Parallel Single Ended 6SN7 [Headphone] Amp

Ever since I grafted a 6SN7 on the line stage section of my hacked Dyna PAS preamp, I've been hooked on the "octal sound." Could it be the higher heater current draw boiling more electrons and/or larger stature that give it a 3-D quality, richer tonal color, detail, dynamics + more, compared to their 9-pin/noval siblings? 

For these reasons, I've longed to hear how the ubiquitous 6SN7 could sound as a milliwatt amp. The Covid-19 2020 lockdown was a good opportunity to try "off the cuff" experiments. An unbuilt Chinese 6V6 preamp kit I bought at an Amazon closeout languishing in the basement for a couple of years provided a nice pre-punched chassis + power transformer for this project.

Ideally, the 3850 ohm plate resistance of the two triode sections of a 6SN7 connected in parallel should drive a 12K primary Z. I considered getting a pair of Hammond 125BSE output transformers, which have the flexibility of reflecting back a 2.5K, 5K or 10K primary Z but I already had a pair of these 5.5K:4 ohm SE OPTs in my parts bin that I was sure would fit under the chassis.  

!!!WARNING!!! 

The voltages in this project are potentially lethal! Proceed at your own risk!

The circuit is a simple 2-stage cascade - 1 triode section of a 6CG7 functions as the input/driver stage R-C coupled to paralleled 6SN7 triode sections.  

The HT supply output of the 6X5 rectifier feeds a 22uf/450V capacitor, which splits into left and right channel B+ rails via two 5H/50mA chokes filtered by separate 47uf/450V caps, provide clean power to the amp. I rectified the filament supply to run at 6VDC for minimum hum and noise on headphones. 

Square waves at 100Hz, 1 kHz and 10 kHz
Top trace = audio generator
Bottom trace = amplifier output

Due to the primary impedance mismatch previously noted and the bonsai stature of the transformer itself,  these square waves show limited extension at both frequency extremes. Subjectively, this was more apparent via speakers rather than on headphones. I should point out that the absolute lack of ringing is indicative of a competently-wound transformer.

Input sensitivity = 0.9 V RMS 

Output = 2.408 V RMS/ 8 ohm load = 720 milliwatts/channel

Between 16 ohms > 600 ohms, I measured 3.5 --> 4.2V RMS output, which was more than enough power for the most inefficient ortho-dynamic headphones I have in my collection.  

This amp draws the listener to focus on the midrange in which most musical activity transpires. It made the JLH HP amp sound comparatively veiled in the midrange, which shifted the listener's attention to the frequency extremes where the JLH actually does pretty well.

Despite the superior bandwidth of the output transformers fitted to the Darling amp, JEL style, the PSE6SN7 amp displaced it musically! The Darling amp just couldn't keep up with the DHT-like harmoniousness and graceful overload characteristics of the PSE6SN7. 

 

Lush and caramel flavored midrange, nuanced dynamics, silky highs and well-defined bass (within limits) are the virtues of this Lilliputian amp. Perhaps due to the high output impedance and low damping factor, this amp can portray the sophisticated qualities I've experienced from the SE171A, 10/VT25 and Western Electric 205D amplifiers!