5E3 screen current too high ??

[jreese2001]

I am working on restoring an original 1958 5E3.

Here's my problem, the screen current seems way too high. Here are the measurements I took to get to that conclusion:


B+ 399VDC

V4 (6V6) pin 3 389VDC
V3 (6V6) pin 3 399VDC

both V4 and V3 pin 8: 16.4VDC
both V4 and V3 pin 4: 286VDC

The cathode resistor is measured to be 240 ohms.

Last filter cap: 210VDC

So, the last resistor in the power supply is 22K, so 286-210 = 76V dropped or 3.45ma for the preamp.

Across the 4.7K resistor I have 399-286VDC = 113VDC dropped, so that is 24ma across that resistor. Subtracting off the 3.45ma for the preamp gets me around 10ma per tube of screen current. The screen to cathode voltage is approximately 270VDC, so that gives me around 2.7W of screen dissipation. The rating for the tubes I am using (TAD 6V6-GT-STR) says 2.2W is the maximum rating. Did I calculate something incorrectly?

The plate dissipation is around 12W which is below the max rating of 14W.


I suspect there are many issues going on with this amp. I am suspecting oscillation is going on. When playing through the amp it seems distorted (even for a 5e3) with that weird paper tearing noise when you play two notes at once. Measuring the output voltage on an 8 ohm load gives me about 7.5VAC, which is around 7W. Not good.

The PI voltages are as follows:

pin 8: 37VDC
pin 6: 174VDC
pin 1: 138VDC



Also, there is some DC voltage on the grid of one of the power tubes, around -1.1VDC. It is related to the tube itself because I noticed that when I switched V3 and V4 the negative voltage was still on the same tube, not the same socket.


Who can help me track down the problem(s) in this amp? Thanks in advance !

[loudthud]

It looks like one of the 6V6 tubes is not drawing any plate current. There was no voltage drop across the OT primary. This will cause excessive screen current in that tube. This is what my 5E3 was doing when I bought it, but the cause was an that the OT was open on one side. The screen dropping resistor was discolored from excessive dissipation. Replace the 6V6 tubes, or at least one of them.

[jreese2001]

loudthud, that was it. Tested the OT and it looked OK, at least not open. I put in a set of 6L6's I had sitting around (don't have an extra set of 6V6's right now but this will do for testing real quick). Sure enough, the problem goes away. Makes perfect sense that the screen current would come up if the plate isn't drawing any current.

Now, the 6V6's I had were brand new, so that's a pretty early failure. What concerns me is when I first turned on the amp, the voltages and currents were all in spec. So the tubes were not DOA. Any reason I should be concerned that another new set of 6V6's will blow out too? Not sure what could have caused it to fail so quickly.

[loudthud]

I bought a quad of 6550s on ebay. One of them had an open plate. You could actually see where the weld was broken to pin 3 inside the tube.

I forgot to mention that before I bought the 5E3 I turned it on and you could see the screen glowing red through little holes in the plate.

[jreese2001]

Wow, I took a flashlight and looked into the base of the broken tube and I saw exactly the same thing. The plate wire is broken right above the weld (a small bulge that almost looks like a solder joint). So I am wondering, was this a manufacturing defect or something wrong with the amp? I know when I turn it on the B+ momentarily goes up above 500V before coming back down to normal levels as the tubes warm up and begin conducting. Would something like that stress the plates?

[loudthud]

Hard to say without knowing what the testing procedure is at the factory. If it's just a quick and dirty DC test where they just measure the cathode current, it could have passed the test. Some Stateside re-sellers do it that way. Probably a weak weld that may have broken during shipping.