It sounds ridiculous I know but they put them on because their customers simply expect that they do. Standby is a totally unnecessary feature on our amps. Firstly, you have a Master Volume control or equivalent which can simply be turned fully down then back up when required. Secondly, valves do not "wear out" under normal running conditions let alone under idling. The need to preserve them in normal use is vastly overrated and comes form a desire to sell more valves by convincing people they are "worn out" when they are in fact fine and in spec. The TSC system does not suffer from the problems of cathode poisoning.
This is the tech side of it. If all of this is of no interest then anyone can just skim over it. Cathode poisoning is caused by having your output valves in a condition where they are still heated, and therefore emitting electrons from their cathodes, while they have no anode voltage to pull those electrons away from the cathode as anode current. The usual method for implementing Standby is to simply remove the Anode voltage from the output valves and sometimes the Phase Inverter valve. The valves are left still heated and emitting ready to go immediately at Standby Off. The emitted electrons form a cloud around the cathode and build up of this forces the growth of the interface resistance at the layer between the cathode metal base and the oxide coating which is the emitting material. That resistance does not reduce and, over time, gradually increases. Yes it is a slow build up but it is very real and documented in "proper" valve reference materials. This of course begins to restrict the valve's ability to pass anode current until it is choked out and does not work as it should.
In the H&K TSC system there is a MOSFET in the cathode circuit. All cathode current, (the same as the anode current under normal conditions), passes through this. The DC bias voltage on the valve grids is a fixed level and adjustment is done by varying the cathode resistance, this is a common way of doing it. The TSC bias control circuitry monitors the bias current on each valve and adjusts the resistance of their MOSFETs individually to increase or decrease the current at each valve as required. In the GM36 case the MOSFET has a maximum on resistance of 0.75Ohm so it plays absolutely no part in the normal action of the valve. When you apply Standby these MOSFETs are put into a high resistance state dropping the valve anode current to virtually 0mA. The anode voltage is left in place. The current is "virtually" 0mA because a very slight current is left passing in order to prevent that electron cloud from building up. It is drawn away as that residual tiny bias current, and cathode poisoning is prevented.
Manufacturers are extremely loathe to be left as the only ones NOT offering a piece of functionality that others do. Remember, Leo Fender was self taught from his uncle's radio repair shop and did most of his early work in that field. His original designs were basically his own tinkering with existing circuits for PA systems after requests from musicians for them. His guitar amps came as an extension from those designs. He was not an expert trained engineer. Even more bizarrely Jim Marshall was a drummer with no electronics background and simply used Fender's Bassman as the basis for his own amps. It is not surprising there were aspects of design they were unaware of. It was a time of plentiful valves and pretty much cheap and primitive. That's why we use ECC83/12AX7 valves so prolifically, they were the most easily and cheaply available at the time. There was nothing subtle about it, they were there, so the circuits were designed around them. (You know my feelings and advice on 12A*7 "family" replacements.
) This issue was not considered a significant problem by them, if they were even aware of it.
If you check you will find that in some simpler designs, still highly thought of such as the Fender Deluxe Model 26, he moved away from having a Standby all together. He also used the method of implementing a Standby by putting a switch in the early power supply circuit. This can be between the rectifiers and the power reservoir caps, or sometimes between the transformer secondary and the rectifiers. This in turn can cause other even more spectacular problems such as explosive rectifier damage in some designs.
Vox actually had an early marketing catastrophe with this issue. The AC30 CC2 model had a rectifier valve and the Standby switch between the rectifier and reservoir caps. They found that a large number of rectifier valves in amps out in the field were blowing, sometimes catastrophically. It was very poor design. They had simply copied the circuits others used and did not understand how their own rectifier setup required a different approach. In the light of this problem they removed the standby for many models completely then reintroduced it in a different form for some later versions.
Manufacturers only do what will sell units! Guitarists nowadays are all electronics experts and they know that their own favourite makers are world leaders in the field of electronics design. Anyone who speaks out against the current "common knowledge" is classed as a heretic and not worth listening to. Most guitarists nowadays are white line followers with product name, fashion and copying their heroes as their criteria. They expect aspects like Standby to be fitted and no matter how much people like myself, with a background in electronics outside of the guitar world, tell them is wrong they will continue to buy flawed designs and stick to their belief in the same old industry myths.
These problems are easily fixable with simple cheap circuit mods or even removal of a feature as unnecessary but manufacturers feel they don't need to address them to keep on selling and guitarists themselves often just won't allow it. For example, if you search here you will find threads where people question why the H&K amps still sometimes have a tiny remaining audible signal left even when on Standby. It is simply because the TSC system leaves a tiny standing current running to preserve your output valves and this allows a residual sound level - with your ear pressed to the speaker!!! Why is that a problem? Some people see it as an affront that the Standby cannot reduce sound to absolute 0!
Why don't we just turn the Master Volume control fully down on any amp and these problems are non-existent?
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