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Conversion of the BREMI BRL-200 amplifier to 4m

by Ian M0BCG

Introduction

The Bremi BRL-200 first appeared in the 1980's and was a 27/28MHz CB/10m amplifier. Bremi is an Italian company, and so large numbers of these amps would have been produced and sold across Europe. Note that here is also a Bremi BRL-210 amp and this is not the same as the BRL-200, so make sure that you've got the right unit: they look different anyway, the BRL-210 is all-black case, while the BRL-200 has sandy yellow outer panels. Switches on the front panel of the BRL-200 include pwr on/off, ssb/fm delay timing, standby/operate, plate & load tuning dials.

It uses a pair of 6JB6A pentodes (intended for 30MHz max frequency), and the original output power was something like 80-150W with drive from a 4-12 watt multimode CB/10m rig (although >15W drive would produce somewhere near to 200W). English versions come with a 240V AC mains input on transformer primary, but I guess that 120V versions were also made. In any event, the transformer secondary voltage is approx 300V AC.

These amps still appear at radio rallies and also on e-Bay on-line auctions. Prices vary, but expect to pay between 10 and 40 for a suitable unit. e-Bay is a good source, but sometimes enthusiastic bidders tend to push the price up to silly levels.

Often the valves in a used BRL-200 amp are well-used or "soft", and so the output may be lower than I have quoted in the conversion details. The "soft" valves can usually be noticed by the output power slowly creeping higher as the amp is either tuned or used on air, but with new valves and after a mandatory 3-4 minute warm up period they will pretty much give full output from the first tune-up attempt. Replacement valves are not cheap, (29.50 each new from Wilson Valves, although NOS types can be found on e-Bay in the USA at reasonable prices (it can pay to shop about though).

Conversion details

Before diving in, ensure that the mains input is disconnected, and all voltages have discharged.
The first step is to change the plate coil to 2.5 turns.
Then modify the plate tune capacitor so that it has 4 plates on the rotor and 3 plates on its stator.
Now change the input coil to 1 turn and remove the two original 82pF input tune capacitors and replace the vertically fitted one with a 10pF capcitor. Both the 82pF capacitors can be discarded.
Now shunt a 15pF compression trimmer capacitor to ground from the RF drive input to the relay (one end of the cap is attached to the relay input and the other to the ground track on top of pcb). Now with a 50-ohm load on output, adjust this trimmer cap to show minimum input VSWR (use an MFJ259 analyser or similar connected to the input socket). This mod will improve the match at 70MHz, allowing full drive into the amp, and also will present the tcvr with a good match in the through position.
The original design used rf sensing for transmit/receive changeover. This does still work on 6m & 4m converted BRL200s, but requires around 4-6 watts drive in order to operate the rf switching. Alternatively, to hard-wire the PTT, fit a phono socket in the rear of amp case, and wire it to the relay coil tag on the back of the pcb.
I did fit parasitic chokes on the anodes of valves, for peace of mind, but its probably not necessary to do this on a single-band amp. The chokes were 47-ohm 2 watt carbon compound resistors with one turn of 16swg silvered/copper wire wound around each.
When the conversion has been completed, adjust the variable capacitor at the input tune section so that you get highest output power.
There is no fan-assisted cooling in these amps, and they do get hot fairly quickly in use, especially at high output levels, so I added an 80mm Papst fan directly above valves, to draw hot air out of the case. This is powered from the meter lamp circuit and is quiet in operation.

Results

Once aligned, 6-10W drive achieves 80-120W output, or 12-15W drive can easily reach the full 150W.

For some additional filtering, I used a quarter-wave coaxial (shorted) stub fitted in-line at the output of amp, using an SO239 T-connector. Emissions seemed clean enough with my converted BRL-200. I used two (not certificated) spectrum analyser/monitors, and had difficulty seeing any harmonics above 350MHz, with lower-frequency spurs better than -46dB down.

My driver is a Spectrum Communications TRC4-IL transverter (144MHz input model) with adjustable output of around 6-18W for 2W input. If I use the full drive from transverter into the amp, I can obtain 160W output for short duration use, which is the UK legal power limit (although I feel that the valves won't last long at this level!). Using 8-10W input, the amp will run nicely at 100W output.

I have found no problems whatsoever with instability at 70MHz, even though these valves are rated at 30MHz tops, they seem to run nicely at 50 and 70MHz in this amplifier.



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