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Old 24th Dec 2019, 11:58 pm   #1
crackle
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Default Bremi BRL200 27-28Mhz Linear Amplifier

I have been trying to sort out a Bremi BRL200 amp which has been severely modified.
The only circuit I can find online is this one drawn by Rick Jackson.
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In the above schematic, is there an issue with the way the diodes are drawn?.

Thanks
Mike
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Old 25th Dec 2019, 8:07 am   #2
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

I don't know this unit, but I think your suspicions are right. Every diode I've quickly looked at has been drawn backwards (as shown, for example, the anode voltage on the valves would be -ve).
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Old 25th Dec 2019, 10:20 am   #3
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Hi Tony, that's what I thought.

There are some other changes because the unit I have originally would have had the 2 valve heaters wired in series.
I believe there may also an issue with the way the "tune" and "Plate" variable capacitors have been drawn.
In reality each cap is connected to chassis where as the schematic shows them in series with L3.
Here is the circuit with the diodes drawn correctly.
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I would be very interested to know if anyone has an original schematic diagram.
In the mean while I will redraw the diodes and post the corrected one on here.

Regarding the BRL200 I have on my work bench, I am looking at it for someone else. It has been modified for use with a single PL519 valve. A second transformer has been fitted to the back of the case to supply the 40v for the heater.
There are other modifications been made to the unit.
The unit was reported to be working following the modifications, but at some point it started to just keep blowing the fuse. So the second owner who bought it following the mods decided to cure the fuse blowing by wrapping it in 4 layers of aluminium foil.
The third owner reported that the transformer was smoking a lot.

The tests I have carried out have shown that both transformers appear to have survived the abuse with no signs of scorching. The main one is giving 367v and 15v on the secondaries.

One item which I suspect may have been the main cause of the fuse blowing was the electrolytics in the PSU. Am I correct in assuming the principle of the PSU is a voltage doubler.
The electrolytics are 100uF 450 volt, They tested on my ESR tester as having quite high ESR, up in the 8 to 15 ohms, which was high I thought for 100uF 450v capacitors.
After reforming them on my reformer at 375v the ESR came down to less than 2 ohms.
The suspect electrolytics was the only thing I have found so far which may have caused the fuse blowing problem.
But I do have to check out the valve to see if there are any shorts, I am going to have a problem testing it fully as I dont think my tester has this large socket for the PL519.

Another very obvious modification is to the "Plate" tuning capacitor. It has had about a 1/3rd of its vanes removed as if converting the amp to 50MHz.

I have traced out the modified circuit with the single PL519 and I would really appreciate some advice on the mods to confirm if they are viable.
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Thanks
Mike
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Old 25th Dec 2019, 10:36 am   #4
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

http://www.uksmg.org/content/bremibrl200to6.htm

http://www.70mhz.org/bremi.htm

References to 6m and 4m conversions. The first one includes the incorrect circuit

MX

Fred
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Old 25th Dec 2019, 11:14 am   #5
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Interesting. Can these be used for lower HF bands with mods? Just CW so not too worried about linearity.
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Old 25th Dec 2019, 4:22 pm   #6
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

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Interesting. Can these be used for lower HF bands with mods? Just CW so not too worried about linearity.
The 6JB6 is the PA in Drake TXs (T4X) so capable from 160 to 10m with the right PI tank and input coils

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Old 25th Dec 2019, 4:50 pm   #7
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

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Originally Posted by MrBungle View Post
Interesting. Can these be used for lower HF bands with mods? Just CW so not too worried about linearity.
Yes, "sweep tubes" are well established as HF amplifiers: a bit of Googling will reveal details for the "Skinnier Linear" and "FRInear" circuits.

if you're only doing CW you can bias the valves back to Class-C, and get rather more RF out than if you wanted them to wotk for AM/SSB duty.
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Old 25th Dec 2019, 5:12 pm   #8
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

There is no safety choke across the output shown on the circuit.
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Old 25th Dec 2019, 8:54 pm   #9
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

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Originally Posted by G8UWM-MildMartin View Post
There is no safety choke across the output shown on the circuit.
Hi
Please can you explain that more to me.

The other thing I find most confusing is all the grids are earthed and the signal is fed to the cathode.
How does that work?

thanks
Mike
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Old 25th Dec 2019, 9:19 pm   #10
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Thanks all regarding HF usage. I may snag one for conversion. Will stock up on appropriate air variables at upcoming rallies.

On choke there’s usually one from the antenna to ground to short DC anode voltage to ground if the matching / coupling cap goes short. Last thing you want is 300V on your antenna feed
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Old 25th Dec 2019, 10:32 pm   #11
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

I remember these amplifiers back in the early to mid 1980s. They actually sounded quite clean on air. For SSB use I think the amp is capable of about 100W PEP if a clean signal is desired.

However, I think a lot of people used them on CB with FM at full power and for long transmissions. The transformer did have a reputation of overheating and failing when used like this. I always kept away from them because of the safety issues when working on them.

The other thing I would add is that schematics for amplifiers like this should really have a voltage hazard warning on them. There are usually interlocks in the chassis to kill the HT if the covers are lifted and a good design will discharge the HT quickly if this happens. I'm not sure how reliable any HT interlock will be on a cheap CB amplifier like this and there really should be a safety choke at the output as well.

Any schematic drawn by Rick J at Euro is always untrustworthy. I'm not sure he has much in the way of technical knowledge because the errors in his circuits are often very obvious and there are lots of errors like this.
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Old 25th Dec 2019, 11:16 pm   #12
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Thanks for your comments guys and safety warnings.

A safety choke has been mentioned a number of times but no real information given. Where should this connect, what number of turns, diameter, length?

Can someone please confirm if the Rick J schematic (coloured schematic) is correct regarding the wiring around capacitors C14 and C15?

And please can someone explain how the valve amplifies with the signal fed into the cathode, and the grids 1,2, & 3 all grounded?
Thanks

Mike
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Old 25th Dec 2019, 11:35 pm   #13
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

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And please can someone explain how the valve amplifies with the signal fed into the cathode, and the grids 1,2, & 3 all grounded?
Thanks

Mike
It's called 'Grounded grid' operation if you want to look it up.

If the grids are grounded and RF is applied to the cathode, there is still rf variation of the grid to cathode voltage, and this, through the Gm of the valve gives RF variation of the valve's anode to cathode current. The anode voltage is a lot bigger than the grid/cathode voltage and you get a lot more swing on the anode than the swing applied to the cathode. So you get gain.

Driving the cathode is a substantially lower impedance than driving the grid of a regular grounded cathode stage because the cathode signal current is equal to the signal current on the anode. Unity current gain. The power gain comes from the difference in voltage swing.

The cathode input impedance is 1/Gm Gm is added for parallel valves, so it's not too far from 50 Ohms and is a nice load for whatever is doing the driving. The pi network on the output transforms the impedance down to suit the antenna.

A grounded cathode amp would give much more gain, but enough to get into trouble. Grounded grid amps are relatively tame and well behaved.

As a bonus, the grounded grids act as screens between the output (anode) and the input (cathode) so no neutralisation is needed to ensure stability.

For a transmitter linear amplifier, grounded grid is a very good choice.

For transistors, there is grounded base mode which has similarities

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Old 26th Dec 2019, 12:57 am   #14
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

That power supply appears to have more problems than just the diodes being backwards.

When the K1 relay contacts kick in, you are effectively putting D1 & D2 directly across the secondary winding of T1.

It looks like it is supposed to be a voltage doubler circuit with the ability to drop the HT for standby, but it ain't going to work.

If you want a clear PDF of that circuit http://www.cbtricks.com/Amp/bremi/brl200/index.htm
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Old 26th Dec 2019, 2:47 am   #15
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

These date back around 40 years and it must be something like 35 years since I looked in a BRL200 and as far as I know there were never any official circuit diagrams released for these or the BRL500 five valve version.

There's no safety interlock on the cases of any of these things and there's a lot more than 300 volts on the tops of those valves - there's well over 1KV on the BRL500 after voltage doubling and smoothing!

There wouldn't have been thought any necessity to include a choke for shorting DC to ground in the event of C13 going short, due to these being generally used to feed an end fed dipole with a tapped loading coil that would have provided the necessary 'short', so not such a good idea when running into a 'wire' antenna. Having a choke here would have probably caused a problem due to it only being set at a narrow frequency band and would have had to have some method of continuous tuning. As strange as it may seem, I don't think that the output coupling capacitors were particularly know for going short circuit, even in the stressed situation that you may think that they're in.

From what's been said regarding the modification of an additional transformer on the back and the fact that the vanes of the tuning capacitors have been cut to use it on higher frequencies, that unless you want it for a project to mess about with yourself, then I wouldn't waste my time on it. If you wanted to play and the PL509 is duff (easily tested with power supplies and AVOs, and a bunch of test leads with croc clips to clip on the pins with the grids all strapped together, which is how I test them), then I'd convert it to run on the much cheaper and more readily available EL509 equiv, but whether you could wangle the heater voltage at over two amps from the original transformer, I wouldn't like to say. The original 6JB6 valves are virtually unobtainable at a sensible price and the last time somebody asked me about them several years ago, Watford valves quoted £120 for two, so well over what one of those amplifiers are worth - and they don't last long if thrashed with FM carriers with long overs!

Terry - I think that circuit you linked to on C B Tricks is the same incorrect version as in the first circuit in this thread.

I've just found in my collection, a couple of circuit diagrams for the similar Zetagi amplifiers, both the twin valve with pre-amp and the single valve version which you may well find helpful to apply to what you've got in front of you and I've scanned and posted them below.

David gave a good description of the 'grounded grid' mode of operation and I well remember when I first encountered it many years ago, that I thought that I had to throw the theory of what I thought I already knew out of the window and start all over again - you'll find that all these linear amplifiers work in this way.
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File Type: pdf Zetagi BV130P and BV131.pdf (1,014.3 KB, 122 views)

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Old 26th Dec 2019, 2:56 am   #16
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

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Terry - I think that circuit you linked to on C B Tricks is the same incorrect version as in the first circuit in this thread.
Yes, that is the original faulty circuit, just much easier to read.

And the revised drawing is still faulty.

The drawing with the single PL519 is correct.
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Old 26th Dec 2019, 3:15 am   #17
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Yes, I think it's as Jeremy said in post #11, that the fellow who draws/re-draws these diagrams is very good at the actual drawing bit, but seems to have very little actual technical know-how. I've found mistakes in his drawings before now, with connections going nowhere, I seem to remember, which is a shame, as they're very clear actual drawings.
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Old 26th Dec 2019, 8:19 am   #18
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

Quote:
Originally Posted by Terry_VK5TM View Post
That power supply appears to have more problems than just the diodes being backwards.

When the K1 relay contacts kick in, you are effectively putting D1 & D2 directly across the secondary winding of T1.

It looks like it is supposed to be a voltage doubler circuit with the ability to drop the HT for standby, but it ain't going to work.

If you want a clear PDF of that circuit http://www.cbtricks.com/Amp/bremi/brl200/index.htm
Hi Terry
We had already established that the schematic drawn by Rick Jackson in your link (which is where I got the the schematic from to base the modified single valve version on) showed all the diodes with reversed polarity.
In reality the transformer is not as shown, it is 2 separate windings of about 350v and 15 volts they are not the same winding as shown and there are no other taps.

Please can you explain/show how the PSU should be drawn, and any other errors you can see, particularly around the output tuning capacitors.
It would be useful to all if we could establish the corrections and post the correct schematic here.

p,s.
so according to the Zetagi drawings supplied by Terry the plate capacitor and the tune capacitor should be connected to ground.

p.p.s. as far as I can tell the PSU of the Zetagi in post #15 and the Bremi in post #3 are in effect the same.

Thanks
Mike

Last edited by crackle; 26th Dec 2019 at 8:49 am.
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Old 26th Dec 2019, 6:09 pm   #19
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

I have found the cause of the initial problem with fuses blowing.
G2 and anode short. I dont know how this is possible to happen they seem quite well separated.
But after a few wacks the short disappeared and has not returned despite rotating the valve in a horizontal plain and tapping gently.
However the valve does not test well on my Sussex Valve Tester, but that may be down to the limits of the tester.



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Old 27th Dec 2019, 4:39 am   #20
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Default Re: Bremi BRL200 27-28Mhz Linear Amplifier

I've been to the pub, but hope my reply is not grossly offensive or inaccurate:

I remember Bremi CB power supplies from the early 1980s with unearthed metal cases.
This amplifier seems to have been designed around the same time to a similar standard...
I consider it to be at least negligent to have omitted the Safety Choke!

Despite my intense dislike of this design, I nevertheless wish you can make something half-decent out of it.

Please see Radio Amateur's Examination Manual by RSGB (author Benbow) pre-1980, Amateur Radio Techniques (RSGB, author JP Hawker), The Radio Amateur's Handbook (ARRL) or The Amateur Radio Handbook (RSGB) for details.
(Various websites have some of the older editions.)

A couple of published designs (grid-driven, but we're concerned with the anode/output circuit really) are:

https://f6crp.pagesperso-orange.fr/tech/PL159.htm
It is RFC 2 : "30 spires sur batônnet de ferrite, fil diamètre 0,8 mm (pas critique)"
30 turns on a ferrite rod, wire diameter 0.8mm (non-critical)

https://pa0fri.home.xs4all.nl/Lineai.../fri150eng.htm
It is RFC (47 microhenry)
"At the output end of the pi-circuit, a RF-choke between antenna-terminal and earth ensures a low-resistance DC-path, as a safety measure in the event of a short between the anode and the output-circuit. The (*Dutch* - Martin) Telecom authorities also require it. When the anode voltage is shorted to the output, without a DC-path to earth for blowing the HT fuse (the wire-wound resistor in the anode-circuit will curtail the flaming arc across the blown fuse)

(*The Bremi has neither, but neither did amateur transceivers, relying on the mains fuse to eventually blow after the safety choke had survived the PSU capacitors dumping their charge into it* - Martin.)

, the antenna cable and antenna will become death traps. An adequate choke is a short ferrite rod with 30–50 turns of 25 SWG enamelled wire. Check it with a GDO for absence of series-resonance in any amateur band."

A PL519 is probably at the limits of the Sussex's capabilities.
(It might be worth trying the 6146 Va & Vg2 settings in the AVO book instead)
They are still available at reasonable prices, you have a very suspect one, and you'd be doing Sussex builders a favour if you reported test results for a good one.

Regards,
Martin.
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