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Topics - R.G.

Amplifier Discussion / More Thomas Vox boards
February 06, 2016, 10:13:40 AM
I just got the first protos of two more PCBs for the Thomas Vox line. These two boards go on the power chassis. One holds a pair of snap-in style electro caps and a bridge rectifier module and replaces the two twist-lock can caps in the original. The other implements the power amp circuit board from the originals, for any of the 30W, 60W or 120W amps. This renews the power chassis on any of the Beatle, Guardsman, Viscount, Buckingham, Westminister, Sovereign, and Scorpion amps.
I'm finally getting some of the Thomas Organ Vox technical stuff into print. I need one proofreader to be ruthless about how confusing my writing really is...   :)

This first booklet is on how to fix your Thomas Vox amp's footpedal or make a new one from scratch.

I vastly prefer someone who has built amps or is at least a part time tech.

In the future this series will flesh out into a series of technical booklets, one per amp, to go with the full-replacement PCBs for the preamp sections of these amps.

Anybody out there that's (1) technically informed (2) interested in Thomas Organ Vox and (3) ruthless in a constructive way?
It's getting on 15 years now since I first noted on usenet, then later the web that the mechanical, not electronic, part of building effects is the hard part. This is even more true for whole guitar amps. Physically constructing an enclosure which will hold speakers, making and fitting a chassis into the enclosure, mounting a heat sink and power devices, and making a good (enough!) looking front panel are far more difficult than building a circuit board and getting it to run.

However, the process of Technological Cannibalism can speed things up a bit. Finding a commercially available product that is **physically** close to what you want to do and then modifying it to be exactly what you think you want is much, much easier. Think about it - even custom car builders don't, as a rule, start by machining up frames, engine blocks, wheels, etc. Instead, they take an existing car and "improve" it, even if their idea of improvement doesn't match yours.

Thanks for staying with me through the intro. We're getting down to the meat of the issue.

I ran into a happy accident that prompted this note. Some industrious searching may make you the happy recipient of something similar. In scanning the local craigslist, I noticed an ad for a "Rogue" guitar amp. I would not normally give a no-name amp a second look, but I did read the ad. The ad claimed a Rogue GS-100R amp, with 2-12" speakers and 100W output. I did a search on that and found that these were a house-brand made for Musician's Friend in 2001-2005. They still sell an upgraded version, the GS-120R. They're around $200 new.

But old and used? They're CHEAP! The craigslist ad had an asking price of $40, and said it worked, with some crackles. $40 might, maybe, possibly buy a single power transformer for a 100W amp. And I get for my money not only a power supply, but also heatsinking (which may need improving!), a chassis, enclosure, speakers which speak if not perfect, knobs, AC power wiring done? Such a deal!

In short, I picked it up and it's a beautiful candidate. The enclosure is as nicely done as I could do. The chassis is big, roomy, and easy to modify. The power amp is a discrete-transistor design on a single PCB with the power circuitry. The power transformer is big (always good for power thingies!). The heatsink is enormous, probably because they neutered it by placing it where it can't get airflow through the box, so they made it huge.

The physical arrangement is such that you could easily put in your own circuitry and have a working, usable and gig-able amplifier. How many of you have ever played a gig with an amp you've built? It's not that the circuits won't work, it's that you can't box it up well enough to survive transport.

In any case, if you can locate a Rogue GS-100R at a cheap price, it's worth your time. Even if it's completely non-functional. They were only $200 new. How expensive can they be 4-8 years old and unloved?

The one I got had a broken input jack. With a replacement jack, the amp was good as (it ever was) new. It's LOUD as you'd expect for a 100W, 2x12. The clean channel is good for effects users, since what you want with effects is to hear your effects, not the amplifier blender-izing them. The built-in distortion is pure, unadulterated crap, to be listened to as a bad example. But good distortions are a dime a dozen these days. Good enclosures, power supplies and power amps are not.

Try it. You may like it.
I've had a number of requests for this, and they're finally in.

Small Bear Electronics has hard copies of "PCB Layout For Musical Effects" in stock.
I'll rephrase myself here, as the forum is new.

Power amps are best thought of as a power *supply* that lets some of the power out under special, carefully controlled circumstances. The power amp itself is really just a highly specialized wart on the power supply. In most cases, the power amp circuits proper are a trivial part of the cost of the unit.

The power supply needs to be the FIRST order of business in designing a power amp. Going to get 50W into 8 ohms? You need to know how to tell what power supply you need before you dig into what power amp or chip you'll use.

After the power supply conversion stages, voltages and currents, etc. you need to consider thermal matters. Again, this is substantially independent of the power amp circuit itself. If you're putting out 50W into 8 ohms with a class AB solid state amp, the power needed from the power supply is 50/.74 = 67.6W worth of DC coming out of the power supply at peak power. But that's not where the peak dissipation on the output devices happens. That peak comes at  forty (something) percent of the max power out. The heat has to be dissipated by heatsinks so the output devices are kept below the boiling point of doped silicon. Otherwise, you have just bought a new set of output transistors - or a new chip amp. The heat generated in the power supply itself also gets released inside the box. Where does that go?

After you have a good power supply design and a good thermal design, run wild with whatever power amp circuit you like.
Preamps and Effects / Preamp thoughts
April 05, 2006, 07:23:43 PM
I have some thoughts on the design of a preamp. Some of these have been stated here, some haven't.

1. Use a high input impedance. 1M is a minimum.
2. Protect your inputs. What happens if someone hooks up 120Vac from the wall socket to the input? The common reply to this is "No one would be stupid enough to do that." Second reply "Well, they deserve what they get if they do." The correct response is "Cool! How can we do that?" because someone can, and if they can, at some point they will. Or they'll hook up the output of their 1000W car audio amp to it; same thing. Protect your inputs.
3. Know what you're doing with feedback - and without feedback. Feedback is a great tool, but it's often misapplied. High open loop feedback gain stages clip sharp as a razor when they clip. So either use high feedback stages where you *want* razor sharp clipping or where it will (almost) never clip. Use no-feedback or low-feedback stages where you want to hear the device itself. In general, soft clipping is the characteristic of a single device, or a high feedback stage emulating a response curve and never clipping.
4. Know what your power amp wants to see in terms of inputs. Give it that. It is rare for a solid state power amp to clip gracefully. Remember all those quips about getting a pig to sing?