• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

It makes me wonder ...

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

:confused: :confused: :confused:
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
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.


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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.
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


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Thanks SY.

NOPE. The ECC99 won't go in any way to that expected bias.

Now the B+ is happily running at 325V and the ECC99 is sitting on top of 4 LEDs at 7.6V. In between just the RC to lower the supply to a suitable value. R can be adjusted from 5k to 15k and C is 10uF.

I changed the bias to 7.6 from 3.7 after a dozen of attemps to bias the tube at 140V, 18mA, 3.7mA. It has been giving 200-240V, 7-11 mA. The history repeated at 7.6V ... the latest point I found was 260V, 9mA, 7.6V.

Apparently all the points were hanging about the ECC82 curves!?!?! but for sure far away from any curve given for the ECC99.


I noticed the tube slowly moves towards the steady state increasing the current, on certain occasions it took half an hour to reach a definitive stable point. But that's a minor issue probably.

I don't what you were thinking when referring to next step, but one option could be to use a VR to set 150V on top of the anode but I would prefer to stay simple. The other to throw away these tubes (brand new actually) and try a 6H30PI.

Don't know. Any suggestion folks?

It has been giving 200-240V, 7-11 mA.

Well, this certainly doesn't fall on the published curves. You've got a choice- the next step is either to see if you can get in-spec tubes or to redesign that stage for another tube type. Increasing the bias voltage is taking you in the wrong direction; it makes the tube want to conduct less, not more.

From the pictures (I've never seen an ECC99), this tube doesn't look anything like an ECC82. I wonder if you've got something like a 6CG7/6FQ7 or 7119 on your hands? Either would be preferable to the high-distortion ECC82.
Yes it is. Infact I checked the overall dimensions of these tubes and they match with ECC99 (ECC82 is shorter). I don't know ... but I imagined JJ assembled the inner parts (plate, heater etc...) into a wrong envelope.

I also checked the heater resistance: it's some 5.5R while ECC99 should be 800mA/6.3V=7.8R

I can't imagine anything else to recognize this valve.

Here is the picture of ECC99:


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