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What would cause the tubes (KT88) to pop at warm up?
Once warm all is ok.
Should I put a delay and warm up tubes for 20 sec?

Greg

Greg,

Does the pop come through the speakers or is it just the tubes making a noise? If it's just the tubes it's most likely thermal expansion as they warm up. That shouldn't be much of a problem. Most of my larger tubes pop and crack a bit, especially when I turn the amps off and the tubes are cooling down.

If it's though the speakers it could be a number of things, but I have had amps that pop occasionally (especially my 832A amp). When I look at them in the dark, I see some very small interelectrode discharges when the pops occur. Amazing but it's true. I've been very careful to put screen dampers in all of my amps to eliminate instability that can cause that, but it still seems to happen sometimes. Just some ideas. As I mentioned it could be other things like coupling caps or resistors (if the sound is coming through the speakers).

Regards,

Hi Ed and thanks for the reply.
The pop is coming threw the speakers.
During the popping I can see there is a blue burst in the centre of the KT's.

Greg

Greg,

I would place a 100 - 470 ohm, 1 watt resistor in series with the screen lead at the base of each of the KT-88 sockets. This is similar to control grid "stoppers" that lower the Q of the grid circuit and prevent oscillations. There is an article on the audioXpress web site about this problem. Something about "care and feeding of power tubes". I've mostly only had this problem with amps that have solid state rectifiers but it could happen in any amp I suppose. If you have a solid state type power supply it would be a good idea to put the screen resistors in right away. One sustained arc and your KT-88's will be toast.

Once I did this on my amps I had no more problems. The 832 amp is another beastie since 832's will work up into the VHF range, but it happens infrequently enough that I haven't bothered with further tweaking.

Hope this helps.

Ed should I do it to both amps?

Greg

Greg,

I'm not sure what amps you have but if you're running KT-88's in both I sure would. I also do this in my EL-34 amps as well.

Ed the amps I'm talking about are the mark 3's with the poseidon driver boards.

Greg

Sorry for this but when you say screen lead are you saying to put the resistor between "G and F" on the Poseidon board?
Or across pin 5 "Control grid" of both the KT's?

Greg

Greg,

Lift the transformer wires that go to pin 4 on each KT-88 socket. These are the ultralinear taps from the output transformers that go to the screen grids. Then attach a resistor to pin 4 on each KT-88 socket as close to the pin as possible. You will need two resistors for each amplifier. Then trim the free end of the resistor and re-connect the output transformer leads to the resistor free lead. The wires from the transformers should be green and green-white. This places the resistor in series with the output transformer lead to the screen grid of the KT-88.

This change is not done on the circuit board.

Hope this makes sense.

Ed thank you again for your help.


Greg

Greg,

Glad to help. Let us know how it works out.

Greg,

BTW - here is the link to the article I was talking about:

http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf

Ed thank you for the link I will be reading it tonight.

Greg

Hi Ed and thank you I left the amps off for a couple of days and then tested them "no pops".
Thank you again for the great help.



Greg

Greg,

Glad to hear it worked. We gotta preserve those KT-88's!