Flea-Power 6N1P - 6P14P SET Amplifier with PIO caps

Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles

1. The Design Idea

I designed this variant of 6N1P/6P14P purist's triode amp for a friend living in Ukraine, optimizing the design for ex-USSR parts. This started as direct clone of my 6N1P/6P14P amp [1], yet I decided to give the driver stage more swing. Hence the design is midway between [1] and Decware Zen [2]. One of the points of the design was to use paper-in-oil caps. Another was to optimize it for the "TWZSH" transformers. Rumors are that TWZSH were manufactured by Dnepropetrovsk RadioFactory. They are butt-ugly and look admittedly cheap but behave as they were master-designed for Zen-style unbypassed, no-feedback, triode-strapped 6P14P output stages. With merciful current and power, they beat stock Decware Zen transformers in sound and on measuring bench [2].

The schematics is very similar to my 6n1p/6p14p amp [1], the only significant change is more B+ to the driver:

This particular configuration runs tubes in a mode that achieves quite low 3rd harmonic; in fact, SPICE promises a cancellation of it right at the middle of the most delicate power range:

This deliver not much undistorted power above 1WRMS, especially when TWZSH is loaded with 8 ohm speakers, yet teh amp promises a very delicate sound below 1W! The speakers for this must be very sensitive, though.

How the value of the load (speaker's impedance) influence output power and distortion? Here are plots for 2,4,8 ohm load and 300V on B+, aligned for better comparison:

you can see how 2 ohms may be the loudest load but not the cleanest one; 4 ohm load delivers the best power/distortion compromise, while 8 ohms offer cleanest sound with lesser power.

2. Design with ex-USSR parts in mind

A long-time friend of mine Alex Bazileev (Алексей Базилеев) lives in Ukraine and this design was mean for him. I suggested him to use tube rectifiers and Paper-In-Oil caps in power supply.

The complete schematic of the amp using USSR parts is as follows:

One trick of the trade I used here is that I use TWZSH transformers as power supply choke. Indeed, what can be better for this application than a 9-10 Henry iron easily withstanding 50mA and having no more than 250 ohm resistance? That's the primary of TWZSH. Any DIYer having obtained a bunch of TWZSH can hand-pick then for best audio quality (lesser parasitic L and C, matching output levels, gaps, etc) and use the remaining ones as well as any with broken or improperly wound secondary as PS chokes.

The power supply was designed with separate rectification for each channel, the power transformer is the only shared part. TAN33 is used here; any other TAN transformer with enough power and secondary voltage can be used.

Al parts including the PIO caps (MGBO, MGBCH) and TAN33 are available in Ukraine. TWZSH is becoming scarce, though, and should not be mistaken for the inferior TWZ-9. Ukrainian DIY audiophiles often scavenge for trashed tube TV sets, an excellent source of iron, 6n1p, 6f5p, 6p14p and full-range paper cone drivers such as 2gd40, 2gd38:

3. Alex Bazileev's implementation

To buid this Amp, Alex constructed a sturdy aluminum case and potted the power transformer in hand-built case. The overal look is excellent:

The TWZSH (all four of them) are under the shassi:

Alex arranged for variable biasing pots in cathodes of 6P14P - excellent - which allows him to dial any idle current he wants. I recommend not to exceed 35mA for TWZSH.

More pics of Alex's amp are in [3].

4. Clean parawatts!

After running into bad luck with avalable TANs (many have quality problems, shorted turns, overheat and underperform) and after some iterations trying to achieve 300..310 V on B+ Alex used a special-wound PS transformer. At the moment he's still a bit short of B+, ruining more at 280V which gives less output power. At this parawatt power the measured distortion is excellent and harmnic spectrum is true SET, all alone the lines with simulations:

The frequency plot is also "typical SET", resulting in mellow tone:

5. References

[1] 6P14P-SET

[2] Decware Zen all-triode amp

[3] Alex's amp pic gallery

Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles