Biasing a KEL84?

[Gromit]

Apologies for the total valve-newbie question (I am learning...honest!)

I fancy trying some different output valves in my 3yr old KEL84 - only thing is that I don't know how to bias them, or even if I need to at all? Does the KEL have an autobias function, or would I have to do it manually?

The amp currently runs NOS Philips btw.

Many thanks for any tips or advice (even if it's 'get someone who knows what they're doing to do it')

[david counter]

you don't have to worry about it, it self biases

you could try some Sovtek M, I think WD sell them now,
I'm very pleased with mine

here's a link
http://wduk.worldomain.net/acatalog/...Valves_22.html

[Gromit]

Thanks David - that's good news.

[Richard]

Hi Gromit,

As David says, the amp is auto/self biasing through the cathode resistors and doesn't need adjusting for new valves.

The only thing I'd add is that the new set should be reasonably well matched for current. That means asking the supplier for 2 matched pairs or a matched quad. Usually this is taken as within 10%. I've tested many new valves of all types and can assure folk that currents in new ones can vary well outside this. Whilst the valves are all "good" they won't all work with each other. Currents of 48mA and 77mA in new 6550's for example and similar variations in EL84s when I ran a leak ST20.

From those days EI are very good sounding, near identical looking, copies of Mullards and JJ's too measure and sound very good.

If you have odd valves and no tester pair and run them at first in a dark room to check for any plate redness or hum which might indicate a poor match. If it occurs swap one around and hope you find pairs which work more happily. The better the match the lower the distortion due to balancing the current in the OPTX.

Rich

[Gromit]

Hi Rich - many thanks for that. Think it might be time to do a bit of valve-dabbling (nice that the 84's are relatively cheap too)

[andrew ivimey]

If you plough your way into eBay's Russian sellers you may be able to find Reflektor EL84M. (perhaps they are what EH / Sovtek are now selling)

When I had a KEL84 I tried a few brands and really was impressed with Reflektors - so much so that I carried on this enthisioasm to a Beard amp that used 16 EL84s! - Marvellous.

(thinks - should really get that Stereo20 up & runnning - I have four Reflektors left ... somewhere)

[Dave Senior]

Hi

I also use reflector EL84M's. They are a fair bit more dynamic than JJ's or EI Golds - drums really have a thwack to them.

Dave

[Gromit]

Andrew/Dave - thanks guys, I'll do a bit of trawling around eBay and see what's out there.