[Top] [All Lists]

[Amps] [Fwd: RE: L7 QSK]

Subject: [Amps] [Fwd: RE: L7 QSK]
From: Garry <>
Date: Thu, 05 Jul 2007 17:29:23 -0400
List-post: <>
Hi Garry,
I only have some pictures of the external power supply when it was under 
construction. I chose to go with the external power supply as that 
allows the 4000 volts under load on the tubes. That is the Eimac maximum 
for RF linear amplifier operation. My bleeder system provides the +120 
volts for cut off bias. It works out to be closer to +140 volts as I 
remember with the present bleeder values.
In my original response to you I stated that the two relay coils were 
placed in parallel. That is incorrect. The coils are in series and the 
reed relay has a resistor in parallel with the coil to make the current 
of the 12 volt reed relay equal to the current of the 26 volt vacuum 
relay. In series with the two relay coils is a resistor that drops the 
difference between the 36 volts dropped across the two relay coils and 
the 150 volt source voltage. The high voltage FET I use is also in 
series with this resistor also. Try <> 
for the vacuum relay. They are tested and inexpensive.

The reason I went to the external QSK box was because it was needed for 
the 30L1 that it was originally designed and built for. There is 
insufficient room to properly deploy the relays and additional circuits 
inside the 30L1. It might be possible to deploy the two relays and 
additional circuits in the L7 with out getting into problems. I have not 
studied this. My main concern would be in not making the amplifier 
unstable because of the RF relay wire paths since it is likely to 
require at least the output relay to be in the RF output field of the PA 
tubes. Since the in and out are 50 ohms the switching is easily moved to 
the external box. I also have the 150 volt relay power supply in this 
box along with the transistor interface to the L7 and the 30L1. Two 
interfaces are necessary because of the way the 30L1 is designed.
Garry, I did not see that you placed my response to you onto the 
contesting .com amplifiers bulletin board as I had requested. I would 
again request that you do this as our dialog might be helpful to others.
Let me know if I can help with additional information.
Russ, NM6DX


 > Date: Thu, 5 Jul 2007 09:16:37 -0400
 > From:
 > Subject: Re: L7 QSK
 > To:
 > Hi Russ,
 > I was re-reading your email this morning and it has renewed my interest
 > in converting mine to QSK again. Do you happen to have any pictures of
 > your modifications? Also, how did you modify the HV supply to get 4000
 > volts or did you build a separate supply for this? I have done away with
 > those two bleeder resistors as all they did was create alot of wasted
 > heat. I added a 25 ohm glitch in either 25 or 50 watt (can't remember
 > the exact rating but was same physical size as the 50 watters I removed)
 > and picked up bias from the B- side with a resistor across the TR relay.
 > I don't know if there is room where the original relay is for a reed and
 > vacuum relay. I would rather have it fully built-in but not much room
 > between those tube sockets so your method may be the way to go with 
an L-7.
 > Many thanks,
 > Garry - WR4R
 > Russell Sherry wrote:
 > > Hi Gary,
 > > I have done this QSK modification. I have constructed an external box
 > > that holds a reed relay and a vacuum relay. The relays are operated
 > > from a 150 VDC power supply to decrease the relay operating speed. The
 > > results are the reed relay (used for input switching) has stopped
 > > bouncing in 800 microseconds and the vacuum relay has stopped bouncing
 > > in 1.3 milliseconds. The relay coils are in parallel and the reed
 > > relay has a 12 volt coil so this has a resistor in series to make it
 > > into a 26 VDC relay. That combination is placed in parallel with the
 > > 26 volt vacuum relay coil. In series with both coils is a resistor
 > > which drops the remaining 124 VDC from the 150 VDC supply. I use an RC
 > > network across the coils to suppress the voltage spike. Using a diode
 > > makes the relays release much to slowly. The relay coils are switched
 > > with a high voltage FET transistor. That is controlled from the solid
 > > state switched output of the transmitter and not a slow noisy relay
 > > output.
 > >
 > > Using the external box also allows the QSK unit to be used with other
 > > amplifiers. I use this with a 30L1 also. The electrical interface
 > > required some thought as the 30L1 in a negative voltage switched
 > > amplifier.
 > >
 > > The internal relay of the L7 is electrically external switch
 > > operated into transmit (and kept switched into transmit for QSK
 > > operation) and the amplifier bias is controlled by another high
 > > voltage FET transistor operated from the solid state switched output
 > > from the transmitter.
 > >
 > > I also have upgraded the HV power supply to 4000 VDC. This requires a
 > > new bias scheme and I use a transistor amplified zener diode to allow
 > > about 90 MA of static plate current when at 4000 VDC. 81 watts of
 > > drive gives just under 1500 watts output. There are several bypass and
 > > plate coupling caps that should be replaced with 7500 VDC units as the
 > > original units are rated at 4000 VDC.
 > >
 > > I have also done diode protection of the grid and plate current meter
 > > movements. Additionally the original PS glitch resistor is increased
 > > from 0.82 ohms to 10 ohms. Use a glass covered 10 watt resistor for
 > > this. This serves as a glitch resistor. For meter movement protection
 > > the meter movements have a resistor added in series with each
 > > movement. The resistor value is selected to drop 0.65 volts across the
 > > meter movement plus resistor combination at the meter full scale
 > > current value. Use an external power supply just operating the meter
 > > circuit to find the value. Do not do this using the amplifier and high
 > > voltage supply. Two 1N5408 diodes wired back to back in parallel are
 > > placed in parallel with each meter movement series resistor
 > > combination. With the plate meter the full scale is now limited to 1.3
 > > amps which is very close to the mechanical full scale. At 0.8 amps and
 > > with the diode protection connected there is a slight inaccuracy
 > > indicated on the meter. Less than 5% as I remember.
 > >
 > > I have also added a third intermediate speed to the fan and slowed the
 > > slowest fan speed. With these changes there is no relay switching
 > > noise and the fan is now very quiet. On SSB the fan mostly runs on the
 > > next to highest of three fan speeds. I originally got some low noise
 > > modern computer fans to lower the fan noise and they were more noisy
 > > than the original! Keep the back of the amplifier away from the corner
 > > of the room and place a strip of carpet under the front edge of the
 > > amplifier. Do not block the air inputs of the amplifier on the bottom
 > > of the case. All this greatly reduces the reflected fan noise.
 > >
 > > The upgraded 4000 VDC power supply uses step start and all relay
 > > control as the amplifier on-off power switch is a unique design and
 > > when it wears out there is no identical switch available. Either a
 > > replacement switch will have to be hand made or a different switch
 > > combination used. The longest switch life is obtained when switching
 > > very low currents such as a relay coil. All this is a direct plug in
 > > for the original power supply.
 > >
 > > By operating two external switches and plugging in the original low
 > > voltage power supply the amplifier is converted back to original
 > > operation. The first external switch removes the amplified zener bias
 > > network and returns the circuit to the original. The second switch
 > > releases the internal relay from continuous transmit and returns to
 > > the original form of relay control. I do not use ALC between the
 > > amplifier and exciter.
 > >
 > > These are the basics of what I have done and I consider the effort
 > > worth the results. The modifications have been in use for 2.5 years
 > > with no problems. Transition from receive to transmit and back is
 > > silent and the fan noise is VERY quiet. During receive periods it is
 > > not unusual to have the fan operate at the slowest speed which is
 > > barely audible.
 > >
 > > Gary, I hope that this has given you some insight into what can be
 > > done with a very nice classic amplifier. Please post this to the
 > > "Amplifiers" bulletin board. Thank you.
 > >
 > > Russ NM6DX.
 > >
 > > 
 > > Invite your mail contacts to join your friends list with Windows Live
 > > Spaces. It's easy! Try it!
 > > 

Local listings, incredible imagery, and driving directions - all in one 
place! Find it! <>

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>
  • [Amps] [Fwd: RE: L7 QSK], Garry <=