People seemed to enjoy my last tube amplifier measurements post, so I decided to do another. I modded this stereo Magnavox 8605 console amplifier to run 12BH7 dual triodes as output tubes, converting it from a single-ended pentode to a single-ended triode (SET) amplifier. I used a schematic by Audiokarma user Flacharlie as a starting point but re-did the first stage. Here is a picture of the schematic:
For those interested in tube amplifier design I go into more detail on choosing the loadlines etc. here.
Without further ado, here are the measurements. Obviously, this amplifier has tiny transformers and is running a preamp/driver tube as an output tube, so we can't expect a lot of power. I usually do 1W, 5W, and then choose additional power levels based on the amplifier in question, but here I did 100mW, 500mW, and 1W. The amplifier doesn't make much more than 1 watt of power.
First, frequency response at 500mW:
It's nice that HF extends past 20 khz before it starts to roll off, but obviously the bass is somewhat weak. I doubt the consoles this amplifier was designed for could reproduce much below 50 hz, so you can't really fault them for economizing on the output transformer. Subjectively, you can definitely hear the bass weakness when using full-range speakers, but the listening experience is still enjoyable.
This is a stereo amp, but I'm not going to bother posting measurements for both channels. I did observe slight differences, but they were attributable to my output tubes, which each have a slightly different distortion signature. Swapping the output tubes would also swap the spectrums. Below are the results for the 12BH7 which had slightly less distortion (within 1.5 dB).
So there you have it: measurements on a very low power SET amplifier. The measurements were exactly what I expected when designing this amplifier. I recently acquired Altec Model 14s, which have compression drivers and are quite sensitive. This amplifier is capable of driving the huge Model 14s to a comfortable listening level without any ugly distortion, which is pretty impressive and kind of hilarious. The bass sounds exactly like the frequency response graph would suggest -- it's mostly there in terms of the audible bass, but the impact is lacking. I was never a SET fanatic, but I picked this amplifier up for quite cheap and it made for a fun project. Because the distortion is relatively high, I do think it has a bit of sonic character, which depending on your perspective is heresy for a power amp or desirable. Second order dominates at the lower power levels, but once you approach 1 watt things get pretty ugly. At 1.1-1.2 watts the waveform is visibly clipping on the scope, so you don't want to drive it that hard. Luckily, I applied enough negative feedback that gain is pretty minimal, so you'd have to crank your source volume to get it clipping. Overall a fun project that yielded a well-behaved and enjoyable, if not exactly hifi, amplifier.