VACUUM TUBE LAYOUT
Top Panel
This section is dedicated to those who do not have an abundant amount of experience with tube amplifiers. The
above drawing is a schematic of the top panel of your JD 502B amplifier. Your amplifier uses a complement of 8
tubes spread over three vacuum tube types. This tube complement is made up of 4 pieces (or two matched pairs)
of 6CA7/EL34s, 2 pieces of 12AX7s and 2 pieces of 12AT7s. Please note that the vacuum tube type KT88 to
KT100 can be used in place of the 6550s. The placement of the tubes in their correct socket is critical. If you
need to replace a tube, consult the tube layout for positioning. The placement of your tubes in your amplifier is:
V1 = 12AX7 tube type
V2 = 12AT7 tube type
V3 = 6550 tube type
V4 = 6550 tube type
V5 = 12AX7 tube type
V6 = 12AT7 tube type
V7 = 6550 tube type
V8 = 6550 tube type
These tubes are readily available at Specialty Audio stores in your local area, mail order houses for guitar and
audio equipment, and local Guitar Amplifier Stores. The majority of professional guitar amplifiers sold are tube
driven since the music industry uses the best sounding amplifiers for professional reasons. The mail order houses
can be found in magazines such as Stereophile, Audio, Absolute Sound, Guitar Player and Guitar World. When
purchasing tubes, matched pairs of 6550s will give you the best sonic performance. A matched pair of vacuum
tubes is literally matching the electrical operating performance of two tubes. Although Jolida recommends
matched pairs for better sonic performance, it is not mandatory to have matched pairs of tubes to operate the
amplifier. The basic problem with matched pairs is that you can pay a significant price of $75 to $125 for the two.
Jolida tubes can be purchased from your dealer or we can help you locate a source for the vacuum tube you need.
The vacuum tubes in your JD 502B amplifier are rated for 3,000 hours of life. The average usage of an audio
amplifier is 500 hours per year or 10 hours a week so changing the power output tubes should be every 6 or more
years. Our experience is that the 12AT7s and the 12AX7s (smaller front tubes) should last 6,000 hours. There is
no exact method for determining the life of the tube since the life expectation is a combination of how the tube was
built, how it is used, and how long it is operated.
Handling a tube amplifier is not complicated even though most people are used to solid state. Replacing tubes
every six years or more, and taking time to bias the amps are not difficult. What you get with a tube amplifier is a
reliable piece of electronic equipment that can give you years of service. Many people remember tube equipment
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