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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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19th Oct 2013, 4:17 pm | #1 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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813 anode choke
I am about to start work on a partially-stripped Labgear LG300. The VFO and wideband multiplier were removed years ago and so I intend to retain the 813 PA but drive it with a GW4GTE Multirock on all bands 80-10 including WARC and 5 MHz.
The (valve) buffer-driver is straightforward enough to design but the PA choke is another matter. Most of the advice from a web search seems to be "... Wind 4 inches of 26g onto a 3/4-inch former and then 'suck it and see' ..." However, this approach can involve fuses blowing and fire breaking out. So . . . does anyone out there have a sure-fire tried-and-tested design for a suitable 813 anode choke (around 90 uH) that does not have series resonances (and attendant pyrotechnics) near these bands? - Peter (G3PIJ) |
19th Oct 2013, 5:08 pm | #2 |
Dekatron
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Re: 813 anode choke
Do you have any way to "suck it and see" at low power? Only time I tried this sort of thing was with a HP Vector Impedance Meter......
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19th Oct 2013, 6:07 pm | #3 |
Hexode
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Re: 813 anode choke
Yes - I can run the PA at reduced volts via a variac on the EHT supply but then the choke will still have to be removed, modified and refitted and retested - possibly many times. I just wondered if anyone had been down this path before and could save me some time (and possible frustration). - Peter
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19th Oct 2013, 8:37 pm | #4 |
Nonode
Join Date: Mar 2008
Location: Virginia Water, Surrey, UK.
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Re: 813 anode choke
Are we talking about the small anode anti-parasitic choke or the main anode choke?
The small choke is about 100 ohms 2W with 6 wound turns over it. The main choke is normally the multi-stage wave-wound type like this one on Ebay. http://www.ebay.co.uk/itm/Trio-Kenwo...item3a7f70059d I don't think it's realistic to hand-wind the main choke. I hope this helps. Jeremy
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19th Oct 2013, 11:04 pm | #5 | |
Hexode
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Re: 813 anode choke
Quote:
- Peter Last edited by AC/HL; 7th Nov 2013 at 12:04 am. Reason: Image uploaded as an attachment |
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19th Oct 2013, 11:18 pm | #6 |
Nonode
Join Date: Mar 2008
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Re: 813 anode choke
OK - that's the multi-stage chappie then. A friend built a 2x813 amp and bought an anode choke on Ebay from, I think, an FT101 for about £10-£12. It worked fine.
regards Jeremy
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20th Oct 2013, 10:05 am | #7 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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Re: 813 anode choke
Most of these chokes will not be OK on WARC frequencies, especially 24.9 MHz. My wish is to build the choke - the information I lack is turns/diameter information.
Peter G3RZP has already suggested elsewhere the following (which may be of use to others on this forum): "... To get a 'feel' if things will work, a good test is to put a resistor equal to the anode load impedance from anode to earth - with the valve in place, and obviously, no volts on it! Then feed in at the antenna socket from a signal generator and using a valve voltmeter or 'scope or spectrum analyser with FET probe, measure than anode volts on each band when tuned for maximum volts. ..." I think I shall wind the usual 4-inch solenoid on a 3/4-inch former and take it from there using Peter's approach, which has the great advantage of not running the risk of pyrotechnics (as the usual 'hot' method does). Many thanks for all suggestions - Peter |
21st Oct 2013, 8:37 am | #8 |
Moderator
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Re: 813 anode choke
The technique was to build the choke as a number of dissimilar sections with a view to not having the whole choke resonant at any frequency in any of the bands.
When things really go wrong, resonance can create multiplied voltages at placs along the sections and produce Tesla coil results for a while. The microwave gang discovered the benefits of winding conical coils for use over wide bands without gaps, which is really carrying the idea of a string of progressively scaled segments to the limit. A sweeper and an oscilloscope would be a great help, or do you know anyone with a network analyser? David
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21st Oct 2013, 12:31 pm | #9 | |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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Re: 813 anode choke
Quote:
Many thanks for interesting points to follow up - Peter G3PIJ |
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21st Oct 2013, 12:56 pm | #10 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: 813 anode choke
When I designed a high-power grounded-grid amp to cover 2 to 40MHz continuously, I resorted to using a two-stage choke wound on a ceramic former, with a vacuum-sealed relay to switch in extra inductance below 22MHz.
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21st Oct 2013, 1:53 pm | #11 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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Re: 813 anode choke
Brilliant!! I have a small vacuum relay which will be just the job - exactly as http://www.rfparts.com/vc2t-26-5.html around 1.5 inches tall. With the relay in close proximity to RF fields, do you feed the energising coil through RFCs (pi-wound 1 mH types)? - Peter
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21st Oct 2013, 2:08 pm | #12 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: 813 anode choke
With my vacuum-relay choke-changer I didn't use any RFCs in the relay coil feed: one terminal of the coil was earthed directly, and the other was fed through a 1000pF feedthrough from a small diode-matrix whose input was the appropriate lines from a Ledex (remember them?) which drove the tank-coil tap-changer switch, so that when you changed to a frequency in the lower-frequency band the relay was energised and switched to open so removing the short on the "low frequency" choke winding.
I did it this way for 2 reasons: firstly the amp spent most of its time on the HF end of its range so the relay remained un-energised, and secondly when on the "LF" range there was then only the capacitance of the short piece of braid from the junction of the 2 chokes to the relay-terminal to worry about. I had the "HF" choke mounted horizontally between two ceramic standoffs, with the "LF" choke vertical on a ceramic feed-through and the vacuum-relay on a PTFE slab mounted on a further 2 ceramic standoffs. 3.5Kv on the anode makes you nervous about insulation. There was also a rather complicated interlock which removed the HT from the amplifier before allowing the Ledex to start turning, and only reapplied HT once it had stopped; I dread to think of the arcs that would have happened if it tried tap-changing with full drive. |
21st Oct 2013, 2:36 pm | #13 | |
Hexode
Join Date: Nov 2011
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Re: 813 anode choke
Quote:
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22nd Oct 2013, 9:11 pm | #14 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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Re: 813 anode choke
Final word - http://www.w8ji.com/rf_plate_choke.htm is about as good as it gets. Many thanks for interesting discussion. - Peter
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6th Nov 2013, 9:23 pm | #15 |
Triode
Join Date: Mar 2008
Location: Huddersfield, West Yorkshire, UK.
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Re: 813 anode choke
Some pix here of my 80m only 813 grounded grid amp.(with normal g1 and g2 voltages)
Important to make sure the grid +screen bypassed to ground properly up to 100Mc/s. Use shortest possible copper strip anode -blocking cap - tuning cap .Make so that path from tuning cap to bottom end of valve is minimum. Pix below have resonance about 120 Mc/s,above possible parasitic capabilities of 813. Observed by sweeper into return loss bridge with small wire loop into test port. The only safe way to have an all band rfc is to short out half of it on the hf bands. The fatal resonances are too variable due stray capacity. In my experience pie sections always burn up, much better is a single layer , spaced with monofilament fishing line.This amp , being qrp is not so spaced. Input pi network last Last edited by AC/HL; 7th Nov 2013 at 12:01 am. Reason: Images uploaded as attachments |
8th Nov 2013, 1:39 am | #16 | |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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Re: 813 anode choke
Quote:
The attached pic shows the LG300 RFC wound in segments - it is possible that the designers managed to avoid the usual resonance at 25 Mc/s without intending to. If I do hit problems later on, then I shall use a vacuum relay to switch a shunt into the existing RFC, or to insert a smaller winding. At the moment, I am constructing a MultiRock II to drive the whole thing (see S9plus.com) - via EF91 / 5763 / 5763. - Peter Last edited by G3PIJpeter; 8th Nov 2013 at 1:42 am. Reason: typos |
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11th Jun 2017, 1:04 pm | #17 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
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LG300 813 anode choke
I have now re-modified my part-cannibalized LG300 to cover Top Band and 60 metres only. The full saga of the DDS - EF91-5763 driver is in a back number of 'Signal'. The circuit in the 1955 ARRL handbook for a 2-30 MHz 813 PA transmitter gives the anode choke as a National R-175A which has an inductance of 175 microhenries.
Does anyone know the inductance of the anode choke (E.5032 from memory) fitted in the standard LG300? I am thinking that its impedance at 1.8 MHz will be insufficient but would be glad to know if it is, which will save me having to wind a new choke. In any case, I shall have to construct a new pi network inductor, with extra 'doorknob' capacitors across the tune and load variables. - Peter G3PIJ Last edited by Station X; 11th Jun 2017 at 2:14 pm. Reason: Threads merged. |
11th Jun 2017, 1:26 pm | #18 |
Octode
Join Date: Oct 2008
Location: Littlehampton, West Sussex, UK.
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Re: LG300 813 anode choke
Peter you have a PM
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11th Jun 2017, 9:50 pm | #19 |
Octode
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Location: Bletchley, Buckinghamshire, UK.
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Re: LG300 813 anode choke
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12th Jun 2017, 10:57 am | #20 |
Hexode
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
Posts: 323
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Re: LG300 813 anode choke
I knew someone would have the figure to hand. Many thanks Dave - I think that your measurement confirms that the existing choke should be OK for Top Band. All I have to do now is fabricate a rather larger pi-tank coil and switch in some extra fixed capacitors across the existing variables. I shall post some photos when completed.
73 - Peter |