KLM 10-140B 2m Amplifier


A new toy arrived. At the Amateur Radio Fiesta this year in San Antonio I was able to acquire a 2m amp. I knew nothing about this model, but the price was right: $35. At home I immediately had to Google it but found pretty much nothing. Apparently it was made sometime in the 70s by a company called KLM Electronics. KLM had been acquired by the well known amp vendor Mirage (which is now owned by MFJ). However, apparently KLM Electronics had in these days a very popular line of linear 2m amplifiers and I found an old manual on the Web. This got me currious and I wanted to know what I had bought and how it compared to the later line of linear amps. Most popular was a 10-160BL model, but also a 10-140BL exists. Notice the "L" behind the name. My model had the number 10-140B without the "L". The amp is supposed to accept 5 to 15 Watt drive with 140 Watt out, as stated by KLM. My assumption is that output is somewhere between 70W and 200W depending on drive and frequency.

From the front the 10-140B looks just like its "L" cousin where the "L" supposedly stands for "linear". In the front there is a PTT switch which has two functions: a) It switches the amp from bypass (Out) to in-line (In) when in operation. And b) in combination with the PTT control wire, when the switch is left in "Out" position, the TX relay can be triggered by the transceiver. At the back, there are two SO-239 sockets to connect the transceiver and the antenna and a big black connector. The connector, in case you are wondering, is a 4-pole Chinch Jones connector. You can find them on eBay for $5-10. You need a 4-pole female plug to fit the male socket whereas two poles are 90 degrees turned. I think the later models did not have a power connector, just the power cable coming out.

  1. 13.5V for driver stage
  2. 13.5V for final stage
  3. PTT to transceiver
  4. GND

Essentially, pin 1 and 2 are connected both to positive voltage and 4 to ground. Not sure what the reason is that there are individual pins for the driver stage and the final stage. Perhaps, this is a mechanism to control drive. The voltage should be somewhere between 13 and 14V for the amp to get the best performance. I measured 21A although the label said 18A. I tested with a 25A fuse which appeared sufficient, but may have to be increased to 30A. In RX mode, the amp appears to draw very little power. You want to be careful. Above, is how mine was wired. I cannot say whether somebody moded it. There is no protection diode. The GND pin 4 should measure low resistance to the shield of the SO-239 connectors. Pin 1 and Pin 3 are connected to each other with a high gauge wire via the PTT switch when it is in "In" position. Do not provide power via the PTT Pin 3 for any other means than for PTT function as you will fry the PTT switch and the fine wire in-between.

The PTT connector is interesting: The function of the PTT front switch is to provide positive power to the relay. The relay is then pulled to ground on its second connector by a Darlington when TX is sensed. When the PTT switch is in "Out" position, the first relay connector is connected to nothing. This is where pin 3 comes in. By providing something around 12V to that pin, the relay can again be triggered by the Darlington if TX is sensed. The front LED is glowing when the relay is triggered. It is interesting as the PTT connector is not directly switching between RX and TX but enables TX, hence in-line mode, when a TX signal is detected on its XCVR SO-239 connector. In the later "L" models, KLM recommends to solder a wire to the base of the relay driver Q4 and trigger the relay directly via a 10k resistor. As mentioned, the connector is a 4-pole male Chinch Jones connector.


The amp can be opened by removing the four screws on the front and pulling out the bottom aluminium plate. The three transistors use the body as heatsink and are fastened to it by screws which need to be removed before you can pull out the PCB. Noticable are the three large transistors from Motorola. There appears to be a class-AB pre-amp to regulate drive with a 2N6084 power VHF transistor and a class-AB final with two MRF245 in parallel configuration. The huge traces on the PCB is micro strip, which is here used instead of discrete inductors for the various L/C filters. Micro strip design is much easier with VHF frequencies where only low values of inductance are required in filters. To note is that all three transistors are of the bipolar type and not FETs. The rest of the amp appears to be matching networks, bias circuitry, and a little control circuit for carrier sense to switch the TX relay. Below Q1, there is also space on the PCB for another transistor. My assumption here is that this was designed to be a third stage to perhaps lower drive to 2W or below. This appears to be a good opportunity to build an amp for a Software Defined Radio (SDR). The rated input is 5-15W. From my tests, it appears to work just fine with my 5W HT.


Parts list amplifier

R1, 8, 9 1 Watt 1 Watt
C42 DM15 DM15
C33 Unelco Unelco
C2 Unelco Unelco
C3, 4, 16, 17, 22, 20,
24, C13, 14, 15, 21, 22 Unelco Unelco
C5 Unelco Unelco
27, 28 Unelco Unelco
C8, 9, 29, 41 Feed Thru Feed Thru
C1, 7, 32 DM15 DM15
C18, 25 disc disc
C10, 19, 26, 30 35V elco 35V elco
RFC3, 4, 5 Delevan Delevan
RFC1, 2, 6 ¼ Dia ¼ Dia
D6 2N6084 Motorola 2N6084 Motorola
Q2, 3 Motorola Motorola


Parts list Rx/Tx switch

R10 ½ Watt ½ Watt
R12 ¼ Watt ¼ Watt
R11 ¼ Watt ¼ Watt
C37 disc disc
C38, 39 Unelco Unelco
C34, 35 disc disc
C36 25V elco 25V elco
D4, D5 1N4148 or 1N914  
Q4 MPSA13  
RY1 DPDT 12V 5A Teledyne T-DG-12C
Connector 4 pole male Chinch Jones


I saw on one forum that somebody mentioned that the "L" stands for linear and that he had traded back in the days his amp for an L-version to gain linearity for SSB operation. I was very curious to see how the two versions really differ: not much. There are only changes in the pre-amp driver section and no changes in the final.

KLM 10-140B

There were a few mods on mine which I marked. The only difference to the 10-140BL are the additional resistors and the trimmers which add up to 3 Ohms to the 15 Ohm resistor R1 and the same to RFC3. R1 pulls the basis of Q1 down to ground, and RFC3 the collector. I am not sure if those changes really make much of a difference in regards to linearity. More tests would be needed here. But I was so more surprised to find out that the two 10-140BL and the 10-160BL are identical which makes the 10-140B almost identical to the popular 10-160BL. I am not sure why KLM claims 20W more than for the 10-140B. The two transisors are rated 80W by Motorola and I found some reports of people getting 90W out of them each. The final power depends on drive. And I assume, at least from an initial design perspective, that KLM was planning to control drive via the voltage applied to pin 1 at the power connector. The manual for the linerar series mentiones that no part needs tuning so that I can only assume that R3's and R6's purpose is to get a precise value out of R2 and R4 and then build some sort of voltage divider and perhaps a neutralization feedback to the input via R4/RFC7 and R7/RFC7. R4 and R7 are 50 Ohms 3 Watt types. It should be easy to convert a 10-140B to a 10-160BL if one desires so. Below is the circuit diagram of the 10-140BL/10-160BL model with differences to the 10-140B highlighted. You can find the complete parts list in the PDF below.

KLM 10-140BL/10-160BL

I do not have a wattmeter for 2m so that I cannot say what the actual output power is, but I can say that with 5W drive I have no issues at all to trigger the D-Star repeater 20 miles away. I was even able to go simplex while testing with my peer going via the repeater with FM. On Both, FM and D-Star, quality appears excellent. I did not test with SSB, though. It definitely was an excellent purchase for $35, even if I had to spend another $10 for the Jones connector and had to remove a fair amount of crud from the case.


  • Circuit Diagrams (PDF) (VSD)
  • KLM L-Series Manual (PDF)