Tube bias current ?

[yaffstone]

I have a question about the proper bias current setting for a modified

Dynaco ST-70. The original design calls for 100 mA accross the

EL-34 output tubes. I replaced the driver board and the

instructions in the replacement kit were to set the bias to 40

mA. Now I have replaced the EL-34 with KT-88 beam power tubes and

am wondering what to do about the bias current.

I had to change a couple of resistors in the bias supply to get it set

back to 40 mA and it sounds spectacular. The question is, what

effect does the bias current have on the sound, tube life, etc?

The KT-88 is rated for 230 mA max plate current so I'm way under specs,

but I wonder is more better or less?

Next question: There is a dual ceramic disk capacitor on the

fillament supply center taps. It's 50 years old and rated at 20

pF. I'm going to replace it, but should I go higher in

capacitance value? I suspect so but opinions will be appreciated.

Finally, I'm waiting for ceramic octal sockets. When they come in

I'm going to change to pure triode mode to see what that'll do.

So far, I just can't believe how good Yo-Yo Ma sounds!

[Marvel]

Craig can probably (make that definitely!) give you more info on the

biasing of the KT88s. The plate current rating and the bias current

aren't the same. The .02 mfd cap can stay the same value.

The heaters on the KT88s pull a bit more current than the EL34. If you

are using the original PS tranny, you may find it warms up even more

than before. I would think it would be quite a stress on the PS to use

KT88s.

Whose upgrade driver board did you go with? I am tempted to replace the

Curcio board I put in last spring, mostly to see what difference there

would be. It sounds okay. Way better than the original, but there are

better designs around.

Bruce

[yaffstone]

I used the Vacuum Tube Audio driver kit with Auricaps and orange drops as well as a Curcio under

chassis mount quad cap replacement board. I left the original

quad cap on the chassis just for looks but it is disconnected

below. I had to modify the Curcio cap design to meet the VTA

requirements. Both upgrades include a bias supply and a solid

state rectifier section. I used the bias supply on the VTA

board since you can set bias individually on each output tube and I

kept the 5AR4 rectifier tube. The original Sellenium rectifier is

replaced with a silicon diode.

As to the filament supply, the KT88 draws 1.6 A while the EL34 draws

1.5. A total of 0.4 amp more load on the filament supply.

The VTA driver board uses 3 12AT7 tubes so that's probably a bit more

current than the original pair of 7199s as well. So far

everythings fine. I leave the cover off for better looks and

cooling.

[Marvel]

I also used the Curcio under chassis ps board. I did replace the power

tranny with one from Ned at Triode Electronics. The attached pic shows

how massive it is compared to the original. Ned's is on the left,

original on the right. It still gets pretty hot, but we have TVA power

here and the voltage pretty much stays at 125 VAC.

Bruce

[Marvel]

Here's the pic. I forgot to scale it down.

[NOSValves]

Yep Marvel has it right the 230ma max plate current is not a bias point. The amount you want to bias the tubes too in this case is dependant on you power transfomer with a stock ST70 transformer your going to want to stay at 50mA or below the KT88's will actually sound better the higher you can put it. The KT88's will last a very long time at 50mA since they can take a heck of allot more then that. I agree that KT88's will sound better. The slightly higher current draw on the 6.3V filament supply coupled with the extra tubes in the front end maybe all its take to make you upgrade that power transformer. Very worth while thing to do in your case.

You want to have some more fun rolling tubes grab you 3 12AU7's and slide them in the front end of that amp. Beware though you will instantly have to crank your preamp volume up more to get the same SPL so don't let this put some preconceived notion that your not going to like the sound. The VTA board has tons of useless gain that is not required the 12AU7's will nock some of that off. You may or may not like the sound give it a whirl.

Craig