Help from tube amp experts or someone whose converted one of these Bremi's would be appreciated with the following:
I have been following the instructions posted here to convert a BRL-200 for operation on 6m.
I have replaced the tubes with a new pair and converted the amp, it appears to operate okay although it takes more drive than listed in the site that I followed, it requires 20w drive to achieve 100w output vs. 12w as described on the website.
Also when first switched on the input is a high swr, it takes a few seconds with drive applied for the swr to drop to a steady acceptable state. I'm really not familiar with tube amps, I have left it with power on and no drive for a few minutes and it appeared to be okay so is this normal warm up that I should allow for before use or is there something else that needs to be checked?
I have another BRL-200 here that I was planning to do the 4m conversion on but want to make sure I have 6m correct first.
hi richard . if you have capability to transmit from about 45 to 55mhz then you can see if the amp is better at another freq at present, [ into dummy load ] and then work out whether you need to lengthen or shorten the plate coil at all .
at present you are right in allowing warmup time, usually it needs around 2-3mins to warm up before using that amp .
you may try altering youre present plate coil by squeezing it closer or widening the coil spacing , sometimes you might have to spread it a fair way wider or closer depending on exact dimensions to ended up at when making the coil .
not sure about the high input swr , i presume you still have the rf sensing working ? , if not look for a 33pf capacitor thats at end of the input track on the pcb below the relay , if its gone then the rf sensing wont work and it will also cause input matching problems due to capacitor shorting .
best to wire an rca jack into rear panel and hard switch the ptt .
the swr rise , if its only at start of tx , might be due to the relay contacts not changing over quite fast enough , or you might have corroded contacts in the relay that arent quite making contact when they touch at first . there isnt any HT applied until the relay goes over .
you will find that different lengths/types of coax leads between radio and amp will make a difference to drive and output achieved, so play around with some RG58 around 4 feet long to start and see what you get . hope this is of help ian
Thank you for the reply, I appreciate the advice and comments.
I can transmit to a dummy load from 45 up to 55, what am I really looking for higher output, minimum SWR after tuning plate and load etc?
It's working on RF sense at the moment but I plan to add an RCA socket. May be you can confirm I am looking to wire it into the correct location, there is an electrolitic cap across the coil, I was going to ground the negative side on PTT. Is that the correct place to tap in? I've tried manualy with a jumper wire and it activates the relay but there's also a bit of a spark when I touch the jumper to ground. Do you know what sort of current this will have as the radio's PTT control has a 100mA limit. It's a little difficult to get in there to measure it without taking the amp apart.
hi richard just take the wire for hard ptt off the relay coil , i think you have the location sorted for that anyway . make up an amp ptt buffer in a little box and use a BT47 type low current relay . bear in mind the BT47 is polarised so you need to have the wiring the right way round for the coil . there are other lower current relays you can buy also . key the BT47 coil from youre tcvr and use the contacts to key amp relay . you can get the relays from rapid elecs or maplin .
the standby switch will still work but you may find it activates just once upon activating the ptt and wont do it again after if in standby , no doubt the capacitor might need removing also .
if you have the approx correct plate coil installed then just spread or make the coils closer and youll see where its best for output etc .
on the drive input to relay you can add a trimmer capacitor [ anything that goes as low as about 5pf approx and it can go anywhere up to 75- 100pf max ] to tune out the relay contact missmatch and this will give you a little better input swr showing to the radio and allow the drive level to remain same as radio is set to when the amp is running or not .
the input coil doesnt make much difference if spread or closed .
ideally you want to have the plate coil organised so you have the tuning capacitors at around mid range give or take a little either way .
it will operate over a fair bandwidth once tuned up , so you wont need to readjust unless you go a fair way off .
they tend to be a bit more efficient on 6m than 4m , are only small amps but fun to mess with .
you may blow the meter lamp , i replaced mine with a 12v capless bulb and socket .
dont rely upon the output meter of amp , it will probably go full scale most of time .
tune up with about 2w or so and peak coils , then raise drive as you go and you should see about 100w output for 10-15w drive . play with coax input and output leads too , it does make a difference to actual results obtained .
Thanks Ian, I'll look through this later in the week, not in the mood tonight. Left the office at 5pm and got home at 10.45pm, normally a 35 minute journey. Missed the whole of the 4m UKAC contest as well.
I'll post again after I've had chance to try a few things.
