Hi Gilbert,

Nice to see your civil reply... please let me make some comments.

: Gilbert Bates wrote:
: Look at it from an engineering perspective Skipp.
: High potting in the commercial / manufacturing environment is
: typically used for design and compliance *verification* and not pass /
: fail *testing*. Obviously, high-potting may find use in some testing
: situations, but if you use it for pass/fail testing, then what does
: that say about your design ability?

When rusky and asian made low cost tubes started showing up as cheap
alternatives, their failure rates (and sound in some cases) were pretty
high. Companies like New Sensor has some reported horible problems with
quality control. People trusted the Dealers to sell them new tubes, which
few had actually tested. Many people experienced amplifier damage and
related tube failures because of short cuts and someone's lack of design
ability on the mfgr side.

Tube people like Groove Tubes, NDB Electronics and others have built up a
faithfull reputation for pre-testing tubes. In a high dollar amplifier,
people will probably pay more attention to the quality of
replacement tubes. In an amplifier with much higher average anode (plate)
potentials, you had better pay attention to the tube quality or the
results can be/often are costly.

: The max rating for a 6L6GC is 500V. If you use it beyond that rating,
: then high-pot testing is used to support your poor engineering
: decision to use a device beyond it's rated specs to begin with.

Some selected & tested tubes are not limited to the max rating of 500V.

: The EL34 has an 800V max rating. If you can't trust it to 800V without
: further testing, then why use it?

I agree, but some Music Man circuits used selected 6L6GC tubes.

: Typical usage parameters and operation are just that, typical. Some
: folks use a general rule of thumb that cut-off usage of a device at
: 80% of it's max rating.

Some folks do. ... and some folks take the wild cards out of deck
by proper testing the tubes for proper operation and spec. Again,
Groove Tubes has made a huge business out of selection, grading and
testing of tubes.

: No offense, as I see were you are coming from and what you feel you
: need to do from the testing perspective. But I'd have to agree with
: Phil, what you are doing with high-potting is just plain wrong from an
: engineering perspective.

I'm not engineering and amplifier over 24 years. Trying to return it
to original operation required you pay special attention to the
replacement tube issues. There is much less tube quality control fudge
factor with the much higher anode/plate voltages used in the MM Circuit.

: If any tube does not perform to it's published specs, then don't use
: it. I don't care if it was manufactured 60 years ago or yesterday. A
: 6L6 or EL34 should perform to its published specs, even if those specs
: come off a datasheet that is older than your grandmother, otherwise it
: is not a 6L6GC, EL34 or it is simply junk. That's the point Phil is
: making.

Nothing wrong with the above... I simply say you should high-pot test
new tubes for breakdown before you place them into the less forgiving
(much higher than average anode/plate voltage) Music Man Amplifiers.

Do you pre-test tubes you place into service? If you know the amplifier
has much higher plate voltages vs the average circuit, would you not
try to test it for breakdown at the higher voltages?

: To add high-pot testing on the rear end of manufacturing is an
: additional costly layer that most modern manufacturers will not
: tolerate.

Of tubes or of amplifier circuits?

: Again, you guarantee performance by *design* (high-potting
: or compliance testing on the front end), you do not guarantee
: performance by *test* on the rear end. Otherwise you end up throwing
: product in the trash when they don't test to specs. Sample testing on
: the rear end is one thing, but if your in the latter position,
: guarantee performance by test on every device. then you fire all the
: engineers and find someone the can design something to work properly
: to begin with. And if you use something at near or above it's
: published specs, then that's perhaps bad decision making on the
: engineers part and you'll have to live with the fragile house of cards
: that you've constructed as you're doing now...
: Cheers!

Possibly, but in dealing with tubes vs a circuit. People will pay
for Wal Mart most of the time.

But people who drive higher performance cars don't often buy tires at
Wal Mart. Owners of amplifiers with much higher than average plate/anode
potentials should not only pre-test new tubes for proper operation. A
high-pot test will give you an indication of the tube will withstand the
higher potentials found in the Music Man Amplifiers.

If you high-pot test a cross section of off the shelf untested replacement
tubes, you'll probably find a few are potential trouble makers.

cheers,
skipp