Wavac Amplifier Repair
The interesting thing about being in this line is, you get to see a lot of amplifiers being sent to you for analysis for free, and the better thing is, they will pay you for it!
Last week, a Wavac EC-300B amplifier were "causing" 300B tubes to have red plates. According to the owner, the tubes were checked and sent back to the tube manufacturer for replacement and it should not be the tube's problem.
Well, okay, let's check the Wavac EC-300B amplifier then. It's a good opportunity to open a high end expensive amplifier like this, like what we did a while ago when we played with Wavac PRT-1.
Before everything else, let's take a look at some pictures.
|Wavac EC-300B repair|
|Wavac EC-300B up close|
This unit is really well built, with ample over-sizing to ensure safe operation and long life. There aren't a lot of audiophile components on it, but it is decent enough. The mechanical design is very innovative as well with very solidly made top late.
In order to debug the unit, we traced out the circuit as well from the input tube WE 435A, to driver tube 6L6 and finally the 300B. It does not use tube rectification though, which is something we like, but everything else are quite high in quality.
WE 435A is direct coupled to 6L6 and Tango NC-20 is used as interstage to drive the 300B. There are no coupling capacitor at all in the design. That is the right thing to do. All the other transformers are Tango's and we've no complain on that, except to wish that it was made with our Level 3 NanoC transformers. LOL.
Visual inspection showed us nothing wrong. All components looks perfect with no leakage, burnt marks, or any abnormalities. A power test will need to be done, to check all the operating points of the tubes.
Upon measurement, we found that one side of the amplifier is showing DC on the grid, and that's causing the 300B to have red plates due to wrong biasing. We traced the DC further and found that it is actually having the same potential as the cathode! Now that's strange!
For interstage coupled circuit, there shall not be any connection between the cathode and grid unless it is shorted. Therefore, we powered it off, and took it out for further measurement to see if it is the tube or the amplifier.
Well, let's have the picture to tell the story! It appears that the 300B tube was causing the short! The grid is shorted to the cathode! It was not the Wavac EC-300B's fault that it red-plates!
|300B short circuit|
|modern 300B quality issue?|
We put on another pair of 300B, PSVANE 300B onto Wavac EC-300B to confirm if it is still working properly, and everything checked out OK. All the operating points are within the designed specifications and left/right matching are within reasonable tolerances.
That concludes our debugging! Time to send the 300B tubes for replacement! Wavac EC-300B works perfectly and no servicing required.
Thank you for sending us Wavac EC-300B for circuit tracing. We definitely learned something from looking at the design.
J&K Audio Design