This is extremely stressful for the Follower triode and can even lead to instant destruction as it arcs across the grid/cathode in a few cases. We have the full HT on the grid and 0V on the cathode for up to 10 seconds until both valves start to conduct fully. That means that the cathode of the Follower is sitting at ground, 0V. Therefore that resistor also has no voltage dropped across it. With the Follower passing no current its cathode resistor, (the lower 100k), also has no current passing through it. Which means in turn that the grid of the follower is also at HT or 300V+. That means there is no voltage dropped across that resistor and the anode of the valve is at HT voltage. If the Gain Stage passes no current then there is none passing through its anode load resistor, (the top 100k). That means that the voltage on the anode of the gain stage is always the same as the grid of the Follower stage.Īt switch on both valves are cold so there is no emission of electrons and they are not passing any current at all. Note that as it is a 'Direct Coupled' follower there is no coupling capacitor to isolate the Follower grid from the anode of the previous gain stage. The diagram shows the general configuration of the V2 valve with a standard gain stage followed by a DCCF. This introduces the V2 valve to HT on its anodes immediately. The HT is applied to the amp immediately at switch on no matter what condition the Standby switch in is. And no, the manufacturers do not know something we don't, they SHOULD implement this simple fix but that would be an acknowledgement that all of their models should be upgraded to include it and which of them would own up to that?įirstly note that the H&K Standby system works by switching only the output valves off via the TSC circuitry in their cathodes. However the basic design of the DCCF suffers from a very bad flaw which is very well understood and catalogued but never corrected. So why is it there in H&K amps? Well, the DCCF has a unique distortion mechanism under some conditions which is extremely "valvey" being even order harmonic based and H&K (and many others) want that sound in their amp. This is usually used to buffer the last gain stage from the tonestack but in the H&K amps the tone controls are around an opamp stage so buffering is pretty much unnecessary. In the bigger 36/40W H&K models there is usually a Direct Coupled Cathode Follower (DCCF) stage.
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