18Watt.com

Just red-plated a set of JJ 84's in my Ceriatone- I'm guessing 30 to 40 hours on max volume. Talked to Nik, he said always to use a matched set, especially in the clones. Also possibly change the cathode resistor to a higher amount.

Similar experience here, about that long, NOS philips miniwatts. Pretty inherent to the design, I'd say.

Jim

I don't get it? What do you mean by "last"?

Do they blow up and fail at this point? Or do you reach some point where you arbitrarily believe they are "worn out"?

Gdavis2265 wrote:Just red-plated a set of JJ 84's in my Ceriatone- I'm guessing 30 to 40 hours on max volume. Talked to Nik, he said always to use a matched set, especially in the clones. Also possibly change the cathode resistor to a higher amount.

Stock value for the cathode resistor is 125 Ohms. I'm using 150 Ohms in my 18watt and liking the sound very much. It looks a little funky on the O-Scope but sounds nice.

I haven't put enough hours on my NOS Polam EL84's (from sgtoverdrive) to comment about the life span, but I'm easily over 60 hours with no signs of trouble.

Maybe hunt down Sarge and see if he's got some tubes that will take the abuse.

str82hvn

I guess what I mean is they are glowing bright red on the outside of the tube, and dropping significantly in volume. So, I'm guessing the tube is not usable any more.

No, the tube could be good. Just might need a higher bias resistor, sometimes 180 ohms cures the red plate and sounds good.

I agree. As long as you don't red plate a tube for a long time, it could still be fine. I highly recommend changing the 125ohm cathode bias resistor to 150 or 180ohms. Back when the 18 watt was designed the wall voltages were up to 20 Volts less than you get these days. This ran the EL84s on the ragged, but generally safe edge. The higher voltages of today are really too high. By making this change you can greatly enhance tube life and the tone is just as sweet.

I've had those phillips red plate on me too. the 150 ohm cathode resistor is the way to go.

I may be totally off base here, but is redplating less a function of the amount of hours, and more a function of B+/biasing?

I was hoping to get more than 40-60 hours out of a pair of Polams (150 ohm biasing resistor at 300-310V B+). Please correct my expectations - what's typical for a good pair of NOS EL84s like the Polam/Telam units?

Please say 1000 hours

On a Spitfire,which is similar to an 18W,I was running 350V on the plates,1K screen resistors,120 ohm cathode resistor,2 NOS Westinghouse Canada tubes......lasted a month worth of 2 or 3 bar dates a week plus home practice.
One of the tubes went microphonic,something loose inside,sounded like reverb when you tapped it or on top of the cab while playing.

Changed the cathode resistor to 150 and two new Sovtek EL84(yes,I know ,there are beter choices but I had them already) and it seems to be running fine for now.
I will get JJs as soon as I can.

Almost forgot to add,it sounded great while they lasted.

Power tubes can last a long time. I think 1000 hours is not unreasonable at all. I'm thinking for like 6 months to 1.5 years of 2-3 nights a week gigging. Preamp tubes can last decades. NOS tubes should generally last longer than new production.

For new production I like the JJs and Ei's. There are people who really like Sovtek (although I'm not that fond of them).

Right on. We'll see about these Poles. I'm going to do the LiteIIb mods that ZP recommends, and not go too high with voltages or too low with cathode resistors.

Aharon wrote:On a Spitfire,which is similar to an 18W,I was running 350V on the plates,1K screen resistors,120 ohm cathode resistor, 2 NOS Westinghouse Canada tubes......lasted a month worth of 2 or 3 bar dates a week plus home practice.

Similar, but it's generally believed that Spitfires are a lot harder on their power tubes due to the way they run the tubes at half the load impedance that an 18W does. So they're approximately doubling the current through the tubes at those crazy high voltages. 18W power tubes should last a bit longer than Spitfire's.

Holy cow - like into a 4k OT or thereabouts?

Yep! Think about it - around 340V and a 4k load. Puts the "fire" into Spitfire.

That was so gonna be my next response (i.e. hence the name)

Thanks for the info.