loewe-opta: Bella Rekord, hot EL84 tube

Good day to all, I have a Loewe Opta, Bella Rekord radio that has has an EL84 tube that gets very hot.

I'm aware that it's not unusual for this tube to be hot however the temperature measures over 100 degrees Celcius when measured with an infrared red temperature device.

The tube has burnt the insulation off the wires that are close to it and has heated the top cover directly above it of the radio enough to crackle and flake off the finish. See pictures.

I am in Canada and my line voltage to the radio is 122 volts. The radio does not have a transformer with different voltage taps (that I'm aware of) like the larger models.So I can't switch it to 127V.

Is the higher feed voltage the reason it's getting too hot or is it a component that needs to be changed or added?

 Pin 7 voltage is 265 v,  Pin 9 voltage is 240 v. The schematics show a voltage for pin9 208v and pin7 225v.

The radio is working OK but has a reverberation or distortion of the sound. Would the distortion be caused by the higher voltage to the tube or is it a separate issue?

I have not changed any components in the radio yet.

Thank you for sharing your knowledge and experience.

Best regards,

Vince Cappuccitti

• Loewe_opta_Bella_Rekord_6725W_Burnt_Wire (232 KB)
• Bella_Rekord_6725W_Top (214 KB)

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Hello Mr. Cappuccilli.
At which mains voltage can you switch your device?
How high is the cathode voltage at pin 3?
I think the EL84 has become defective due to overloading.
The fault is usually the permeable capacitor C73/22nF on the control grid.

Regards WB.

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Hello Mr. Bauer, The answers to your questions are shown below.

 - The EL84 tube checks out good on my tube tester and was previously tried on another radio. I did not pay attention to the temperature at that time. I will try another tube if you think it will help.

- I have checked the two 22nf capacitors in the vicinity of the tube and they both have good values of 21.5nf and 22.5nf. It's very difficult to trace the wiring in this chassis. If you think C73 is leaking I can attempt to replace it. It will be difficult to do with the limited space.

Thank you for your assistance.

Regards,

Vince

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Dear Mr. Cappuccitti,

maybe You already have a correct Supply Voltage, but under the following Circumstances:

if You not only "Neutral" and "Life" in Your House, you may have L1, L2, and L3, which often is used for large household Appliances like Dishwashers etc. They often run on L1 and L2, which is 1.7 times the Normal Line Voltage.

If You can switch the Radio to 220 Volts, this might just fit.

Please ask a local professional Electrician, because i don't know the Rules and Regulations and Safety needs in Canada!

Just an Idea, that came to my mind. I often have to deal with Instruments, which run on 208 Volts ( L1-L2) in US, and on L1 and Neutral in Europe.

Best Regards from Germany,

Henning Oelkers

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Mr. Oelkers, I must say that is very creative. Unfortunately the radio is a low end model and was made for North American market so only has 115volt  with no other option. The larger German models all had the option for 127 volt plus others.

I correct my last post stating 110 volt,  it is written on the back cover 115 volt.

Do you believe it is the higher voltage that's causing the hot tube?

Thanks for your help.

Regards,

Vince

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Update:

As an experiment I tried powering the radio using a dim bulb tester with a 100 watt bulb in it. Since I don't have a variac I thought this would reduce the voltage enough to see what affect it would have on the radio.

The result was a better sound quality without distortion and a much cooler tube. Voltage on pin 7 and 9 were reduced to 250 and 230 volts respectively.

I'm measuring these voltages with the tube out.

Thank you.

Vince

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Hello Vicent,
please send me a picture of the front, back and chassis of your device.
It may be necessary to create a new model in the RMorg if your model is a only 110 volt version.

Regards WB from Austria.

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Goodmorning Wolfgang, I have taken a closer look at the back cover and applied some polish wax to clean it and you are correct this is a different model. It is a 6726W.  My apologies for my poor eyesight.

I will take more pictures this weekend and submit them to the model pages.

For now here are a few to look at. I look forward to your comments.

Regards,

Vince

• 6726W_back cover (137 KB)
• 6726W back open (123 KB)
• 6726W back (121 KB)
• 6726W (120 KB)
• 6726W_front (129 KB)
• 6726W_dial (135 KB)

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Hello Vincent,
Yes, this is a new model 6726W.
The export version of the 6725W for 110 volts.
Can you create the new model in RMorg or should I help you with that?
I saw that you have never done that before.

Best regards WB.

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Hello Wolfgang, Thank you for your offer to help. I will give it a try and definitely ask for your assistance if I hit any snags.

While searching I found a German website with examples of this radio at Radiomuseum-Bocket.

One of the pictures shows the identical heat damage of the top cover as my radio has. It also has a picture showing a burned hole in the back cover . My radio also had a hole in the back that I somewhat repaired. This seems to be a common problem to this export model.

I've noticed that as the tube temperature rises the sound distortion increases relatively.

For now, I believe this radio is a dangerous fire hazard until I find a solution to this very hot EL84. Would you agree?

Best regards,

Vince Cappuccitti

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Hello Vincent,
It's not normal for the tube to get so hot.
Pull the tube out of the socket and measure the voltage on pin 2 / g1

Regards WB..

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Testing the VALUE of a capacitor on a valve grid is nearly useless. You need a current limited (2 M Ohm series) 300V to 400V  voltage and see the leakage. One way to test is a 10M input impedance DVM on grid to chassis when the EL84 (or other output valve) is removed. The voltage should be zero.

The paper dielectric capacitors had only about a 10 year life. Not all are in waxed card tubes. Only hermetically sealed types in cans with seals that have not perished remain good.

The leakage on capacitors from g2 to ground (screen grid decouplers) reduces gain but unlike g1 leakage doesn't risk damaging valves or transformers. There is also often a capacitor on the primary of an output transformer or on final anode to ground often called a "tone corrector", it's a snubber and needs to be much higher voltage rating (HT plus any spike voltage). The actual values of any of these three groups of capacitors are not critical.

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Wolfgang, I'm getting a very low fluctuating mv voltage reading on pin2 of 0.2-0.5 mv.

Based on both yours and Michaels's comments I will follow your initial instructions and change out the C73 capacitor and perhaps a few more to see what the effect will be.

I thank you for continuing to assist and will let you know the results.

In regards to uploading a new 6726W model, My attempt was rejected by radiomuseum with the comment that it already exists in their database?

Have a good evening,

Sincerely

Vince

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Michael, thank you for joining in. As I have stated to Wolfgang I will replace the capacitors.

I don't have the necessary equipment to do the testing you mention so the alternative is to replace the parts.

Best regards,

Vince

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Measuring with the tubes/valves out on the g1 pins (0V) and g2 pins (should equal HT with no valve) if it's a parallel heater radio with valve/tube rectifier or any kind radio with metal rectifier/germanium/silicon will work as the HT provides the test voltage. As long as the DVM is 10 M Ohm input impedance. Some cheap ones are only 1M and Analog meters are range dependent unless the FET or VVM type.

I use a converted flash from a single use camera. The 300uF replaced by 1uF metalised plastic and 2 x 1M ohm in series to the "+ test terminal". The "- test terminal" has a neon with 100nF (0.1uF) metalised plastic 250V minimum in parallel across the neon. The neon won't light if the test capacitor is "good". It will flash slowly with a "not good enough" capacitor and typically flashes so fast as to appear solidly on  with a typical paper dielectric capacitor, which may check out perfect and greater than 20 M Ohms / no leakage on a DVM as they only use 3V to 9V. Don't test Electrolytics with it, aluminium or tantalum. Use a 30V current limited bench PSU to test leakage or reform HT electrolytics. Lower voltage if needed. They can safely leak maybe a quarter milliamp when reformed, though usually drop to 20 uA. Obviously even 1uA is a fail for a grid capacitor to 1 M Ohm load as that would increase bias more positive by a volt. 10uA leakage at 300V will mean a short valve life if the output transformer doesn't burn out first.

A leaky capacitor to 0V / Chassis / Earth on a g2 (screen grid) will dramatically reduce the gain. A small number of radios actually use variable voltage on g2 for IF or RF manual gain or for AGC via an AGC amplifier as it causes less distortion than the usual approach of variable mu valve (remote cut off) and DC AGC on the signal g1 pin. It also allows use of valves/tubes without the variable pitch grid that gives variable mu / remote cut off.

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Now the new model has been created by Mr. Cappuccitti.
See ==> Bella Rekord 6726W.

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Vince, I am working on a Mullard valve radio that had low high voltage and the output valve was getting vabnormally hot. The output tube is a 6M5 (equivalent to EL84). I stood the valve in a shallow pool of mildly acidic cleaning fluid to clean off the black deposits from the pins. Now all is well with the temperature. I think that the screen grid pin was not connecting at the socket.

Regards.

Wayne Cockburn.

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Hi Wayne, thanks for your input. I changed the capacitors as initially recommended by Wolfgang and the temperature subsided on the tube. It is still a hot tube but much less now.

It was difficult to get to the capacitors to change them which was why I was reluctant to do so in the first place.

Cleaning the tube pins is always a good idea. I have run into various problems caused by poor contact of the pins.

Regards

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