Chapter Two: The Amplifier

I recently bought a used Chinese amplifier that was advertised on the local Craigslist. It features a single-ended EL34 per channel with a 6SN7 front-end tube and a common 5U4 rectifier tube. The front of the chassis says “Nobsound EL34-b” and the going price on ebay is around $400. I paid half that, with upgraded tubes.

You’ll find similar amps under GEMTUNE, OLDCHEN and other generic Chinese manufacturers. The basic chassis is the same, but the amps may have a slightly different front-end circuit. For example, the GEMTUNE uses a 6SL7 instead of a 6SN7 and the internal circuit is definitely different, although the sound is surprisingly similar. So where do these products cut corners?

First, all the tubes are Chinese. Some of them are fine, some are so-so. It is very cheap to replace the 6SN7 or 6SL7 with a decent vintage RCA or Sylvania. Prices from reputable dealers are around $15 a piece. The same goes for the 5U4 rectifier. That’s the good news.

The not so good news are that you can get a much better performance out of these amps by opening the bottom plate and replacing the WIMA coupling caps. Why WIMA got a great reputation is beyond me. These small value WIMA caps are the cheapest metalized polyester (MKP) you can find. They are simply not good enough for audio. My go-to replacement caps are Dayton film-and-foil (DFFC-series) capacitors at $1.50 a piece, but you couldn’t go wrong with REL-CAP RT-series ($10 for 0.22uF/400V) which are slightly better and would give the best caps available a serious run for the money.

The pictures below show before (red WIMA caps) and after (black and much larger Dayton caps). Notice that the solder joint on the amp were covered with white glue. You must take your time and scrape the capacitor solder joints clean before soldering the new caps in place. I would also recommend that you add a ½” strip of museum wax (like Blu Tak) to the length of these new caps to reduce the little microphonics they may have. That tip is universal and not unique to the specific Dayton caps.

You’ll probably notice that the volume potentiometer is tiny and looks generic. You can replace it with an Alps unit, but I did not find its sound objectionable. It turns smoothly and does not have scratchy noise while doing so. But I won’t stop you from upgrading it while the soldering iron is hot. The same applies to the RCA jacks and speaker posts. The RCA connectors are insulated and minimal and can be left alone. Speaker posts are not expensive. You can replace the six posts with minimalistic Rhodium connectors for around $15. If you prefer WBT at $50 a piece then go for it. My preferred speaker connection is via minimalistic banana plugs rather than lugs. Avoid thick lugs and heavy cables that put a severe mechanical stress on the post itself.

For a budget system, I find a 14AWG or 12AWG OFC pair just fine. These do not require a heavy-duty connector or post, so you should install parts that use the least metal possible. The best speaker connection is a continuous wire from the output stage of the amp to the speaker crossover. The connectors are a matter of convenience only, so the less you add the better!

Another Option: vintage console amps

Recycling a vintage console amp is doable, but it generally takes many more hours than you imagine. We will focus on tube amps not because solid-state is bad. Tube circuits tend to be simple and generally decent sounding while a lot of vintage solid-state gear does not. You have to know which piece of gear stands a chance of sounding good and which one doesn’t. In addition, fixing solid-state gear is usually more involved than fixing tube gear. 

Many of these old units are in need of repair and it falls on you to figure out the failure and fix it. If a transformer is shot then the serious job becomes a very serious job. The first order of business is to get a copy of the schematics or the service manual of the unit you buy. You can fix tube equipment without the schematics but it’s much easier with them.

Generally speaking, many of the vintage console amps show sub-standard construction and materials (flimsy tube sockets, cheap wiring and solder, mediocre grounding, bad capacitors, etc) to the point that they are not worth fixing for a high-end sound.

I recommend that you look at pictures of the candidate amp in detail. If there is a major mess of “spaghetti wiring” or flimsy tube sockets (especially riveted brown ones, see pictures) then stay away. There are better units to be had. The pictures show the component side of the Zenith 5K29 chassis. It’s a single-ended stereo amplifier with tone controls (that I usually remove). While Zenith is a reputable manufacturer, they chose a cheap Canadian (yeah, blame Canada) sub-contractor to assemble the 5K29 chassis – and it shows. While I finished the job, I remember cursing the moment I started it, and not just once. It brings a smile to my face writing about it, but it wasn’t cool at the time.

The simplest approach to re-purposing console amps is to replace parts but keep the same circuit. As can be seen in the schematic diagram, there is a lot of nonsense circuit, like tone controls. A more purist mod would remove these and clean up the circuit to the bare essential. An even more purist approach would be to convert the output stage to a triode-connected tube. Triodes are more linear than pentodes and tetrodes, which allows you to lower the level of negative feedback without adding excessive amount of distortion.

I’ve done my mods in stages and the unit sounded quite good even without the triode connection, but without a global feedback. I also removed the cathode bypass electrolytic on the voltage gain tube (who needs that much gain?!). But then I somehow convinced myself to do the triode-connection mod. Yes, the sound was more lush by a slim margin and the tone resembled more real-life, but suddenly I noticed a tiny hint of hum that wasn’t there before. You get the point? The more mods you make, the more you shift from a hobbyist to a full-fledged engineer, with all the headaches that come with that dubious title. For a newbie it is best to stick with a known formula. It’s the only sure path to success.

To see what’s involved in recycling a console amp, I recommend that you watch the three part series on Youtube channel BlueGlow Electronics, starting with part 1. This is not for the faint of heart. Another very involved conversion of a console amp delivers a real flea power. Think of it as a stereo single-ended 45 for $200. Once again, not for the faint of heart!

In the end, you may save a few bucks but the console amps are not always a great starting point and definitely are not the first choice for your very first DIY project. But excitement is guaranteed (just watch out for that big rocket exploding upon separation).

I really should add a word of caution in regard to old tube equipment. Sometimes it’s a bargain, other times it’s more than you bargained for. Tubes burn out and in some cases they take out other circuit elements, such as expensive transformers. Capacitors age too and sometimes they take out tubes that take out transformers.

A real-life example is a Dynaco ST70 that I picked up cheap. It was supposed to have a bad 5V winding in the power transformer. That is a very minor issue, that can be remedied by adding a tiny 110-to-5V transformer or possibly even taking apart the covers of the original PA060 power transformer and re-winding that section.

I took the amp home and took some connectivity measurements. The power transformer was fine. I then tested the output transformers. One channel had an open winding, probably due to over-current from a tube that failed and shorted. The other channel had connectivity but one side showed double the resistance than the other, hmmm. A little research showed that the A470 transformer has two parallel windings. Double the resistance means one of these windings is open. So an amp that was supposed to be an easy fix ended up in need of two output transformers. Ouch!

Bottom line is, do not assume anything regarding old tube gear unless you can test it thoroughly. If you don’t know the history of the gear, then assume that it’s in need of a complete overhaul. It’s a little bit more work but you can bring old gear to its original new specs or better with a little bit of TLC, a smart move in my book.

Another Option: low-cost kit amps

AliExpress and Ebay are full of such products. For example, the Gemtune amp that was mentioned before is available in a kit form. In general, these kits are not bad and are easier to put together than say fixing a vintage amp that needs many parts replaced. These kits are sometimes also offered fully assembled for not much more. You do your own math. One advantage of buying the kit version is that it gives you the option to upgrade certain parts during assembly or customize it by painting the chassis or adding wood panels. That way you have full control over the quality of the final product and the overall budget.

An extreme approach would be to buy a chassis and transformers and build the whole circuit point-to-point. It takes more work but the result can be outstanding despite the limited budget. There are some solid-state kits that are worth building. Most are copies of quality products, but don’t expect the same sound quality. Kits may use cheaper parts than the “real thing” to save costs. Do your homework before you commit to this route.

If you have questions, please leave a comment below.