2 channel amp OD question

Catthan

I have a question regarding 2-channel amps: When you switch to the gain channel there's more gain on tap, is it 'cause an additional gain stage gets engaged or are there other electronic bits like FETs and diodes that change the sound? What if there is just one preamp valve like my old HT5? I assume each channel shares half of the valve so what happens when you change to the OD channel and you get "more" ? Higher signal? different pots, caps and resistors?? (I understand pots are basically attenuators so different values will let more-less pass through) What about Mesa amps where you flick a switch and you get, say, "Burn" or "Red" or whatever.. Whereas with a single channel amp it's simpler, you crank the volume and you get OD. Is it really more valve-tone or other bits as well (like pedal circuitry)? Sorry if this is a silly question but I've been interested in Mesa amps since I got my LSS and all these switches make me think that after a point I am just engaging pedals built in the circuit. The tone is lovely and "amp-like" obviously but is it as pure valve saturation as most people think? I'd appreciate any info pls.

ThePrettyDamned

The HT5 uses a hybrid solid state/valve circuit.  Other amps use pure solid state, others use digital modelling (I consider that different enough to analogue solid state) and others (most valve amps) use cascaded stages of valve gain.  

I always think of water hitting a dam.  The first valve might be used for the clean channel and boosts the signal up a bit.  When you engage the drive channel, that might hit 2 more preamp valves.  So an already boosted signal hits the second, and it's too much for it to handle - some of it 'spills over' into overdrive.  That signal is boosted by the circuit, then hits a third one, which gives more drive. 

Power stages are kinda different - I think of them as controlling how much signal (or water, in my analogy) they let through - like a massive dam.  The more you open it (the higher you turn the volume) the more of the signal it lets through.  This isn't strictly true, but it's a rough analogy that helps me understand the difference between preamp and power amp gain, and the position of effects loops.

That's how I think of it, and I might be totally wrong.  But that's the basic 'cascaded gain stage' idea, where rather than driving the hell out of a single valve, or FET/transistor, you use several to build the gain up. I think that's the common method.

Other ways use clipping diodes, but I don't know how they work to be honest.

Hopefully @Martinw, @ICBM will comment soon.  They'll be better equipped than me to explain.  

fretmeister

Depends on the amp I think. I'm sure it's done in many ways.

Catthan

Thanks guys,
the dam analogy is really good. are >1 gain stages a feature of dedicated dirty channel amps or they work in single channel amps too? clarifying that would help my understanding of it I think. I'll do some digging.

Catthan

Sorry but the Mech. Eng geek kicks in. the dam is a good analogy but in reality the dam restricts flow whereas valves amplify. but yes, amplified signal from one valve gets to the next one which saturates more easily. and since you need to spend energy (electricity) to amplify, a more appropriate analogy is a compressor. although in reality it is just that... an amplifier. the dam is like a passive tone stack (cuts).. apologies for the gig fest, back to topic

ecc83

fretmeister said:

Depends on the amp I think. I'm sure it's done in many ways.

Indeed it is. And it is not just about boosting the signal voltage, OD stages need careful "voicing" to make the right noises.

I can't tell you in any detail how it is done in the HT-5 but I CAN say that despite the number of op amps in it, the vast majority of the distortion comes from the ECC83.

Dave.

ICBM

There are almost as many ways of doing it as there are amp designs...

Some use the same signal path and number of gain stages, and switch resistor and cap values to change the gain and voicing.

Some add in extra valve stages.

Some use more than one completely separate audio path with valves.

Some add in solid-state diode clipping.

Some use solid-state gain stages.

Some use combinations of several of the above!

Amps can be all-valve (ie no solid state components in the direct audio path, although reverb and FX loops are usually tolerated if the main signal doesn't go through them when the loop is bypassed), all-valve-gain (ie no solid state gain stages, but use diode clipping - diodes do not add gain), all-valve *distortion* (ie solid-state gain stages, but which do not distort), or full hybrids where the solid-state parts contribute to both gain and distortion (these are not the same thing and need to be distinguished if these things bother you!). 

There is a lot of marketing obfuscation about these categories… since there is such a perception that valves are superior that most companies don't want to admit to any solid-state in there, even when there is. And which is not necessarily a bad thing anyway.

Catthan

Thanks @ICBM