Electronic States
Work blog of Atomium Amplification in Philadelphia, PA
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Ampeg SVT-4 Pro. Had the same problem I often see in these, damaged solder connections on the rear of the main PCB. Bass heads get lots of mechanical stress because they’re always sitting on top of a box full of speakers. Solder joints on flexible and poorly-supported PCBs go intermittent, making the amp cranky/possessed/unresponsive/etc.
This amp in particular wouldn’t turn on (no blown breaker) because the solder under the IEC AC mains jack had crusted and flaked away. I reinforced it with buss wire and epoxy, and resoldered every pin on every jack along the back. This is a needlessly expensive repair, because of how time-consuming it is to get the whole board out, for such a silly problem.
How to get the Fender sound from a V-4. I ran a SPICE simulation; this will get you ‘Twin with all tone controls at 5’ within 1-2dB throughout the audio spectrum.
Anonymous asked: Hi, I have used your anchor states blog post for my tsl602 reading a lot. Have done 68pf on lead vol pot and am going to put a 390pf on r74 for complete that. My question is about the C9 clip, people on anchor states said it is the same as c49 on a tsl60, but that is only true on the mainboard, your C9 clip is for the lead board right? So far I haven't been able to figure out the equivalent in the tsl60, any help would be amazing, though I understand if it will be too time consuming. Thanks
Without a TSL60 in front of me, I can’t tell you. The JCM2000 schematics are universally awful and impossible to read without having the amp in front of you to take notes on.
C9 is on the lead board on the TSL100, yes, but regardless, it’s just a cap from the signal input to the wiper of the lead gain pot. So it’s next to that pot and easy to find if you know you’re looking for a treble bleed cap. There is also another cap to ground from the wiper of the gain pot, at least in the TSL100 (C8), a treble shunt cap. At this point, I don’t know that I’d recommend changing anything around there. It has basically no effect on ‘fizz’ to cut that cap out, it would only reduce ‘quack’ (or ‘articulation’ if you like) when the gain pot is turned down. It’s located before any real distortion happens.
No guarantee that the TSL60 is actually arranged the same way — they are very different amps. In fact, I’m almost certain that there isn’t a direct equivalent — because I think the gain pot is in a different location in the circuit in the 60. The closest thing to the TSL100’s gain pot network in the TSL60 is the stuff around R90/91/92.
Trace Elliot V-Type bass head with the entire power stage removed. The ‘Bipolar Bear’ power modules in these amps are notoriously unreliable, and this one had shorted all the outputs. In this case, the client deemed a board swap too expensive, so it’s now a preamp only. Rewired the line-out as a master output from the master volume control, which can drive a rackmount power amp. FX loop functionality is unchanged, and all the front panel controls still work the same.
I did enjoy that on the bottom side of the power module, Trace Elliot had printed into the PCB: “Make it so, Number One.”
The 5150II from earlier, after repair. Peavey sent a new power board to drop in, which significantly reduced what would have been a prohibitively time-consuming repair. Just needed to replace a few other resistors on the main and preamp boards, and the amp plays good as new.
For how much people turn up their noses at Peavey gear, you have to respect how easy they are to work on, and how easy the company is to deal with. They sell modular replacement parts to anyone (affordably!), and they’re always helpful on the phone. And I love the 5150 sound anyway. It’s probably the best sound-for-dollar of any high-gain amp. When I’m on tour sitting backstage, I can identify one being soundchecked in the other room without ever seeing it.
In case it wasn’t obvious, if you have a dead 5150 or 5150II, I’ll happily take it off your hands.
Anonymous asked: I saw your post of the JCM 900 2100 fitted with a choke and a new OT. Is it your opinion that the 900 series benefits from these changes?
Yes, in very specific ways. The JCM900 series has Dagnall transformers that are wound with a type of ‘self-stripping’ wire. In many (but not all) cases the wire looks red instead of the usual gold color (clear enamel). The problem is that the enamel vaporizes when exposed to heat — convenient for quick soldering but a terrible idea when used in an output transformer. The net result is that JCM900 transformers very commonly fail with partial shorts. I see this all the time, and it happened with the recent 2100 that I upgraded. It doesn’t happen with earlier or later amps, just the 900s.
The choke is just a cheap way to improve the power supply at the margins. If you don’t crank the amp it will make no difference. It stiffens the screen supply during high-power transients, so that the amp feels like it has more ‘umph’ at high volume. This isn’t appropriate for everyone. Metal players who play loud seem to like it a lot.
Mercury OTs that I’ve seen are overbuilt and therefore quite reliable, but also usually have more iron so they can deliver more clean power and bandwidth (power supply and tubes allowing). In some cases the primary impedance might be adjusted (with this 900, 1.7K instead of 2K, which adds crunch harmonics and increases power with EL34s). Their power transformers sag less under load than most others. A lot of people buy Mercury stuff for the ‘magic’ factor or the name, but in my experience there are actually specific measurable ways that their units outperform others. But I think that to see those performance gains in practice, you have to be pushing the amp really hard. For some clients it’s exactly what they need, for others, it’s overkill. It depends on the application. I play loud and have a ‘100W’ Marshall TSL with the Mercury MAR100OM and MAR100P transformers in it. It does 135W before clipping.
For the 900s, there are other considerations. You can’t just go upgrading parts and expect great results. Many of the component values in the power stage of the amp were tweaked to allow for the use of 5881s during a period of poor EL34 production. The amp’s power output is limited by these changes, particularly when using EL34s. Many ‘100W’ JCM900s actually only do about 65W unless their power stages are revised to more ‘standard’ Marshall values — 1K screen resistors, hotter phase inverter biasing and a shorter tail, 220k grid leaks, etc. In a lot of cases, the easiest way to get a better sound out of a JCM900 is just to put 6L6s in it and not worry about it. It’s probably the ‘least Marshall’ Marshall anyway.
andrewjamesmacgregor asked: Can you give me a bit of info on what exactly the differences between Silver Knob and Red Knob Model Ts are?
They’re exactly the same except for the tone stack and master volume. The 1st-gen has a pretty standard Bassman/JTM45 stack driven by a cathode follower. When the controls are at midpoint, the amp has a mid cut, just like all interactive TMB stacks.
The 2nd-gen (red knob) uses a plate-driven James-Baxandall network for the treble and bass and and active negative-feedback arrangement in the recovery triode for mid control. That allows for a selectable mid frequency and far less interactivity than a TMB stack. This is basically a poor man’s Ampeg V4 tone stack, because it tries to do all the same things but with fewer stages and less complexity. It’s not as good as the V4’s stack in terms of linearity and predictability. With the controls at midpoint, the amp will have a mostly-flat response with a slight downward tilt.
The master volume in the 2nd-gen is a dual-gang pot that turns down the volume both before and after the tone stack. Since the ‘before’ part of this is right on top of the pre volumes, it’s redundant, and it means that when you turn down the master, the amp will be clean even if the pre volumes are cranked.
You can get most of the sound of the 1st-gen amp in the 2nd-gen by disconnecting the 2nd gang of the master volume and setting the mid frequency switch to the left with the mid control at about 2 or 3. Set bass and treble to taste.
1970s Sunn Model T Super. This is a transitional model with the 2nd-gen preamp but without the red hardware. It needed to have the bass side of the James-Baxandall network rebuilt, since a previous repair attempt hadn’t been wired correctly. I rewired the master back to a standard configuration, without the dual-gang setup that makes dirty sounds tougher to get out of the 2nd-gen version. Both coupling caps were a little leaky and got replaced.
Now that the preamp is working properly, it’s still not producing the rated power. May just need new tubes. These amps are so simple that it usually doesn’t take long to narrow down the problem.
Two film capacitors of comparable value and voltage. The one on the left is new, the one on the right is from the 70s. Graphic demonstration of the improvements in material and fab processes over the past couple of decades. Often when you fully rehab an old piece of equipment, it’s actually better in many respects than when it rolled off the assembly line.
Warning: graphic imagery. Gutted 5150II. Still trying to figure out what to do with this one, since the power tube PCB is burned through in multiple places and completely shot, and there are component failures all over the amp. Basically, one initial flashover carbonized enough PCB area to permanently couple the plate voltage into the heater circuit, which blew every single tube in the amp, preamp cathode circuits, the LV relay supply, heater reference resistors, and a few sundry other bits.
I suppose the moral of the story is that you should always take an amp that “makes lightning” to a tech immediately. Don’t assume that because it turns on again that it’s ok, and don’t sell it to an unsuspecting buyer without telling them.
