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Old 3rd Jun 2018, 12:10 pm   #1
Andrew B
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Default Junk sale linear amplifier

I picked this little toy up yesterday at an amateur junk sale, it appears to be a well constructed linear amplifier, it has 3 PL509's along with all the power supplies inside the case. I need to do a bit of "reverse engineering"to trace out the grid configurationon, it but judging by the quantity of transformers inside (6) 1 for each of the 40v heaters, a whopping great HT transformer 612-0-612 and a large unkown transformer for (screens?) it's probably set up in an AB class which should provide a large amount of gain, possibly built for a 50mW SSB exciter.
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Old 3rd Jun 2018, 2:37 pm   #2
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Default Re: Junk sale amplifier

I don't know much about amateur transmitting gear as I was only ever an SWL. However I do appreciate good construction. The builder certainly went to town on the construction.
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Old 3rd Jun 2018, 2:50 pm   #3
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Default Re: Junk sale amplifier

Just wish he had posted a circuit diagram inside
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Old 3rd Jun 2018, 4:07 pm   #4
Sean Williams
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Default Re: Junk sale amplifier

Could it be the PA0FRI Frinear?
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Old 3rd Jun 2018, 6:35 pm   #5
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Default Re: Junk sale amplifier

Not sure but I fired it up !!!
There's 450V HT on the anodes, which seems a bit low. There's a push button for testing the standing anode current and ~I got the following readings, TX readings are in ( )

Anode 480v (390v) sagging HT?
G2 272v (269v)
G1 -246v (-65v)
K 0v (0.475v) has 10 ohm resistor to ground, tied to G3

Main HT transformer 612-0-612 @ 450Ma (CT GND) is putting 1240v AC into bridge/smoothing caps.
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Old 3rd Jun 2018, 6:51 pm   #6
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Default Re: Junk sale linear amplifier

Could it currently be in some sort of tune-up mode with reduced/ballasted HT, something maybe to do with the relays etc. that are discernible? I see what appear to be transistors on the underneath tag-strips- they must feel a bit like the rabbits next to the lions' enclosure with all that HT around!

It certainly looks competently put together (as one would hope with something of that nature....)- must weigh a ton. Good luck with what I would find a rather intimidating beastie.
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Old 3rd Jun 2018, 7:29 pm   #7
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Default Re: Junk sale linear amplifier

There are 3 Sockets on the thing, there's a SO259, a Belling Lee and an RCA phono, which I suspect is for TX/RX switching. There's a panel meter on the front which is scaled 0 to 100 micro amps (will look for a shunt). There are 3 relays in it, 1 is a large Omron and 2 smaller GPO type change over. These are in the in/out circuit and change the antenna over between RX/TX and switch the bias, I can find nothing that "up's" the HT. Weight is 20+ Kg

Grounding the RCA does nothing and there's no voltage on it.

The transistors appear to be screen voltage regulation and seem to be holding up well, just a puzzle why the HT is low.
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Old 3rd Jun 2018, 8:56 pm   #8
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Default Re: Junk sale linear amplifier

I've wrapped it up for tonight, just need to calibrate the Ik standing cathode current meter, as I mentioned before each valve has a 10 ohm cathode resistor, measuring each cathode in turn I got 0.478v, 0.267v and 0.505v, look like valve 2 could be low in comparison but total current for the 3 is about 125mA, a bit high (the meter read 200 microamps, so a dabble with the shunt should sort that, along with a wind down of the standing current to 80 or 90 mA hopefully should be a ball park figure. Does that seem right for 3 PL509's ?
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Old 3rd Jun 2018, 9:06 pm   #9
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Default Re: Junk sale linear amplifier

Hi Andrew, I think we were at the same sale, we were talking outside about that amplifier, and also microwave oven transformers etc. The linear looks different with the covers off, but certainly looks well made. You were hoping it was going to contain a single specialized valve with a bespoke chimney and cooling system rather than TV line output valves. However, there's nothing wrong with 'sweep tube' in linear amplifiers for non-continuous use. Have you checked whether the valves are conducting hard dragging the HT down?

I think that amplifier was the bargain of the day - I certainly missed that one!
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Old 4th Jun 2018, 12:48 am   #10
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Default Re: Junk sale linear amplifier

aha, we certainly were Techman, I was with Fuzz and Jim (knew him from the 11m days).
Basically I turned the knobs (thought it had a rollercoaster), lifted it up and thought hernia, I just knew it was an amplifier., pot luck it had 3 X PL509's instead of a 2CX/3CX or an 813. HT is too low for an 813 mod but it's extremely well made. Has regulation on G2 screens, hard switched G1 bias and choke smoothed anode HT with 700 volts, it's a well made item. I'm looking for a low power HF transceiver to hook up to it, maybe test a CB (into a dummy load). It's hard switched, so need a TRX with a relay output or incorporate an RF switch, Should be good for 100+ W output, wonder if 509's will work on 6M ?. For £9 it was a good buy.
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Old 4th Jun 2018, 1:50 am   #11
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Default Re: Junk sale linear amplifier

Something about that amp is tickling a bell and Radcom magazine comes to mind, can't quite put my finger on it.

If you do a 'net search of PL509 rf amps, most are running in the region of 1100V HT and 3 valves should safely give you around 300W, even more with forced air cooling.

It looks (in one pic) that you have at least one cooked resistor in there (left hand side under valve socket).
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Old 4th Jun 2018, 2:05 am   #12
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Default Re: Junk sale linear amplifier

I don't think this is your amp from your description of some of the components, but may be helpful.

Radcom February 1990 page 30.

It uses PL519's but other articles mention PL509's and 519's as being interchangeable.

And there are mentions on the net of PL509's being used on 6m.
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Old 4th Jun 2018, 10:29 am   #13
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Default Re: Junk sale linear amplifier

The PL519 was a later version of the PL509 used as a colour TV line output valve. I have a box of them I saved from the last scrappers, they are complete with bases and top cap connectors.

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Old 4th Jun 2018, 10:39 am   #14
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Default Re: Junk sale linear amplifier

Quote:
Originally Posted by Andrew B View Post
For £9 it was a good buy.
At that price it was a steal, let alone a good buy.

Just the transformers..........

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Old 4th Jun 2018, 12:17 pm   #15
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Default Re: Junk sale linear amplifier

HT does sound low: check that you haven't got an O/C rectifier diode.

Also you say it's got various relays: one trick I've seen in amplifiers is to have the screen-supply from the centre-tap of the HT transformer and to switch the "HT" for the valves to the same point for tuneup (so screens and anodes are at the same voltage) so reducing output power and the risk of doing something nasty to the valves/PA network if the antenna-matching is way out of kilter. The "Tune" switch which does this is bypassed and full HT hits the anodes when operating normally.

You may find that you get full HT with the right relays closed!
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Old 4th Jun 2018, 1:15 pm   #16
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Default Re: Junk sale linear amplifier

Interesting bit of kit! Nicely constructed as well.
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Old 4th Jun 2018, 8:03 pm   #17
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Default Re: Junk sale linear amplifier

I'm going to post a bit of the HT diagram using "pencil and paper" MK1 scanned. the rectifier circuit IS full wave with an 8H choke in the centre tap to ground. so the "700 and a bit volts" HT is all that I have. The screens have a separate transformer and regulator, hence the transistors. The slightly "cooked" looking resistor is a 1K 3 Watt and it's in the common screen supply rail, with a max rating of 7W per screen then maybe a 10W WW would serve better?
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Old 6th Jul 2018, 7:36 pm   #18
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Default Re: Junk sale linear amplifier

The gettering looks a bit 'skinny' on the PL5x9 closest to the camera in the first photo. This may or may not have a bearing on the operation/performance.


HTH


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