Anonymous said: what do you think of 5881 tubes? I heard an amp using them recently and really liked the tone. I've had kt88s in my amp for a while and am not too pleased. I like the sound of 6550s and kt66 tubes as well, but havent used them.
It’s a more complicated question than you might realize.
In my experience, power tubes don’t have a “sound” of their own. They might impart certain subtle characteristics to a particular circuit, but they definitely DON’T have “EQ” in any real sense. Tubes’ non-linearities show up in different ways depending on circuit parameters like plate voltage, screen voltage, output transformer primary impedance, supply stiffness, etc. In most cases, I can’t tell the difference between different power tube types in the same circuit until the power stage of the amp is clipping, and even then, it’s very difficult.
If you’re comparing two different amps, then power tube comparisons are largely meaningless. You’re more likely hearing the difference between the two circuits and not the tube types. An obvious example is the fact that the prototypical Fender Bassman used 5881s. In the 90s, Marshall started installing 5881s in JCM900s. It didn’t make the JCM900s sound like a Bassman, they still sounded just like a JCM900.
Moreover, the case of 5881s is an interesting one in itself. What is marketed today as a 5881 (any that are “made in Russia”) is not in fact one. It is a Soviet military surplus tube that happens to have characteristics similar to a 6L6, and also happens to be extremely reliable and resistant to microphonics. It’s cheap because there are so many of them laying around. It works in basically any 6L6 circuit at the cost of a small amount of power output, so you see it all over the place.
If you liked the sound of an amp with 5881s, think about acquiring THAT amp rather than changing tube types in another amp to match it.26 notes
Not really a pedal, since there’s nothing to step on. This is a completely passive Varitone-style control designed to extend a guitar’s tonal range without modifying the instrument. It goes first in the effects chain, making it part of the guitar’s resonant circuit (placed after a buffer or used with active pickups, it does nothing — it relies on interacting with the LCR network formed by a passive pickup and controls). The knob is a 5-position rotary switch that selects a capacitance value to place in parallel with the guitar’s output. The 3-way toggle switches between high-range, low-range, or bypassed.
Guitar tone controls are a fixed capacitance in series with a variable resistance. They allow you to lower the height of the pickups’ resonant peak. This control allows you to lower the frequency of the resonant peak instead. Kind of like an X-axis to the tone control’s Y-axis. It’s pretty much the same as changing your cable length to alter the guitar’s high end, but far less kludgy and with much greater range and precision. You just stick to a reliable, low-capacitance cable of a convenient length, then dial in the peak from there.
I wouldn’t use it with most humbuckers, since it can only lower the resonant frequency, not increase it. But it’s very useful with bright pickups, particularly when trying to match them with different amps to eliminate harshness in the clean sound, or increase clarity/attack in overdrive.64 notes
I’m back from tour and taking new work. Get in touch.
Fender Twin Reverb, model AA769. This might be my favorite Twin. It’s the late-60s SF version that has most of the older circuit values from the BF version, with no master volume. You get all of the tone of a $6000 museum amp, often for less than $1000 on the used market. If I had a regular use for one of these (I’m sure I could invent one if I tried hard enough), I would buy one in a heartbeat. The last two Rosetta records have used these amps as part of the clean-tone mix, along with my Model A and my TSL.
This amp just had a couple bad bypass electrolytics in channel 2, so that there was a nasty fuzzy overtone when using the reverb. No big deal to replace. I’ve worked on this particular unit before, which belongs to a family friend.
The AA769 does 80W from four 6L6GCs. On a scope you can literally see why people love to crank these things up. There’s a lot of 2nd-order harmonic distortion (compression of the positive voltage swings relative to the negative) well before the power stage clips, so the clean tone is noticeably thicker at high volumes than lower volumes, yet without what we would normally call “distortion.” You don’t hear as much of this effect in the later models that tweak the LTP for a more hi-fi sound.
Bias circuit is the later ‘balance’ control and is not actually adjustable.14 notes
Peavey VTM-60. Pretty common problem of cracked solder on the control PCB, since the only thing supporting it are the pots and jacks on the front panel, which themselves are only connected to the board by their solder pads. Easy enough to fix, and otherwise very reliable amps. Precursor to the 5150. You would be surprised at the number of important metal and hard rock recordings these amps were (and still are) used on. They’re the “Mississippi Marshall.”2 notes
Hybrid compressor that I keep wanting to call the Atomsmasher (after James Plotkin’s seminal 2000 recording of the same name, a big influence on me). Controls from right to left: threshold, ratio (big knob), attack, level (big knob), release. As with all my pedals, the controls are arranged in their order in the circuit.
It’s not really my design, since it’s basically a hybrid of the Engineer’s Thumb and the Diamond Comp (the yellow one). Full-range, suitable for guitar or bass. It’ll do the Ross/Dyna Comp ‘click’ sound that everybody likes but it can get a lot more hi-fi than that. Works as a limiter too.
I had a stash of vintage phenolic knobs that seemed perfect for it.44 notes
Traynor YBA-4 combo. Needed new power tubes and had an intermittent screen resistor and an open plate load in the PI stage. Those Mallorys are still good!
Very nice little PTP amp for bass, clocking in at around 50W. It has a kind of Tweed Bassman topology but with tweaks to the gainstaging. The treble ‘range expansion’ is a standard guitar amp presence control, and the bass expansion is actually in the traditional mid pot position for a Marshall TMB stack. Oh, and the chassis is shock-mounted to isolate it from chassis vibrations, reminiscent of vintage Ampegs. Nice touch. Tubes last longer in here than in guitar combos, and for a bass amp, that’s exceptional.10 notes
Boss CS-1 compressor. Unlike the later Boss comps, this one uses two optocouplers to attenuate both the opamp input and its gain. They’re driven by the output. Response time is pretty slow/bloomy compared to the later pedals with VCAs.
This pedal had a leaking zener diode in the opto driver stage, causing the optos not to conduct. It was acting like a bad distortion pedal since there was nothing to limit the opamp’s gain.
Schematic is here.4 notes
Silverface Fender Quad Reverb (early 70s). This is exactly the same circuit/chassis as the 100W CBS Fender Twin Reverb, but fitted to a 4x12 combo cabinet instead of a 2x12. Speakers are 16-ohm instead of 8-ohm so the same OT is still driving a 4-ohm total load. It’s a monster in every imaginable way.
This amp had some incorrect tube substitutions, cruddy sockets, dirty pots, and some compromised soldering. It had been recently recapped, so that was a time- and money-saver.
There’s some misinformation floating around about the 100W CBS circuit’s bias supply. People are constantly talking about “biasing” this amp, using the trim pot that you can see in the middle-left above. This is NOT a bias adjustment; it is a balance adjustment to match the idle DC draw of the two sides of the push-pull output stage. The absolute bias level is fixed at a pretty cold spot. If you want to change that, you have to replace the 15K resistor to ground with a smaller value (or a trimmer in series with a smaller value). But in most cases, it’s not worth doing because the reliability tradeoffs don’t come with commensurate sonic benefits. In most cases, you don’t even need to adjust the balance control if you use matched sets of power tubes.9 notes
After I posted that I wouldn’t be taking new work after June 8th, I got hit with a tidal wave of amps. Right now I have more than enough to keep me busy until I head out on tour at the end of June, so I will not be taking any new jobs until I get back in late July.