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 Post subject: TRF Tuner Project
PostPosted: Aug Wed 03, 2016 7:28 pm 
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I'm thinking of building this TRF tuner just for the fun of it. I believe I have the 3-gang variable cap - but wanted to ask if anyone can suggest a suitable donor source for the "High-Q RF coils" (and the Antenna coil while w're at it).

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TRF Radio.pdf [167.18 KiB]
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Thanks


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 03, 2016 11:16 pm 
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Hi everyone. AES in phoenix has the coils you need order the pc70 rf and pc70 a. the a is the antenna coil use 2 of the rf coils for the others. https://www.tubesandmore.com/


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Thu 04, 2016 2:52 am 
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Perfect - thank you!


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Fri 05, 2016 1:36 pm 
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Quick question on variable caps and trimmers. The air variable cap I have does not have trimmers. The 'plans' call for tweaking each section as part of initial set up. Can I simply tack on a mica trimmer for each section? If so, recommendations on value? I assume something as low a value as possible - like a 2.7-40pf trimmer is adequate? The plans call for the variable cap to be 365 mmf for each of the three sections.

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vari-cap.JPG
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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Fri 05, 2016 6:06 pm 
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Hi all. Yes those trimmers have plenty of range and maybe a little more. you track them at the highest frequency end of the band and the coils I mentioned are adjustable with slugs and are used to track at the lowest frequency. Adjust the low end first as changing the slugs will change the high end markedly. That capacitor looks like it will track nicely. Also pay attention to your layout as getting the coils too close to each other can make it unstable. You can mount them at right angles to each other and that will help out.
also if you do not have an alignment tool set get one from aes as you do not use metalic tools to adjust the slugs.


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Fri 05, 2016 7:48 pm 
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Perfect - thanks.

The idea behind the trimmers is to adjust/keep capacitance of each gang as close as possible, right? IE, if I find that each gang is within a pf or two, I may not need the trimmers? Or are the trimmers for final tweaking regardless of measurement of each gang?


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Fri 05, 2016 10:14 pm 
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mbear2k wrote:
Perfect - thanks.

The idea behind the trimmers is to adjust/keep capacitance of each gang as close as possible, right? IE, if I find that each gang is within a pf or two, I may not need the trimmers? Or are the trimmers for final tweaking regardless of measurement of each gang?

They also compensate for differences in the wiring, tubes, and coils. So you still need them.

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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Sun 07, 2016 5:36 pm 
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I'm now working through my power supply now while awaiting parts to arrive. The transformer I have is 136v no load. My goal is 90v B+. Using PS designer, I came up with this configuration. The components are actual measured values with my target of 90v B+ at 33.8mA.

Questions:
What's the best way to calculate B+ load for this circuit (see full circuit below or in PDF at top of thread)? I used the 'typical operation' values from the tube data sheets and arrive at a combined/total of 33.8mA for the two 6BA6 and one 6AV6. Changing PS Designer load to resistive, I arrived at a test load of 2.47k to arrive at my goal of 90v.

However - setting up on the bench, using this configuration I am at 112v B+ and a current draw of 45mA. Is a resistive load not a good (close enough) representation of the actual circuit? I believe if I have a better/validated estimate of the current requirement, I can adjust the circuit accurately.

Original PS section:
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trf-ps.jpg
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Proposed PS:
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trf.JPG
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Full schematic:
Attachment:
TRF-Schem.jpg
TRF-Schem.jpg [ 109.74 KiB | Viewed 2452 times ]


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Sun 07, 2016 8:54 pm 
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Throwing out PS designer, I made R1 150 ohms and R2 1k and get exactly 90v @36mA (at 121.5 vac in). So providing my current draw estimate is close, I think I may be good.


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Mon 08, 2016 12:03 am 
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Where did you get the 33.8mA? I don't see it on the schematic or in the text of the article (but I may have missed it). Be aware that the current drain isn't constant and will vary considerably (almost a 2 to 1 range) with the adjustment of the "bias" control.

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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Mon 08, 2016 12:13 am 
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Understand. I'm looking to get somewhat close as I assemble parts.

I got the 33.8mA as a combined estimate using each tubes 'typical operation' values from tube data sheets. Roughly 15mA for each 6BA6 and around 3mA for the 6AV6. I'm open to a better estimate here!


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Mon 08, 2016 12:34 am 
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Quote:
around 3mA for the 6AV6

That is impossible. Even if the 6AV6 had a direct plate to cathode short, the 100K resistor would limit the current to 900uA.

The 6BA6 data sheet I am looking at, http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BA6.pdf, says that at 100V the plate plus screen current is 15.2mA when using a 68 ohm cathode resistor. Your schematic shows 330 ohm resistors so the current through these tubes will be a lot less as well.

Time to recalculate.

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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Mon 08, 2016 1:20 am 
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That's what I was missing! My new 'calculations' now have me at 9mA total.

- Plate of the 6AV6 with a 100K resistor to B+
- Cathode of each 6BA6 with a 330 ohm resistor to B+
- B+ is 90v


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Tue 09, 2016 4:55 pm 
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My Antenna and RF coils arrived yesterday. I had planned to find aluminum cans to mount these inside of and opened a WTB in the for sale forum. A comment was made there that these coils are designed for chassis mount installations and mounting them in cans may be impacting.

Attachment:
coil.JPG
coil.JPG [ 29.61 KiB | Viewed 2386 times ]


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Tue 09, 2016 6:42 pm 
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mbear2k wrote:
My Antenna and RF coils arrived yesterday. I had planned to find aluminum cans to mount these inside of and opened a WTB in the for sale forum. A comment was made there that these coils are designed for chassis mount installations and mounting them in cans may be impacting.
Your PDF shows those coils are to be mounted inside J. W. Miller #S-32 shielded cans.

Attachment:
diagram.JPG
diagram.JPG [ 83.18 KiB | Viewed 2380 times ]


.


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 10, 2016 2:25 am 
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They need to be shielded from each other, or else they will feed back and you'll get uncontrollable RF oscillations.


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 10, 2016 2:49 pm 
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Thanks guys.

Next question:
The parts list is specific with the coil part numbers.
Attachment:
parts.JPG
parts.JPG [ 17.76 KiB | Viewed 2356 times ]


The data sheet for these coils:
Attachment:
coils.JPG
coils.JPG [ 106.01 KiB | Viewed 2356 times ]


As drawn in the schematic:
Attachment:
coil-caps.JPG
coil-caps.JPG [ 39.1 KiB | Viewed 2356 times ]


Per the schematic, the coils show caps integrated in each coil. Since no value is indicated for the caps, I assume the intent was that they are part of the coil - or the draftsman used a coil template here when drawn. Opinions on whether a cap is needed? I know most of the IF cans I've done silver mica repairs on used about a 100pf mica cap as replacement.

Thoughts?


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 10, 2016 3:35 pm 
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Try it first without the caps. You will probably find that you don't need them. If you want to try caps, use something in the 3 to 10 pf range, 100pf would be way to big!

Those are coupling capacitors, not resonating capacitors like the ones used in IF transformers. Smaller is better for these coupling caps.

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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 10, 2016 5:11 pm 
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Thanks - very helpful info.

Another question:

At the top of the thread, on my variable cap I am adding trimmers as the one received from AES does not have the mica compression trimmers installed. This cap looks to have an option to have had these compression trimmers installed on either side (or both?).

As I am adding external trimmers, is there a preference which side of the variable cap these are connected? Are there any issues with mounting and wiring my trimmers maybe 1/2" to 3/4" away from the variable cap?


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 Post subject: Re: TRF Tuner Project
PostPosted: Aug Wed 10, 2016 6:34 pm 
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With two high-gain RF amps like you have I would keep the trimmers away from each other to avoid feedback. You may want to put some shielding between them. It is best to locate the trimmers close to the tuning cap and coil.

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