300b lovers

Since this is (mostly) a 300B thread, I’m still curious about the general circuit of Ralph’s 300B amp. He’s been doing this a long time, so his design choices are of considerable interest.

I have my own way of doing things, and that was strongly influenced by my research when I was writing for Glass Audio and Vacuum Tube Valley. John Atwood, in particular, showed me the Bell Labs archives and other primary sources. Charlie Kittleson, the magazine’s founder, had a treasure trove of working 1930’s electronics, which sounded very different than anything I’d heard before ... not like the Fifties sound at all. They clearly had different priorities back then.

John Atwood and I have a lively interest in early technology, particularly early monochrome and color TV in the USA, the UK, France, Germany, and Russia. Developments in color TV filter technology went on to influence Neville Theile in Australia, modern crossover design (Laurie Fincham in the UK), and the time-switching technology used in the GE/Zenith FM Stereo multiplex system.

Hi Lynn and Don,
What do you think? Is it a good idea to use driver-input power transformer taps to feed 300B filament? This power transformer should be less pushed then the 300B power transformer.
I don't use only electrolytics capacitors for B+. My DIY friend taught me to use a mix of electrolytes, polypropylene and vintage industrial oil capacitors (Siemens MKV, Tesla, KBG-MN and new Obbligato). All this capacitor bank is bypassed by 0.1-0.22uF PIO and 0.001-0.01 uf Soviet silver mica. This bunch of capacitors doesn't smare sound. The only drawback I can hear is the relatively long warm up time (at least 2-3 hours). Probably a 200+uF set of good quality vintage industrial oil capacitors will sound better. But it will be more expensive.

The 300B filament circuit is a very delicate circuit node. The high voltage windings of the B+ transformers see switching pulses of hundreds of volts with very steep rise times. It only takes a few pF of winding-to-winding capacitance to transfer that 120 Hz switching noise straight into that expensive 300B. Unless you know that the low-voltage winding is electrostatically screened (with copper foil), don’t do it. Use a separate transformer just for the 300B alone.

You wonder where low-level buzz comes from? Winding to winding stray capacitance. Whenever you hear buzz instead of low-frequency hum, that’s a capacitive coupling, not magnetic. The spectrum gives it away.

I would NEVER use vintage caps in a power supply. Never never never. Use modern parts. They’re not that expensive, and used in air conditioners all over the world. Vintage is OK in a crossover, where failure is no big deal. In an amp, just say no.

Not sure I see the merit of mixing films and electrolytics. If you have the space, use the industrial parts, and bypass caps to personal taste. In terms of location, the cap bank can be several inches away from the tube socket, but the little caps (0.1 uF or less) need to be close by, an inch or less.

You can do a lot of sleuthing just by listening to the spectra of noise. Magnetic induction is going to be pure 50/60 Hz and fairly hard to hear. Capacitive coupling is high frequency only, and will sound like buzz, usually harmonics of 100/120 Hz switch noise from the rectifiers and transformer secondaries.

Ground loop noise can be isolated by shorting the input plugs of your preamp or power amp. If the input is shorted, and the noise persists, it is inside the component itself, and is usually a design or layout error.

If the noise is the result of two components connected together, that is a ground loop. This can be confirmed by disconnecting the interconnects between them, turning both on (with volume down), and using a DVM to measure the AC voltage potential between the two chassis. Scrape through the paint or anodize if you need to, then measure.

The AC potential between the two should be less than 1 or 2 volts. If it is more, then you have a ground loop. This is caused by capacitive leakage from the power transformer to the chassis. It can cured by reversing the AC polarity going into the power transformer on ONE of the components, but this is not a DIY job.

What causes this is that consumer AC power is not balanced; instead, there is neutral, which is only 1 or 2 volts away from safety ground, and hot, which is 120 volts in North America and 220 to 240 volts elsewhere. Power transformers are not symmetrically wound; one side has lower capacitance to ground than the other, but unfortunately, the leads are not marked, so they can randomly assembled in production. Ideally, the low-capacitance side of the primary should go to HOT, and the high-capacitance side of the primary to NEUTRAL.

If all your components were assembled this way, you would never have ground loops. Unfortunately, the phasing of the power transformers is random. The capacitive leakage from primary to transformer case will let the chassis float to a high value relative to safety ground, which is the true ground. The only real solution are medical-grade power transformers, which have extremely small leakage to chassis.

Short of that, you can hire a skilled technician to wire all of the power transformers in your system for minimum AC HOT to chassis leakage ... which is a good idea from a safety perspective anyway. No more little shocks when you touch a component (which should never happen in equipment built to code). Safety code requires that the fuse, then the power switch (in that order), always be on the HOT side of the line.