>
>I have an opportunity to purchase a Tentec Titan 425 amp. The reviews
>on eHam seem quite favorable, although they do warn that the finals are
>expensive and are very sensitive to excessive grid current draw. Anyone
>here have any opinions pro or con to help me decide whether or not to
>purchase this unit?
>
/\ I repaired a TenTec 425 with reportedly c. 20 op. hrs. It had an
intermittent HV flashover to gnd problem. I tested both tubes for
leakage with a high potential tester. The anode-grid leakage @ +5kV was
c. 50uA. Normal leakage is under 5uA. With -5kV, leakage reduced to c.
10% of the leakage with + polarity. Thus, the problem was probably being
caused by loose gold meltballs in the 3CX800A7s. (see Figure 24 on my
Web site for a photo of an autopsied 8877 with loose gold). According to
an Eimac rep., the grid's gold plating boils and evaporates during an
"oscillation condition". ('Eimac Letter' on my Web site) The fix was to:
1. install lower Rp parasitic oscillation suppressors.
2. settle the meltballs into the base of each tube by tapping firmly on
top of the anode cap with the flat side of a 4oz ballpeen hammer. (yes,
it sounds meshugah) Periodically check the (+ pol.) leakage @ 5kV during
the tapping process so that you can learn how hard to tap with the hammer.
/notes -- Hold the tube base-down in one hand and wield the hammer with
the other hand. Inverting the tube after the loose gold is settled in
the base will cause the gold to recoat the anode-grid insulator - so the
tapping process must be repeated.
>
/\ The 425 needs c. 80w of drive. Mo' is not mo' betta -- So a variable
output transceiver is preferable.
>
cheers, Joe
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
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