Gradual HT ramp up

[merlin]

Well my direct experience tells me otherwise ,
The standby switch is far from perfect , but its proven to increase tube life when opperated correctly , otherwise why else would it exist ?

I believe you have had to replace tubes over the years, but I don't believe it was because of the 'speed' of the HT ramp up. Fender added the standby switch to protect the PSU caps and possibly the DC-coupled cathode follower, not the other tubes.

Ive seen many examples of tube amps with solid state rectification and no standby, tube life is definately reduced

Yep, usually because SS means higher voltages all round, especially on the screens grids. It's true that the switch-on can be an issue for fixed-bias if the bias supply takes too long to come up, so I guess a standby is a 'solution' to this, albeit a sub optimal one. Reminds of that old joke:
Patient: "Doctor, doctor, it hurts when I move my arm!"
Doctor: "Well don't move it then."

 

[Tubetec]

Thats a fair point about fixed bias tubes , earlier Marshall 50W amps took the bias supply from the cold side of the standby , so when you switched to on the powertuibes run flat out until the bias voltage builds , that ate tubes and often caused blown fuses . The effect is also quite clearly audible in the speaker output too , hum vastly increasing at the moment of switch on and fading away after a short time .

Neither of the tube amps I added a standby to are fixed bias ,
and Ive never had to change the tubes in either since , before the tubes became noisy after a short time .

Im really happy with my new electro-thermal ramp up control , the circuit values, thermals and time constants can be adapted to any situation
it requires three extra components .

By the time you add a Mosfet circuit , the ramp up and the protection circuit ,how many components is that ?

 

[merlin]

Why exactly is inside one of those thermostats like the KSD01F? I can't seem to find a straight answer. A triac? An actual mechanical switch?

 

[Tubetec]

Its a set of bimetalic switch contacts as far as I can tell .

These are commonly used as battery pack thermal cut outs ,
I had one of those opperate once in a nicad pack with shortcircuit , luckily it saved my RC car from turning into a blob of molten plastic

 

[abbey road d enfer]

By the time you add a Mosfet circuit , the ramp up and the protection circuit ,how many components is that ?

One may think that less components equal more reliability, but when you do a basic comparison like evaluating MTBF, the weak link is the mechanical thermal switch.
A properly dimensioned MOSFET-based circuit would have an MTBF vastly superior to a thermal switch. However, I believe it doesn't constitute a decisive element, since we'll all be dead when anything fails.

 

[beatnik]

I've read on an hi-fi book about using thermistors on the power transformer primary.

From what I understand they don't provide exactly a soft ramp up of the voltage but they rather limit the inrush current, which maybe is all it's needed in order to give an easier life to the power transformer, filter caps and possibly the tubes ?

It seems they also have the advantage of allowing to rate the mains fuse closer to the actual value required.

What do you think ?

 

[abbey road d enfer]

I've read on an hi-fi book about using thermistors on the power transformer primary.

From what I understand they don't provide exactly a soft ramp up of the voltage but they rather limit the inrush current, which maybe is all it's needed in order to give an easier life to the power transformer, filter caps and possibly the tubes ?

It seems they also have the advantage of allowing to rate the mains fuse closer to the actual value required.

What do you think ?

Spot on! Nothing to add. Beware though that they are not eternal and could fail.

 

[trobbins]

From what I understand they don't provide exactly a soft ramp up of the voltage but they rather limit the inrush current, which maybe is all it's needed in order to give an easier life to the power transformer, filter caps and possibly the tubes ?

It seems they also have the advantage of allowing to rate the mains fuse closer to the actual value required.

What do you think ?

Imho only the latter advantage has a chance of being substantiated, and only then for an engineered design, or someone prepared to confirm by testing, or for an amp with a fuse that already has a lot of headroom such that there is a lower rating value available.

 

[merlin]

I've read on an hi-fi book about using thermistors on the power transformer primary.

It seems they also have the advantage of allowing to rate the mains fuse closer to the actual value required.

Correct, this is very common in big SS amps. Big transformers have brutal inrush; they don't just blow the fuse, they trip your house breaker! Soft start is essential for living with them.

 

[bibi]

I think it is unfortunate that people perpetuate this myth of standby switches prolonging tube life. It has gotten to the point where people regularly ask for one on amps that were even designed without them. When asked, I always put this question back to them: can you name a single piece of professional test equipment, military equipment or hi-fi equipment that included a standby switch?
If delaying the B+ voltage truly extended tube life to any significant degree, even marginally, it would have been omnipresent on the best equipment where longevity and reliability were paramount over cost.

 

[Matador]

When I was routinely gigging, the *only* good use I found for a standby switch was that it would cut the background hum from my 4x10 cabinet between sets.

It's not hard to sequence the power rails in a tube amp, so that the fixed (negative) bias comes on quickly, and the HT rails slowly (and also the reverse, so that the bias falls slower than the HT rails when discharging). Yes it takes some more parts, but you also end up with measurably 'cleaner' supplies in the process.

 

[beatnik]

If delaying the B+ voltage truly extended tube life to any significant degree, even marginally, it would have been omnipresent on the best equipment where longevity and reliability were paramount over cost.

Perhaps B+ delay or slow ramp up doesn't make a difference, but lowering / cutting out B+ when the amp is not in use should provide some kind of longevity I guess ?
However there maybe issues with leaving the amp on standby too long ? (Cathode poisoning ?)

I think military and scientific labs tend to have more resources than the average musicians, it wouldn't be such a pain in their wallet to change out some tubes - or swap the device entirely and dump the faulty one.
I also have the feeling the military cares not so much about topics like sustainability and environment.

 

[beatnik]

It's not hard to sequence the power rails in a tube amp, so that the fixed (negative) bias comes on quickly, and the HT rails slowly (and also the reverse, so that the bias falls slower than the HT rails when discharging). Yes it takes some more parts, but you also end up with measurably 'cleaner' supplies in the process.

Tell us more

 

[Tubetec]

Placing the feed resistor for the bias circuit before the standby switch ensures your output tubes dont run balls to the wall at turn on .

Bibi were talking about a gradual ramping of of the HT supply not a delay ,
what tube amps do you know of that are reliable with solid state rectification and no standby ?

I have a couple of themistors here , they measure around 15 ohms cold , just as a quick test I applied a single lithium ion cell (4v)across the terminals , took about 20 seconds for the device to drop to less than an ohm
My 15-20 va transformers have a primary reisitance of around 200+ ohms , the 15 ohms of the thermistor are useless to prevent inrush ,
I was thinking of trying the thermistor in the LT secondary winding with slightly higher volts

 

[Tubetec]

Ive got little more than resentment for my stupid simple ramp up circuit , Im in the amp repair game a long time and I see the guys who dont leave time for the cathodes to warm before applying HT more regularly with amp problems , case closed .

 

[beatnik]

For sure the standby is pretty much the norm on solid state rectified tube guitar amps.
But as its been pointed out it this sector is not immune from marketing bs.

Tube manufacturers should come up with a proper scientific research on this subject.
But I guess tube longevity is a little bit against their interest ?

What is happening in the hifi side of things btw ? Do these amps also use standby switches ?

In terms of pro audio gear the only thing I know of with a standby switch is the Phoenix compressor by Thermionic Culture.
The switch actually just lowers the B+ voltage without cutting it out completely and meant to mitigate wear on the input vari mu tubes.

 

[Tubetec]

Definately Beatnik ,
Morgan Jones suggests not leaving tubes starved of plate voltage as they warm , but allowing the anode trickle away at a lower plate volts initially ,

 

[abbey road d enfer]

My 15-20 va transformers have a primary reisitance of around 200+ ohms , the 15 ohms of the thermistor are useless to prevent inrush ,

In this case I agree, but it just shows that this NTC is inadequate for this xfmr.
These 15 ohm NTC are ommonly used on Twin-Reverbs, with a 300-ish VA xfmr.

 

[abbey road d enfer]

When I was routinely gigging, the *only* good use I found for a standby switch was that it would cut the background hum from my 4x10 cabinet between sets.

Not only that. It somewhat preserves tube life when the amp is overbiased and they glow red.

 

[reinw33]

....

Im not even incorporating an on/off switch , you connect it too the device , plug it in , heaters come on , HT ramps up then switches out the series resistor automatically ,

I think the doubts about reliabillity are totally unfounded , the thermal switch is much more robust than any mosfet based ramp up circuit and it only requires two components , it also wont self destruct due to an inadvertant short of a probe tip .

Theres no electronics involved here , its thermal/electrical considerations , backround or ambient temp will play a part in the switching time but its all good compared to full unloaded HT at start up ,

@Tubetec
I think you found sth. excellent Tubetec. Of a soft switch start.
Everything else seems like deepfrying gear like a turkey.