My new 45 SET (6SN7, direct-coupled)

[E. Wong]

I'm a new member and I'd like to share my latest completed project: a single-ended triode amp using the 45 triode. The driver is a choke-loaded 6SN7 that is direct coupled. This allows the 6SN7 to be placed underneath the 45 triode, and steal some of the 45's cathode current (see the crude schematic below). This idea was outlined in an article on tubecad.com, about a "Safe Loftin-White Amplifier." The power supply, not shown in the schematic, is a simple pi filter with solid-state rectification, using a motor-run capacitor as the last capacitor. The filaments of the 45s are each powered by a dedicated transformer, so I can switch on the filaments to warm them up before I switch on the B+. Everything came together smoothly, and the amp started playing music immediately upon power-on. I got the 45 tubes on Ebay for $40 for the pair -- they weren't tested by the seller, but they looked fresh and beautiful. They sound great to me.

(My signal chain: Apple lossless and 256kB iTunes files on my MacBook, Stereo-Link USB DAC, amplifier, Axiom M3 speakers (which have no crossover components on the woofer, and I replaced the tweeter's electrolytic capacitors with plastic film types).

This amp is amazing. It puts out about two watts, which is plenty loud for me in my smallish room. Detail is tremendous, the transients of orchestral snares and timpani are startling in their realism, and reverb sounds very lush and detailed. Voices are natural. One of my favorite test tracks for a lower-watt SET is "No One But You," from Yo-Yo Ma's Goat Rodeo Sessions. This track has a male and female vocalist, singing quietly, accompanied by piano, mandolin, violin, and cello, all recorded very naturally. On my system, the music is downright spooky -- soundstaging is excellent and everything sounds immediate and present. Noise is actually very low -- I can hear hum with my ear placed inches from the speaker cone, but in the room I can't hear anything. Bass is weaker than the amp I had before (Gary Kaufman's direct-coupled 6EM7 amp), but everything else is better. Of course, this amp can't roar like my Scott 299C, but it handles even modern, highly detailed and dynamic orchestral recordings very well (For example, Rattle conducting Mahler's 2nd). I've read so much about how great the 45 is, and now I can believe the hype.

Anyway, I'm a new poster, but I though I'd share. What a great, screwy hobby this is.

[Geek]

Welcome!

Intriguing design. Love the 45 sound.

[EWBrown]

Very unique, and interesting circuit topology!

I would surmise that this circuit could be easily modified (by changing the power amp cathode resistor values), in order to be able to use 6DN7s,
or 6SN7 and 6BL7s, or possibly even 6CG7 and paralleled 6S4As, for those of us who like to use the inexpensive TV vertical deflection tubes.

Just another SWAG, on an early spring evening...

/ed B in NC

[Geek]

Very unique, and interesting circuit topology!

Like a Loftin-White with the boringness removed

[Brik]

This circuit reminds me of the original Lotus motor cars design by Colin Chapman.
Simple and elegant, yet it takes some figuring out to understand how it works and why.

Great build!