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Old 14th Jul 2006, 10:44 am   #21
paulsherwin
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
What re-adjustment of the bias might get the best out of that output valve?
If the original circuit specified 270 ohms for the 6V6 I'd stick with that. You may find you just don't need all that output power though, in which case increasing the resistor to say 400 ohms would reduce the current drawn. This would make the valve run cooler and last longer, and reduce the load on the PSU. You can experiment once you have enough signal to drive it properly.

Good luck, Paul
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Old 15th Jul 2006, 9:22 am   #22
Neil Purling
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Default Re: Regarding home made TRF

I have increased the AF signal through the circuit byt fitting a B7G base on a bit of ally sheet over where the Octal base of the detector was.
I have a EF91 as detector now, well it is marked CV138 and carries the government broad arrow mark. Said valve appears to be internally coated? The EF91 seems to be giving more output than the EF39 and its American equivalent, the 6K7. When I whack up the volume it influences reaction at the extreme of its travel so there's some degree of feedback along the HT line. I have resistors & capacitors to decouple both AF stages. It does not motorboat thankfully. Whether it is the extra drive or due to the charactersitics of the EF91 I don't know, but the set sounds more trebly.
Last evening's exercise with the soldering iron was fruitful!
Another forum member suggested trying frame-grid EF183 and EF184 IF strip valves.
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Old 15th Jul 2006, 2:29 pm   #23
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Default Re: Regarding home made TRF

Neil, I would focus on your strange volume problem. Is it the wrong type of volume control - I guess it should be a log type? Otherwise it must mean something is hitting a limit as you must be able to get the 12.5V swing from the 6J5 with a 250-300V supply. What are the cathode/anode voltages of the 6J5, 6V6?
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Old 15th Jul 2006, 4:53 pm   #24
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Default Re: Regarding home made TRF

Getting to the party a bit late here, but generally I would consider it very optimistic to expect good performance if you want to go from a regenerative detector and to speaker volumes in only two audio stages.

As a general rule you should use as 'weak' a pentode in an electron coupled regenerative detector as possible, not relying on it for audio gain. An EF184 or EF183 will certainly have quite a bit of gain, but it will be very hard to make a smooth receiver using it. Almost anything can be made to oscillate, but that doesn't mean you want to do so.

The best regenerative detector I have ever made was designed after careful study of circuits that those that came before me had come up with. The best of the modern valves fulfilling the requirements that I could find was an EF86(!). A near perfect regen detector in the 7MHz ham band with only 12V on the plate/screen supply.

I believe I have previously posted the circuit and the design requirements here on UK-VR. Yep, here they are.

Hope this doesn't confuse matters too much.

Frank N.

Last edited by YC-156; 15th Jul 2006 at 5:02 pm.
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Old 15th Jul 2006, 7:15 pm   #25
Neil Purling
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Default Re: Regarding home made TRF

Do I might need to rein in the 6V6 a bit, or do they always run rather hot?
I started with a NOS metal envelope 6V6 here that whiffed for a bit as the oils baked out of the paint. I used the circuit of the AF stage right out of Radio for Boys, and the 6V6 was the specified valve in this location. I assume that it is biased correctly.

It has also been suggested that I change cathode resistor of the 6J5 from 3K9 to 4K7 and/or alter the anode resistor to around 150K.
The 6J5 was substituted for a EBC33 and if I need to make any changes in the biasing of the 6J5 I will de-couple itat the same time.
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Old 15th Jul 2006, 7:24 pm   #26
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
Do I might need to rein in the 6V6 a bit, or do they always run rather hot?
What anode supply voltage are you running it at, please?

If you told us before, then I missed it. The cathode resistor is only appropriate at a particular supply voltage. Go above that and you will cook the 6V6.

Best regards

Frank N.
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Old 16th Jul 2006, 4:30 pm   #27
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Default Re: Regarding home made TRF

I have been making some measurements
The HT voltage is 250 volts.
Vk on the 6J5 is 3.2V and on the 6V6 it's 12.3V
I have measured the anode voltage on the 6J5 to be around 75 volts and on the 6V6 it's anode measured 240 volts under no-signal conditions.
The AVO's internal resistance was possibly a influence.
That was without the detector valve inserted. Having a signal tuned in didn't make any apparent difference.
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Old 16th Jul 2006, 5:28 pm   #28
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Default Re: Regarding home made TRF

Then the 6V6 cathode current is 12.3 V / 270 ohm = 0.0456A = 45.6mA.

Total anode and screen grid dissipation is then (240-12) * 0.0456 = 10.4W.

The datasheet I have here for the 6V6 says it has a maximum anode dissipation of 12W plus 2W for G2, so you are running it within the limits, if only barely so. I would probably back off the anode dissipation a bit, say to two thirds or half of the present level. It hardly pays off to run the valve at 90-100% of full dissipation. You almost certainly don't need the full capabilities in any case.

Try increasing the cathode resistance until the cathode current is 25-30mA. The valve will be much happier I suspect, and you won't be able to hear the difference.

The cathode current of the 6J5 is 0.8mA, which is fine, but the cathode decoupling capacitor is too small. It should be increased to 1uF or perhaps even larger. As it stands you will not get the full amplification from the 6J5 at frequencies below 500Hz.

The 10nF anode blocking capacitor from the anode of the 6J5 to the 6V6 grid is barely large enough. You will get a bit of cut off of low frequencies (which might not be a bad thing, since it suppresses hum to some degree).

Be mindful that just because you saw a circuit in an old book doesn't at all mean that it works as intended! I have seen more mistakes and lousy designs in old books that I care to think about.

Hope this helps a bit.

Frank N.
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Old 16th Jul 2006, 6:13 pm   #29
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Default Re: Regarding home made TRF

Need to backtrack on my own words here for a moment.

My previous comments regarding wringing the most performance from the detector didn't take into account that you are not trying to make a receiver for weak amateur signals.

For receiving comparatively strong AM signals on the MW broadcast band construction and actual circuit values will be much less critical. In particular you will not need to have the detector oscillate nor are you trying to get the lowest bandwidth possible, so it will be much easier to control in your situation.

I still wouldn't use a high slope detector valve though.

Frank N.
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Old 16th Jul 2006, 6:40 pm   #30
Neil Purling
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Default Re: Regarding home made TRF

I will put another 270 ohm resistor in series with the other on the cathode of the 6V6. Hopefully that will make the valve last longer.

Well, I have an assortment of low voltage electrolytics.
I will stick a 1uf electrolytic in place of that 0.1uf 6J5 cathode bypass presently there or go up as far as 10uf?
Like you say I probably won't notice the effect of changing the bias of the 6V6 if the 6J5 is producing more gain.

Is it a characteristic of the high slope EF91 that the audio output apparently favours higher frequencies. Would changing the 0.01uf between the 6J5 anode & 6V6 grid make much difference to the frequency response?

I have just had to change the 47K reaction pot as it had a intermittent wiper contact.
Was the circuit of the amplifier built for maximum gain (assuming usage of a EBC33)? I have no idea of how many people made the 4 valve mains superhet from Radio for Boys but I know that Hunts Smoothing Bomb used it too.
I ought to PM him and find out what his sounds like.
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Old 16th Jul 2006, 7:26 pm   #31
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
I will stick a 1uf electrolytic in place of that 0.1uf 6J5 cathode bypass presently there or go up as far as 10uf?
I would add a 4,7-10uF if you have one, that will certainly be enough. Then move the spare 0.1uF up to the 6J5 anode instead of the current 10nF unit.

Quote:
Originally Posted by Neil Purling
Like you say I probably won't notice the effect of changing the bias of the 6V6 if the 6J5 is producing more gain.
You won't change the gain much by adding an extra 270 ohm resistor at the cathode. The cathode resistor is bypassed, so the only parameter, which really suffers, is the maximum peak output power available.

Quote:
Originally Posted by Neil Purling
Is it a characteristic of the high slope EF91 that the audio output apparently favours higher frequencies.
No, I think you should make a few mild changes to the basic design of the detector:

*) Disconnect and utterly remove(!) the 100 turn antenna coupling coil. You can loose your Certified Valve DIYer License by using a large coil like that. Instead experiment by directly connecting the end of the antenna to the chassis(!) near the main coil, that will be plenty coupling.

By using the large coupling coil you are almost certainly utterly loading down and thus ruining the positive qualities of the tuned circuit.

Properly executed these detectors are extremely sensitive and contrary to common beliefs you don't need anything like a 'hard' coupling between antenna and detector to get good output. Read: it is not the degree of antenna coupling, which dictates the available audio output voltage.

*) Connect a 1uF/63V electrolytic or film capacitor in parallel to the 0.1uF screen grid decoupling capacitor, while keeping the 0.1uF in place. The value of 1uF is fairly critical. Any larger and it will be hard to adjust the regeneration. Any smaller and low frequencies suffer in the reproduced audio.

*) Add a 220pF / 350V capacitor straight from the anode of the EF91 and to ground. This decouples the anode for RF, something which is often forgotten in these detectors.

Quote:
Originally Posted by Neil Purling
Would changing the 0.01uf between the 6J5 anode & 6V6 grid make much difference to the frequency response?
If will make a small difference, and won't hurt, so I'd do it.

Quote:
Originally Posted by Neil Purling
Was the circuit of the amplifier built for maximum gain (assuming usage of a EBC33)?
It was apparently made for components at hand to the author.

Using a completely different triode (or two) would have increased the possible gain tremendously. By using a 6SL7GT, also an Octal valve though a dual triode, the sky would almost have been the limit when it came to the maximum gain. The voltage gain of the first stage isn't terribly impressive, about 15 or thereabouts.

By using half a 6SL7GT a gain of around 40 is easily achieveable, so two audio stages like that in series would give a maximum gain of about 1600 in a single valve envelope, compared to the 15 you got now. Only a difference of a factor 100.

You might get hum and stability issues instead, but that is a different set of problems.

Let us know how things progress for you. More suggestions for tweaks available upon request.

Best regards

Frank N.

Last edited by YC-156; 16th Jul 2006 at 7:32 pm.
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Old 16th Jul 2006, 10:45 pm   #32
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Default Re: Regarding home made TRF

One of the first things I found with the design was that the 100 turn coupling coil heavily loaded the tuned circuit so that even with the vanes of the variable capacitor fully open I couldn't tune the very bottom of the medium wave band. The aerial is 30 ft of wire on a spool stuck on the highest shelf in my bedroom. Coupling to the main tuned circuit is 3 turns of insulated wire very loosely wound around the bog roll coil former. Well it works.............
Another forum member wondered if the later frame-grid EF183 and 184 valves would be worth trying in the detector. I used to have tins of the little sods and I chucked them all out.
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Old 16th Jul 2006, 10:57 pm   #33
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
... I used to have tins of the little sods and I chucked them all out.
Ack! Please don't do that in the future. I will happily take any and all frame grid valves off your hands if you have too many.

I still believe they are a bit too much of a good thing for a regenerative detector, but they are certainly fantastically good valves with many interesting uses. High gain IF strips being one of the less spectacular applications.

Best regards

Frank N.
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Old 18th Jul 2006, 5:43 pm   #34
Neil Purling
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Default Re: Regarding home made TRF

Well........
I have discovered a pretty spectacular mistake when delving into myTRF radio and found the 6J5 cathode bypass capacitor to be a whacking 22uf.
I was told by a member of this forum it ought not to be any more than 1uf, so the old one was duly clipped out and replaced with a 1uf.

I have also changed the cathode resistor of the 6V6 output valve from 270R to 470R. It doesnt seem to chuck out as much heat and there is no audible difference after changing the bias of the 6V6. The circuit in Radio for Boys did specify 270R which is strange when 470R would let the valve apparently run cooler and presumably last longer.

I have also put the EF39 back in place. The high slope pentode EF91 worked and had higher gain, yet didn't sound quite so good.
The EF91 was also somewhat microphonic.
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Old 21st Jul 2006, 1:16 am   #35
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
One of the first things I found with the design was that the 100 turn coupling coil heavily loaded the tuned circuit so that even with the vanes of the variable capacitor fully open I couldn't tune the very bottom of the medium wave band. The aerial is 30 ft of wire on a spool stuck on the highest shelf in my bedroom. Coupling to the main tuned circuit is 3 turns of insulated wire very loosely wound around the bog roll coil former. Well it works.............
I've been mucking about making a TRF (using a single 6sn7gt) recently and found that the aeriel loading could make a big difference. I like you had a spool of wire chucked on a high shelf (well an old petrol pump actually). I found that cutting off the spool and just having the wire worked a lot better at some frequencies.

Also the "best" (i.e. at most frequencies) seems to be a _very_ loose antenna coupling - both for broadcast and amateur bands!

Just a quick thought on the low gain - I also had problems with RF frequencies getting into the second audio stage and feeding back via the HT and reducing the reaction at higher volumes - a bit of extra decoupling on the detector stage improved matters.

Dom
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Old 25th Jul 2006, 7:37 pm   #36
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Default Re: Regarding home made TRF

Is it possible to alter the tuned circuit of the electron-coupled circuit to get Short Wave of around 6 Mc to 10 or more?
For Short Wave I believe that I ought to use a smaller value of tuning capacitor, but I was wondering about the changes I ought to make to the coil.
For the Medium Wave broadcast band it is 65 turns, tapped at 5 turns with a 500pf var capacitor.
Yep, i'm getting the urge to make another TRF radio.
I'm thinking of using a ECL80, driving it's pentode section with a 6J5 or one half of a 6SN7GT.
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Old 25th Jul 2006, 11:27 pm   #37
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Default Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
Is it possible to alter the tuned circuit of the electron-coupled circuit to get Short Wave of around 6 Mc to 10 or more?
Technically, yes it is possible, but you might find it tough going.

First there is the stability issue. As the frequency rises, the problems with staying on frequency increases in proportion. So for proper operation, even for AM operation, you must think bullet proof mechanics and good shielding.

Secondly, there is the issue of band spread. The AM broadcast stations are spaced equally around the 8MHz broadcast segment as compared to the MW BC band. That means that if you cover 6-10MHz with a single capacitor, the stations will be spaced around eight times closer on the dial. This is assuming your MW radio covers all of MW. If it doesn't, then it will be even worse.

Third, signal strength. Some of the most powerful BC stations are hovering around 8-14MHz, and you might get problems with keeping them out of your speaker instead of trying to reel them in. Good shielding and decoupling of power supply lines is what is needed here.

But as I said, yep, it is doable though personally I wouldn't push my luck very much past 10MHz. You just have to be very careful with the construction. No cardboard or open frame construction allowed. And if you have any doubts whether you have made it physically strong enough, then you haven't.

Quote:
Originally Posted by Neil Purling
For Short Wave I believe that I ought to use a smaller value of tuning capacitor, but I was wondering about the changes I ought to make to the coil.
It depends on a number of factors, in particular which frequency range you would like to cover, and, say, if you have some components and coil forms you would like to use.

Frank N.
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Old 26th Jul 2006, 7:52 am   #38
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Default Re: Regarding home made TRF

So; Not a good idea to try Short Wave then.
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Old 26th Jul 2006, 10:20 pm   #39
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Exclamation Re: Regarding home made TRF

Quote:
Originally Posted by Neil Purling
So; Not a good idea to try Short Wave then.
Oh dear me! Where's the pioneering spirit? In the bottom of a glass? Please don't be put off by what you've read - technically accurate as it may be! Remember: all great discoveries were made by mistake - and (IMveryHO) that an ounce of practical experience is worth a ton of theory! They all laughed at Marconi with his attempts at cross-Atlantic comms. attempts - and look where we are today!

Have a go - read up on the basics by all means - and experiment! You may be quite surprised at the results!

And good luck!

Al / G8DLH
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Old 26th Jul 2006, 10:27 pm   #40
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Question Re: Regarding home made TRF

Quote:
Originally Posted by YC-156
. . . EF183 / EF184 . . . High gain IF strips being one of the less spectacular applications (with these valves).
Frank N.
I'm a bit puzzled by that comment, Frank. Just out of interest, what do you have in mind, may I ask?

Regards,

Al / G8DLH
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