A scattering of what I have read/learned over the years:
1) Lowered Voltage Filaments:
At normal rated heater voltage a tube meets its spec with respect to gm etc. However that gm drops with age. Running at -5 to 10% will give you the gm of that same tube with 1,000 hours on the clock (at normal heater voltage) but will hold that gm out to 10,000 hours while the full spec heater tube's gm continues to drop. That is, passed 1,000 hours the -5 to 10% powered heater tube actually will have higher gm.
SO LONG AS you design the circuit with that lower gm in mind then marginally reducing the heater voltage gives worthwhile tube life time improvement.
Twisting AC Heaters:
3 to 5 twists per inch. Less is not enough, more is a waste of time. The final connection at the tube socket is critical - see Merlin for advice The Valve Wizard
Elevating AC Heaters with a DC voltage:
I always found it helps to elevate the AC supply. Take the elevation voltage from the cleanest node in the amp power supply which in a well designed power amp is often the output tube screen supply. RDH recommends at least +70V but most implementations I see on this and other forums seem to go for +20 to +30V. I tend to trust RDH so always go at least +50V.
Have I used DC Heaters in a Power Amp? - Yes, once, The heater winding just did not have enough capacity. There was however an unused 5V @ 2Amp winding. Schottky Bridge on that with a 10,000uF gave +5.97V when powering 2 x 12AX7 heaters BUT I did elevate that by +50V. So making use of an uncommitted 5V winding is one place a DC Supply is useful/justified.
You may also like to read what Tim says in this article:
http://dalmura.com.au/projects/Hum article.pdf
That's my shotgun at the barn door for today.
Following this with interest as my next project is my first foray into DHTs and believing in diving in the deep end it is a Parallel Push Pull 300B.
Cheers,
Ian
1) Lowered Voltage Filaments:
At normal rated heater voltage a tube meets its spec with respect to gm etc. However that gm drops with age. Running at -5 to 10% will give you the gm of that same tube with 1,000 hours on the clock (at normal heater voltage) but will hold that gm out to 10,000 hours while the full spec heater tube's gm continues to drop. That is, passed 1,000 hours the -5 to 10% powered heater tube actually will have higher gm.
SO LONG AS you design the circuit with that lower gm in mind then marginally reducing the heater voltage gives worthwhile tube life time improvement.
Twisting AC Heaters:
3 to 5 twists per inch. Less is not enough, more is a waste of time. The final connection at the tube socket is critical - see Merlin for advice The Valve Wizard
Elevating AC Heaters with a DC voltage:
I always found it helps to elevate the AC supply. Take the elevation voltage from the cleanest node in the amp power supply which in a well designed power amp is often the output tube screen supply. RDH recommends at least +70V but most implementations I see on this and other forums seem to go for +20 to +30V. I tend to trust RDH so always go at least +50V.
Have I used DC Heaters in a Power Amp? - Yes, once, The heater winding just did not have enough capacity. There was however an unused 5V @ 2Amp winding. Schottky Bridge on that with a 10,000uF gave +5.97V when powering 2 x 12AX7 heaters BUT I did elevate that by +50V. So making use of an uncommitted 5V winding is one place a DC Supply is useful/justified.
You may also like to read what Tim says in this article:
http://dalmura.com.au/projects/Hum article.pdf
That's my shotgun at the barn door for today.
Following this with interest as my next project is my first foray into DHTs and believing in diving in the deep end it is a Parallel Push Pull 300B.
Cheers,
Ian
I changed the "DC" on my 6SN7 to AC on a driver for 300B.
The DC came from simple DC-DC converter (14V-6.3V) and yes it measured 6.3V DC but there was oscilation from the switcher visible on the scope, even on the secondary. Inserted a choke on input and output of switcher, improved ripple. But. Sound was so-so. It was sort of meagre (the opposite of mellow and rich?), .
Went back to AC and now the sound is perfect, good balance. Have a 25 uH choke in one leg. During warming up the harmonics improve (initially some harshness in the highs, in time reducing 3rd I guess - but have not checked) and then after about 30 minutes the sound is just perfect for me.
Long time ago I also - for quickness - used DC-DC in phono and the switch frequency was visible in output. Yach. But I did that because the simple rectifier/CRC I had gave audible rectifier noise . . . in the output when I turned the vilume up 20 dB@Lowther
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