal. Wouldn't that just lower the volume of that distorted signal, and not do anything to get rid of distortion?

Tommy

[gearhead]

Early guitar amps were designed to -not- distort. Had to crank them up a bunch to do so; and many couldn't take it. Hendrix pushed quite a number over the edge.

One of the design characteristics is biasing in the preamp triodes. If your input signal gets large enough, the signal waveform can be clipped due to plate-current cutoff or plate-current saturation. Depends on where you set the bias.

Good explanation is NEETS module 6, pages 1-27 to 1-30:

http://www.phy.davidson.edu/instrumentation/NEETS.htm

[muchxs]

Basicially what I'm asking: What is the difference in construction, between a guitar amp and Hi-Fi amp....if there is any?

It almost comes down to a difference in philosophy... hi-fi amps are supposed to be clean, guitar amps don't have to be.

Marshall used to advertise as "The World's Most Powerful Distortion Free Amplifier". If they advertised as "distortion free" right now they wouldn't sell anything!

If would be great if there was a simple answer like, "it's the preamp" or "it's the power amp" or "it's the power supply" or "it's the OT" or "it's the tubes", the truth of the matter is that with very few exceptions it's all of it.

Hi-fi designs tend to be conservative, take my old Altec Voice of Theater for example. Around 20 watts from a pair of 6550s might not seem like much and it isn't. At 20 watts they're not being pushed at all so the amp will be very clean, in fact without radical mods it's difficult to make it dirty.

Next, let's look at the power supply on the old Altec. It's an overbuilt regulated power supply. I can't think of a single guitar amp that uses a regulated power supply. Regulated means that the voltage can't swing around wildly in relation to power tube drive levels so the amp is very clean and stabile. Overbuilt means that no matter how demanding your bass frequencies are the amp has ample power available to reproduce them. "There Ain't No Such Thing As A Free Lunch." It doesn't matter how nice your power amp design is, if there isn't lots of power available from your power supply you amp is going to sound like an oversized whoopie cushion.

Next, let's look at the small signal section of the amp, the voltage amp otherwise known as the preamp. Hi-fi designs are very conservative and use low gain tubes like 6SN7s, 12AU7s, 6CG7s and other similar types. The lower gain tubes are able to handle larger signals at the input, for instance we tweak Leslie 122s using 12AU7s on the input to handle 7.5 volts at the input, that's unthinkable for a guitar amp. Guitar amps usually see less than 1 volt peak to peak, they start to smoke at around 4 volts. Literally if you exceed that bad things will happen.

Transformers figure in. Lots of iron is clean, light iron tends to be dirty.

It's all about the weakest link. You can build an amp be clean clean clean by seeking out all possible sources of distortion and eliminating them or you can do what we do here by seeking out all possible sources of distortion and nurturing them! Seriously, you can create distortion at your preamp, create distortion interstage by using "dirty" sounding components, distort your PI, distort your power amp, or you can keep everything clean through the power amp then squeeze it to saturation through an undersized OT, you can make your amp quack like a duck with a saggy underbuilt power supply or you can layer aluminum oxide style grit over it with a low fidelity speaker.

Did I leave anything out?