Grey smoke from a KEL84 - Help!

[bikerhifinut]

Personally I don't think playing around with the cathode resistor is of any benefit. The KEL84 is not, of itself, an inherently unreliable design. The OP just got unlucky I think with a set of dodgy valves. He never said if they were new or "Previously enjoyed".
If it was my amplifier I'd be satisfied with a replacement of the 270R cathode resistors, perhaps uprated to 2 or 3W, and their associated bypass capacitors x4 as they literally cost pennies for good quality non "boutique" components. Along with the 100R screen grid resistors.
Plug in a set of known good EL84 and measure the voltages on anode, screen grid and cathode to check all in tolerance.
and enjoy.

Now on the Stereo20, which is only similar to a KEL84 in that it uses the classic 270R cathode resistor on its output stage along with 50uF bypass and theres a reason for that, there were substantial mods made to the St20 namely a reduction in HT volts,grid leak reduced to 470k, and a reworked output transformer the 8778 which had the UL tap moved from 50% to 25%, all of which made it an easier life for the EL84.
Owners of the older Stereo20 that didnt get modified with expensive mains and output transformers can just use the modified circuit values and that usually suffices along with a coupling cap mod on the LTP phase splitter and also reduce the first stage anode R to 47k from 100k. Some also had a frequency correction network on the first stage anode but this was by no means universal.

Andy

[misterc6]

Many thanks to all who have responded to this thread. Special thanks to John for sending a pack of very helpful resistors. Thanks to Richard for the offer of capacitors, but the single spare I had did the job.

I was trying out a set of Mullard EL84s from my spares box and one of them turned out to have an internal g1 - g2 short. The total damage was a 100uF/35V capacitor, a 270R resistor, a 100R resistor and the fuse. Thanks to John, I could replace the 100R and 270R resistors on all four valves.

I normally use military specification 6n14n EB Sovtek output valves which have 14W plate dissipation and a life span of 10,000 hours, I'll be sticking with those in the future.

Malcolm

[bikerhifinut]

Ah the 6P14PEV (to translate from the cyrillic).
Nice meaty valve, however don't be lulled into thinking they'll last any longer than a bog standard EL84 type. My set of said Ex Soviet mil spec toobs went noisy on me. and relatively quickly too.
Whereas my much maligned set of Chinese made "Golden dragon" brand (PM components trade mark) Valves are still in the St20 some 20 years or so down the line, the amp sounds fine and dandy to me and the cathode voltages are still in spec so make of that what you will.
I could have just got lucky with those and unlucky with the Roosian jobs.
Stuff happens as they say.

As to old Mullard EL84 "pulls"............. well if you must, but get them tested or measure all the electrode voltages carefully whilst running up on a Variac.
I can't recommend a variac highly enough if you're into servicing and constructing Valve gear. I have a huge one bought at a swapmeet for peanuts, of which there are a few scattered round the country along with Ham radio fairs. These can be very happy hunting grounds with prices based firmly in the real world that eludes the eBay fantasists.
Oh and some jackal dealers that I'm sure slap the word "Audio grade" or "audiophile" with some weasel word flowery descriptions in the expectation they'll lure in some unsuspecting soul who's spent too much time browsing some of the more extreme internet forums.

Sorry guys am I on a rant?

A.

[bob orbell]

You do put a good point Andy and I am with you ref. variac, got one. Bob