>........ I guess my next question is how much trouble is it to add a second
>tube to a 77D?
Adding the extra tube is relatively easy. Correcting the ramifications
is not. The tank L needs to be decreased for each tap. There isn't
enough tune C to make it work with a Q of greater than 6 on 160m, so
either a C-padder and vacuum relay needs to be added, or the original
vacuum-variable needs to be replaced with a double the C unit. The load
padders need to be beefed up. Since the tank circulation current
doubles, the tank L ESR loss increases--as does the heat dissipated by
the bandswitch contacts. The tuned input circuits need to be redesigned
for 25 ohms out instead of 50 ohms out. Another failure associated with
adding a second tube is filter-C failure. When the second tube is added,
the 120Hz ripple current in the hv filter C doubles, which in turn
quadruples the (P=I^2*R) dissipation in the filter capacitor. Mostly
during contesting, case seams sometimes split and the oil leaks out.
Peter W. Dahl reportedly has a retrofit upgrade.
-It adds up to a lot of dollars, grief, time, and less than a half
S-unit.
> I will also need the info on converting the Bias circuit, ...
An easy conversion of the bias circuit is to replace it with the
optoisolator ECBS circuit in the Jan. 1994 issue of QST. (*The Nearly
Perfect Amplifier*). This circuit will never allow the 8877 to switch to
non-linear bias during transmit. An improved version of this circuit is
available at:
http://www.vcnet.com/measures
See Fig. 7B. There's also an Excel spreadsheet that you may find useful
for recalculating tank L taps.
If it were my amplifier, I would beef up the grid-overcurrent trip
circuit. IMO, the device which interrupts the flow of grid current in an
8877 should be capable of interrupting 50A. (sic) I have heard of
numerous cases where the existing device shorts during a glitch.
Rich---
R. L. Measures, 805-386-3734, AG6K
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