only one side of powertubes conducting - other side is dead

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violetv
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only one side of powertubes conducting - other side is dead

Post by violetv »

hello,

I've got a strange problem in the powersection of my 36 watt. And I can not grasp the reason why it is happening or what is causing it. (man... the houres that I've spend on trying to figure it out....)

I've used the schematic that is posted on this site. The connections of the powersection of the amp (screen, anode, cathode, etc...) are exactly the same.

It seems that only one side of the powertubes are conducting.

I'll try to explain: if you have the OT, I get 333v at the central tap (where the B+-line is connected). One side of the OT (left side) is 297 volts. This is the side where the 2 powertubes are conducting. The other side (right side) of the OT stays at 333 volts, meaning no current is going through these two tubes.

I have about 24 volts on the cathodes. Ive used 180 ohm resistors, so 133 mA of idle current is going through 2 powertubes. Way to much according to the datasheet of JJ EL84.

Another strange thing is that it's not always the same 2 tubes that are conducting. When the amp is switched off and on again, it can happen that the other side (right side of the OT so to speak) is conducting and the left side is 'dead'.

If I raise the cathoderesistors (say from 180 ohm to 280 ohm), the voltage goes up keeping the idle current at 133 mA. So baising this thing is impossible.

I never get all the 4 pentodes to conduct at the same time.

Has anyone experienced this type of strangeness. Or has anyone have a clue of why this could be happening.

I've changed the powertransformer (now a hammond 275v - ct - 275v at 200mA), I've resoldered all of the joints, I've looked over all of the wires, groundings, etc... but I can not find the problem. I've even swapped powertubes in the hope that some kind of order of powertubes could make this thing going, but no dice. I've measured the OT for any broken windings, but everything seems ok.

I really could use some help here. All possible suggestions are welcome.
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kleuck
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Post by kleuck »

8O
Sounds like a wiring/connectivity issue.
I encountered this problem once (and fried a good RFT :( ), because I forgot to connect one side of the OT.
But your story is weird : not always the same side ?!?!
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violetv
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Post by violetv »

yeah, weird problem... but solved.
Thanks to the guys at ampgarage. :lol:

Seems that I've soldered the grid-to-ground resistors before the grid caps (between the anode off the driver-tube and the cap so to speak). And they should be after the grid-caps, so the grid of the powertubes have a reference to ground.

Still, despite the many houres of pulling hairs out of my skull, a very interesting lesson learned.

So if anybody encounters this same strange thing I had... the above is the possible answer.
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