Yesterday, I told the story of my runaway reverb unit and how I solved the problem. Here's a little more about my '65 Fender Twin Reissue.
Since I was 16 years old I've lusted after a Fender Twin. My father had one when I was a kid. It was a Silverface 135. He liked the amp well enough but its heft finally got him and he sold it around 1988 or so. This was before I really started playing a lot or I would have begged him to let me "buy" it from him. It took me probably around 24 years to finally get one of my own. Unfortunately, like so many things I've lusted after in my life, once I had it I realized it wasn't quite what I wanted.
The Fender Twin Reissue is a 2-12 beast of an amplifier running at around 85 watts. It has two channels, an amazing tube driven reverb unit and tube driven vibrato, and weighs as much as an economy line Ford car. I learned this when I went down to Nashville for a week and carried that beast all over Broadway - up and down stairs, in and out of cars, on and off stage, etc. I vowed that it definitely needed wheels … and a roadie.
But, I digress.
The problem I have with the amp is really one of concept. It was envisioned and designed for a much different musical environment. This amp is loud. It will over-power an aggressive drummer and can easily compete with your most self-confident bass player. All without even breaking a sweat or overdriving the power amp stage. This made it a great amp back in '65 especially if you were playing a 200 - 400 person club or hall using a Shure Vocalmaster and pushing with stage volume. Unfortunately, today it's significant overkill.
It's been a long time since I ran an amp without micing it and running through the front of house speakers. Even the tiny clubs in Nashville all have sound people to mic the amps. As much as I long for the days of palm rolling my high wattage amplifier and letting it scream like a banshee in heat, those days are gone. This means that while I love the amp and what it is, to be useful in a modern context, it needs some tweaking from stock.
The first thing I decided to do was reduce the overall power output of the amp. This is actually a very simple thing to do on this type of design, which is most things Fender from the 50s, 60s, and 70s with 4 power tubes (this also tends to apply to Marshalls of the day as well). Yank out two of the power tubes either the inner or outers and you effectively half your power consumption. This brings my twin from 85 watts down to roughly 40 watts (give or take). The problem is that this circuit was designed to drive 85 watts into a 4 ohm load. We now have 40-ish watts driven into a 4 ohm load, which is potentially taxing on the output transformer. I say potentially because given the variance of component design and the tolerances built in, there is a very wide range of allowable values.
As an example, the Fender Twin uses two 8 ohm speakers wired to present a 4 ohm load to the output transformer (8 ohms / 2 = 4 ohms). At least it does on paper. In reality, when verifying the values of my specific speakers, I had one at 6.5 and another at 7 ohms. This would provide a combined load of 6.75 ohms. Kind of an interesting play on tolerances, eh?
Getting back on track, something that's recommended as conventional wisdom is to disconnect one of the speakers to provide a proportional load given the reduced output. In theory, this would shift the load from 4 ohms to 8 ohms. However, as was seen in my personal example, if I disconnected the 7 ohm speaker, I would only increase my effective load by .25 ohms. Definitely not the 4 ohm difference we would expect. My own opinion is to use common sense with it. If you have the tools, evaluate your own numbers and make a determination. Given the differences involved and the tolerances of components, your chances of hurting anything are minuscule (don't come back to me if you blow something up though).
So, I've halved my wattage and gotten closer to making my amp usable in a modern setting. But, I'm not quite done yet. My goal is to get a less than clean response out of it. No, I'm not looking for JCM900 high gain madness. All I want is a little growl when I push it. Unfortunately, even at half wattage, the amp is still kind of clean for my tastes.
Here's a couple things I did to get it closer to my range of liking. Some people may be comfortable with this and others may not. It works for me and I'm happy with taking a risk. Use your own judgement and discretion.
The cool part about the Fender Twin is that it has six preamp tubes. Four of these tubes are 12AX7 while the other two are 12AT7. While I'm not going to get into a whole dissertation on tube theory, I will say that these tubes are substantially similar. They do different things and operate differently, but changing things out can make for some interesting combinations.
The first thing to understand is how the classic Fender circuit works and how there are subtle types of influences the two seemingly distinct Normal and Vibrato channels have on each other. If you've ever seen a classic bassman or an old Plexi using the patch cord between the inputs, you kind of understand. The twin is very similar. There is excess signal that is "bled" off into the other unused channel even when the volume is off. This design allows the amp to stay as clean as possible for as long as possible. My goal is to drive through the preamp and hit the power amp hard to coax out some of that sweet, sweet overdrive.
Since I use the vibrato channel (I like reverb), the simple way of removing this bleed over into the normal channel is to remove the 12AX7 from socket V1. No tube, no channel. No place for the signal to go but forward. So, what do we do with that AX7 I just pulled out? Well, let's put it into V6. This is the phase inverter. Replacing the AT7 with an AX7 reduces the amount of headroom in the amp before it begins to break up. Essentially, the AX7 is a tube that provides a lot of signal amplification therefore it absorbs current and increases it. The AT7 does the same thing just not as much. The AT7 will take the volts passed by the preamp and just keep taking them while providing a nice, steady current out at a moderate pace. The AX7 takes the same current and hits everything downstream that much harder. This means it takes less signal from the upstream stages to produce much more downstream current.
To recap, I've reduced my overall wattage, created a much stronger preamp signal on my preferred channel, and I've changed up a tube in my phase inverter to drive the amp harder at the same relative volumes. All of this together means that my formerly very squeaky clean Twin is now a bit grittier and can get just a tad rough at a realistic stage volume.
Don't get me wrong, this amp is not, nor will it ever be a screaming dirt monster like a pegged Deluxe, Plexi, Mesa, or Soldano. It is what it is and will always have that character, but it's nice to let it flex its wings a bit and let its hair down.
No comments:
Post a Comment