>Hello,
>
>are there any 3CX3000A7 amplifier schematics for an HF amplifier out
>there? Think I have all the parts to start.... :-)
>
>Thank you, Carsten - DL6LAU - J75T
>
>PS: L I F E I S T O O S H O R T F O R Q R P ! ! !
>
- I agree, Carsten. An old timer in Newington said that the awards in
QRP contests should go to the people who copied the weak signals from the
QRPers.
- I remember one Summer evening on 75m SSB. Most of us were running
high power due to the 20db over S-9 bursts of static. A weak breaker was
heard. We pick him up. He tells us his name and that he is running 5w.
He asked how much power the rest of us were using. When he found out
that most of us were running over 1kW, he launched into a long,
lock-to-talk sermon condemning the evils of running high power. About a
third of the time he was buried by static. He was so weak that people
began doubling on top of him - and the general conversation continued as
before the interloper came upon us.
- A schematic for a 3cx3000A7 linear amplifier is virtually identical to
that of a 3-500Z . The main difference is in the current and voltage
handling ability of the components. ... ... Some free advice:
1. There is no need to buy a 51A bifilar filament choke if you:
Insulate the filament transformer from the chassis on a sheet of plastic,
RF-bypass the secondary and primary windings to the frame, and put a
~10uH 3A bifilar choke in the transformer primary. The C between the
frame of the transformer and the chassis figures in the tuned input
calculations for C2. Since there will be RF on the secondary winding's
CT, a ~10uH 3A RFC will be needed between the CT and the neg. HV
circuitry.
2. A number of 3cx3000A7 builders have experienced
parasitic-oscillations between 60 and 70MHz. Part of the problem seems
to be that the typical globar-type resistor has too much inductance for
satisfactory VHF-suppressor service. A lower VHF-Q/VHF-Rp suppressor can
be made entirely from resistance-wire and resistance-ribbon, or from
stainless-steel shim stock.
- "The combination of both resistance and inductance is very effective
in limiting parasitic oscillations to a negligible value of current." -
F. E. Handy. 1926 Handbook, p.72.
3. At 28MHz, there is a considerable amount of tank RF circulating
current that passes through the DC blocking capacitor due to the ~3500v
rms across the tune-C and the built-in anode capacitance of the tube. Be
sure that the blocking cap. is capable of withstanding the 29MHz
circulating current. Non-current-rated garden-variety 'doorknob' caps.
that may work okay in low power amplifiers, may Not work ok at high
power.
Rich---
R. L. Measures, 805-386-3734, AG6K
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