Amp Mods for Better Reliability & Protection

Amp Mad Scientist said:
Amps without a standby are class A amps. No bias voltage to control the current.
In a Class A amp the output runs at 100% full time.
In Class A amps, the tube life is shorter.

In Class AB, the bias voltage and filament are turned on before the B+ voltage is.
This changeover to class AB was probably to increase reliability and tube life span.
In Class AB, the bias voltage causes the output to idle when there is no audio going through. This reduces the heat overall.
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Ok, I should have stated "audio equipment" & excluded transmitting equipment.
The change to class AB was to get much greater output power from the same number of tubes than is possible with class A, no "probably" about it. I have owned, & seen many class AB amps without standby switches, both guitar amps & audio amps. If we look to proper valve text books, none recommend or suggest the use of a standby switch in audio equipment, regardless of the class of operation of the output tubes (I've been using text books like my original 1953 print copy of 4th edition of The Radiotron Designers HandbookIMG_20200411_220245.jpgwhen looking for information, for the 30+ yrs I've worked with tubes, not internet lore).
Yes, standby switches are used in transmitting equipment but to prevent "cathode sputter" it is recommended that the filament voltage be reduced to 80% & that a minimum amount of cathode current (10 ~ 20%) be allowed to flow while in standby, also that the equipment be shut off if expected standby time should exceed 15 minutes.
Do you know of any guitar amps using transmitting tubes or using HT voltages of several Kilovolts?
None of the valve authorities (Valvewizard, Kevin O'Connor etc) recommend the use of standby switches in guitar amps. Cheers
 
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Yes, in class A operation the output tube or tubes are biased close to maximum plate dissipation. A class A amp flows its most output tube plate current at idle ("quiescent" current). Does class A operation shorten tube life?? Only if you leave it idling all the time & not playing through it. How long does the output tube last in say, a Fender Champ, a Vox AC4. Are they known for short output tube life? Cheers
 
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So... Are there any benefits in adding the simple mod of an in-rush current limiter to our amps? Or is it just a big waste of time?
 
Mr Grumpy said:
Not in my class A it doesn't... :sick: Well, not 100% to the places it's supposed to go anyways...
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Definition of class A is:
100% (90-100%) and no reduced idle current.
Or it isn't class A technically.

There has been claims of "simul-class," which runs class A and Class AB at the same time.
However technically due to design simulclass does not qualify for actual class A operation.
Many techs besides myself have said so...
 
Amp Mad Scientist said:
Definition of class A is:
100% (90-100%) and no reduced idle current.
Or it isn't class A technically.
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Actually, the definition of class A is that current flows in the output devices (in this case the anodes (plates) of the power tubes) for the full 360 degrees of the signal. "That" is the criteria for class A operation. (Feel free to look this up).
The tube can never be driven to the point of cut-off or it is no longer operating in class A (saturation, yes, but cut-off, no as current is no longer flowing.
To speak of "bias" in class A operation, the tube is biased to the center point of linear section the anodes characteristic curves. Cheers
 
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Amp Mad Scientist said:
Sure there is advantages, that's why Fender and so many other manufacturers do that.
It's cheaper to use in rush protection than to replace parts under warrantee.
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Without getting into discussion on the different reasons manufacturers use these, yes, it is never a bad idea to add this type protection to an amp (even if it doesn't "need" it).
As a precaution, use some type of tubing or heatshrink to enclose the body of the thermistor. These things can go off somewhat explosively. Cheers
 
Yesterday I accidentally posted the following info at another amp thread of mine. But it's still relevant on both threads:

As a follow up with the results of adding a soft-start inrush current limiter to this amp: With my meter connected, I can clearly see a much more gradual buildup of DC voltages just before the first filter cap. Without the thermistor, immediately after flipping the power switch to on, full voltage is hitting the caps. For a couple seconds, I saw the voltages of around 10% to 20% past the normal operating voltages before settling down. With the 200 ohm thermistor installed, in about a 10 second time span, there was about half the voltages initially and from there it was a linear climb to the proper operating voltages. This has to be better for the amp's internals, right?

Another result from adding a thermistor is a 2% to 4% reduction in the B+ voltages. The Spiffy amp lost about 5 to 10 vdc. I installed another thermistor, this time in my Chihuahua amp, and it lost about 15 vdc. Very minimal losses and I cannot tell the difference if the amps performance has changed any.

Update to the update:
Also, Ivan H just reminded me to check if the heater voltages are still in proper specs!
 
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