Pre Gain & Post Gain - Amp setting?

Wes Hamshaw · From: Pacific City, Oregon * R.I.P.

This is probably a dumb question and the answer abvious
but: just what is the purpose of the PreGain and Post Gain
controls on many amps? I notice they both have an effect
on the volume but why the two? Thanks much.....Wes

Smiley Roberts · From: Hendersonville,Tn. 37075

Wes,
I really can't answer your question per se,but I do know that you should ALWAYS have your pre-gain LOWER than your post-gain. I would put my post-gain wide open,then,adjust the pre-gain accordingly.

If you want a natural distortion ("fuzz"),place your PRE-GAIN wide open,& your post-gain adjust accordingly. At least,that's what used to work for me.

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  ~ ~


  ©¿©  It don't mean a thang,   


  mm   if it ain't got that twang.

Ken Fox · From: Nashville GA USA

It is nice to drive the preamp a little closer to distortion sometimes (that sweet spot!) and use the master to keep it at a decent level.

I have already agreed with what Smily said above. Lately I have been driving the preamps a little harder and getting away from a wide open master volume. On the old Session 400 and LTD 400 amps I like the Volume at around 7 and the sensitivity (fancy name for master volume) at a level suitable for the ocassion!

[This message was edited by Ken Fox on 28 June 2006 at 01:17 PM.]

David Doggett · Posted 28 Jun 2006 2:32 pm

Wes, you're getting some operational suggestions, but I'm not sure they answered your original question. The pre-gain controls the output of the preamp. The post-gain (sometimes called the Master Volume) controls the power amp output. The preamp shapes the tone with the tone controls, but has very little output. Maybe 95% of the output comes from the power amp, which only has one control, its volume or gain. Each of these amps usually has some distortion in its upper volume range. Very clean solid state amps may not let you drive either of them into much distortion. Some tube amps will start distorting if either volume is more than about half way up. Cleaner tube amps will have a "bloom" around 2/3 to 3/4 volume, and will distort beyond that. The tube harmonics and distortion sound different for the pre and power amp. You can hear the difference by first turning the pre all the way up, and then adjusting the volume to a listenable level with the power amp gain. Then do the opposite, turn the power amp all the way up, and adjust to a listenable level with the pre. The combination of the two, both near max and distorting is a third sound. This is usually the ideal distortion for a tube amp. Finally, of course there is the clean sound when both gains are kept in their clean zone. Usually the pre will distort much more than the post. What you are typically supposed to do is find the level of the pre that gives you the distortion you want for a particular situation, and then use the post to adjust the overall volume to the appropriate level for the situation. When you want things clean, keep the pre volume in its clean zone, and adjust the final volume with the post.

On many clean solid state amps, the post will not distort at any level. Many of these have very little distortion on the pre also, you just need to keep the pre a little below the post. But some SS amps give you some SS distortion (for what it's worth) at high pre levels, so you can use that by turning the pre up high, and adjusting the final volume with the post.

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Student of the Steel: Zum uni, Fender tube amps, squareneck and roundneck resos, tenor sax, keyboards

Ken Fox · From: Nashville GA USA

I hate to disagree, but the pregain is in the early stages of the preamp and post gain controls are usally at the final output of the preamp(before it goes to the power amp). At least that is the way it is in Fender tube amps and most Peavey transistor amps I have seen. The volume control of most amps (pregain) is usually located at least one gain stage down stream after the input signal. After several gain stages the preamp is done processing the signal and ready to be applied to the power amp. The master volume, sensitivity control or post gain is usually located at that point to control the power amps input signal. Typically that is a signal in the range of .03 to 1 volt RMS. The power amp is typically fixed in it overall gain.

A lot of folks think the first thing the guitar signal sees is a volume control. Wrong! Too much noise would be induced at that level and usually one or two gain (amplification) stages are applied before the signal see the pregain control. So if you have a hot pickup that is distorting the preamp, the pregain volume control will not help that much! It is already driving the first gain stage crazy with all that signal. That is why there is a second, padded input available on most amp to knock down that signal typically -6bd.

[This message was edited by Ken Fox on 28 June 2006 at 06:33 PM.]

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