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Sloan wrote:This is seriously probably the most text-per-post thread I've ever witnessed during my internets. Keep up the awesome!

Best thread on Shortscale ever.

He brings the info, he brings the lols.

Kudos.

Sloan wrote:This is seriously probably the most text-per-post thread I've ever witnessed during my internets. Keep up the awesome!

That's right, and we haven't even gotten to the part where I start killing corpses!

Thanks, peoples.

PART 4 -- Stupid Power-Amp Tricks

Now things get fun... I hope. Now that I've replaced the speaker and some worn-out parts, I think I have a basic idea of what amp sounds like. Now I can start experimenting to see how far I can nudge the amp towards some even better sondz.

Like I said before, the amp has a very tight-bright-and-responsive sound that's nice as-is, but from another angle I might say it sounds a bit stiff. I'd like to get a little more compression going, and maybe try to balance the brights in the tone even more too. I also want to rein in the really high wattage that the power tube is running at -- to save on component wear-and-tear, and hopefully improve the sound as well.

This means working around some of the highest voltages in the amp, so I always switch the amp off and check to see that the voltage has been bled away before I start. Here's my one-hand chopstick voltage-checking move, Vibro-Champ version: from the rectifier tube to the chassis.



First I added a screen resistor to the output tube. Lots of places among the internets recommend this for Vibro Champs. Here's where it goes in the schematic diagram.



I know very little about tube screens. If anybody can enlighten me on these and tell me what difference they might make sound-wise, have at it. For what little I do know, click here....
I soldered in a big 2-Watt screen resistor, hanging it in mid-air with a cunning double-right-angle bend to hold it above some of the other power-supply resistors.



The way I wire amps, be thankful I'm not designing freeways. I had to re-route another wire from the vibrato tube, and I gave that a square bend too. Major boutique-amp-square-wire-bending points for me, yo. Right-angle wiring doubles an amp's value. Ask anybody. If they say it doesn't, ask somebody else.

I A/B tested the resistor over and over. Aside from longer tube life, there may be a subtle difference in sound. I thought I heard slightly more distortion without the resistor, and slightly less with it in place. Again, if anybody can tell me what difference in sound a screen resistor will make, I'm curious.

I'm chopping this post in half because otherwise it'll be a real marathon. The next part is a lot more interesting, in my geeky opinion.

Right-angle wiring doubles an amp's value. Ask anybody. If they say it doesn't, ask somebody else.

He's right, it totally does. Filtercap, I love you.

Actually, you can think of a curve as a series of infinitely small right angles. So really any spaghetti-wired amp is right-angle-wired too. Though I don't know if Budda or Victoria or whoever have sold anything with that approach. I haven't, but then again I haven't tried.

i love this thread, but when do we get to hear a coupletones?

I love filtercap so much. Definitely one of my favourite posters. Awesome stuff.
All my marshalls have had screen resistors so I dunno what they do, other than stop tubes from burning out so quickly, for example if you switch from 6L6s/5881s to EL34s you're supposed to up the screens from 470 ohms to 1K in my 6100.

well shux.

Yes, some tonez would be the scientific icing on the cake. Right now, doing the modz and writing them up is about all I have spare time for, outside of this new "baby" project we have on our hands. (She's all stock, no mods. Do they have Fender baby clothes?). I'll set up some mics after the solder smoke clears.

the internet just got a powerup. this needs to be wiki'd

PART 5 -- Stupid Power-Amp Tricks II

On to better things, such as getting that high voltage down to a sane level. I decided to try something that's worked really well in my Twin Reverb, because there may be a sonic payoff to boot. Enter the 10K-ohm 25-watt wirewound rheostat.



This is a lot like a garden-variety pot you have in a guitar, except it's made of metal wire wrapped around a hunk of white ceramic, and (if you do your math first!) you can jam relatively high wattage through it without burning it up. It can still produce a lot of heat though, depending on how you set it. Careful careful.

Careful careful +100, because I'm putting this in the krispiest part of the amp: right after the rectifier tube where it's 410 volts in the shade. All safety procedures apply!



Actually, I didn't install this in the amp. I'm testing with it, not stuffing it in there permanently. Not wanting hi-voltage wire ends where I can blunder into them, I made a test panel out of a genuine cardboard United States Postal Service box.



Then I connected a wire to the rectifier tube, another wire to what the rectifier normally connects to, and ran these two wires out of the amp into my cheapo panel... like so:



That isn't a stomper switch on top of the box, it's the twistable rheostat shaft.

Now for massive infusions of FUN. I set the rheostat on zero ohms, flipped the switch, and dove for cover. Phew, okay, the amp's on. True to my calculations, the world did not explode.

I consulted my handy-dandy Vibro-Champ Schematic, which told me that the 6V6 power tube got 342 volts to its plate back in the good old blackface days, not ninety mazillion silverface volts. Okay then. With my meter probe on the 6V6 pin (397 volts!), I caaaarefully started turning the rheostat dial (just a shaft really -- knobs are for knobs) until I got the voltage down to 342.

With the power tube's volts dialed down old-school to 342, I checked the 6V6 cathode voltage. 21 volts, just like the diagram. The first 12AX7 plate? 205 volts on the nose, just like the diagram. Like, instant 1964 specs, maaan. It was eerie.

Now the power tube was passing .04 amps and dropping 321 volts. That means a plate dissipation of about 12.8 Watts instead of the 18 watts it was getting force-fed before I started. (See my Part 2 post.) A much better number, and close to "safe" operation even for older 6V6 tubes.

I picked up the guitar to try it out, and right off I could tell I was starting to get the Vibro-Champ version of THE SOUND. Before, the VC was giving me a straightforward sound with every last molecule of pick attack, followed by a very present yet stiff-sounding sustain. The voltage-tweaked VC had a little bit of "give" during the pick attack, and after that the sustain sort of welled up and gave each note a subtle lift. It's a warmer yet springy sound that seriously made it hard to put down the guitar and keep working.

Hah. And lots of the Internets said it couldn't be done on a Vibro Champ.

Why not, and what's going on? Details here....
I A/B'd the new sound with the old by dialing the rheostat back to zero, then back up to the Leo-Fender-approved 342 volts. I did this several times. Definitely a difference, not just my ears/brain telling me something's there. Then I dialed the volts down to 325. Even more of that spongy/springy response, with more warm distortion happening (the amp's volume knob was on 7). I settled on 342 volts as good medium between no rheostat and lots of rheostat.

With the amp switched off, I checked the rheostat with my meter. It was set to about 770 ohms. A number I'll want to remember later on.

i would never have thought about messing with the power supply voltage like that. that's pretty cool that it works.
this thread is making me want a champ.

filtercap is legend.

Fucking A as normal. When I read your unhidden description the first word that came to mind was also 'sag', I hadn't even thought of the Class A implications of it. I'm clueless, your two guesses sound plausible. Are you getting compression + sag?

There was probably a little bit of cathode-resistor compression to start with. I'd credit the rheostat with the rest of the compression I'm hearing. I don't see how I'd be getting more compression from the cathode resistor just by turning up the rheostat. But I could be wrong about that.

By the way, I've placed the rheostat where it will affect the preamp B+ supply as well -- if the power amp can get some sag going, anyway. So it could be that I'm getting a little bit of sag from the power amp, and that's making the preamp compress a little too -- strengthening the effect. Guess #3.

EDIT -- Actually, the lower preamp tube voltages are likely to make the preamp compress a little more, even without any sag. Didn't think about that.

PART 6 -- Stupid Power-Amp Tricks III

In addition to lowering the power supply voltage and getting some nice compression in on the bargain, I thought I'd experiment with the power tube's cathode resistor and bypass cap. These are more or less the "bias control" on a cathode-biased amp like the Vibro Champ.

Changing these parts seems to be the preferred way to lower the wattage that the silverface Vibro Champ cranks through its longsuffering 6V6 power tube. It should also affect the power amp's compression, volume, and bass response.

The diagram, again....



The sondz I was getting were already pretty good. I set things up so I could flip back and forth between a few resistance values and a couple cap values, and return back to what I started with. Here's the amp, with resistors and caps sticking into the air all over the place. (The elevated ends are low enough voltage to be safe to touch with the amp on -- not that I make a habit of doing that.)



I added a couple switches to the control panel.



One switch flips between 500 ohms and 1000 ohms of cathode resistance. At the same time it switches from 25uF of capacitance to 50uF, to keep the bass response more or less the same. The other switch puts another resistor in parallel with all that business to give me 333 ohms or 833 ohms. I also re-soldered things to get 1500 ohms on the cathode.

Basically, as I went to higher and higher resistance,

-- The sound got more spongy/compressed, with less pick attack.

-- There was slighly more soft overdrive if I turned up guitar or amp (I generally tested with Stratocaster volume on 7 and amp volume on 7.)

-- The upper-mids pulled back, with lows getting warmer.

-- The overall volume/presence decreased a little bit -- probably due to the mellower upper-mids (which human ears are especially sensitive to).

-- The current streaming through the 6V6 tube decreased (40 milliamps at 500 ohms, down to 28 milliamps at 1000 ohms).

-- The voltage across the tube INCREASED (321 volts at 500 ohms, UP to 350 volts at 1000 ohms). The voltage on the plate also increased.

-- The tube's plate dissipation decreased (12.8 watts at 500 ohms, down to 9.8 watts at 1000 ohms).

-- The voltage at the rectifier increased back up to where it was before I started messing with things. With a 1500-ohm cathode resistor, the voltage went up to about 425. (This is with my rheostat still doing its thing).

So increasing cathode resistance definitely increases compression, even with a bypass cap in place.

I like what's happening when the cathode resistor's at about 1000 ohms, with a 50uF bypass cap. Dark, mysterious, with a nice spring to individual notes. I also like the 500-ohm setting, with more presence in the mids and a little more pick attack. I'll probably set up the amp so I can switch between these two settings. I might goose the dark-n-springy setting up to 1200 ohms to emphasize the difference a bit more.


...............


One last little experimento before I called it a night. While doing the above, I was especially keeping an ear out for what the lows were doing. The filter cap can I added (first post on this thread) gave me an extra, unused section to play with. What would happen if???

I jumpered the spare section to one of the used sections (the one wired closest to the rectifier) with some alligator clips and switched on. I could tell that a little bit of the low hum (Part 2 again) was gone. I picked up the guitar and tried it. More bass? I AB'd by disconnecting/re-connecting the jumper a half-dozen times. Yep, a little less hummm and a little more bass. Kewl.

Here's the top of the cap can with that last section wired in permanently.



I'll redo that dodgy-looking black ground wire later. Along with.... other things......

Loving the in direct reference to the Fletcher Munson curve.

Another great great great post. Seriously filtercap this stuff is better than Torres.

euan wrote:Loving the in direct reference to the Fletcher Munson curve.

Another great great great post. Seriously filtercap this stuff is better than Torres.

+1.

Legend.

Heavy.

I had the Fletcher Munson response for both my ears modelled a couple of years ago. It was pretty cool/weird.

Filtercap's like the internetz Bill Bryson at making tube amps funny and interesting. Even though I don't understand half the terminology.