Author Topic: What causes amp sag? (Read 5535 times)

Underground_Player · « **on:** January 05, 2006, 12:34:38 PM »

First off I'll say that I know dangerously little about how amps work.
However, I thought I knew where sag came from - valve rectifiers. Or sag circuits, or whatever.
But I recently read the view of a Matamp forumite who strongly suggested that amp sag originating from valve rectifiers was a myth.

Although I've never experienced any significant degree of sag from an amp without a valve rectifier, it did strike me that the last 3 valve rectified amps I've had the chance to crank responded very differently.
A 50 watt JCM800 head with volume and gain wound up 'a lot' ( :twisted: )didn't seem to have much sag at all (is it even valve rectified?!). A Bluesbreaker reissue combo had more, but an old WEM Clubman, with all knobs on 10, was the most incredibly squishy amp I've ever played or heard.

So, if it's not solely down to the rectifier - what's the real cause of amp sag?

Thanks in advance :)

HJM · « **Reply #1 on:** January 05, 2006, 01:37:43 PM »

Plate voltage, amp design, valve choice, wattage, filtering, rectification - probably more!

Lots of factors come into play, no one factor determines response. That's why you have experienced different responses with the amps you mentioned. A JCM800 has a fair amount of filtering and solid state rectifier. The reissue Bluesbreaker has a good amount of filtering but valve rectification - the filters store charge so resisting the rectifier sag. And the WEM probably had a valve rectifier with poor filtering as many cheaper amps do, so the sag was more evident.

We messed around with this at Matamp a bit a few years ago, most Matamps have a lot of filtering, helps keep the mains noise down among other things. We found that a simple swap to a valve rectifier made hardly any, if any difference at all unless the filtering was also reduced!

Cathode biasing lends to a softer compressed tone (AC30), as do lower wattage valves.

Hope that helps a bit!

Underground_Player · « **Reply #2 on:** January 05, 2006, 05:07:18 PM »

Crikey I'd better stop pestering Jeff about a valve rectified Chino then!

Dave's sag circuit seems pretty bulletproof, I think I'll go with that. He might take a bit of convincing though, I know a really fast response is kind of Matamp's USP!

Thanks as always :)

HJM · « **Reply #3 on:** January 05, 2006, 05:46:21 PM »

Tell me about the fast response.....you can't get away with anything!!

Underground_Player · « **Reply #4 on:** January 05, 2006, 05:52:53 PM »

:idea: :idea: :idea:

Switchable sag circuit PDT_049

Why didn't I think of that before?!

HJM · « **Reply #5 on:** January 05, 2006, 06:27:42 PM »

Dave will love that - lots of volts there!

Underground_Player · « **Reply #6 on:** January 10, 2006, 04:36:21 PM »

Quick update for anyone who's interested :)

According to Dave (Matamps design engineer) the 'sag' I've been talking about is simply compression.
I suppose this ties in with how difficult Matamps can be to play - the lack of compression explains both the speed of response and the sensitivity to pick dynamics.
Interestingly (anyone still reading? :lol: ) Mesa Boogie apparently patented sag, as opposed to compression, in the eighties. Whatever the hell sag is I still don't know :lol:

Anyway I'm visiting the factory again on Thursday, hopefully to put down a deposit.
Dave said - in his traditionally long and winding technical manner - that the switchable compression circuit can indeed be done. However, Matamp are currently working on a new amp which he says I might be very interested in............

HJM · « **Reply #7 on:** January 10, 2006, 07:11:02 PM »

What time will you be there?

Underground_Player · « **Reply #8 on:** January 10, 2006, 08:10:27 PM »

About 10 in the morning till sometime, no later than 330 or maybe 4 though.
I'll prob need all the time I can get - I mean deciding on a colour scheme should use up at least half the time :!:

It would be cool if you could make it. I was actually gonna go in last Friday, but this mysterious new amp wouldn't have been ready - I can't wait!