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 Post subject: Tube IF transformers
PostPosted: May Mon 10, 2021 1:10 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Some might know I like to home brew AM sets.
I have a couple of scrap chassis's, and I've tried to put together a superhet receiver with tube type If transformers using transistors and a Ne602 as the mixer.
Tube IF's are different to tranny ones as they are double tuned, and it seems the impedance is higher.
Whats a good way to measure up IF transformers, I recall hearing somewhere you measure it with a sig gen and find the 6db points either side of the peak, the impedance here is the operating impedance of the transformer.
Which I'm guessing is the desired Q / 1/2 the anode sorry plate resistor.
I guessed at 22k for a lashup I made using just one IF stage and it works, both sides of the coil peak but Q seems low, so the resistance is probably too low.
I have a nano vna, I keep meaning to put together some baluns to measure higher impedances.


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 Post subject: Re: Tube IF transformers
PostPosted: May Tue 11, 2021 4:08 am 
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Joined: Feb Sun 05, 2012 5:12 pm
Posts: 1128
Location: North Syracuse NY 13212
The December 1965 issue of Radio-Electronics has an article titled "Transistorize Your Tube Portable".
It describes how to approximate the input and output impedance of a tube by using emitter coupling.
I haven't tried it yet, but it may be what you need.
Here's the schematic:
Attachment:
TR.jpg
TR.jpg [ 172.77 KiB | Viewed 942 times ]


Bob

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 Post subject: Re: Tube IF transformers
PostPosted: May Tue 11, 2021 8:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2691
Location: Saskatoon
In order to maintain maximum Q of an IF stage, the tube plate is loosely coupled to the IF transformer primary, and the IF secondary is loosely coupled to the grid of the following stage. Loose coupling is the opposite of matched impedances. If you try to match the impedances, you kill the Q. You'll typically see plate resistances for the tubes used in this application in the megohm range for this reason.


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 Post subject: Re: Tube IF transformers
PostPosted: May Tue 11, 2021 1:12 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Thanks thats what I'm looking for.
I had been playing with fets, 2 tranny's would also give high impedances, its understanding how the IF's are designed into the circuitry that I've been looking for, the article goes into that.
I'll read that at work tonite as long as they dont pester me too much.

Yes even in tranny sets impedances are high to maintain high Q.
One thing I've found is tube IF's have very lossy coupling, to avoid the dip in the centre of the passband you get double tuning because of the phase effect.


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 Post subject: Re: Tube IF transformers
PostPosted: May Tue 11, 2021 2:52 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Yep I see where they are going now.
Tube IF's are high z in and out, tranny ones are medium z in lo z out.
My fet design would probably work by using a source load resistor and coupling the primary to it through a small cap, instead of using the primary as the source load as per the tube design.
I'll try that first and if theres till issues I put together the emitter coupled circuit as per the article.
I guess they didnt have J310's & J113's back in the mid 60's.
Thanks again.


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 Post subject: Re: Tube IF transformers
PostPosted: May Tue 11, 2021 11:09 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1014
Location: Montreal, Quebec
JFETs were cutting edge around 1965. They were certainly in ham radio converters, and not ultra expensive. But it takes time to trickle down.

There was an article in the late sixties where someone modified a surplus tube radio, something like the BC-1206. But he did it two ways, with bipolar and then with JFETs. I can't remember the date or author.


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 Post subject: Re: Tube IF transformers
PostPosted: May Wed 12, 2021 12:59 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Yes I have a telequipment 'scope thats from the early 70's that has the black blob ceramic base tranny mosfets, I guess they'd be willing to spend more for a 'scope though.

Heres a simulation of the emmitter follower / grounded base amp using 2 'modern' trannies the the bc547b's.
The gain is similar around 40, capacitance is lower and bandwidth wider, I'm going to try it when I get chance.

Attachment:
radpic.png
radpic.png [ 338.13 KiB | Viewed 840 times ]


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 Post subject: Re: Tube IF transformers
PostPosted: May Thu 13, 2021 1:33 am 
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Joined: Nov Sun 22, 2015 4:51 pm
Posts: 36
Bob wrote:

> In order to maintain maximum Q of an IF stage, the tube plate is loosely coupled to the IF transformer primary,
> and the IF secondary is loosely coupled to the grid of the following stage.

In the schematics I've seen, a mixer or amplifier plate is series-fed through the tuned primary of a two-tuned-circuits transformer, and the grid of the following stage is series-fed through the tuned secondary. That being the case, I don't see how either the plate or the grid can be "loosely coupled" to its associated tuned winding. The transformer primary and secondary may well be loosely coupled, depending on the selectivity v gain tradeoff desired.


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 Post subject: Re: Tube IF transformers
PostPosted: May Thu 13, 2021 7:00 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2691
Location: Saskatoon
My definition of loosely coupled is perhaps more broad than what people are used to. I simply meant that we are not trying to match impedances to get maximum power from the plate into the primary of the IFT, and we are not trying to match impedances to get maximum power from the secondary of the IFT to the grid of the following stage. Trying to do that by matching the impedance would kill the Q. We simply want to inject a small signal into the IFT without loading down either it or the tube plate. Likewise on the secondary side.


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 Post subject: Re: Tube IF transformers
PostPosted: May Thu 13, 2021 1:07 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Not only would close coupling kill the Q it would make a dip in the passband, so it would resemble a letter 'M', as some of the secondary ends up reflected back to the primary, and seeing as the phase is shifted at resonance it creates a dip.
This is something I learned in this tube technology venture, solid state IF's tend to be single tuned.

I'm wondering whether to dig out a scrap tube chassis and see if I can transistorize it, I think I have a chassis that does trawler band, that might be interesting.


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 Post subject: Re: Tube IF transformers
PostPosted: May Fri 14, 2021 4:42 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1269
If you have a tube radio and you want to convert it to semiconductor, and keep all the same coils in the IF and RF section, the best move is to use a jFet like the MPF102.

There are a few traps, in the osc stage you need to connect a series resistor and diode across the gate and source terminal to conduct when the gate gets driven positive with respect to the source, (If the osc stage was self biased as many are where grid current on peaks sets the bias). The reason is that when the jFet is pushed into gate current, the effect is not soft as it is with the tube, and serious harmonics result. Tube IF transformers are quite suited to use with the MPF102 and also excellent with Fetrons too.


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 Post subject: Re: Tube IF transformers
PostPosted: May Sun 16, 2021 9:31 pm 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
Duly noted, I've used fets in comb marker oscillators before.
I was thinking of using a bipolar tranny for the osc.
For the prototype to make things easier I've used a Ne602 for the front end so I can get the IF right.
I tried to get the emitter coupled amplifier to work, couldnt it either ate current or oscillated.
So I kept the emmiter follower for the first stage, then used a common emmiter amp for the second, and coupled to the IF primary with a small cap to keep the Q high.
I guessed the values for the Agc, seems to work well.
Now onto the antenna coil for the front end, the osc coil has 3 wires but one appears to be open.


Attachments:
radpic.png
radpic.png [ 137.15 KiB | Viewed 630 times ]
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 Post subject: Re: Tube IF transformers
PostPosted: May Fri 21, 2021 6:38 pm 
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Joined: Apr Sat 22, 2017 8:04 pm
Posts: 12
Attachment:
Mosfets replacing tubes.PNG
Mosfets replacing tubes.PNG [ 174.74 KiB | Viewed 515 times ]
What about using dual gate mosfets to replace IF tubes?
The attachment is from Dec 1969 QST.


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 Post subject: Re: Tube IF transformers
PostPosted: Jun Wed 02, 2021 7:31 am 
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Joined: Nov Sun 22, 2009 9:21 am
Posts: 197
Location: UK
You mean dual gate jfets, I tried single gate ones with small <10pf cap in series to up the o/p impedance.
It worked howver the trans performs better if the high impedance comes from the active device rather than a series passive component.
Nevertheless if this was a commercial product I'm sure they'd put the time in and get something like that to work.


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 Post subject: Re: Tube IF transformers
PostPosted: Jun Mon 07, 2021 3:44 pm 
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Joined: Apr Sat 22, 2017 8:04 pm
Posts: 12
No, actually dual gate mosfets are not the same as JFET dual gate or not.
Dual gate Mosfets are not junction FET but isolated gate and they have order of magnitude higher impedance than JFET
Good luck with your trials


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