[Amps] Re: Drake L7 newbie - Summary

[Thomas R. Williams]

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 <><