Homemade Tube Amp

Arcticguitar

I made this because I wanted a loud singe ended tube amp for Gypsy Jazz. Here’s a video of the amp I came up with

https://youtu.be/9xugoOQiL2g?si=qPX_txX4Tz3Zz7bJ

The amp is pictured at the end of the demo. I recorded it into a AEA N13 positioned as pictured. The input goes into a 5751 Tube that drives an Ampeg inspired tonestack, into a 6SN7 driving a KT88. The amp uses a 15 inch speaker and is built into a tweed pro style cabinet. I’ve already gigged with it and it works fantastic. Just wanted to share this here because I am really happy with how this turned out.

Buco

Sounds great. Did you design the circuit or used something existing? How much does something like that end up costing?

voutoreenie

Very cool to hear 'Round About Midnight like this, very pretty arrangement.

bbwood_98

Sounds so good! bravo!

Arcticguitar

Thanks.

Buco, the circuit design is my own. But there is a narrow range within which these things work, most of the famous old amps by Fender, for example, are circuits lifted directly from RCA’s tube handbook. So there’s nothing really “original” in any tube amp circuit. Starting back in the 1930s RCA published handbooks of sample circuit ideas for their tubes, they did this first and foremost so engineers of the day had easy to follow guides for using RCA products and they also published, as public domain, every reasonable permutation of working tube circuits so that no other company could patent or monopolize a particular design, the thinking being that they would sell more tubes if all the best designs were public domain. Also because tubes are non-linear calculating the operating points without a modern computer is really difficult, the RCA tube handbook provides specs for neutral operating points for every tube made by RCA (which is basically every tube in existence) at various voltages. I have the tube manuals published in the ‘30s, ‘40s, 50s, 60s. I used those and some ideas from existing amps to come up with the circuit. Then I built the circuit on a bread board and tweaked things until it sounded good to me. The tone controls are almost exactly copied from an Ampeg guitar/accordion amp except for a capacitor change based on what I had on hand vs the schematic. Anyway, the design is my original design, but with this kind of old technology nothing is really original.

As for the cost, I’m not sure. I’ve been tinkering with tube amps for a while and almost all of the parts in this are salvaged from older non-functioning tube gear that I’ve collected over the years. Years ago I bought a bulk lot of resistors, and I don’t use any of the fancy expensive capacitors that are trendy with tube amp builders. So the internal components literally cost me pennies. If I were building it from scratch the expensive parts would be the transformers, choke, chassis, cabinet and speaker. Out of those parts, a usable power transformer can be found cheaply on eBay, the choke could be omitted and replaced with a mosfet filter/regulator (better and cheaper, but I had a choke on hand), a chassis can be made cheaply, or purchased expensively, as can a cabinet. The output transformer is the only part that I think needs to be purchased new, but that is because you absolutely need to know the manufacturers spec for how much current it can handle. In my amp the output transformer needed to be new because single ended KT88 guitar amps never existed back in the golden age of tube amps, so finding a used transformer for this application isn’t realistic. A suitable output transformer can be had for $50 or so, a new power transformer is probably $75-100, or used transformers can be found for $50-ish. I think a person could buy all the electronic parts, new, without much bargain hunting, for less than $200. The chassis, cabinet and speaker can be super expensive, the best route there is to find a cheap or non-working amp and repurpose the parts. That’s what I did because I am not much of a woodworker/metalworker. Also vintage electronics are like vintage anything, so when I say the electronics are less than $200 (with most of that being transformers), I am using modern components, if it were built with modern reproductions of vintage parts or NOS parts it would be more expensive (and noisier).

billyshakes

Thank you for that thorough post. I did not know that RCA put nearly all design circuits into the public domain. That's a very interesting idea and one that I doubt would be followed today. They'd want to corner the market on parts but also then monetize the various patents, defend them with their gnashed teeth, and leave nothing for anyone else.

littlemark

It is my understanding that Elon Musk put his electric engine design in the public domain ten years ago or so. The idea being to encourage others to make electric cars.

A similar thing is true for Linux computers.

Lots of open source programs.

Buco

That's interesting about the RCA circuits, I don't know that I was aware of that. I knew that Jim Marshall took a Fender design and modified it to create the first Marshall amp. But I don't think I knew that it was all so influenced by these RCA circuits.

Maybe there's a narrow framework in which you work but there are still infinite possibilities. I suppose you could use lapompe as analogy. It's a narrow target where most of us think of something as good sounding rhythm. But within that target, there are many, many variations.

I didn't know you know a lot about tube amps and tube amp building. Now it makes sense. Every time I looked at doing something diy (I can follow diagrams and solder and am generally handy but at this point I know next to nothing about amp design), the cost added up to be too much. When you put everything together it's not too far off from buying a brand new amp. Maybe a budget brand but still a nice amp.

Arcticguitar

Buco, the whole history of tube design is fascinating. Western Electric, another early tube manufacturer, also published circuit designs that ended up in many fender amps, and other amps. It is like la pompe, the variations within the target are numerous, but the target is narrow. In the end tube design is just math and the numbers only work out in a limited number of ways.

I’ve always thought that publishing sample circuit designs was less about modern notions of open source sharing, and more about early 20th century cut throat capitalism. I think RCA wanted to prevent any single company from patenting working tube designs, so they could sell more tubes.

In the end the subtlety matters. As you mentioned the first Marshalls were copies of the Fender Bassman. Marshall changed a few little things here and there based on the availability of components in England vs the US, and ended up with an amp that, at least to me, sounds quite different.