300b lovers

Although I don’t enjoy "tuning", it is an unpleasant necessity for speakers and power amps. For reasons that are not clear, various brands of metallized polypropylene capacitors sound quite different from each other, and there is little correlation with DA and DF parameters. Based on measurements, they should all sound the same.

On a system with moderately high resolution, subjective differences appear that can mimic crossover balance shifts and driver swaps. I found during development of the Ariel, back in 1993, that cap substitutions required 0.5 to 1 dB crossover adjustments to subjectively offset the colorations ... and this was with pink-noise test stimulus, not music.

You can really get into the swamp comparing silver vs copper wire. This should not be audible at all, and I have heard of no convincing argument why any differences are audible. You can go out on a limb and compare silver oxide vs copper oxide, and various weird sources of corrosion, but it’s all very speculative, and again, no useful measurements to be had.

On the other side of the objective/subjective fence, I have heard of well-known speaker designers who never audition their new speakers ... they do it all by numbers, then walk away. I frankly didn’t believe it when I first heard that about fifteen years ago, but other folks confirmed it, so I guess it happens. So it is possible to ignore "tuning" and let the product sound like whatever.

But in the speaker world, it is widely recognized that a "perfect" zero-coloration speaker is impossible at the current state of the art, so it comes down to choosing which set of parameters are most important. Speakers are still very imperfect, compared to any other audio component.

In principle, it should be possible to design and build a zero-coloration amplifier. I started my career in audio design in 1973, and haven’t heard a "perfect" amplifier yet. They all have a sound, and a little bit worse, topologies tend to have distinctive sounds. But that’s my personal experience, not necessarily the experience of others.

If a customer, or reviewer, is in the fortunate position of finding that all well-engineered amps sound alike, that’s great! You can sure save a bunch of money, skip over tubes entirely, and just buy the latest Shenzen-made confection for a few hundred dollars.

Let’s talk technical about DHT filament power supplies. My friend John Atwood built a low-RF filament supply a few years back (100 kHz or so), but he made the discovery that the filament inductance of DHT tubes was all over the place, making it difficult to assess how much power was actually going into the filament. This is a big deal because DHT tubes require very tight control of filament power, preferably 5% of specification, or better. In practice, the RF supply had to be individually tuned for each tube ... and it didn’t sound any better.

An aspect of even very clean sinewave AC heating are "hum sidebands" ... not hum per se, but IM distortion harmonics that are displaced by 100/120 Hz on each side of the fundamental and about 60 dB down. It is clearly visible on a good spectrum analyzer, and is caused by small temperature fluctuations on the filament modulating emission, which in turn modulates the forward gain of the tube ... not much, but it is measurable. (The discovery of hum sidebands led to the experiments with RF heating.)

An aspect of DC heating is where the virtual center-tap appears. If the virtual center-tap is in the middle of the filament, the inherent balance of the filament can give a free bonus of 30 dB or more of additional noise rejection. Considering how difficult an additional 10 dB of noise rejection can be, 30 dB is not to be sneered at. And the whole DC supply has to float, relative to ground, while appearing symmetric from the viewpoint of the filament (mimicking an AC supply in that respect).

As I said above, there are little tricks to regulated DC filament supplies on DHTs....

But there are choices that affect the sound.  If you need a 1 watt 1K resistor you can use any type on a cathode, but different types have different sounds.  The cathode bypass cap might need to be 100 uF.  Different 100 uF caps sound very different.  All will satisfy engineering standards, but parts and layout choices have profound effect on the final sound. 

When you run zero feedback, the circuit has no ability to reject things like this. So everything makes a difference. However, for something like an electrolytic bypass, I think you'll find that as long as the part is good quality, the big differences you hear will be more about the part forming up over time: they will arrive at the same place sooner or later.

For reasons that are not clear, various brands of metallized polypropylene capacitors sound quite different from each other, and there is little correlation with DA and DF parameters. Based on measurements, they should all sound the same.

Sometimes you have to do your own measurements because the specs of the manufacturer don't always tell the whole story. If you use a precision differential amplifier to drive the caps in question, you can measure how they behave and differ from one another while in circuit.