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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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4th Jun 2020, 9:52 pm | #21 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Hi Jerry, thanks again.
Tomorrow I will make a video of the set playing, upload it on youtube and send you the link to have your advice. I believe it is great now, but i'd like to hear from you. Cheers, Diniz from portugal. |
5th Jun 2020, 12:14 am | #22 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Advice on output transformers
Because the pentodes in the ELL80 share the same cathode and the output transformers both go direct to HT, the voltages will not tell you accurately if the two halves of the valve are running at the same current. The best option would be to measure the anode current with your meter but be careful as the meter will be at a high voltage. You can measure the transformer primary resistance and the voltage across the primary when running BUT the winding resistance changes with temperature.
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5th Jun 2020, 3:47 pm | #23 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Thank you for the tip. I'll make those measurements and let you know.
Diniz |
5th Jun 2020, 4:03 pm | #24 | |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Quote:
https://youtu.be/U8L65gUHh6c https://youtu.be/oybSdsp5hzM https://youtu.be/cFsky1OGoEY Hope to have your feedback. Cheers, Diniz from Portugal. |
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5th Jun 2020, 9:34 pm | #25 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, thanks for posting the videos. You've done a great job! Listening to the sound on my Samsung tablet with its tiny speakers doesn't give a fair impression but it sounds like, as you said, some distortion seems to creep in when you turn up the volume,and also possibly the bass control.The treble response sounds very good. It will be interesting to read your ELL80 anode current measurements. If that doesn't reveal any problems, then maybe you are ready to give the oscilloscope a try. Hopefully it is a working instrument! If you need more advice on connecting it up to the speaker outputs, and feeding in an audio tone via the pick-up sockets, I'm sure we will be able to guide you through the process to find out what is happening to a sinewave input. Kind regards, Jerry
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5th Jun 2020, 11:14 pm | #26 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Here are the measurements:
Anodic current (pin3) 15.45mA Anodic current (pin8) 18.16mA Hope to hear your opinion! Cheers, Diniz |
6th Jun 2020, 2:51 pm | #27 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, I compared your anode current measurements with the ELL80 specification here http://www.r-type.org/pdfs/ell80.pdf. As you can see the anode currents are about 60-70% of the expected value (24mA) and so the valve emission is reduced. That also explains why the anode voltages are a bit higher than expected (250v). That said, both pentodes are reasonably well matched and so I'm not sure if the reduced emission is causing distortion - others might wish to comment. You said the speakers measured 5 Ohms - if that is the dc resistance measured with a multimeter then their actual impedance could be 8 Ohms. Therefore, as an experiment, you could try a different OT connection, for a 9k anode load and 8 Ohm speaker. For this use 1 - 3 for HT and anode and D - B for the speaker, and see if it sounds better. Otherwise, the oscilloscope is the best way to figure out what it causing the distortion. Kind regards, Jerry
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6th Jun 2020, 10:12 pm | #28 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Hi Jerry, thaks for the datasheet of ELL80. Although it states a anodic current of 24mA, the datasheet of the radio specifies 18.2mA in FM, so I'm assuming the ELL80 is not that bad (hopefully!!)
Regarding the speakers: since the original speakers are in the console (which is at my parents house in the countryside) I am using others from Telefunken (see attached image). I thought they were 5ohm impedance but actually they are 4ohm. I just measured the DC resistor (with the speakers disconnected from the chassis) and I get the value of 7.8ohm in one of them and 7.0ohm in the other... is it normal? Regarding the scope, what should I look for? The signal in Pin 8 and Pin 3 of ELL80? Regarding the online tone generator, do you suggest any specific frequency for the sinewave? Hope to hear from you. Your help is being much appreciated. Thanks in advance. |
7th Jun 2020, 1:01 pm | #29 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, concerning the speakers, since they are marked as 4 Ohms then most probably that is their correct impedance unless, of course, the loudspeaker drivers inside the cabinets have been replaced with 8 Ohm units! The difference between the 4 Ohm marked impedance and your measurement is possibly caused by your multimeter - some instruments are not great at measuring very low resistances and any slight corrosion or dirt on the probe connectors (where they plug into the meter) can sometimes cause higher than expected readings on the low Ohms range. Try shorting the Ohm meter leads together and check that it is reading zero. If it is, then the meter should be OK so you might want to have a look inside the speaker cabinets to see what impedance is marked on the actual speaker drivers. If the speakers are definitely 4 Ohms, it is probably best to leave the OTs connected the way you have done it. As far as the scope is concerned I would connect the Y-input (vertical) to the speaker output, while the speaker remains connected. This is safer than connecting to the anodes where there is high voltage. Be careful to make sure that the earthed side of the scope signal lead is connected to the earth side of the speaker output otherwise you'll short circuit the output which won't be good. I would try injecting a selection of different audio frequencies say 50Hz, 400Hz, 1000Hz. If you check the training links I sent you should be able to figure out that the timebase of the scope then needs to be set to 20msecs/scale division, 2.5msecs/scale division, and 1msec/scale division for those frequencies, and the sync set to internal. Use AC coupling for the Y-input and be prepared to adjust the Y-input sensitivity control to get the trace to the best height. Then adjust the sync control until you see a stationary sinewave. Turn the Grundig volume up some more and see if the waveform distorts. (you might need ear plugs ) Good luck!
Last edited by cathoderay57; 7th Jun 2020 at 1:16 pm. |
7th Jun 2020, 11:03 pm | #30 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Hi Jerry,
Thanks for your detailed comment. Since the meter is ok (zero ohms lead against lead) i followed your advice and i opened one speaker cabinet and it seemed untouched (see attached image). I was not expecting to find this kind of transformer inside... I dont know if the reason is that they are external speakers. But I assume it is ok to use them, since the set has this option to connect external speakers. I measured the DC resistor inside: twitter--> 6.5ohm; bass -->7.6ohm. I will know try to introduce myself in the "oscilloscope" world to see if I can perform the tests you suggested. At least I'll be learning something, that is my main purpose, indeed. Thanks again for all the effort. Cheers, Diniz |
8th Jun 2020, 1:04 pm | #31 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, that's interesting. The transformer, coil and capacitor form what is called a crossover network whose purpose is to split the incoming audio frequencies, low frequencies to the bass driver and high frequencies to the tweeter. Maybe the inductance and capacitance of these components is somehow affecting the resistance reading of your multimeter. I've never found this myself - always the dc resistance reading in my experience has been lower than the marked impedance, not higher. Never mind, we can only assume that the speakers are 4 Ohm as confirmed by the chart on the RadioMuseum website here https://www.radiomuseum.org/r/telefu..._41_hi_fi.html. There's also a U-Tube video on these speakers that says they were expensive back in the 1970s. Kind regards, Jerry
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8th Jun 2020, 4:26 pm | #32 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Hi Jerry
Here I am reporting my findings. See the attached images for the 3 input signals (50Hz, 400Hz, 1kHz). I am not sure I am measuring it right, because the scale/division are different from the ones you suggest. The measurements were done with the volume at its maximum. The 50Hz signal is very strange, indeed. Hope to hear your opinion!! Thanks again for all the support. Cheers, Diniz |
8th Jun 2020, 5:55 pm | #33 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, well, the shape of the 400Hz and 1000Hz signals look good - I'll come to the 50Hz signal in a minute. The point to note is that the distortion will probably depend on the input signal level. You should set the Grundig volume about mid-way and adjust the volume of the phones output of your smartphone or computer until you can hear distortion in the Grundig speakers. Then turn the smartphone or computer volume down until the Grundig output sounds clear. Then turn the Grundig volume up and have another look at the waveform on the scope. For the 50Hz signal I'm not sure what is going on. It looks as if the earth connection to the scope signal lead has come off. Try putting the scope leads onto the smartphone or computer phones output and see if you are getting a clean 50Hz sinewave going into the Grundig. Then do the volume adjustments, as before, to check that the input sensitivity to the Grundig isn't being exceeded, and recheck the output waveform at the speaker. Hope that makes sense. Looks like a nice scope! Kind regards, Jerry
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9th Jun 2020, 10:58 am | #34 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Hi Jerry, I followed your advice and performed some more measurements.
I must say that I didn’t noticed much difference on the curve shape for different levels of Grundig or PC volume. In Fig.1 you can see the curve for the 50Hz signal with Grundig and PC at maximum volume. Fig.2 illustrates the measurement process in the secondary of OT. After that I measured the signal at the PU socket (Fig.3). Finally, I disconnected the plug from the radio and measured the signal as illustrated in Fig.4, and the result is in Fig.5. So, I am assuming that the signal is not a perfect sinewave itself, and that the shape of the curve measured at the secondary OT is dependent on that. In addition, the distortion is not noticeable with the 50Hz signal. On the other hand, for the 400Hz signal it is possible to hear the distortion, despite the perfect shape of the curve (Fig.6 in the following post). I would like to have your opinion, but I think it is fair to assume that the problem is being caused by the speakers and not the set... I should try the original speakers when I have the opportunity. Thanks for all, I'm learning a lot! Cheers, Diniz. |
9th Jun 2020, 10:59 am | #35 |
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
The missing Fig.6.
Cheers |
9th Jun 2020, 2:04 pm | #36 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, well done with your measurements. The 50Hz signal is not producing a decent input waveform so it's best to ignore the 50Hz result. Because the signal looks OK at 400Hz, maybe you are right and the speakers are not performing well at high volume. Have you plugged them into another radio or amplifier to test them? Or do you have some different speakers to try on the Grundig? Unfortunately I don't have a reason for the distortion at this point. Maybe somebody else might have a new idea? At least the Grundig works now and sounds good at low to mid volume levels! Kind regards, Jerry
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9th Jun 2020, 4:00 pm | #37 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
Posts: 1,378
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Re: Advice on output transformers
The distorted signal looks like a clipped waveform with the fundanental attenuated so it has a large 150Hz component and a lower 50Hz component than it should be. This could be due to insufficient inducance. I do not see how a speaker could affect this.
What is the waveform like at the grid of the output valve? Put in a nice low level sine wave. |
9th Jun 2020, 9:01 pm | #38 | ||
Pentode
Join Date: Nov 2016
Location: Lisbon, Portugal
Posts: 187
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Re: Advice on output transformers
Quote:
I will try plug the speakers to a different radio (probably tomorrow) to see if the distortion persists. I have some more speakers, but since I recently moved to a small apartment, I left them in my storage room. When I have the opportunity I'll try them and let you know. Quote:
Nonetheless, I'll take a look of grid 2 and grid 2' of the output valve. Thanks for you comment. Regards, Diniz from Portugal |
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9th Jun 2020, 9:22 pm | #39 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hi Diniz, thanks for the replies. I think Trevor meant put the scope onto grid 1 and grid 1' of the output valve. There is another easy way of generating a clean 50Hz input signal - use the Grundig 6.3v heater supply via a resistor and a potentiometer, so that you can vary the signal level. I will hopefully post a sketch of the small circuit to do this in a minute or two as my scanner is refusing to work right now. Cheers, Jerry
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9th Jun 2020, 10:17 pm | #40 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,388
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Re: Advice on output transformers
Hello again. Scanner now works again - never had trouble before but some Windows 10 updates seem to have caused problems..... Please see attached sketch. It is very simple - Portuguese mains is 50Hz and therefore so will be the 6.3v heater supply on the Grundig mains transformer. Any potentiometer in the range 100k - 1M will do, it isn't critical. Just connect one of the outer contacts to chassis and the other to the 6.3v terminal of the heater supply. Connect a 10k resistor to the slider (centre) contact of the potentiometer and connect the other end to the PU input. Turn the potentiometer to minimum and slowly increase, monitoring the 50Hz signal at the speaker output first then try grid 1 of the ELL80.
One last thing - we still don't know why the 4 Ohm speakers are measuring a dc resistance of between 7 and 8 Ohms. As an experiment, you could try a different configuration of the OT, to match the output valve to an 8 Ohm speaker. For this the HT and anode need to be connected to 1 and 3 and the speaker to D and B. It would be interesting to find out if the sound quality is any different at high volume. Cheers, Jerry |