One of my favorite books of all time. Best part of the advice on airflow was that there is no such thing as too much airflow.
So long as it does not dislodge the tube from the socket.
Makes ya think the author didn't want to leave any doubt.
73
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Single 4CX250B base amp for the masses!
- Thread starter Master Chief
- Start date
One of my favorite books of all time. Best part of the advice on airflow was that there is no such thing as too much airflow.
So long as it does not dislodge the tube from the socket.
Makes ya think the author didn't want to leave any doubt.
73
If I wanted to drive one of these amps with a JB150, could I use a 't' connector on the input, with a 50 ohm 25 watt dummy load on one end and the other end connected to the jb150?
your radio would then see a 25 ohm load, since you've paralleled two 50 ohm loads.
no, that's not going to work.
how about some specifics?
what amp do you have, and what do you want it to do?
LC
no, that's not going to work.
how about some specifics?
what amp do you have, and what do you want it to do?
LC
I am building/modifiying a SB200, into a 4cx400 grid drive using a dx300 schmatic; I know they also marketed the DX300 as a KW-1, by using resistors in the input circuit, but can't find that schmatic
AAAha, so the resistor would have to be inserted into the input line feeding the tube, huh|?
Using the grid as the input element on a tetrode tube like the 4CX250 or '400 requires insanely little drive power. What the '250 needs is voltage, but at a lower current than a 50-ohm drive source will deliver.
The DX300 input circuit uses a three-to-one toroid transformer to step up the RF voltage from the radio. On the output side of this transformer, the impedance should be nine times fifty, or 450 ohms. Pride used a parallel pair of 1.5k resistors in parallel as a load between the step-up transformer and the tube's grid. Those resistors present a 750-ohm load, and soak up around ninety percent of the radio's drive power. The tube only needs that remaining ten percent.
Not sure why Pride use 750 ohms for this. We use 470 when we rebuild that circuit. Also increase the inductance of the resonating coil in parallel with those resistors. Improves the input SWR.
Since 9/10 of the radio's power is used to heat up resistors, there is no need for additional drive power. The JB driving this type tube is a waste of time, and just won't work well.
The KW-one model skips the step-up transformer and uses a bigger pile of resistors to soak up 99 percent of the drive power, more or less. There is enough RF voltage to do the job without the step-up ratio, but the power that reaches the tube is still just as small, but an even-tinier percentage.
Never have quite figured out just what was the point of that.
73
The DX300 input circuit uses a three-to-one toroid transformer to step up the RF voltage from the radio. On the output side of this transformer, the impedance should be nine times fifty, or 450 ohms. Pride used a parallel pair of 1.5k resistors in parallel as a load between the step-up transformer and the tube's grid. Those resistors present a 750-ohm load, and soak up around ninety percent of the radio's drive power. The tube only needs that remaining ten percent.
Not sure why Pride use 750 ohms for this. We use 470 when we rebuild that circuit. Also increase the inductance of the resonating coil in parallel with those resistors. Improves the input SWR.
Since 9/10 of the radio's power is used to heat up resistors, there is no need for additional drive power. The JB driving this type tube is a waste of time, and just won't work well.
The KW-one model skips the step-up transformer and uses a bigger pile of resistors to soak up 99 percent of the drive power, more or less. There is enough RF voltage to do the job without the step-up ratio, but the power that reaches the tube is still just as small, but an even-tinier percentage.
Never have quite figured out just what was the point of that.
73
I use the jb150 for local talking, only a matter of time before i forget to turn it off, hence the reason for wanting to pad the input of dx300
If the Pride is in the coax line downstream from the JB, you'll fry the Pride's keying circuit this way. Even with the Pride powered down. That much RF passing through the Pride's standby side still feeds into the keying circuit.
Padding down the input of the DX300 that much would just turn it into a KW-one. Would have a power gain of only five or six times the power you drive into it.
A radio with 20-Watt peaks would only show 100 or 120 Watt peaks on high side. About half that on low side.
Probably not what you're after.
Or you could use a MFJ 1700C, a dual-circuit coax switch. One side for the two amplifiers' inputs, the other side for the amplifiers' outputs.
Actually it has six sockets on each side. You only need two.
But it would allow you to have only one amplifier in line at a time.
73
Padding down the input of the DX300 that much would just turn it into a KW-one. Would have a power gain of only five or six times the power you drive into it.
A radio with 20-Watt peaks would only show 100 or 120 Watt peaks on high side. About half that on low side.
Probably not what you're after.
Or you could use a MFJ 1700C, a dual-circuit coax switch. One side for the two amplifiers' inputs, the other side for the amplifiers' outputs.
Actually it has six sockets on each side. You only need two.
But it would allow you to have only one amplifier in line at a time.
73
