Valve gone microphonic - my fault?

[bigfathairyvika]

Been reviving a BC5645 , doing the usual cleaning, oiling, greasing, replacing knackered components.
Also in between all this, powering up and seeing what the result was.

I had just replaced the 32uf smoothing caps and noticed the radio had gone microphonic.
A few taps later and traced it to the dh77 DDT.

Could it be something I've done or just coincidence and the valve needs replacing?

FM works fine, AM is a bit insensitive and there is an annoying hum on low volumes.

Note, I haven't changed out all the hunts 0.01 and 0.005 etc caps yet.

Mark

[peter_sol]

A bit of DC on the control grid due to leaking coupling caps can cause the microphonics.
SeE what it's like when you have done your replacements.

[bigfathairyvika]

OK, will update once checked.

[paulsherwin]

It's really only a 6AT6 in disguise, so there should be no problem replacing it if necessary.

[bigfathairyvika]

Replaced couple of caps and a few way out resistors and the AM and FM working fine.
Still a hum and valve still microphonic, but not as much, and the grid measured -0.6v DC. Zero with valve removed.

OK on 6at6.
Have eye on one for 8 quid delivered. (Apparently never used ;-) )

[paulsherwin]

You should be able to find a good pull from a scrap set easily enough. They don't fail very often. Try asking if anybody has one: https://www.vintage-radio.net/forum/...splay.php?f=28

[bigfathairyvika]

Ta Paul,
I will probably get around to it after I've given the socket a better clean and tightened up the pins.
I found a thread on here where someone describes that cleaning the socket and pins etc fixed the same problem.
I suppose that if I put the chassis back in the cabinet and the issue isn't noticeable then I'll leave as is.

[cathoderay57]

If you're interested I have a used Brimar DH77 that tests about 75% on triode and diodes. I'm unable to test it for microphony, but you can have it for £3 postage. You have a PM. Cheers, Jerry