Which Amp?

[Richard]

If the paper in oil bypass caps are in parallel with the standard coupling caps they will be seeing the same voltage (blocking 285Vdc) and if they breakdown and leak electrically it will have the effect of turning the valve on.

Kel84 is cathode biased and should show 11Vdc at cathode k pin 3 and 0V at grid 1 pin 2.

You can check for coupling cap leakage easily with a multimeter set to mVdc. Clip the meter neg lead to signal 0V ground then probe grid 1 pin 2 carefully with one hand. The grid should be 0V and anything more than a few mVdc (paper may leak a little) needs changing.

[Shane]

The Jensens are the cathode bypass caps. They are not in parallel with the coupling caps. They have 11V across them.

[Richard]

Morning Shane, so are they 100uF, or small value bypass to the 100uF electrolytics, or are the cathodes just partially bypassed? Either way I suppose the answer is the same, if they go short they turn the valve on, if they go open there will be a loss of stage gain. They can't be tested in situ though in the way described for the coupling caps so best to change them if old or in any doubt

[Pingushome]

Hi Richard,
Here’s a photo of the caps if that’s any help

https://i.postimg.cc/C5WcQhkP/kel84-inside.jpg

Greg,
your comments greatly appreciated I’ll look into both tomorrow. I’m leaning towards the 88 myself.

I finally had clearance to have my jab late yesterday.
No major probs yet but feeling a bit “spacey” atm however maybe that’s the anti histamines I’ve taken
I’ll have a good look tomorrow when I can concentrate better
Martin
PS Shane thanks for the info

[A Stuart]

Difficult to tell from photo, and not familiar with the amp, but does something not look right with the black wire? (selected from picture) It looks as if it is floating free.

[Greg.]

Those Jensen PIO caps are C8,9,10,11 0.22uF. They are coupling caps. Manufacture date stamp is 1.05 and 1.09 visible on three caps. That makes them 12 to 16 years old. I would replace them regardless of measurement or performance.

[bob orbell]

Originally Posted by A Stuart

Difficult to tell from photo, and not familiar with the amp, but does something not look right with the black wire? (selected from picture) It looks as if it is floating free.

You have good vision Alastair, it dose look as if it is not connected. Bob

[bob orbell]

Agree with Greg, paper in oil are more bother than they are worth, in my opinion. Bob

[Greg.]

Originally Posted by bob orbell

Agree with Greg, paper in oil are more bother than they are worth, in my opinion. Bob

Jensen PIO caps are good and they work particularly well in KeL84. Replacing them every ten years doesn’t seem much of a bother to me.

That black wire looks attached to me. Is it not connected to the remote volume pot? I’m not familiar with how these hook up.

[Richard]

Hi Martin, yes you can check them for leakage as in post 11. If they need changing I'd cut the leads near the cap body to leave long legs to solder the new cap legs to, which will save taking the board out. (A quick hot iron and clip a heat shunt near the pcb.)