A question about Marshall tube power sections

[protoangel]

Hi amp guys,

Please would you help me out with a bit of info:

I'm thinking about getting a power amp, and like the idea of getting a second-hand Marshall tube amp or power amp.

I have a Sansamp amp sim pedal, which I routinely plug into various "power amp in" or FX return sockets according to the various amps I encounter in rehearsal studios:

I used a 50 watt channel switching JCM 800 with a greenback-loaded the other day:

I Plugged into the FX return socket.

With the amp sim bypassed, its got a pleasant lively sound with plenty of level... not too much treble. However it seens far from flat: The sold state Peavey Bandit I plugged into has a much mellower sound going into the "power amp in" socket

However, when the amp sim pedal is active, there's quite a presence boost, especially off-axis. Plus its either slightly too quiet or too loud (sigh)

A thought occurred to me that the Celestions might be contributing to the harshness?

Are all power sections essentially the same, or are there components in the FX return path which might degrade the signal?

Is there an advantage getting a "power amp in" socket installed?

[Shea]

I don't think it has anything to do with the effects loop. The effects return jack in that particular amp just taps right into the circuit, and doesn't pass through any additional components other than the same stuff that the preamp always goes through. Some amps have buffered returns, but that's supposed to be an improvement over this design.

But here's what's going on. The output section of a tube guitar amp colors the tone quite bit. Part of it has to do with the higher output impedance of a tube amp compared to a solid state amp. Part of it has to do with the output transformer. And a lot of it has to do with the speakers. A speaker's impedance varies at different frequencies, so the speaker's impedance curve interacts with the tube power amp to shape the tone you hear.

The amp sim is designed to be run through something with a flat frequency response, so it's eq'ed to mimic the way a tube power amp shapes your guitar tone. But when you run it through a tube power amp, your signal getting colored twice instead of once, so it doesn't sound right.

Shea

[protoangel]

Ah OK!

That totally makes sense... thanks for that explanation!