To: <amps@contesting.com>
Date: Sun, 10 Aug 97 19:41:21 +0000
Rich Measures wrote:
> >
> > - For example: When the second tube is added to a 77dx, and the tubes
> > are driven to 2A of anode current, the stock tank's operating Q on 160m
> > drops to about 5.9. IMO, this is not good engineering practice.
> >
> >That may be your opinion, but it is most likely not a real
> >concern. There is no rule or law that says the Q needs to be
> >"twelve". ...snip...
------------------------------------------------------------
> Eimac recommends a Q of 10 to 15.
>
> According to Vince Fedele, a problem with adding the second tube to the
> 77dx, is low efficiency and reduced output on 160m. A tank that is
> designed to work properly with one tube is not going to do so if a second
> tube is added. IMO, an efficiency of 45% does not indicate good
> engineering practice.
> Rich---
>
> R. L. Measures, 805-386-3734, AG6K
------------------------------------------------------------
8/10/97 from Vince Fedele:
Hello Rich, Tom, Phl, et al:
I have extensive notes in my engineering notebook Re> Changes made to
improve the 77sx which I have had since 1988. It was purchased new from
the factory and was one of the last 77sx's made, serial 3419. Yes,
indeed, the tank Q drops from 12 to about 6 when adding the second tube.
The result remains acceptable with the Pi-L design, but I would
certainly have preferred a higher Q, like 8-11 as well.
I agree with Phil, that it is one of the finest desktop amps ever built.
Having said that, problems arise when correcting some of the
shortcomings.
First, the internal plate supply is very limited. Even with the proper
internal transformer from either Alpha or Dahl, the plate voltage drops
from 3,900Vdc no-load to 2,900Vdc at 2 amperes (CW single tone key
down). This is about 5,800 watts in, 3,200 out at about 55% efficiency,
typical for this amp. The 8877(s) will not perform as well at this
voltage, and requires more drive (120-150W) and also more grid current
(approx. 60mA per tube).
Raising the plate to 4,500Vdc no-load / 3,600Vdc full-load (2A) results
in much lower drive requirements (100-110w), and the grid current drops
markedly (40-50mA** for both tubes). **See note on Grid metering, below.
With corrected p/s (I used a Peter Dahl external 2A ccs xfmr and his
replacement filter cap of 35uF at 5KV.), the input power is 7.2KW,
output is 4 - 4.5KW on all bands, into a 50ohm dummy load, depending on
efficiency. I have never seen a stock 77sx deliver more than about
3,200W out (single tone CW key down), but only on bands above 160m. On
160m about 2,000w or less output is typical. I used a Peter Dahl
external 2A ccs xfmr and his replacement filter cap of 35uF at 5KV.
Now that the p/s limitation is remedied, other problems arise. For
example, the amplifier is intentionally mis-tuned on 160 meters and does
not deliver full output (L tot=about 38uH, should be about 23uH). Fixing
this increases output from about 2,000 watts out on 160m to 4,500 w out.
The amp was mis-tuned to limit output power for several reasons:
1) the 160m padders on the loading were sized too small to handle RF
currents at normal efficiencies at 1.8Mhz;
2) the plate choke only exhibits about 960 ohms (86uH) of reactance at
1.8Mhz. This should be closer to 2,000 ohms (200uH), minimum, plus the
plate choke bypass caps need to be increased substantially in order to
bypass all (i.e., most) RF current from reaching the HV supply;
3) the cathode chokes are also too low in reactance to keep RF out of
the grid metering circuitry at full power and normal efficiencies on
160m;
4) the 160 meter toroid (L7) is too small to handle full output at
normal efficiencies, and will overheat and fail.
After fixing these items, output on 160m is the same as other bands, and
efficiencies are normal (55-61%) on all bands.
The loading caps are far too small on 160m IF one has increased the
output from 2,500 to 4,500. They will heat up, drift (tuning will
change), then fail catistrophically. My solution was to use 5 200pF N750
HEC doorknobs purchased new from the factory, after calculating the
total RF current being handled by the loading cap (at actual mesh) and
padder combination, then simple ohms law for the current needed to be
handled by the padders. The caps need to be sized at 160m, as they
handle less RF current at low freq?s. I increased the size of the (L7)
toroid (using T250 cores, and teflon insulated #12 wire), reset it from
25uH to 11uH (total L @ 160m = L5 + L6 + L7 = 6 + 6 + 11 = 23uH), and
moved it to the side panel. This opened the front panel up so that 5
15KV door knobs in parallel (1,000pF total) at the proper current rating
could be fit up with short straps to the bandswitch.
The RF plate choke is fixed by adding a second choke in series, at right
angles to the existing choke. It is switched out on all bands except
160m. I used a small vacuum relay rated at 7.5KV, 12 amperes. The relay
is switched on/off using unused contacts from the input band-switch in
the 160m position. Be sure to filter and bypass all control leads
into/out of the input compartment. In addition, I doubled the bypass
caps using several values--all rated for high RF current. Finally, the
Plate Tune vacuum variable is at MAX C at 1.8Mhz (0.00 turns). It would
be nice to vacuum relay switch in about 100pF at 160m, but I have not
yet had time to do that.
**Note on 77dx/sx Grid Metering
The grid metering in the 77sx is actually 200mA full scale--not 500mA as
marked and as indicated in the manual. This was an intentional "design
error", or so I was told, by Ray Heaton, to make users think they were
running more grid current than actual. Going through the calculations of
the grid metering ckt there is a 1 volt drop across a 5 ohm metering
resistor at 200mA (i.e., full-scale) and a 1k divider to the 0-1mA
multimeter, most 77's are 200mA f/s. At 3,500Vdc CW, my 77sx runs less
than 1/4 scale, or 50mA for both tubes.
Hope this has been helpful to clarify some of the issues raised
regarding this amp.
Vince
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