Mojave Dampening

[Pressed_Rat]

I picked up on this thread over at AX84 earlier this week. Eric Johnson would never go for this. I am certainly not an Eric Johnson.

Anyway I wired it up like the Hoffman schematic, trying both 25k and 50K linear pots. Works pretty darn well. It really isn't power scaling, but for $1.50 in parts, and five minutes of work, two of which were spent waiting for the iron to warm up, it is far more.... cost effective.

This is a 45 watt 5F6-A clone amp. Straight up these have a big beautiful sound, but are painfully loud in a domestic situation. With this modest investment installed, it is a little louder than bedroom level at max attenuation and max volume, but doesn't alter the tone much. Highs roll off a bit, within my treble adjustment range. If you are in an apartment and hate not being able to crank your JTM45, Bassman or Plexi - this is well worth trying out. The crunch is there. If you need more, try Kevin O'Conner's methods/kits, attenuator/brake, etc.

YMMV, but I'm satisfied. I had been planning a reduction in tube power to 6V6s, but now I'm thinking about getting some KT66s!

Note that I also tried this in series with the 10K tail resistor. Nothing.

Whoo Hoo!

[LeeMo]

Update:

I ran by the "shack" at the mall and picked up a two pole six position switch (275-1386) and an assortment of 1/2 watt carbon film resistors(271-306) 5% tolerence. A whopping $10.
Since I had good luck with the 47k pot I started from 56k and worked my way thru to 0 ohm (straight wire). I used the following: 56k, 22k, 15k, 10k, 4.7k, and a piece of cut-off wire from the end of a resistor.
I slipped the resistor wire thru the hole from the inside until the body almost touched the terminal. I soldered it and waited for it to cool before bending it straight up out of the way and trimming the excess. I worked my way around the switch soldering one at a time until I had all five resistors and the jumper done. Then I started just above the resistor bodies and bent the unsoldered end at a right angle so that it would contact the one to its left. I trimmed the tails off and then worked my way around the semi-circle soldering .
The two wires that I had run to interrupt R15 (the PI cathode resistor)were then soldered to the common terminal on the switch and the top semi-circular buss, respectively.
Now I have six positions that range from stock to bedroom level. I'll give it a two thumbs way up. It works much more solidly than the pot.

LeeMo

[Raja]

That's great LeeMo, can you post pic's and clips. I saw the forum at ax84 and it looks like you and other's are having good results with this. Do you think it would work with a Lite IIb circuit? Where did you place the switch?
Raja

[LeeMo]

Coco from Trinity drew this schemo. I put the switch in , instead of the pot (red).

No clips yet (lazy me ). I'm too busy playin' it . I still use a little attenuation (4dB) along with it. It's pretty cool. It compressesso when you hold a note, it'll sustain. It goes into controlled feedback on some of the middle settings as the volume swells back up. I'm really liking this mod a lot.(as you can prolly tell. )

LeeMo

[billv]

So - do you get any hum from the V3b heater? (Just yanking your chain )

Thanks for posting that. I'm going to give it a try in an AO-44 build and will report back.

[phsyconoodler]

OK,here's the deal:I used this mod in my 6v6 TMB and I used a 56KRA pot in series with the 470 ohm resistor.I use fender PI values.
It not only works,it works extremely well.If the volume is cranked and the master is about half or less it lowers the volume significantly and the sound quality is better than an attenuator.Of course it doesn't drive the speakers as hard,but it does work well.It changes the bias of the PI and it adds a bit of grind.If you use a 50k pot it isn't as smooth a sweep as a reverse audio pot.The best way is with a six pole shorting switch and use resistors.With DC on the pot it is scratchy when turning it and it takes a second or so for the coupling caps to catch up.It works ok without the reverse audio pot but I am going to use a wirewound pot instead of a carbon one.All in all it works great like any attenuated volume control it does suck some tone,but it works better than an attenuator in my opinion.

[LeeMo]

So - do you get any hum from the V3b heater?

I didn't notice that before.
Now I have to point fingers at Coco. Looks like an honest mistake. My chain feels, uh ,I don-know umm, longer. like it's been s-t-r-e-t-c-h-e-d.
Has someone been yankin' onnit?

LeeMo

[Coco]

LeeMo..How's this? Chain link repaired..

[LeeMo]

Tres frigid! That's redneck French for very cool.

LeeMo

[rjgtr]

I was working on a Sundown amp last week. The idiot owner put 6550s in a 6L6 amp and when the fuse blew he put foil over the fuse! Fried caps and OT1

What was also fried was the 'RMS' control. This was clever way to do power reduction on a fixed bias amp. It is a pot raises the cathodes from ground. I didn't see any adjustment to the bias supply.

But at any rate it was a simple and cool way to reduce the output of the amp. I'm not sure how you'd adapt it to an 18 watt though...

[tarzanalog]

All of this sounds like you'd be reducing the signal to the output tubes, correct? An 18W gets most of it's grind from the power section, correct? I'm thinking it would be ill-advised to install this on an 18W?!? Unless you're going for max preamp distortion at bedroom levels (if you dig that) then I'm sure it would work swimmingly.

FWIW, I put a 250k stereo pot in series with the 470k resistor pair in my 36W and can effectively "cold-bias" (correct terminology?) the PI to get more of a Vox-like response with a little more "clean" headroom out of my EF86 channel. But, as Z-P said... it's kinda fizzy at that setting when dimed. That whole mod seem like the polar opposite of the mod discussed in this thread.

Back on topic: I think you'd be OK installing this "dampening" mod on something that didn't rely on power tube distortion so much, but not on an 18W type amp. That EL84 grind is where it's at...

.

[Pressed_Rat]

Well, I thought it caused the PI to clip.

I have no idea how this sounds with an 18 or 36 watter. I used a 50K linear pot in a Bassman clone, with a 1987 style 1st stage preamp & 12AX7. Sounds pretty good to me. As I increase the value of the clamp - I can open up the preamp volume knobs more and get a decent grind without irritating the wife.

I also tried a pre-PI master volume (1M audio). Nowhere near as nice as this trivial little mod. Haven't tried a post-PI MV yet. Don't think I'll bother. Simplier is better IMHO. Don't get me wrong, this is not as versatile as power scaling is advertized to be.

[s2]

Don't get me wrong, this is not as versatile as power scaling is advertized to be.

Power scaling is not as versatile as it is advertized to be. True, it works pretty well, but it has its limitations.

If you scale the whole amp, PS is useful until you turn the voltage control down to about 3.5. Below that and it starts to sound pretty bad as the preamp tubes are starved at that point.

You can get around this by scaling only the power amp, but this has its own set of issues. First, you have to use a drive compensation control (which is really just a MV). As you turn down the voltage, you also have to adjust the drive pot--just one more thing you have to mess with. Also, you have to create a dedicated power supply node for the preamp. If you have a choke in the amp, it will only filter for the screens, so the preamp does not benefit from the choke any longer.

If the amp is fixed bias, you have to use the bias tracking circuit. IMHO, this causes the amp to be too complicated for the average tech to service.

Finally, the MOSFET generates a lot of heat and is somewhat less rugged than the rest of the amp. It hasn't happened yet, but I'm afraid those things are going to blow at some point.

I've built at least a dozen amps with power scaling and I like it. But I'm going to try this little trick in an amp and see how it works.

[rjgtr]

It sounds to me like you're 'scaling' the response of the PI only, so for an amp like the 18 it isn't as good. But for a larger amp it probably sounds at least as goods as a PPIMV.

Could you actually do this on the power tubes in an 18 as well? how high can you go with the cathode resistor ?

[Pressed_Rat]

how high can you go with the cathode resistor ?

Well, I don't know, but I can tell you that there is a diminishing return with increasing resistance. There is a little more limiting / damping / clamping with a 50K pot vs. a 25K pot in a 5F6-A. I think 25K is about as far as I care to go with this; accomplishes what I was looking for. I don't know about an 18 watt, but if the effect is produced by clipping the PI output, the power amp will be amplifying a square wave - which won't really sound like power amp distortion. Maybe this is the compression 57 Chevy was referring to in his November.

You know, if this works for an 18 watt, it might be a cool little addition to the 2061X PA amp . Hmmm...

[thegipper]

I shot over from the plexi palace to look at the power dampening thread stuff here!

I've tried it, with some mixed results. I am thinking of trying the rotary selector switch in this thread. I'm yet to try power scaling and it sounds way too complicated. One pot would be cool, or a rotary switch.

There are some negative's I heard of this from this thread at the gear page. Go to page 18, and read the quotes from k.o.c. that mikey posts in #258, about half-way down.

http://www.thegearpage.net/board/showth ... +dampening

Is this true? Doesn't sound good!

We've linked that thread back to the plexi-palace too!

[Raja]

I have a 18 watt Lite IIb and I hooked my substitution box up between the PI and resistor and it worked as others have found but it didn't sound as good as using an attenuator. It might be a good candidate in another type amp though.
Raja