You need to check the voltage across the 470 ohms resistor. since this is an autobias circuit, you need to check the voltage across R21 and R22, normally it should be around V=IR (assuming EL34 running at about 35-45ma, the voltage should be .035x470=16.45V), the 47u should have a voltage of about 50V. What is the exisitng voltage of the cap ?
Do not worry as sometimes the cap need to charge up if it sits without working too long. The circuit will work without the caps as this is the cap for tuning the sound. You may beed to disconnect the blow caps, check the voltage and see what happen. Normally, we check the first stage ( with small tube first) and make sure it is ok, then put in the power tube and check it. Sometimes, the resistor on the Grid of the power tube malfunction, it will drove up the voltage too. Look at the monoblock, they are using default resistor, it may be one or two resistor malfunction.
I do not like EF86 as the first stage enlargement , it is a 五極管，which does not sound as good as triode. I perfer 12Au7 or 12At7 or even 6SN7 is better than EF86.The material in the mono block is really bad. DO you get a chance to switch on and listen to it, I cannot really tell the sound by the photo. :0
yes, R19 and R20 should not connect to V1 's 1. If you do so, you shorting the Grid with the cathod, so the caps blow. C12 and C13 should work in pair.
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.
The capacitor is quite common, it is 0.22U 500V, you can use Mit which have some 0.22U 600V rating. But I do suggest you to clear out the noise before upgrading, look like the noise is not from the componenet but from the wiring and gounding. Did you get my email?
[quote:44dab73dfb="alfcat"]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[/quote]
I have a very old tube test which is ok, if you really to test some tube, you are welcome to use it.
Just my two cents. running EL34 at 350V is a little bit low, this why you only get 35W, if you want more power, you may consider to run EL34 at about 400V, this will give more punch to the machine. Also consider to change auto bias to fixed bias, this will make matching of tube less effort.
