Single-Ended 6B4G Amplifier
This is my current favorite and latest build. This is the cure for any single-ended amplifier addiction. It is built on a 17″ x 10″ hammertone-painted steel chassis, and features Transcendar output transformers and an Edcor power transformer. This is the first directly-heated triode amplifier I built for myself, and had the time to wrap my ears around. It’s an amazing synergistic match to my sealed-enclosure speakers, and sounds outstanding at moderate listening levels.
This is as simple as such an amplifier can be, and I always believed shorter is sweeter when it comes to audio signal paths. The two 6SN7 triode sections are directly coupled, with one capacitor coupling to the output tube. All stages are cathode-biased and bypassed to ground. A worthy experiment might be to connect the output tube cathode bypass cap from the cathode up to B+, a connection often referred to as ‘ultrapath’. The power supply is very ordinary, but has a high inductance high current choke to keep hum down. The heaters and filaments are all supplied a rectified and filtered 6.3 VDC. Hum balance potentiometers are used across the output tube filaments so any hum there can be adjusted out at that point. See the global feedback? Nor do I! There isn’t any! This runs open-loop. The large output triode should help to keep output impedance down, but it will still be somewhat high, so damping will be poor. This requires the right sort of speakers to use effectively. When it does work out, it has punch, bounce, and puts you right there with those musicians. The bad news? This amplifier will reveal all of your poorly recorded music, and change your musical tastes to well recorded music.
Evil Science Audio 
What is the voltage rating on the coupling capacitor?
Probably 630V, because that’s what they sell. Anything higher than the power supply voltage always works!
And what are the FWB squares on the filament supplies ?
Thanks!
Full-wave bridge rectifiers.
Thank you Bill, for being very helpful!
hi,what is the input sensitivity please?…thanks
The input sensitivity is fairly high, almost excessive. I’m sorry I do not have an amp to measure anymore.
Hello–
Thank you so much for posting this. I have constructed a few kits and am preparing to do my first schematic-to-amp build. This looks like a great schematic for a first-time build.
The schematic indicates that an OPT is used. Why, then is the output impedance high? Isn’t the job of the OPT to get it as low as possible?
The “Output Impedance” of an amplifier at the speaker terminals is a different measurement than the rating o the OPT secondary. The OPT secondary is a “Matching Impedance”. To measure the real amplifier output impedance, you give it a signal that generates a specific output voltage across the open secondary. With no speaker connected, dial a signal generator to give 1 volt RMS or 2.8V peak-to-peak across the 8 ohm speaker terminals. Now connect an adjustable load, like a 25 ohm 1 watt rheostat, and adjust until the output voltage as measured drops in half. Measure the rheostat. That is your output impedance. It would ideally be 8 Ohms but with this amp will be higher (I forgot it but it was higher than 8). Anyways, the higher this is, the worse the damping factor will be. This stuff interacts with all of the variables in speakers to produce changes in the sound.
Also, in a setup like this, how important is it to have “matched” 6SN7 tubes?
Probably don’t need to be terribly matched. I have substantially changed this amplifier though, and no longer use 6SN7s in mine. I can say I was not using matched tubes, and all was well.
What are you using in place of the 6SN7’s?
I changed to 6EM7s and a very different direct-coupled circuit almost identical to something I found on the ‘net. Mostly because the circuit here has too much gain.
DC coupling is intriguing. Could you point me to your online inspiration? Thanks.
Is the B+ really 450V? If so your 6B4Gs won’t last very long…
This is an example of “Better to keep quiet and be thought a fool, than to open one’s mouth and remove all doubt”. I would have asked this as a question: How do you run 6B4’s at 450V? Well, I do that by only using the large, modern, monoplate 6B4G from Sovtek that is rated for continuous duty at 450V and 40 Watts dissipation. Yes, ordinary old-skool 6B4G from grandpa’s old radio don’t belong in this.
It turns out that these monoplate 6B4 and 6A3 tubes are, in fact, exactly the same as the 300B made by the same company on the same production line at the same time. They probably change the filament, and care a little less during processing. I bought a handful of these, but the same two are still in the amp after all these years.
All the data I have found states 15 watts as maximum plate dissipation. Are you certain of the 40 watt number? Thanks.
Please reread my comments above. I am certain.
Thanks for the confirmation.