Help with tube data sheet, ecll800

[wozt]

Hello and thanks for any input /advice.
The tube data sheet for the ECLL800 shown here http://frank.pocnet.net/short/001/e/ECLL800.gif shows 26ma for the two pentodes and 16ma for the shared screen. What are these values of, desirable bias, maximum bias, absolute maximum values over the range of an amplifiers performance?
There are no data sheets that include the graphs for working out bias etc. So I sort of ball parked an amp together, this has served me well for 2 years. I have gone back inside to take measurements as I was hoping to get more power as it is quieter than my 5 watt marshall,

With tubes in I have 290v HT, I have a cathode resistor 360ohm that shows 10v across it =28ma total
It states max power for each pentode is 6w, so if I was aiming for 70% bias then (6/290)x0.7=14.5ma per pentode, so is it biased cold. What do i change to get it nearer the 70% bias? DO i lower or raise the cathode resistor?

Thanks.

[Chuck H]

That's an unusual (to me) data sheet. It looks to me more like they've set it up as an example circuit. In that case I would assume using this circuit has the tube at nominal operation, not max.

You can ignore the 70% bias (rule?) because you're cathode biased. This tends to bias the tube colder as it conducts and just overall holds the bias in check. Typical cathode bias currents would be 90% to 100% of max dissipation for EL84's as an example.

You would raise the bias voltage (cooler) by raising the resistor value or lower the bias voltage (hotter) by lowering the resistor value.

Since you're only a little higher than the example circuit specs for voltage, and the example circuit is likely nominal operation, I'd say go ahead and try a 180 ohm shared cathode bias resistor. I'd also have a 220 ohm on hand for adjustments.

Since your voltages are reported to be about right it could be something else causing the low output. Are you using the same speaker/s for both this amp and the Marshall? Speakers can be a real deal maker or breaker for actual volume, power not withstanding. What is the rest of the circuit for this amp? If you don't drive the grids of the ECLL800 with enough signal you won't get full output power. There are many possibilities. Most of them would be related to peripheral circuits and method of operation which we (the forum) have no information on.

[wozt]

Thanks for the reply. I have a marshallesque parallel input V1 12ax7, into a typical cathode follower v2 12ax7, into a ax84 tone stack and onto the power valve.
If my tubes are only conducting approx 11ma on each pentode could I have crap (cool?) tubes? Not sure where I picked up the 70% bias rule.

[Chuck H]

Yes, your tube is biased cool. Try the lower value cathode resistor and if you like it better, it's better. Provided the tube is operating within it's limits.

I will speculate by your preamp description that you have very little drive at the power tube grids. The parallel first stage doesn't have significant signal voltage over a single triode. The cathode follower has gain of less than one and the tone stack eats considerable gain. So, you basically have much less than one single triodes worth of gain for the guitar signal before the power tube grids. That's not much gain. You might try series triodes for the first tube instead of parallel to bump it.

[wozt]

I have never considered that. I'll lower the bias resistor 1st and then rewire one side of the amp so v1a/b are in series, I'll be able to compare the two, can't wait. Thanks for your time.

[wozt]

Cathode resistor lowered and a nice volume increase on both sides. Then I just put one side v1 in series and YEP that's done what I needed.
Many thanks chuck, your time was appreciated.