Some precautions about using high voltage power tubes:
Unlike other vacuum tubes such as KT88 or EL34, which typically run at 450V plate voltage, high voltage power tubes such as 845, 805 and 211 run at plate voltage 900V or even higher depending on your amp design. Due to the high voltage in the circuit, biasing an 845 / 805 or 211 amp improperly may expose you to lethal high voltage causing instant death. If you haven’t owned any tube before, we recommend you to be cautious when starting with a 845 /805/211 tube amps and get yourself comfortable with maintaining such high voltage tube amps.
Always bias your amplifier by strictly following your amp manufacturer’s instructions. Do not bias the plate current over suggested reading (usually in mA) in any case. This will cause tube overheat, tube failure, shorten tube life significantly and even burn out other parts inside your amp.
High voltage tube has maximum rating of plate heat dissipation. Do not run your amp at a plate current that could cause the tube plate turn red hot (it is a cherry-ish deep red glow in a dark room. Normal tube glow is more towards orange-ish). When you start using a new tube, try it first in a room with dark light so that you can observe the plate color closely. If the plate turns red hot, turn down your amp bias immediately until the plate no longer show overheat symptom. Bias the two channel’s plate current readings to the same safe level for both tubes. It’s not recommended to have two channels biased at different level.
Plate heat dissipation (measured in watts) = plate voltage (in Volt) x plate current (in mA) / 1000
For example, if a tube amp has plate voltage of 960V on the 845 tube by design, and if the 845 tube has max plate heat dissipation rating of 75w, your maximum plate current bias reading should be 78mA. If the amp has plate voltage of 910V by design, your maximum plate current bias reading should be 82mA. It is not recommended run a tube constantly near its peak rating so ideally you should set your plate current bias at about 10-15% below the maximum. This will prolong your tube life and ensure a safe and long term enjoyment of your amp.
Although each tube amp has its designed plate voltage for tubes at a fixed input power voltage, a variation of more than 5% of your input power voltage may cause the plate voltage for tubes vary greatly (for example, swing from designed 910V to 950V or even higher). In such a case, your tube could be overheated and fail in a very short time. It’s strongly recommended to use a power conditioner properly designed for audio with your tube amp to reduce the possibility of such an accident and financial loss.
An excellent tube amp design along with poor selection of parts and poor production quality control could also cause your amp to overheat a tube and cause it fail prematurely. Always choose amplifiers made properly for your local voltage from a reputable reseller. In the past, we have seen many after-market modified Chinese tube amps which have been sold into North America with main transformers not designed for the right voltage, and cause not only inferior performance, but also tube failure or burning up components. Be very cautious when buying ‘cheap’ dual voltage tube amps from overseas.
Last but not least, never leave your tube amp unattended for extended period of time. Never leave a 845/805/211 high voltage tube amp on without anyone in the room.
We usually do not recommend people to chase so-called ‘high wattage output’ 845/211/805 amps. Some of these amps were designed to run output tubes at its marginal peak rating in order to squeeze a few extra watts – those watts you as a consumer may never actually need because most tube amps only sound excellent in their ‘sweet spot’ output ranges, that is – not too low and not too high. Allocating your funds to a pair of properly matched higher efficiency speakers to your 845/211/805 amps is a much much better bang for the buck, and it’s a much enjoyable musical experience too. Do NOT waste your money on chasing the last few watts by driving your tubes hard, instead focusing on get the BEST first few watts!
Due to the complicated feature of a tube amp’s design and user’s settings and maintenance vary greatly, tube manufacturer only warrant a vacuum tube for 90 days. If a tube has inherent manufacturer defect, it will show in the first 90 days of normal use and it will be covered by warranty. Improper usage including amp voltage not properly matched to your local power voltage, using tubes in an amp not suitable for the specific tube or set bias too high or too low, input power spikes etc will result in tube failure prematurely – such incidence will not be covered by tube manufacturer’s warranty.