How long to pre-heat filaments on 6SN7s?

[Brian Beck]

It's hard to conclusively lay out general rules about warm-up sequences for audio tubes to prevent stripping and cathode interface generation. Tube types vary, as do the operating voltages, etc. I will say that for most audio uses, the B+ levels are too small to cause much of a stripping problem, but over time, cathode interface generation may be the bigger factor in tube aging, unless tubes designed for long periods with no cathode current are used (see my post #8 above). For this risk/aging factor, there is ample documentation. With that in mind, you don't want to always wait too long (the "minutes" that Nick referred to) to bring up the B+ after heaters are powered, at least not as a practice every time. But, a small delay of the B+, after the heaters are powered up, can't hurt, and will allay any lingering concerns over stripping, as well as allow voltages to settle more gently in circuit (B+ applied with no heater puts full B+ voltage across coupling caps at a time when B+ may be higher than when under design load). Whether that delay should be 15 or 20 seconds, or the 45 seconds that Chris recommends, we can't say for every case. But generally I'd prefer to see a tube amp with simultaneous heater and B+ power up (in most cases of low B+) than one where the heaters are on for a long time (minutes) before the B+ is turned on. With NOS prices being what they are, a bit of coddling seems reasonable.

 

[duderduderini]

Thanks for the tip

Hi
Point taken
will turn on b+ sooner...
I read somewhere its the turn on oand power down that causes tube degradation.. no i wont leave mine on all the time but is this fact or fiction?
Cheers
Nick

 

[Wavebourn]

Brian,

That was my guess. I suppose that the only risk is cathode-stripping

Do you mean huge transmitting tubes?

Considering that some of the popular tubes such as 6SN7's are getting expensive, fully switching off could extend tube-life.

When tubes were used in computers (thousands of tubes per computer!) people got a lot of experience, such as to switch off filaments as rarer as possible because each on/off cycle shortens life of tubes.

 

[Wavebourn]

A TV damper diode gives a good delay and soft start.

I have a better solution, it waits for tubes to heat-up and start consuming current:

It has advantages of both solid state and vacuum tube power supplies, but don't need vacuum tube rectifiers to buy and replace.
A first, it starts softly, raising a voltage up slowly, waiting for tubes to warm up and to start consuming a current. As the result of a current, due to a positive feedback it more quickly increases output voltage up to a regulated value, after that a positive feedback stops working, but a negative feedback starts to dominate, stabilizing output voltage on a given level, and decreasing a ripple level, without a costly and heavy chokes, without vacuum tubes to regulate output voltage.

Please be advised that no one polarity of a rectifier and filter cap must be grounded!
Also, it produces a negative bias voltage that may be used for good, otherwise leave it as is (it is a positive feedback loop for soft starting).

Vout=12V*(R5/(R6+R7)) so you may vary output voltage.
Minimum input voltage should be no less than 5V + ripple + Vout + 12V.

 

[jnb]

I have a question about standby mode, or running the heaters at half power to keep them running but avoid cathode poisoning.

Are they warm enough this way to protect from cathode stripping if the full heater power and B+ is applied at once on a 6SN7 running 7mA with over 250V on the anode?

 

[Brian Beck]

Probably. In Morgan Jones' book Valve Amplifiers, there is a bit about keeping the heaters partially heated. I've never felt so inclined, but he offers a good rationale.

 

[jnb]

Yes, he does. He also has a PS with standby mode and HT delay. My issue is just that I seem to be wanting more relays than valves and it's starting to make my head hurt 😉

 

[duderduderini]

running heaters at 6V

Hi
With respect to tube life, is it not current wisdom (pardon the pun) to supply heaters with slightly less than specified heater voltage. I read an article on Steves tube pages http://members.aol.com/sbench102/dht.html
which I know refers to DHT's and the order of magnitude less in distortion achieved, but also many ttransformers supply more than 6.3v as specified etc etc.
Anyone run there tubes' heater voltage at 6v?
With regards to tube life, i think the on from cold causes mechanical stresses on grids and posts etc.. hence the protocol of high power tubes to be kept on as it were.. I suppose the tubes we use at such low values of b+ would not undergo the mechanical stresses of warm up and cool down but it might be a factor
Nick

 

[cbutterworth]

Actually, I run my 6SN7's at 6VDC and not 6.3VDC. Again, I remember a posting which suggested that slightly less filament voltage could be beneficial in prolonging tube life.

Charlie