Question about other options of power tubes for the express.

[redshark]

What should be the correct resistor change to allow the express to use KT66, 5881 or 6L6 type tubes and have it properly biased?

Are there any real wrecks running this types of tubes?

Just curious.....

[funkmeblue]

I changed the 220k in the bias supply to 150k so I could run kt66 and kt88 in an amp I built for a good friend

[martin manning]

6V6's will work with little or no circuit change, but I don't think the KT66, 5881, or 6L6 will be very happy (or sound very good) with the 6k6 OT impedance. The KT66 might do pretty well with the impedance switch indexed up to reflect 3k3 (set at 16 ohm and run into 8 ohm load), the 6L6 not so much, but probably still usable, and the 5881 might be too close to red-plating.

[M Fowler]

I've run 6v6, EL34, 6L6, and 6550 in my express only adjusted bias.

I liked the 6L6's but went back to EL34's.

[Clyde]

I would think that 3k3 would be awfully low for KT66. Isn't 6.6k fairly optimum for 5881 6L6?

[martin manning]

I would think that 3k3 would be awfully low for KT66. Isn't 6.6k fairly optimum for 5881 6L6?

"Optimum" depends on the plate and screen voltages and how they vary with signal level. I looked at the load line location with respect to the knee of the scaled 0V Vg1 line and max power dissipation running at Express voltages (400V plate, 385 screen is where mine runs with rlguitars Pacific PT). If the load line is below the knee the screen current shoots up at peak signal, unless there is a lot of screen voltage sag. With the exception of the 6V6, the beam tubes mentioned above would all have the load line placed well below the knee at 6k6 Raa. The EL34 will have the same issue but the screen current rises more gradually in pentodes. Its interesting to note that the Mullard EL34 data sheet recommends a 3k5 load at 400V with a 1k5 Rg2 (the Express Rg2 is close as it has 2k between the screen grid and plate voltage).

[Clyde]

Thanks for doing that Martin, very interesting. I have to admit I didn't plot load-lines but have been running KT66's into 8k for years at 490v B+ with no problems. As well, KT77's into 3.4k and 6.6k with the same voltages. Is the RJ OPT 6.6k as well?

[martin manning]

Yes, rj's pacific OT is listed at 6k6. If you don't drive the power amp hard enough to pull the Vg1 down to zero, or there is a lot of resistance in the screen grid circuit to take the screen voltage down along with the Va-k, then the screens won't experience the high current. Maybe the Express has enough Vg2 sag to keep out of trouble, but it seems scary to me since the Express is all about driving the power stage hard. What kind of screen voltage do you have to go with your 490V Va-k, and how much screen circuit resistance?

[Clyde]

Sorry, offhand I'm not sure of the screen voltages, but I'm using 1k 5w screen resistors. With the KT77's, B+ is a little higher and I think there's about 489v on the screens.

[redshark]

Very interesting guys!! thanks for the answers. My question was more if there were any resistor changes in the bias circuit to take 6L6 style tubes so I will put a pair and read current with bias probes. I though the readings were gonna be very high.

[Richie]

I changed the 220k in the bias supply to 150k so I could run kt66 and kt88 in an amp I built for a good friend

I like using a 180k, just seems you have more range for biasing over the 220k.

I also like the addition of the presicion 10 turn locking pot on the outside back of the amp, with test points on the back. It makes biasing the amp much easier with easy access..