It makes me wonder ...

[Gluca]

ok I am not Robert Plant and this is not the problem. The problem is with the ECC99 from JJ I am using in the driver stage of my amp.

I would bias the tube at 150V anode/20mA/3V bias but it seems it likes 210V anode /15mA/2.5V bias (the best I found) and that point is not anywhere in the ECC99 curves. I am wondering if that tube is something else, for example the ECC82???

BTW could you give me any advise and what I should look for to find the cause of this strange biasing?

Some info:

Power supply: GZ34 + CLCRC. I just use a 10K pot for the R in order to find the proper setting. At the moment it's ca 4000k. Changing the R does not shift the ECC99 that much ... it just lowers its anode current to stay at ca 210V vith different biases.

Driver: ECC99 Push-Pull into an interstage LL1660 from Lundhal. Its biased at 2.5 V with a common 85R. No bypass capacitor. Anode-Cathode voltage is 210V ca. Halves of the ECC99 are balanced within +/- 3mA with this bias.


Ciao Gianluca

[SY]

At 150V, 20mA, the curves for this tube look like a 4V grid bias is necessary.

[Gluca]

Yes thanks.

The problem is that the tube won't go to that bias ... as you can see it find its balance around a point very far away from any datasheet curve. It seems it's not the ECC99 but an other tube!
The problem occured also with an other spare ECC99 I have in my hands.

Ciao. Gianluca

[Tekko]

Make sure the resistors you used is the right values, it could also be a bad tube.

[Gluca]

R's are OK and, yes, I tried with 2 different tubes.



Ciao

[SY]

Have you tried changing the value of the cathode resistor? Maybe 85 ohms isn't right.

A schematic of that stage wouldn't hurt.

[Gluca]

Yes I did from 60R to 150R. I tried with a pair of red LEDs today to fix the bias at 3.7V but the tube just goes to around 200V anode and 15mA and that point is nowher in the ECC99 datasheet.

Here comes the schem. PSU is GZ34 + CLC to give 325V followed by a simple RC (10K adj + 10uF) to supply the driver stage.

Ciao. Grazie.
Gianluca

[SY]

OK, if the plate voltage goes to 200V with an LED bias, that means you need to re-adjust the driver supply. A 10K adjustable seems rather high for a target draw of 40mA.

But let's eliminate the tube as a suspect (or convict it)- set up a little jig with two series LEDs in the cathode and your driver supply running the B+. Just pick one section for this test. Adjust the variable resistor until you get 140V at the anode. Check the supply voltage to the resistor.

Power down, let all the caps discharge, then disconnect the variable resistor (being careful not to disturb the setting) and measure it. The current = (supply voltage - 140)/measured resistance. If this is within 10-15% of 0.020A, the tube is probably OK and we can go to the next step. If not, the tube does not meet spec.

[Gluca]

Thanks SY.

I carried out some tests with only half of the valve running (tried 2 different valves). Bias at 3.6V with 2 LEDs, GZ34 + 10uF + 4H + 200uF in the PSU (+325V) followed by the adjusting pot + 10uF.

Well, I just got one point close to the ECC99 specs (blu circle in the attached plot) all the others are meaningless. I got that point by chance as when I later tried to set up the same test condition the valve biased in that cloud you can see in the chart.

Just a thought. Might the B+ (325V) be too far away from the desidered bias leading the valve away to a strange unexpected point?

The output stage works very fine BTW.

Ciao. Gianluca

[SY]

I keep coming back to that rail dropping resistor- something tells me that you've misadjusted it. Set everything back up, adjust that dropping resistor until the B+ is correct, then check the voltage on the cathode resistor.