tube life, soft start, cathode bias

[pforeman]

I just finished my tubelab simple se! I have read ( I think ) somewhere in this forum, but can't find now, that it is easier on output tubes to run them with the cathode bias on a lower cathode resistor (?more current in the cathode, less on the plate?)
Do I have this backward, it seems wrong.
For now (only listened a total of about 2 hours to this amp, but I don't hear much of a difference with the el34 tubes and the cathode resistor at 680 ohms, or 600 or 560. If I don't hear a difference, which would be easiest on the tubes or how much does it matter.
Also, I was wondering if it might be easiest on the tubes to start it up with the cathode setting on the low setting. And, if thats the case seeing is how I have a 6 position cathode bias selector switch, to use one of these settings as a extreme value just for start up, (sort of a soft start setting).
If anyone cares to reply, could you also answer a question I've had for years, but can't seem to find the answer to. Is preamp, input rectifier tube life affected by how loud you play your stereo. I believe almost all tube preamps are class A so it shouldn't matter?, how bout the rest of these tubes?
I did read that input tubes need to be replaced about twice as often as output tubes. Comment please
Thanks,
Paul

[Ty_Bower]

Higher cathode resistance = lower tube dissipation = longer tube life.

It would be preferred not to switch cathode resistances while the amp is running.

[Frank Berry]

It's been my experience that the output tubes fail far more often than the input tubes ... They typically work much harder.
I have an HH Scott amplifier which was built in 1963.
The output tubes have been changes many, many times.
The Telefunken input tubes are the originals and they're still going strong.

[Tom Bavis]

You'll get a very loud THUMP if you change cathode resistors while it's on - not a good idea! Slow turn-on via 5AR4 rectifier is good for cathode biased amps, as the cathodes should be hot by the time the B+ comes up.

Small signal tubes should outlast MANY output tubes. And yes, in class A, tubes wear out just as fast without signal present. Even in AB, the average current isn't much higher than idle.

[richwalters]

Originally posted by Frank Berry

The output tubes have been changes many, many times.
The Telefunken input tubes are the originals and they're still going strong.

Not unusual. Telefunken diamond logo stuff is worth a fortune. No need to change.
I still use 807's KenRad from the 1960's still good emission in my Linear amp. No need to change.
My other MI amp DID have Manley KT90's. All changed in one session..scoop for clay pigeon practise.


richy

[whitelabrat]

pforeman,

just curious, did you wire up for feedback. covered myself with deet and fought off some large moths in the garage and almost finished wiring up my gxse opts.

in any case decreasing the bias resistor value will be tougher on the tube, but reduce distortion. 600 probably is a good value if you can't tell the difference. 560 will let you know if your tubes are wimpy when they start turning red..

for a soft start put a cl90 thermistor on the power transformer primary and you're good.

my shift key stopped working.

[Miles Prower]

Originally posted by pforeman
more current in the cathode, less on the plate?

Not right, and a big violation of Kirchov's Current Law. For triodes, cathode current always equals the plate current. For pentodes, cathode current is the sum of plate and screen currents. There are some odd types where the cathode current can exceed the plate current due to the use of an electron accellerator between the cathode and control grid (FM demodulator type) You won't see one of those unless it's in an FX box.

Do I have this backward, it seems wrong.
For now (only listened a total of about 2 hours to this amp, but I don't hear much of a difference with the el34 tubes and the cathode resistor at 680 ohms, or 600 or 560. If I don't hear a difference, which would be easiest on the tubes or how much does it matter.

VTs are quite "forgiving", and cathode bias resistors can vary quite a bit. Back in "the day" you might find cathode bias resistors with tolerances of +/-20%. +/-20% resistors were common, +/-10% regularly used, and +/-5% resistors were considered "precision". They didn't have +/-0.1%, laser trimmed, metal film resistors back then.

680R -- 560R is almost +/-20% of a 600R resistor. If you don't hear a difference, and you want to take the stress of the finals, stick with the 680R cathode resistors. This will get the plate current down, and be easier for the cathode capacitors to bypass.

Also, I was wondering if it might be easiest on the tubes to start it up with the cathode setting on the low setting. And, if thats the case seeing is how I have a 6 position cathode bias selector switch, to use one of these settings as a extreme value just for start up, (sort of a soft start setting).

Not necessary at all. If you're using hollow state rectification, you already have "soft start" built in. One project I did used a solid state power supply, with DC coupled VT stages. For this project, I separated the heater and HV DC power. That way, I can preheat the VTs so that they're all good to go when the HV DC hits 'em. That way, nothing over volts while cathodes are warming up. Otherwise, makes no difference (unless the spec sheet says otherwise: 6AS7s require cathode preheating).

If anyone cares to reply, could you also answer a question I've had for years, but can't seem to find the answer to. Is preamp, input rectifier tube life affected by how loud you play your stereo. I believe almost all tube preamps are class A so it shouldn't matter?, how bout the rest of these tubes?

Not for audio amplification. It's a different matter for RF. Here, modes like AM, SSB, and CW are a good deal easier on RF finals than "brick on the key" modes such as FM and digital. That's because these RF amps are usually Class C, Class B, or very deep Class AB and develop very little power without a signal. Audio amplification is almost always Class A or shallow Class AB. Here, the no-signal current is as great, or nearly as great, as the max-signal current. Of course, finals are a good deal tougher than you might expect, and unless you really abuse them (guitar amps and players) they should last for years, unless there was some sort of defect that slipped past inspection (it happens).

I did read that input tubes need to be replaced about twice as often as output tubes. Comment please
Thanks,
Paul

If you did, it was from some idgit. Small signal tubes should last for decades. It's the power finals that operate at high current densities, or the high voltage types (CRTs and their hollow state diodes) that lose emission due to cathode poisoning and/or depleation.

[rknize]

Originally posted by Tom Bavis
You'll get a very loud THUMP if you change cathode resistors while it's on - not a good idea!

It depends how you wire it. If you have a "permanent" cathode resistor and use the switch to add parallel resistors, you won't get the case where there is no cathode resistor while turning the switch (which could be bad news for the bypass cap). You can also get a make-before-break or "shorting" switch, but then you are relying on the switch entirely.

I find that the bypass cap on the cathode resistors slugs-out the switching and, in fact, it takes several seconds for the new value to "settle-in". I wouldn't go this route for the reasons the original poster stated, but it is fun to quickly try out other tubes at different operating points and see how it affects (or doesn't affect) the sound. I spin the switch on mine all the time and it makes no sounds whatsoever. Now the triode/UL switch on the other hand...don't do that.