[Amps] Fw: L7

[Thomas R. Williams]

----- Original Message ----- 
From: Russell Sherry 
To: N2CU at magtrol.com 
Sent: Friday, March 26, 2004 12:16 PM
Subject: L7


Hi Tom,
My name is Russ, NM6DX. I, like yourself, have recently acquired an L7 in 10+ condition. I have several comments about your amplifier as you indicated on the AMPS reflector. 

Power supply voltage for my unit: No load in CW 1900, in SSB 2750. I have confirmed the accuracy of the volt meter and it indicates low by about 100 volts at 2750. The voltage divider drifts slightly with temperature. It indicates lower by about an additional 50 volts.

Power Output. With 100 watts of drive on 40 I get 1300 watts peak as measured with a scope (calibrated 2467B) and with my peak reading Autek RF-1 (also calibrated). I tune the amplifier for maximum output with high speed dots in the SSB mode with 100 watts of drive applied.

These output conditions are at 246 VAC input that drops to 245 VAC under full load (50 amp circuit). Slightly less power was obtained at 123 VAC (30 amp circuit) dropping to 120 at full load.

Modifications to amplifier. I have measured the filament voltage and it would be proper at 240 VAC input. I added a resistor to the secondary of the filament supply to reduce the filament to 5.0 VAC at 246 VAC input measured with a calibrated meter at the filament pins.

I have added QSK to the amplifier. I have an external box using a vacuum relay and a reed relay to switch RF the input. There is no room under the chassis so I elected to use an external box. I had previously used the external QSK box with a 30L1 that I have. The QSK modification to the L7 requires the addition of a switch transistor to electronically switch the bias. I use a low resistance enhancement mode FET that is controlled directly from the band data jack of the FT-1000 series radios I use. I also use a FET interface to protect the radio if all goes wrong. I also used an electronic interface to key the 30L1 (negative 130 switching was needed) that I have. The L7 was modified to have a small external box (metal) that contains the FET, a grid meter protection set of diodes and a switch along with two RCA jacks that connect to the FT-1000 radio interface in use and to the QSK box. The mechanical switch switches between the normal relay control of the L7 and electronic QSK control. The wires exit the amplifier through the hole in the RCA jack used for the RCA connection for the ALC. I do not use ALC as it only dose bad things if the operator tunes and operates the amplifier correctly. I have not protected the plate current meter at this time but I have the parts (0.39 ohm resistor and 3 amp diodes) and will install that the next time I open the amplifier up. The box size is 0.75 by about 2.5 by 3.5 inches. This is Velcroed to the rear of the amplifier near the HV terminal.

Fan Noise. I discovered that to reduce the fan noise do not place the amplifier in the corner of a room where the sound bounces from the two surfaces at 90 degrees and returns to the operator. I tried "modern less noisy fans" and returned to the Drake fan. It is less noisy. I did add an intermediate fan speed making three speeds. Most of the time is is in the second speed when talking on SSB. It occasionally gets to high speed (the Drake maximum speed) but soon returns to the intermediate speed. It takes some 10 minutes to return to the lowest speed when the amplifier is placed into stand by. This was done by adding an additional thermostat and resistor to the amplifier next to the existing thermo switch and resistor. The low speed resistor was increased to provide a slower minimum speed. The added thermo switch was about 5 degrees C lower than the original. I measured the original by measuring the output temperature of the fan going from low to high (Drake original). Additionally I discovered the air intake at the bottom of the amplifier makes noise that is reflected from the hard mounting surface that supports the amplifier and from that surface into the operators ears. This was cured with a piece of carpet slipped under the amplifier to block the path to the front of the amplifier. I left about 6 inches of space between the carpet and the amplifier air intake slots.

The power supply should have the 0.82 ohm glitch resistor replaced with a 10 ohm unit. I have not replaced the diodes or capacitors as of yet. It is likely I will do the diodes at one point using modern 1000 V PIV devices. I have checked the DC voltage across the individual caps in the cap bank and it is presently acceptable so I will not change the cap equalization resistors.

I am in the process of putting step start into the amplifier power supply. I have no issue with making modifications to the power supply. While doing this I am also operating relays with the SSB/CW switch and the AC power switch. That will allow the switches to last a long time as they are not available and an exact replacement is not generally available. I use DC relay coils (no hum). I use an RC timer operating a high current set of relay contacts (3rd relay) that switch the step start. The relays that switch high current will be wired with 10 GA wire. This will reduce the voltage drop in the wiring harness. The step start is intended to reduce the inrush current to the filaments in the 3-500Z tubes. I am less concerned with charging the cap bank but it will reduce both. This circuitry is contained in a 2 inch by 5 inch by 7 inch chassis mounted to the non serial numbered end of the power supply. The two fans that I had left over (the modern low noise units with ball bearings) are mounted on a carrier frame and rest on top of the power supply. They are operated in series with an additional resistor used to reduce their speed. These are 120 VAC fans so I will run them from the step start box instead of a separate 120 VAC source. The fans provide excellent cooling for the PS and mane no noise that one can hear. All these power supply modifications require NO wiring modifications to the L7 amplifier section of the system.

Please contact me if you have any further information needs about any of the items I have spoken of.

Please share this message with the AMPS reflector as while I read the digests I do not have an active account.

Russ, NM6DX



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