Wha's 'em things do?

[Boy Howdy]

I've been looking at schematics a lot lately, trying to learn a little. I have a couple of Sunn heads (Sceptre and Sentura II). I love these amps.

I'm curious about the normal vs. bright channels. They don't sound like most amps in that instead of just being duller sounding the normal channel sounds bigger. It just gets this really big sound. Looking at the schematic (http://sunn.ampage.org/site/schematics/), I see the bright cap on the bright channel inputs. I understand that well enough.

What I don't understand exactly is what the resistors on the inputs do. I find that the normal channel gets too muddy to use at anything above normal bedroom levels, but I really like that channel for jazzier sounds. I was wondering if I might get a sound somewhere in between the normal and bright channels if I were to change to values to something in between the 270k and 47k resistors (with or without the bright cap).

And yes, I understand that just adding a cap to the normal channel may get me there as well. I'm really just interested in what the input resistors do as regards the way the amp sounds. Why do other manufacturers not use different values for their various inputs?

[Tom Phillips]

Gene,

What I don't understand exactly is what the resistors on the inputs do.

The 270k resistor along with the .001µF cap and the 150k resistor form a voltage divider circuit. Since the reactance of the capacitor goes down as the frequency goes up AND the values of the resistors do not change with frequency (at least at the frequencies we are worried about) the overall input circuit operates as a frequency dependent voltage divider. This is the same principle that makes a passive tone circuit work. In your amp the bright channel input circuit attenuates the highs less than the lows. Therefore, it sounds “bright.” Hope that makes sense.

Why do other manufacturers not use different values for their various inputs?

There’s just lots of different ways to do things. Each may result in a different response and each designer has their own preference to the overall circuit layout. The method used in the Sunn amp is a way to have bright and normal inputs while sharing one triode section for all four inputs. Any changes that you make to the first stage triode amp will affect all of the inputs so you can’t put the bright circuit there without using an additional triode section.

I find that the normal channel gets too muddy to use at anything above normal bedroom levels

If it were my amp I’d try reducing the value of the first stage 250µF cathode bypass cap. I’d reduce it to at least 25µF and go down from there as I did listening tests. 10µF or 5µF might be good values. This will reduce the overall lower bass response. Maybe enough to eliminate the muddy sound while still giving a good sound through the bright inputs.

I was wondering if I might get a sound somewhere in between the normal and bright channels if I were to change to values to something in between the 270k and 47k resistors (with or without the bright cap)

Yep. That would work if you pick the right values. Like I said, there are lots of ways to change the overall response. Changes made at or after the tube will affect all inputs. Changes made to the input circuit will affect just the bright or normal inputs.

Have Fun,
Tom

[J Martin]

Something else is wrong if you get muddiness at "bedroom" levels. My Sunns stay crisp and clean up to earplug levels. I love them too and have accumulated about ten of them over the years including two I bought new in the 70's.
jm

[Tom Phillips]

Something else is wrong if you get muddiness at "bedroom" levels. My Sunns stay crisp and clean up to earplug levels. I love them too and have accumulated about ten of them over the years including two I bought new in the 70's.
jm

Good point.
It's best to make sure that an amp is operating properly before jumping into mods.
Thanks,
Tom

[Boy Howdy]

Whoa, ya'll gettin' ahead of me.

Tom and JM. There's nothing wrong. The Sentura II and the Sceptre are the same amp, both work fine and the Sceptre was recently fully restored (just before I bought (STOLE!) it).

The muddiness I mentioned might better be described as 'overly puffy', and in any case is a relative term. Also, I use big ported cabs that tend to be a bit bottom heavy. So, no issues there. I just always want what's not available, and in this case, lies between the two inputs.

Thanks for the detailed info, Tom. Just what I needed. I will give some thought to the cathode bypass cap - it is a bit bottomy, but again that could be my cabs (Music Man 2x12s with a big port - they're almost as big as 4x12 cabs - and were originally used for both guitar and bass. I have Celestion GK12-85s in them. Like this one: http://cgi.ebay.com/MUSIC-MAN-212-RH...QQcmdZViewItem ).

But I'm basically just in love with these amps - and have been for the 6-7 years since I got the first one - and don't want to make any major changes. Just a little messing with the inputs, if that. I figure I have four of them, why should they only make two different sounds, right?

So I'm okay to just change one of the resistors to something around a 150K (or anything between 47K and 270K), and take it from there?

Thanks for the replies, guys.

[Tom Phillips]

So I'm okay to just change one of the resistors to something around a 150K (or anything between 47K and 270K), and take it from there?.

You won’t hurt anything doing that. If you want to know more, you can also calculate the response of the little input circuit made up of the 2 resistors and the cap.
Another possibility that’s reversible is to remove an input jack and install a switch in the resulting front pane hole to change component values.
Have fun,
Tom