My New 45 SET (choke-loaded 6SN7, direct-coupled) - diyAudio
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Old 24th April 2014, 04:34 PM   #1
EWong is offline EWong  United States
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Default My New 45 SET (choke-loaded 6SN7, direct-coupled)

I'd like to share my latest 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 attached). This idea was outlined in an article on, 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 great. 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. The sense of air and the hollow resonance of the mandolin really come through.

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.

Best of all, this was a budget-friendly amp. With the cheap tubes and budget transformers, I spent maybe $250 - $300 total, including the "enclosure:" a cheap silverware chest.

What a great, screwy hobby this is.
Attached Images
File Type: jpg 45 Amp.jpg (115.2 KB, 325 views)
File Type: jpg 45 Amp Schematic.jpg (134.3 KB, 328 views)
File Type: jpg 45 Amp Guts.jpg (200.9 KB, 322 views)
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Old 24th April 2014, 05:09 PM   #2
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Nice! I have already seen this kind of schematic, called "Free lunch" or "Monkey". Glad to hear from you that it sounds good. What output transformers did you use?
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Old 24th April 2014, 07:00 PM   #3
disco is offline disco  Netherlands
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Bypassing the LEDs might be beneficial, have a go with Oscons if you can find some. Bass cut off is a function of your outputs alone or in combination with the anode choke (lacking the necessary inductance). The 45 was my first build in 2001 and I still recall the thrills. At that time my psu was generating some extra low notes that were not in the music but impressive it was. Enjoy it man!
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Old 27th April 2014, 03:02 PM   #4
EWong is offline EWong  United States
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I used the cheapest, smallest Edcor transformers (I previously built Gary Kaufman's 6EM7 DC amp, and I dismantled it to built this one). Because this is only a 2 watt amp, I'm pretty sure that the bass response is adequate. Speaking of bass response, I've been analyzing my music using GarageBand's spectrum analyzer. I've come to the conclusion that for everything but techno and electronic pop, there's little energy below 40Hz or so, and even bass-heavy tracks peak at 40-70Hz. Because human hearing sensitivity drops off precipitously as you go down in bass, any low bass content (<40Hz) must actually be much higher than mid-bass to be audible. So, I'm not a stickler for super bass.

Thanks for the tip on Oscons! I'd never heard of them. Too bad they don't come in, say, 80uF at 500V for power supplies.
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