Replacing Output Tubes
The output stage of the Gremlin is fully cathode biased, and no bias adjustment is
necessary when changing output tubes. The design of the Gremlin’s output stage
allows a fair amount of margin for variation in tube characteristics, so any known
brand of KT66 can be installed, and will work properly and be appropriately self-biased.
Other popular power amp tubes such as 6V6, 6L6, EL34, 6550 or KT88 or others for
example can also be substituted.
Tube Quality
Currently manufactured tubes are built in China, Russia, or the Czech Republic, and
are not built to the same quality standards as tubes manufactures by the U.S.
manufacturers in the “glory days” of tube manufacturing.
We musicians choose to use to vacuum tubes because of their tone, but we need to
accept that the tubes available to us these days can be imperfect devices, and are most
certainly the least reliable component in the entire amplifier.
It would seem that the obvious solution would be to use new-old-stock tubes that were
made back in the “glory days”, when tubes were properly made. However, we’ve lately
been finding that much of the available stocks of such “new old stock” tubes are either
gassy, noisy, or unreliable – we do believe that we’ve reached the bottom of the barrel of
NOS tubes.
Tube problems generally reveal themselves as a crackling noise or microfonics which
can occur continuously, sporadically, in response to mechanical vibration, or –
especially in Combo amplifiers - in response to your playing (e.g. a crackling or other
type of noise which occurs only when you hit a note).
We subject all tubes to a thorough burn-in and test procedure to ensure that they are
fully up to spec and operating perfectly. However, you must be aware that the majority
of tube failures occur early in their life, and may come about as a result of the jostling
and jarring that an amp can receive in shipping. In spite of the exhaustive testing we
perform at the shop, early-life tube problems cannot always be found in such testing.
The first two months or so are the most tenuous period for any set of tubes. Most
manufacturing defects will be revealed in the first two months of their life, but many are
not detectable in initial testing, even after a burn-in period.
If you notice any noises, crackling, humming or any other odd behavior of your
amplifier in this period, note that it is most likely to be the result of a defective tube,
and should be debugged as such.
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