PART 8 -- Eek You!
Next up, a few EQ adjustments, and a brilliant (and easy) tremolo mod.
So far I'd been testing the amp with the Treble on 4 and my Stratocaster tone knob on about 6 (neck or bridge pickup) because the amp's so bright.
Not that there's anything wrong with that. But it seemed like the amp might be filtering out some useful low-mids through that smallish .022uF coupling capacitor. Here it is in the
Vibro Champ schematic diagram.
So I tested a few things and decided to change this up to .047uF. A fuller, rounder sound.... less presence, but I still have brights to burn on this amp if I turn the guitar's tone knob up. No problem.
About coupling capacitors.... Clicky clicky.
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A coupling cap passes your guitar signal (AC) from one tube stage to the next, while blocking the high DC voltage each tube needs to operate. This means that the tubes don't interfere with each other but teh rockzorz still get through from tube to tube. Each coupling cap also works as a high-pass filter, so changing cap values can make an amp brighter or, uh, dimmer.
At this point I noticed that with the guitar volume all the way up, hitting a low E string hard sort of interfered with the way the high E and B strings sustained. This was the case with only the very lowest notes, from low E up to maybe low G. It sounded like the booming, slightly overdriven low string was cutting notches in the sound of the high strings. This is that "modulation" I mentioned in Part 7... where what's happening in one set of frequencies (the lowest lows) changes how other frequencies (the highs) sound.
What caused it, and why I wasn't surprised to hear it....
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It wasn't too surprising, for a couple reasons. First, I'd been doing things to increase the bass and low-mids. Second, traditional Fender preamps are good at amplifying low frequencies. So good that unless the power supply is beefed up or a bunch of high-pass filtering is done (smaller coupling caps, for example) the preamp can start to overwhelm the power amp with bass. The result can be flabby-sounding lows, "farty" distortion, speaker pops, or the kind of modulation I was hearing.
The offending low frequencies might not even be very audible -- they might even be below the effective range of the speakers, down in the range that only blue whales and Tchad Blake can hear. But they still require a lot of =electrical= power from the amp, robbing power from the more audible frequencies. Thus those sonic gaps in the sustaining high strings -- the power amp was busy dealing with power spikes in the overdriven bass frequencies.
To deal with this without losing the lows I'd been working to get, I started changing capacitors in the tone stack. I ended up reducing the bass cap from .1uF to .022uF, and reducing the mid cap from .047uF to .022uF also. (In the photo, the biggest orange cap is the new coupling cap.)
This pair of 22's gave me a tone stack similar to some of the more modern Fenders like the Blues Jr. It's a pretty common fix. Really, I didn't lose any audible bass. I just shifted the low end of the amp's response up out of the whales-only frequencies that don't have much to do with guitar anyway.
Funny, because I'd ordered a handful of nice "orange drop" capacitors to do direct replacements of the tone and coupling caps. The original blue-black caps have a bad reputation, and I was going to replace each with a new cap of the same value and see how much of an improvement there was. But now that I'd changed cap values around, I had an orange .1uF left over, and a shortage of .022's. I ended up recycling the old blue .022 coupling cap as my new mid cap. It sounds good as-is, so Science will just have to wait.
Back when I replaced all the caps in my Twin Reverb with orange drops, I do recall a vague feeling of wellbeing afterwards. But I was doing lots of other mods at the time too. Has anybody replaced tone/coupling caps in a Fender amplifier and noticed a change? What did it sound like?
.......
I think I've pretty much got the amp where I want it, gain-wise and tone-wise. While it's open, I thought I'd try slowing down the tremolo ("Vibrato" in Fenderese). It sounds good at moderate speeds, but it would be really handy to get an even slower pulse.
As I mentioned before, the Vibro Champ vibro is set up differently from most other Fenders. Here are some deeetails.
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Some time ago, I did some vibrolo work on my Twin Reverb and posted some wordage about it. The Twin has a pretty standard Fender trembrato, with a photocell "roach" that modulates (modulation again!) the signal going to the power tubes. The Vibro Champ is partly similar to the Twin, and partly different. The similar part is the "Low Frequency Oscillator" that makes the whole thing go. The LFO works like really really slow feedback. It takes the output from the plate of a 12AX7, sends it through a loop of capacitors, and feeds it back into the tube's grid. Here's the loop in the Vibro Champ schematic diagram.
Around and around it goes. The capacitors help reinforce the feedback and form it into pulses. The timing of the pulses is set by the caps, along with some resistors. The Speed control is one of these resistors. The Vibrato footswitch sends the looping signal to ground and ruins everything.
Here's where the Vibro Champ goes off in its own direction. A bit of the looping pulse is sampled by another 12AX7 grid, where it makes a stronger pulse jump through the tube. This pulsing current creates a pulsing voltage level on the tube's cathode. A bit of this pulsing voltage can be passed to the second preamp stage (through the Intensity control) where it wreaks havoc on the bias in the preamp stage, basically reducing or even shutting off the surf guitar audio going through the tube and then letting it go back to normal.... over and over and over again.
After a little internettery, I found a forum poast with the solution I needed. Change one of the trembrolato vibroscillator caps from .01uF to .02uF (there are three: two .01's and one .02). Great. Yet another .022 cap needed. Fortunately I had a baggie of funky old caps harvested from my Twin Reverb. I fished out a beat, waxy old .022 and soldered it into the Vibro Champ instead of the .01 disc cap.
Perfect. I can get a nice slooow vibrothrob now, and it's taken some of the hard edge off the faster chop too.
Good Internet. pat pat pat.
Gooood Internet.
At long last, I think I have everything tweaked the way I want it. Time to get it all nailed in place inside the amp.......
But I do think that a Twin (for example) needs its Treble knob on about 4 max once it's balanced with a loud drummer.
