Tom,
I want to thank you for the summary. So offen there is much good
information that is passed directly to the person asking the question and
the Newsgroup gets left out..
Good luck, the L7 is a great amp and will serve you well.. I'm planning to
convert mine to a GU84B Russian tube.
Thanks and 73's,
Dudley
WA5QPZ
jhurry@austin.rr.com
----- Original Message -----
From: "Thomas R. Williams" <n2cu@magtrol.com>
To: <amps@contesting.com>
Sent: Tuesday, March 30, 2004 11:58 AM
Subject: [Amps] Re: Drake L7 newbie - Summary
Thank you to all who responded to my questions about the Drake L7. So far I
have found it to be an awesome amplifier, easily outdistancing my old
reliable SB-201. The comments received ranged from doing absolutely nothing
to the amp, to completely rebuilding the power supply and performing
extensive mods to the RF deck. I will try to summarize the comments I
received in some sort of order.
"Necessary Mods"
1. Soft Start - Less wear and tear on the tubes, filter caps and power
switch. The Harbach SS-100 can be used here.
2. Soft Key - The switching voltage and current may be too high for some
modern rigs. Play it safe and install this. The Harbach SK-401 can be used
here.
3. Filament Voltage - Should be 4.9 V. Install series dropping resistor to
reduce to reasonable value.
4. Filter Caps - If they are dried out, they need replacement. An indication
of bad caps is low high-voltage, or a white deposit around the pressure
vents.
5. 10m - Of course.
"Possible Mods"
1. QSK - Squeezing vacuum relays into an already crowded chassis could be a
chore. One respondent used an external QSK box. Everyone who did Rich
Measures' mod loved it.
2. ECBS - Electronic Cathode Bias Switch, used along with the QSK feature.
3. Cathode Bias Supply - Replacing the power wasting divider network on the
HV supply with something more efficient. One suggestion was to insert a 100
kOhm 2 W metal film resistor from the filament center tap to ground. Use the
middle set of contacts on the three pole TR relay to shunt the resistor
during transmit
4. Glitch Protection Resistor - The 0.82 Ohm resistor is the source of some
controversy. Some say to replace it with a 10 Ohm, others pointed out that
you may be sacrificing the expensive plate choke if there is a problem and
the resistor doesn't open.
5. Relay Speed Up Circuit - Easy mod to make the standard relay operate
faster.
6. Rectifiers - Replace with 1000 PIV, 3 A diodes.
7. Rectifier Equalization Resistors - Another controversy. Remove them based
on information found in newer technical papers, or leave them alone unless
you have replaced the rectifiers with new, modern ones.
8. Meter Protection - Installing 0.39 Ohm resistors and 3 A diodes across
the meter movements.
9. Grid RFC - Replace with a 30 Ohm 1/2 W resistor at each tube. Acts as a
fuse to protect against excessive grid current. There is also a suggestion
to replace with two parallel lengths of nichrome wire at each tube.
10. Grid fusing - Install a bayonet fuse holder on the back panel. Use a
fast blow 3AGC fuse between 250-400 mA rating. Wire in series from grid to
ground. Install a 100 kOhm 2 W metal film resistor in parallel with the
fuse.
"Operation"
My original post about Plate Voltage was incorrect. The values came from the
overly-conservative Drake manual. Actual measurement with the internal meter
showed 2200 V no-load, 2000 V full-load in the CW position and 3200 V
no-load, 2900 V full-load in the SSB position. This is with a rather high
251 VAC input (I've contacted my electric company about that).
The CW position on the amplifier uses the full primary winding, yielding a
lower step-up ratio. This provides a lower high-voltage setting. Seeing that
the amp comes from the days of the legal limit being 1 kW INPUT POWER, you
tune for around 0.5 A at 2000 V to be legal.
The SSB position uses half of the primary winding, yielding with a higher
step-up ratio. This provides approximately 2900 V. With today's legal limit
at 1.5 kW output, there are no problems when operating CW in the SSB (HV)
position. The amp was designed for those peak power levels anyway.
I operated a little over the weekend in the CQ WPX SSB Contest. The amp was
used just as I received it, without any mods. It performed exceptionally
well, and I received many unsolicited comments about a beautiful signal.
Cool. I plan on using it only lightly until I perform some of the mods, but
it's so new to me I hate to start tearing it apart already! Comments are
welcomed on any of the suggestions, and if you need more information feel
free to contact me.
73,
Tom N2CU <><
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