Oh no, a cheap chinese SE Amp

[JacquesD]

All,

Seeing multiple offerings for a complete PCB with tubes for a 6P1-based power amp, I thought I could give it a try for 16 US$ and (as so often) free shipping, as I had all the required iron around. If you look for something like "JCDQ-11" on that auction site or Ali's fast engine or many other places, you'll find several offerings, from +/- 15 US$ to more than double, also several assembled units.

It arrived rather quickly (10 days) and the PCB is well made. The tubes, two Chinese 6P1 'power' pentodes and one 6N1 double-triode, were old and used, judging from their appearance. I've put everything inside an old amplifier chassis and voilà, from the first switch-on everything looked OK and sounded reasonably, but nothing more. Some slight hiss and a difference of level between the channels were the first thing that appeared. As I had NOS 6P1P-EV and 6N1P-EV tubes around, I tried these first. According to the specs, they should be equivalent to those far-eastern 6N1 and 6P1 tubes, only quality control in the USSR was quite superior to what was often practised in the Empire of the Middle at that time. The sound became quite a bit more interesting, more open and airy. Both channels now sounded about equally loud.

After a few hours, the amp became rather pleasant to listen to, very detailed soundstage, only some uneasiness in the bass area with some material. Especially MP3s of heavily mastered material (pop music) were sometimes unforgiving, but small combos (like "Ella and Louis" with the Oscar Peterson quartet) are a delight, even the plucked double-bass. The damping factor of an SE circuit is usually not too high, which can cause the issues around the speaker's resonance area. 

With the Soviet tubes, the amp is very silent, no audible hum or hiss. Without an input source, it makes some noise unless you plug something in or you put a 220K resistor (or potentiometer) in parallel with the input.
I can't go back to the Chinese tubes as I dropped them on the floor (something I haven't done in 50 years )...







I might add some negative feedback as the sensitivity seems high enough (haven't measured anything yet), some variants of the scheme (see, for instance here and here) use it. Also, the bridge rectifier is a simple 4 times 1N4007, either faster diodes or snubber capacitors could enhance it. And 2 RMS watts is a lot of power for the living room. I have used the amp with old Wharfedale Dentons (test speakers I can afford to burn) and some better bookshelf speakers, like my dear AR-15s.

I have some suspicions about the circuit, you can find the very same with 6N2P (characteristics of an ECC83/12AX7) and 6P14P (equivalent to an EL84/6BQ5) tubes, with identical component values, not sure which was first...

As a conclusion, I'd say, for the total cost (you have to add transfos, chokes and you might prefer to plug in Soviet equivalent tubes, depending on what you receive) there could be better circuits; I suspect that one would better look into an EL84/6BQ5/6P14P equivalent.
But this board demonstrates clearly the pros (and maybe cons) of a single-ended circuit and could be used as a first venture into building a tube amp yourself. No, it wasn't my first.

[vanakaru]

I am going to try this as well. I have some russian EV tubes in my drawer. What output transformer you used. I got two SE trannies out of an old Swedish radio, but I have not measured these yet. The said radio is long gone but as I remember it had two EL84 parallel in a channel. Transformers have primary thicker wire with center tap. What you think - do these work?

[JacquesD]

Hi Erki,

Difficult to judge transformers from a photo, just a few ideas:

A small capacitor was often placed across the primary side of an output transformer. I'd  think that this is the thinner wire and the thicker wire is on the secondary windings, carrying more current. The "center" tap could be a tap for 4 Ohms and the further one for 8 Ohms (or for 8 and 15). To be sure, you better measure all the connections with an Ohmmeter to see which ones go together. The highest resistances would be on the primary side. Then, apply some AC voltage across the "highest" resistance winding, say around 40 - 50 volts AC, and measure the voltages between all taps. From the ratio of voltages you have the ratio of winding turns and the square of that is the ratio of impedances.

There are many descriptions on the Web of how to find out transformer characteristics, for instance, https://www.diyaudio.com/forums/tubes-valves/199833-measure-output-transformer.html

A parallel output tube arrangement in a SE circuit is somewhat uncommon for a vintage radio, two pentodes would often be placed in a push-pull circuit because of the higher efficiency. But for two EL84 tubes in parallel the primary impedance would be in the 2.5 to 3.5 KOhm range then, which is probably too low for this Chinese PCB, unless you have a 4 Ohm winding that you connect to an 8 Ohm loudspeaker...

In any case, try to find out what your output transformers are like.

Incidentally, mine come from an old tube radio as well, I got them from a former maintenance technician, have several more. They were from a local belgian manufacturer, made in the early 1960s. The same manufacturer made a few excellent tube amplifiers as well.
You can see the transfos in this circuit: https://nvhrbiblio.nl/schema/Carad_FTPAS48.pdf

Enjoy your quest!

[vanakaru]

Thanks for the link.
I measure 10.1V into primary gives 0.42V to secondary. Transformers were feeding 8Ohm speakers. So I calculate ratio 25x25x8=5K. It should be OK.