Drake L7 Linear Amplifier is a little weird part 2. Input circuit, 10 meter mods and Grid Grounding
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- čas přidán 10. 09. 2024
- Here is a short and precise link from a Ham on the Ten Meter Mod: www.ea1ddo.es/l...
Already did the top side of the Drake L7. The tank circuit of that amp is very weird. Underneath with the input circuit, not so much. However, it does have its quirks.
First of all, the amp has 2 protection circuits. Both are related to having the cover off. With the cover off, there is a switch in the main ac power circuit, that cuts main power when the cover is off. The 2nd circuit is a crowbar switch that mechanically shorts the high voltage off if the amp is powered up with the cover off, and somehow the main power cutoff switch doesn't cut power. Drake really didn't want anyone running this amp with the main cover off.
Back in the day, many amp makers, usually on the better amps, floated the grids on 3-500z amps. Drake was one of them. They use a large uh value choke and .01 caps going from grid to ground. This floats the grid a little above ground with RF. It gives the amp very slightly better specs, but floating the ground does the opposite for reliability. Just like grounding a normal CB radio or amp, good grounds are good. Floating, indirect grounds are bad. Lots of info on that on the net. I directly grounded the 3 grid pins.
This amp also has a few tricks to keep you off 10 meters and another extra trick after that to keep you off 11 meters. First is the band switch has a mechanical stop or block that won't allow you to turn the band switch to the 10 meter position. The band switch has the 10 meter tap in it, but the stop keeps you from turning to it. On mine, the stops were already removed, but I see the 2 mounting holes for the stop right behind the Input circuit cover area and looking up into that area, you can see the big metal bracket arm of the band switch swinging around to that bottom point in the ten meter position. Remove the stop to enable the 10 meter position on the band switch.
Even with the block removed, the amp doesn't have any parts connected to the 10 meter input. All the other bands have a variable coil and a fixed cap for tuned input circuit for each band. Not so much for 10 meters. It has just a wire across it. No cap or coil. From the schematic, it shows that for export versions of this amp, which would have factory 10 meters in it, there is only a 68pf fixed mica cap going to ground for the input tuning. Also, since often the 15 meter band can be tuned for 11, they made sure that didn't happen either. On 15 meters there is only the adjustable slug or coil, no cap. They did that so that so trying to use the 15 meter input tuning would not be wideband enough to tune to 11 meters. In the Export version, the 15 meter input coil has the cap. It is another 68pf fixed mica cap. Hence, if you want to convert the amp to full capability, get 2 68pf mica caps and add one to the 10 meter position and another to 15, and of course remove the stop.
However, that's not all. Similar to the Heathkit SB-221, the Drake L7 has a so called 'Filter' that is connected inline to the input circuit. This 'Filter' is also not in the export version. This so-called filter is I guess really a filter all right. It filters out 11 meters only. It is kinda like a TVI filter that filters out the TV frequencies, but this filter only filters out 27mhz. It will pass 10 meters or 28mhz though. During production of this amp, 12 meters wasn't available yet. So, if you want full frequency capability of this amp, the filter needs to be bypassed to. One is to just run a straight coax from the relay to the input circuits completely bypassing the 'Filter'. However, on the 'Filter, there is a blank path right under the In and out coax connections that you can just move the coax connections to. So, the options are to leave the filter in and run a new long single jumper from the relay to the Input, or do the same but take the filter out, or leave the filter in and move the original coax connections to the straight trace just above the original connections.
Funny that the amp with the blown weird plate choke cap was the same amp that had the 'Filter' in it and the filter was still connected. I've told the story a few times, but I believe that that 'Filter' in a buds SB-221 is what gave him brain cancer and killed him. He was driving his SB-221 with the 'filter; in it with a Phantom 500 for a driver. 500 watts in, 1000 watts out all on his desk right in front of him for years. I believe there was all kinds of bad harmonics going on and those harmonics are what caused hi brain cancer. The Drake L7 is a very interesting amp. I like it, but maybe not as much as I like the L4B though. More to come.
I know a guy picked up a L7 from Drake factory directly, back in 1980 (I think). He's English, but was licensed and working in the middle east, so got the export version.
He's still got it and uses it, nearly 45 years on. 👍
Nice explanation Doc! It’s enjoyable to see these old amps and radios explained, some of which I never got my hands on.😊
Glad you enjoyed it!
Thanks for the video. I just aquired one of these at the shelby nc hamfest. It looks like new but the output is not what it should be. modifying for 10 meters is in its futre. Hope you make more videos on this amp.
73 DE KW4EMF
they also play games like making the plate choke resonant at 26-27.5 so it catches fire basically if you use it on 11m
I think Ameritron did exactly that until 12 meters came out. Then they found that that choke would blow on 12 meters too, so they redid the choke to not resonate on 11 or 12 meters.
Great information
Thanks
Wasn't floating grounds,
For mobile radios? & amplifier.
Can it be used on 11 meters? Thanks