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Old 30th Jul 2019, 4:29 pm   #21
davidgem1406
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Default Re: Unknown Home Built TRF radio

I have been working away on this radio and making good progress.
Today the progress slipped. First I found the output transformer primary had gone O/C since removing it from the chassis.
Then I went and dropped the 349A output valve, it did not bounce.

Fortunately I had a 6V6GT available so no wiring to change about. Not that I have done any as yet, other than drawn it all up.

The question is about replacing the output transformer. I have several available that I can make fit in place of the original. However I do not know what the primary DC resistance should be.

The data sheet, page 2, for the 6V6GT states the load to be 5K with anode and screen volts at 250V.
Does that refer to the primary DC resistance of the output transformer?

My H.T. volts for the anode is 240V, and the screen is 90V.

Thanks,
Dave.
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Old 30th Jul 2019, 4:33 pm   #22
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Default Re: Unknown Home Built TRF radio

Don't worry about the primary resistance as such, the more important thing is the impedance-ratio to get a good match between the 6V6GT and the speaker.

Pretty much any generic output-transformer designed to match a 5KOhm source-impedance to a 3Ohm speaker will do - the biggest issue can be whether the replacement physically fits the available space.
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Old 30th Jul 2019, 5:00 pm   #23
Mike. Watterson
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Default Re: Unknown Home Built TRF radio

The DC resistance can be from 200 Ohms to 600 Ohms for an audio out transformer depending on size. The the AC load resistance is the voltage ratio squared x speaker ohms (The DC resistance of that can be close to AC impedance).

You can't tell from DC resistance. One way is to drive transformer primary from 115V shaver transformer and measure AC on secondary

Vin/Vout = turns ratio.
Z primary = (turns ratio) squared x Speaker impedance.

The actual DC resistance depends on the size of the transformer, not the turns ratio. A 100V line speaker transformer, one for 16 Ohms and one for 3 Ohms could all be the same DC resistance on the primary.

A 20W 240V to 6V mains transformer is about 40:1. With 3 Ohm load that looks like 4,800 Ohms. That will work to test. DC bias current is an issue with single ended transformers. That cancels on Push Pull, however a larger transformer than audio power suggests mitigates it. Never test single ended with a toroid transformer as they will saturate at a much lower DC current.
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Old 30th Jul 2019, 10:55 pm   #24
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Default Re: Unknown Home Built TRF radio

Thanks very much chaps.
I think that solves my problem.

I,m sure I have at least 1 transformer that will suite and fit the space.

Very much appreciated,
Dave.
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Old 31st Jul 2019, 9:48 pm   #25
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Default Re: Unknown Home Built TRF radio

Quote:
Originally Posted by davidgem1406 View Post
Today the progress slipped. First I found the output transformer primary had gone O/C since removing it from the chassis.
Before scrapping the transformer have a careful check of the connections. It seems odd that the transformer should go O/C just by removing it from the chassis. Quite possibly one of the connections to the transformer tags have been disturbed when it was disconnected and it may be possible to 'remake' the connection. Worth a try before trying to fit another transformer.

Quote:
The question is about replacing the output transformer. I have several available that I can make fit in place of the original. However I do not know what the primary DC resistance should be.
As has been said, it's not the DC resistance that is important. It's only given in service information as a guide but in reality the DC resistance has no bearing on the transformer ratio.....basically thickness of wire, size of transformer....there is a lot more to it than that....

The transformer ratio you require is 40:1. This is worked out by knowing the anode load (usually shown as Ra, not the same as ra) and the speaker impedance. You already know it's 5K (or 5000 ohms) and the speaker is 3 ohms, so if you divide 5000 by 3 this equals 1666.6666. Take the square root of that and you get 40.824....just ignore everything after the decimal point, so just 40:1


Mike has given you a way of measuring the transformer ratio. Basically you feed a known voltage into the primary and measure the secondary. Don't put too high a voltage in since it's only an output transformer. I use an audio generator connected to the primary with a scope connected to the primary and secondary. Adjust the generator to give 1V p-p on the secondary. Measure the voltage going in to the primary....for instance if the primary says 25V p-p and the secondary shows 1V p-p then the transformer ratio is 25:1
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Old 12th Aug 2019, 4:25 pm   #26
davidgem1406
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Default Re: Unknown Home Built TRF radio

Hello Sideband,

I did check the connections, but they are wire feed outs, no tags. I cannot get any reading on these wires right at where they enter the winding.

Using a generator and a scope to get the ratio is a nice easy way to do that, thanks for the tip, I will remember that one.

Best regards,
Dave.
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Old 17th Aug 2019, 4:23 pm   #27
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Default Re: Unknown Home Built TRF radio

I am now looking for some pot to replace the gain control (See above threads).

I have found that the GEC AC mains 4 uses almost the same circuit, but this has a normal pot with a value of 7K.
From what I can see, there seems to be no reason why this set I am rebuilding cannot use a normal pot of an appropriate value.
The value I will try to decide from what I can measure of the tracks on the original. Maybe 47K.

Using a normal pot may also solve the problem of fitting a DP on/off switch, although I have solved this by fitting a pot with a DP switch, with the track and contacts removed, to the back of the wave change switch. That does introduce other problems though.
A normal pot with a DP switch would be much better.

Any thoughts on this would be appreciated.

Regards Dave.
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Old 12th Oct 2019, 3:14 pm   #28
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Default Re: Unknown Home Built TRF radio

I have now completed the refurbishing of the chassis and it is now working on both wavebands.

The gain pot (mentioned earlier in this posting thread #27) I have replaced with a standard pot with a DP switch, as I was hoping could be done. The pot itself works fine as it has no problem handling whatever current flows in that part of the circuit.

I do however have 2 problems to solve.
Firstly with the gain pot as this goes from zero gain up to full gain on about the last ľ of the track, that makes it very touchy to control the output volume, which is quite substantial on a strong station.
I have tried 2 values, first I used 47K, that was so touchy it was difficult to control as all action took place nearly at the end of the track. I now have a 25K, that is a little better, but still to touchy.
I also found that the HT feed to V1 and V2 goes up and down quite significantly with the gain pot.
Is there any way to better this and spread the gain over a larger portion of the track? Or do I need to further reduce the value of the pot?

The second problem is that we have distortion that I can only describe as like a dog growling behind the stations. It is more prominent on music than on speech, and is there from low to high levels of audio. At zero gain there is total silence.
As, yet I have not tried to investigate this so any ideas would be helpful.

There are just 2 preset capacitors available for alignment, such as it might be, these are C2 and C4 (The circuit diagram is attached).
This is my first meeting with a TRF radio, so I guess that both trimmers are set for maximum output at some frequency at either one end of the scale, or midpoint.
Could someone direct me as to where these adjustments should take place?

I have attached the circuit diagram and valve voltages in pdf format, and 3 pictures of the refurbished chassis.

Thanks,
Regards Dave.
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Old 12th Oct 2019, 3:53 pm   #29
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Default Re: Unknown Home Built TRF radio

The pot you replaced - was it a logarithmic or linear-taper one?

Getting all the "action" happening close to one end of a pot's travel is often a sign that you used a linear where you should have used a log, or the other way round.
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Old 12th Oct 2019, 5:44 pm   #30
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Default Re: Unknown Home Built TRF radio

I'd have expected the gain pot. to be a linear type, having looked at the circuit, but I may be wrong. The type (law) of the original pot. should be marked on it's body alongside the value. This may be shown as 'Lin' or 'log', or by an 'A' or 'B' after the resistance value.
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Old 12th Oct 2019, 5:53 pm   #31
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Default Re: Unknown Home Built TRF radio

Where you have a gain control which is also controlling regeneration, as it seems to do in this circuit, itís pretty normal for all the action to happen at the top end. This is certainly the case when a variable capacitor is used as a reaction control. Back in the 1920s achieving a good audio level from a TRF with regeneration was regarded as something of an art!

How long is your aerial? The set might perform better with a longer and higher aerial that would give it a stronger signal requiring less regeneration.

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Old 12th Oct 2019, 8:40 pm   #32
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Default Re: Unknown Home Built TRF radio

That chassis is now a thing of beauty! Has the set got a cabinet to do it justice?
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Old 12th Oct 2019, 9:00 pm   #33
davidgem1406
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Default Re: Unknown Home Built TRF radio

I have just checked the pot with an Avo meter it is definitely Log.
The original is an ex WD wire wound twin track item with no markings except a military part number 10C/1888. I suspect this would have been linear.
I don't think I have a linear pot to try in its place, I would need to check that, it would also need to have a DP switch.

The set performs well, with a volume well above normal listening, on strong signals, it gets all the MW and LW stations that are available here.
The aerial is about 6 metres long, I am unable to get it higher or longer.

Thanks Dave.
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Old 12th Oct 2019, 9:02 pm   #34
davidgem1406
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Default Re: Unknown Home Built TRF radio

Hi Chris,

Thank you, it is somewhat better than it's original state for sure.
I have the cabinet that is yet to be done, it is in a poor state at the moment.

Dave.
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Old 12th Oct 2019, 9:08 pm   #35
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Default Re: Unknown Home Built TRF radio

I have one of these Premier radios made from a kit by my father when I was 3! I have been using it with an isolating transformer for safety. Very little has required attention over the years. Good luck with your's.

Peter
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Old 14th Oct 2019, 3:39 pm   #36
davidgem1406
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Default Re: Unknown Home Built TRF radio

Hello Peter,

Is it possible for you to be able to tell me the value of the gain pot and what type it is, Log or Lin please.

Dave.
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Old 20th Oct 2019, 4:54 am   #37
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Default Re: Unknown Home Built TRF radio

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by davidgem1406 View Post
Hi Lawrence,
Yes, it seems to be doing something like that. The centre is grounded, one end to the aerial coil, the other to HT via 100K and 220R grounded. Seems rather strange to me!
It might be similar to the arrangement in this:

https://vintage-radio.net/forum/atta...0&d=1144184530

R1 being the 100k and R2 being the 220 Ohm?

Lawrence.
Now that's a real TRF!

The term has been devalued in recent years with people labelling things like ZN414s with only a tuned aerial input stage as "TRFs".

It brings out the grumpy old man in me!
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Old 27th Oct 2019, 5:11 pm   #38
davidgem1406
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Default Re: Unknown Home Built TRF radio

Yes it is a real TRF for sure, but as it says here has 2 problems, neither of witch I have solved as yet.
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Old 27th Oct 2019, 8:25 pm   #39
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Default Re: Unknown Home Built TRF radio

Variation on a theme

This is 'as found'. I have had it for around 35 years. Lineup is 6K7, KT263, 6V6 and metal rectifier.

The front end is basically the same as yours and the potentiometer on this one is 15K LOG

This one has a 0.05 across the top of the transformer this could cure the mains hum growling that you mentioned.
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Old 28th Oct 2019, 10:21 am   #40
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Default Re: Unknown Home Built TRF radio

That should have been modulation hum, not mains hum.
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