6П14П Amplifier build

[admin]

Originally Posted: Mon Feb 08 2021 20:12:48 GMT-0500 (Eastern Standard Time)


One of several projects in work here at Linuxslate, is a basic tube amplifier built around actual Russian military surplus 6П14П (6P14P) tubes, and a Chinese SMPS that is at least theoretically made specifically for tube power amplifiers.

Here is a picture of the amplifier during a recent "glow test".  While it looks pretty complete, nothing is wired except the tube heaters.



The area that looks like a window or display will house an LED VU meter.

I have discussed the particular power supply used in this project on my "Modern Power for Tube Projects?" thread on AudioKarma, and I will not reproduce it here.  The unit in question can be found on sites such as AliExpress by searching for "tube amplifier switching power supply".  This is the 250W version.

The short version is that the SMPS will produce the appropriate voltages for a modest tube power amplifier, but is struggles to power the heaters of these 4 power tubes, and since the feedbacks of all of the outputs collectively feed the drive (primary) of the SMPS, it will not produce the correct voltage unless all outputs are loaded appropriately.  In order to take the picture above, I had to use a 1.5K ohm power resistor to load the B+ (300V) output.

In reviews on AliExpress, others have mentioned that the transformer at the center of the SMPS gets very hot with even a moderate load.

At least in theory, this power supply should have some benefits for someone looking to build a small tube amp project on a budget.  The SMPS itself sells for about $40-$45 (not counting shipping), and has the advantage of operating from 100 - 240VAC.  That means that your project will not only work anyplace in the world, but it should have consistent behavior even if the line voltage changes.

So While at this time, I cannot recommend this power supply, the build shall go on.  I plan to work at a very leisurely "hobby" pace, so I wouldn't recommend compulsively reloading this page looking for an update.

I also have a couple other Tube Projects going on.  You can Read about a larger, more powerful SMPS 7591 Tube Amplifier build here, and Read about a concept for the "Does Everything" Tube Table Radio" in this thread.  (Links to be added soon).

[admin]

Originally Posted: Sun Mar 14 2021 20:49:49 GMT-0400 (Eastern Daylight Time)

This project is complete.

First, here is the Switch Mode Power Power Supply (SMPS) that this amplifier uses:


Photo from vendor ad

...And here is the completed amplifier:

Photo: linuxslate.com 2021  - Click for a larger image.

The amplifier includes a built-in USB DAC (Analog RCA inputs are provided, too.)  Using (2) 6[ch1055]14[ch1055]'s per channel running at almost 300V should provide about 12-14 Watts/channel.  A Realistic APM-500 audio power meter confirmed at least 11.5W/channel without noticeable distortion.

I build my amplifiers to produce bass, and while the budget Output Transformers (OPTs) used in this build are not the beefiest iron available, it is capable of producing bass you can feel as well as hear.

Conclusions:

• The power supply does work to run a basic power amp that is no bigger than a very basic "small bottle" Push-Pull amp with no other tubes.
• All of the outputs collectively drive the feedback, which means that the load on one affects the others. There is a collective adjustment only, which in a manner of speaking makes it work somewhat like an unregulated linear supply. Strange and possibly damaging output voltages will result if the output loads are not "typical".
• The "group regulation" also causes startup transients that may momentarily apply excess voltage to the heaters and B+. Without significant external clamping circuitry, I don't think there is anyway around this, and designing/building such circuitry defeats the purpose of a compact, $40 PS module.
• The SMPS definitely has some advantages. In addition to the wide input voltage, it is also very compact. I think it would be very difficult to build an EL84/6BQ5/6P14P P-P amp in this small of a chassis with a conventional power transformer. (Note that the chassis also includes a DAC and LED VU meter)
• It also runs fairly cool. The SMPS transformer does get warm to moderately hot, but I have run it for several hours with everything closed up, and nothing reached a temperature that would cause me any concern. In fact it's the coolest running tube amp I've ever seen. Nothing but the power tubes themselves get significantly hot.
• This cool operation is also shown in very low power consumption for an P-P tube amp. I show ~55W AC Input draw with the amp playing music very low (essentially idle).
• It's certainly not "HiFi Clean". Even with external Inductors and Capacitors, switching artifacts are present. Keeping the SMPS further away from audio circuitry would probably help, but again, that negates the advantage of the compact size.
• The SMPS has an over current shutdown. This seems to work very well, but it has the side effect of preventing the amplifier from starting if the Power tubes are hot.

Even though I now have a compact, nice looking, efficient, and very inexpensive tube amplifier, I do not recommend this power supply for the reasons mentioned above.

For a full detail of this build, please visit the build thread on AudioKarma.com