HAMPTONe9. OPERATIONAL NOTES / THERMAL CONSIDERATIONS / LONG TERM RELIABILITY
The pad switch serves a dual function. When using a mic, it provides a –14dB pad. When using the ¼" jack, it switches it from an instrument to a line input. For high output condenser and dynamic mics, the pad may be needed to prevent overloading the input (unless that is your desire). For ribbon mics, you will typically leave it out. With the pad switch out, or "Inst" mode (pad out of circuit), the ¼" jack is a 2.2M high Z input for low level sources (basses, guitars, piezio/transducer pickups without preamps, unbalanced microphones…). Do not use the pad for these sources. When switched to "Line" (pad switch in), the ¼" input has an input Z of 250K, with a –14dB pad in front of the first gain stage. This is for line level signals, keyboards, tape machines, computer interfaces…. If you want to change the pad value, it is simply a matter of changing the ratio of R3 and R4, while keeping the sum of the two the same. A table is provided on our website (follow link above) that gives R3 and R4 values for pad values ranging from –14dB to –24dB.
When powering up the unit, it is best to wait a few seconds to allow the power supply to reach full/stable voltage, before turning on phantom power to microphones. Each channel card has a two stage LP filter to prevent power on "pop" and to provide added filtering. These filters provide the proper turn on profile for microphones.
The tube filaments for all 4 tubes (2 per channel) are run in series. This was done to eliminate the need for a third regulated supply, minimizing assembly time / cost, internally generated heat from the psu…. The downside is if one tube filament fails, both channels will stop working. In reality, the tubes under normal wear and tear should last 5-7 years at least, so this shouldn’t be an issue. In addition, all of the tubes in the micpre are under $10 each, so having a spare set is a good idea. In a pinch, you can substitute a 12AX7 or a 12AT7 with either tube in the preamp. Though it won’t work exactly as designed, it won’t hurt anything, and can get you through a few sessions before you have time to replace them with the proper type. Even brand new tubes sometimes fail short of their expected life, this is a part of owning tube gear. In essence, tubes are like light bulbs, they are fragile, and eventually fail. So if you plan on using the micpre in a working setting, you should have spare tubes for it, as well as any other tube gear you own.
If you plan on putting the HVTP2 in a rack shelf, I recommend using a 3U. Though the unit will fit in a 2U shelf with the handle off, it doesn’t leave much room for ventilation on the top. Generally, all tube gear in racks should have space above them, to decrease internal temperature. Heat is the enemy of electronics, and generally speaking the hotter something is forced to operate at, the shorter its life span will be.
Also, don’t keep the unit where it will be subjected to vibration, like on top of a speaker, amp, or monitor of any kind. This will not only generate noise in the tubes, but will shorten their life.