Mam pytanie w sprawie attenuatora THD Hot Plate, który jest szeroko polecany jako jeden z lepszych produktów.
Ostatnio na forum Marshall natknąłem się na informacje, mówiące że używanie tego tłumika może być szkodliwe dla wzmacniacza, gdyż attenuator nie dopasowuje zbyt precyzyjnie impedancji i podczas pracy zamiast 8 ohmów, działa jakby miał 4.
Czy ktoś może posiada wiedzę w tym temacie i mógłby zweryfikować te rewelacje?
źródło tutaj :
http://marshallroadhouse.com/viewtopic.php?f=2&t=2825
"It is the Hotplate that's the problem, because it has the wrong impedance characteristic. (...)
Amps are designed - by theory, trial and error, or otherwise - to be stable into these kinds of loads. So in order to load the amp no differently, an attenuator should mimic that curve. The problem is that the Hotplate doesn't - and in fact, the actual impedance it presents is dependent on the switch settings. Instead of a steady rise to high impedance with rising frequency, the Hotplate goes the *other* way, and actually has too low an impedance at high frequency - substantially too low if the Deep and Bright switches are turned on, down to around half the nominal value. For many amps, this isn't a problem - valve amps can usually cope with too-low loads, unlike solid-state ones. But *some* amps - and most Marshalls are an example - don't like it, and heavily overstress the valves, even to the point of failure. When I had my Hotplate, it was easy to make the valves in a 50W Marshall I was testing go into very bright glowing screen overload and even a bit of red plate glow, which is one step away from blowing the valves and at the very least will shorten their lives. (This is also likely to blow screen resistors and is potentially dangerous to the output transformer.) Because it seemed dependent on the switch settings, I measured the impedance curve of the Hotplate and found the problem.
The solution is actually very simple - the Hotplate's overall impedance is much closer to *half* the claimed value. In other words, if you have a "16 ohm" Hotplate, treat it as an 8-ohm load, and set your amp to that. (Or 4 ohms with an 8-ohm Hotplate). When I tried this with the same amp and settings, there was no unusual valve stress - no worse than cranking the amp into speakers, anyway.
Why such an apparently well-designed and made unit should have a major mistake like this, I really don't know. I once discussed it with someone who works for THD, who denied there was any problem, but admitted that he ran his Marshall into a 16-ohm Hotplate with the amp set to 8 ohms! (He said, because it sounds better... I wonder why!!)"
