Blown Capacitor in EL34

Dear Friends,

I was working on this EL34 project for my colleague for the past two weeks. Following the manual, I tried to set everything right.

After two weeks of hard work, the Mono Block finished last night. When I turned it on, tubes (EL34, ECC 83, etc) all lit up normally. DC voltage measured at the speaker terminal is also 0.0V (meter set at 20V DC).

Music played alright for 30 seconds. Then the Capacitor C12 got BLOWN and exploded.

I re-checked all wirings; seems to be okay to me. I measured the output voltage at the transformers, ALL give recommended readings.

What could have caused the explosion of the capacitor C12?

Please help.

CAT

Dea Accphoto,

I should put in the tubes before doing the measurements, am I correct? Or else I won't be able to have that reading as I need the tubes to scale down the voltage, right?

I am really worried about blowing another "item" on the PCB. Honestly, this is my fear .... sigh.

C12 is rated at 47uf / 63V. "Theoretically" it should be okay. Just don't know WHY it is blown. I blew BOTH C12 for the two Mono Blocks. I just hope the caps, resistors are all okay as they had been sitting inside the box for over 10 years.

CAT

Dear Accphoto,

Thanks for your advice.

I keep asking this question. Why was C12 blown but not C13 as well? As you have said, it MUST be the voltage supplied to C12.

First thing I did just now is to check again the Ohm reading of the cement resistors R21 & R22. They read 468 Ohms or so (Both). That should be okay.

Then I de-solder the Blown Cap out. I also checked the reading of R19. It should be 2K2 and it reads close enough.

Then I check one end of the R19. It is soldered to Pin 1 of V3. From my limited knowledge, I have the feeling that C12 and C13 should work in pair. Why is R20 not connected to Pin 1 of V4?

Therefore I took the manual out and I "think" I got the answer. I mis-interpret the diagram .... the other end of R19 should not be connected to Pin 1 of V3. Because I have it connected, the voltage supplied to C12 becomes too high.

Do you think I got the answer to my problem?

I hereby attach the photos for your reference.

Many thanks.

CAT

Dear Accphoto,

One more thing. After reading the design / schematics of this amp. Do you think it is a good design.

As this toy belongs to one of my best friends, I am wondering whether I should upgrade the components for him. It seems to me that the materials used here are not of very good quality.

What do you think?

CAT

Dear Accphoto,

Maybe I used the other mono block to test out the sound last night. As I don't have 47uf / 63V on hand, I move the un-burned one to one single block. Now I have all the working C12 on one mono block.

After disconnecting the resistor R19 from PIN 1 of V3 and resolder back all the necessary points, I turned the power on.

This is the exciting moment. I stand far away from the block, see the heaters lit up and waited 5 mins before I move forward.

No blown capacitors now BUT no sound as well. All I can hear from the speaker is the sound of electricity.

I checked the speaker terminal out, it says 0V DC but last time it varies between 0.1 and 0V.

I then check the Voltage at the Valves. The two EL34 gave CORRECT readings at pin 5 (0V), pin 8 (29V), pin 3 & pin 4 (<410V) as stated in the manual.

HOWEVER, the EF86 and ECC83 gave terrifying readings.

EF86:
Pin 1 -- 160V (should be 90)
Pin 6 -- 180V (should be 80)
Pin 3 -- 1.9V CORRECT
Pin 9 -- 0V    CORRECT

ECC83:
Pin 1 -- 355V (should be 305)
Pin 2 -- 180V (should be 80)
Pin 3 -- 0V     (should be 81.5)
Pin 6 -- 350V (should be 305)
Pin 8 -- 0V     (should be 80)

What could have happened to these TWO tubes? How come there is 100V DC difference and for ECC83, no Voltage at all for Pin 3 & 8.

This is indeed a "challenging" (but frustrating) project ....  :cry:

CAT

I checked R8, it should be 1M. Put a multi-meter at its lead, there is NO resistance at all ....

No wonder I got 180V for Pin 2 ECC83. The point before R8 gives 180V and the point AFTER R8 is also 180V. R8 is dead, right?

Second Pin 8 of ECC 83 should be 80V. I think the capacitor C9 is dead. The front lead of R9 gives 0V and R9 goes to Pin 8 of ECC83. But Pin 7 of ECC83 has a voltage of 185V going into C9. C9 gives nothing out.

Sigh .... Terrible.

I totally agree with you .... "The materials in the mono block is really bad".

CAT

Dear Accphoto,

I checked all the resistors in that particular block. Two of them were dead. I am not sure whether I blew them or they came in dead.

For the first block I construct, I test each and every resistor and capacitor before I solder them in to avoid mistake. But once I got that right, I didn't check the components for this one.

One of the capacitor in this block is also dead. I went to Ap Liu Street yesterday and it is really hard to find a substitute for it. It is 100n at 500V, Polypropylene type, sigh ....

CAT

Dear friends,

I finally finished this EL34. I turned it on and it is not impressive at all. The "hum" sound is quite obvious, compared with my ASL AV-25. Some said that it is because the capacitors had not yet run in.

The unit is now back to its owner. He will find some way to improve it when he has time ....

CAT

Dear Accphoto,

The unit is not with me now. I will ask my colleague to take some photos ... How do we ground the 6.3V ?

CAT

Dear Accphoto,

My mistake. My colleague said the noise should be from electric current, not indeed "hum". I don't know what is the difference tho', ha..ha..

CAT

Dear Cheaplow,

Thank you very much for your information.

Your photos are blurred because of focusing problem. You have to switch your Digital Camera to MACRO mode (Flower sign) before you can take that close shots of a book. And of course, sufficient light and a steady hand make the job even better.

So what do you think caused the "Deee" noise? Is it because the capacitors haven't yet run in? I TRIED to find replacement for those capacitors but ended up with almost nothing compatible.... sigh.

CAT

Dear Accphoto & Cheaplow,

My friend finally get rid of the "noise". The unit is now perfectly quiet (He is using Mullard 196x production EL34 !!).

I asked him how did he eliminated the noise. He said he reduced the number of wires connecting to the tube socket to TWO instead of the original FOUR. He said the extra two wires leading from the PS unit created the noise.

I am so glad he solved the problem and he said with the $$$$ Mullard EL34, the unit sounds pretty okay. The Tubes are "n" times more expensive than the amplifier .... ha..ha...

Cheaplow, I have signal generator & oscilloscope at the college I am working with. I myself got an AC voltmeter and a capacitor meter. However, I can't find a reliable Tube Tester.

Thanks for your info.

Accphoto, I got your email already; just toooo busy these few days preparing for the Open Day on Sunday. Will try to reply you asap. Thanks for your advice.

CAT

Dear Accphoto,

I have thought of acquiring a Tube Tester myself. However, I learned that these toys need to be calibrated before it can give accurate results. I don't think this kind of service is available in HK .....

My friend told me that you can get a new one made in China at GuangZhou. The price is around RMB 1000.

If I need to test out some tubes, I will drop you a message. Thanks for your offering.

CAT