Gooped PT

[paggerman]

You may remember in an earlier post 'Bad case of GAS' I talked of a Nippergram portable phonogram that i bought at a garage sale. Well, I've taken the advice that I was given and stripped the guts in preparation for a low watt SE amp build. Steve Conner at the time said that the wax paper PT is fine as long as it doesn't have goop all over it - an indication of a bad component at some time in the past. Well, it's got goop on it. The side where the secondaries are has what looks like hardened resin over the wax paper and has run along and hardened on the leads. So I replaced the previous 2core with 3core mains lead and earthed the chassis and plugged in with nothing else in the chassis. I measured 300VAC between each of two green leads going to the 6X4 rectifier socket and earth, 210VAC between an orange lead (I suspect that this was to power the turnatable motor) and earth and about 7VAC between the heater lead and earth. There are two yellow leads to the chassis. And of course the primaries at 230VAC. Or 240VAC. Depends on who you trust.
Question 1: despite the goop, is this PT OK to use?
Question 2: If the answer to 1 is yes, is its longevity jeopardised by the goop leak?

Many thanks to everyone - as ever, you are a source of inspiration and education.

[Enzo]

There is goop and there is goop. The point is, if it looks like some sort of goop melted inside and ran out, that is a bad sign. On the other hand if they made the thing and squirted some sort of potting goop into it, that is fine - it was made that way.

[paggerman]

update to gooped PT

So I took Steve's advice and removed everything and left it to run with no load for a couple of hours. Nothing happened. No warming, nothing. So I built a little SE amp. Did the usual check, re-check and check again. Tested all the tags for shorts to adjacent tags. Feeling confident I plugged in the 6X4 rectifier (no other valves) and the mains. The heater started heating fine and. I started measuring voltages. 283AC on each of the anodes and about 17VDC on the cathode. Uh? Checked the B+, B++ and B+++ and each had a value (300 or so) that I might have expected in but in mV!!! Started to check other things. Checked pin 5 of the rectifer which had a varying DC voltage in the order of 180-200VDC. And that's when I heard the crackling. A quick look at the rectifier and it looked somewhat brighter than other valves I've seen and there were a few arcs going on on the central stem. Unplugged straight away. Checked the VDC on the filter caps and all was safe. So I picked up the chassis - the PT was warm. So I suppose the 6X4 is the cause of the problem.

Question - if I replace the the 6X4 with 1N4007s could I expect the PT to be usable?

Many thanks

[Steve Conner]

Yes. But do check to make sure your circuit is correct and the B+ isn't shorted to ground. (Even if your circuit is correct, are you reusing old filter caps?)

If it is shorted, that's one explanation for the rectifier arcing, and when you install those 1N4007s, they could pass enough current to burn out the PT.

Of course another likely explanation is, the 6X4 is simply old and worn out.

Also, your readings aren't consistent. I could take from your post that the rectifier is putting out 17V, 300mV, 180V, or 200V. Which is it?! I suppose if it was arcing, the voltage could jump around crazily, so it might actually be all of them.

[paggerman]

Hi Steve. Found the root cause of those odd readings. Let's just call it a beginner's learning experience with no harm done (except to the filter caps - oops). I have now put in the 1N4007s, a 100R dropping resistor and I'm getting decent voltages all round. Plugged in today for the first time. Very, very quiet. My strings are louder than the output. The output is clipping, so I think my pre-amp stage is OK, I just need to make sure I have the power section right. The output valve is a 6M5 for which there is very little info available. Apparently it is similar to an EL80. Even that has scarce information. I have obtained a couple of schematics using a 6M5, but the bias point is shown as being on the grid rather than the cathode. The datasheet shows the grid as needing about -7v for the voltage I have on the anode. I have not yet checked my wiring compared to the schematics I have so that will be my next check.

The overall design is very much experimental. I have used a number of circuit diagrams, datasheet and Merlin's website as references: ampmaker WF-55, Gibson GA5 and Dynaco's AA4. The power amp has been designed according to the scant information I have found for the EL80 pentode and referring to other SE designs. I am using two 6AT6 for preamp, the original chassis, PT, OPT and 6M5 from the donor. I have created a schematic and will post it in the event of either success or inabilty to resolve!

I'll let you know how I get on.

Many thanks for your advice, I really appreciate it.

[paggerman]

Plugged in again this evening - all voltages OK, plugged in a guitar - big sound! I don't like having a non-repeatbale problem, but for now, it's alive. Full story and circuit diagram under 'Bad case of GAS'. Cheers