Decreasing headroom...on purpose

[wcrankshaw]

I was wondering if there's any general easy-ish way to decrease the headroom of an amp. I just did a bunch of work on an AC15 (MM output, new caps, GZ34 conversion) and it sounds wayyy better, but has A LOT more headroom. It would be nice to get back the somewhat earlier grind that it had before. Any ideas?

[Sweetfinger]

If you've converted it to a tube rectifier, sub a 5U4 or 5Y3. I haven't looked at the schematic for one of these, but apparently they have a "sag" circuit. It might be possible to re-insert that into the topology.

[wcrankshaw]

Well it was actually a 5Y3 to begin with, but I lowered the filtering, put in a choke, and upped the rectifier to a GZ34. Adding the GZ34 should have increased the headroom quite a bit (as it appeared to do). So, I assume putting the 5Y3 with it's larger voltage drop would return it to previous conditions. But, since I lowered the 47uF cap to a 16uF, would that cause adverse reactions?

[Wilder Amplification]

Use a zener diode on the center tap of the HT winding to drop the B+ down. The zener voltage = how much voltage it will take off the B+.

Either that or use a power transformer that yields a lower B+.

[R.G.]

Headroom is an almost terminally misleading term. The general idea is that there is some "maximum signal" that the device will get and work properly with. Headroom is the extra signal above that maximum that the amp can work with before distorting.

The problem is that there is no "maximum signal", especially to guitarists, and so headroom is a meaningless term. The way it's used is kind of the inverse of "how soon does the amp distort?", which is how you're using it.

Maximum signal before distorting is something that has a meaning. You want the amplifier to start distorting on smaller signals.

Every single active device in the amp is a place where distortion can start. It's only when no device before the output amplifier causes limiting distortion that you get the biggest output that the amp is capable of.

So there are two paths to "lowering headroom". One is to limit/clip the signal in the preamp path before it hits the power amp. The other is to lower the power supply voltage to the output amplifier, which makes the output amplifier clip earlier. This is what several of the bits of advice you've been given amount to.

[wcrankshaw]

It's not a misleading term in this situation. It's really a pretty simple concept when it comes to the specific scenario of an AC15 that doesn't distort as early as it used to. Nominal level is my guitar plugged into this amp at a non-distorting level. The distance between there and a nice sweet overdrive is the headroom. I'd like that distance to be a little less than maxed out.

[Steve Conner]

Maybe it was breaking up early because one of the components you replaced was faulty? A leaky capacitor or worn-out rectifier tube could cause that.

You often see this, vintage amp owners get used to the funky sputter of an amp that's technically broken, and call it "sweet overdrive". Then it goes to get repaired and comes back, now giving full power, as a roaring monster that has them cowering behind the couch. Well, maybe not quite, but it does happen

Also as others said, the GZ34, choke in place of a resistor, and Mercury Magnetics OT will probably all increase the output power. If you wanted your amp to break up early, these were all the wrong choices. Did you try putting your old rectifier tube back in? The 16uf isn't an issue: too-small capacitance won't hurt a rectifier, only too big.

[mooreamps]

I wonder if one could power scale the filament power to a rectifier tube ? or would that be another one of the "mad scientist" mods ???


and for those wondering , since there are no issues with the so-called "cathode stripping", ya I really would try that and I already have a fix for the waste heat issue as well. There would be no wasted heat generated, regardless of the current load.
-g

[Steve Conner]

I wonder if one could power scale the filament power to a rectifier tube ?

Well you're just the kind of guy we need to try it!

I imagine if the tube had an oxide-coated cathode, either directly or indirectly heated, reducing the filament power would kill it pretty quickly. A thoriated tungsten filament would be fine, but how many rectifier tubes have those?

[mooreamps]

I did not catch this, the GZ-34 does have a cathode. That would be a problem. You don't really see that on schematics, but it's there on the data sheet.

-g

[R.G.]

I imagine if the tube had an oxide-coated cathode, either directly or indirectly heated, reducing the filament power would kill it pretty quickly.

Dead-on correct. Never, ever suck away all the electrons that an oxide cathode can produce. This removes the space charge cloud and stops it from shielding the cathode from positive ion bombardment.

[wcrankshaw]

To be honest, I did all the gutting and replacement for a friend (the owner of the amp). I played it a little before doing it. Personally I think it sounds awesome with the upgrades, and has a ton more openness. He wants it a little more grinding though. I guess throwing the 5Y3 back in is the best advice. Actually, the best advice is probably that an AC15 isn't the best amp for him. Thanks for the ideas.

[mooreamps]

Well you're just the kind of guy we need to try it!

I imagine if the tube had an oxide-coated cathode, either directly or indirectly heated, reducing the filament power would kill it pretty quickly. A thoriated tungsten filament would be fine, but how many rectifier tubes have those?

ya, I was never real big on playing with varying filament voltages.

-g

[mooreamps]

I was wondering if there's any general easy-ish way to decrease the headroom of an amp. I just did a bunch of work on an AC15 (MM output, new caps, GZ34 conversion) and it sounds wayyy better, but has A LOT more headroom. It would be nice to get back the somewhat earlier grind that it had before. Any ideas?

I use a power brake for the power tubes. Seems to still work fine on some of my earlier builds. To that note, I am getting one of my early 18 watt amps back for upgrade. going to pull the plate resistors out of the first preamp gain stage, and replace with plate chokes. No more room on the chassis, so i'll have to mount them in the cab, somewhere.......


-g

[shredhead]

Get a variac but don't go too low. You don't want the heater elements to get too cool or your tubes won't last long. Best is to mod the amp so that the variac only controls the plate voltage, while everything else gets the normal voltage. Awesome sag will result, without any damage to anything.