DRRI bias sweep.

[martin manning]

Ok, so to increase the bias voltage (and idle current) you need to reduce the value of R59 to bring the bias pot's wiper closer to ground (note the other end of R59 is grounded). Try putting a 10k across the original 22k (that makes R59 6.9k) and see what you can get for bias current. The exact bias voltage doesn't matter, as long as you can get the current you are after.

Leo is a good guy, and I think he had you on the right track. I only stepped in to see if I could clear up that higher-lower negative thing.

[Sea Chief]

Ok, so to increase the bias voltage (and idle current) you need to reduce the value of R59 to bring the bias pot's wiper closer to ground (note the other end of R59 is grounded). Try putting a 10k across the original 22k (that makes R59 6.9k) and see what you can get for bias current. The exact bias voltage doesn't matter, as long as you can get the current you are after.

Leo is a good guy, and I think he had you on the right track. I only stepped in to see if I could clear up that higher-lower negative thing.

Ok Im with you martin- I'll try a 10k across the 22k instead of t'other 22k.

Leo was saying he was concerned tho about the bias voltage was it, being less than -30V mark.. and that the 6V6's were possibly damaged.

I dont get where he's gone- a very good guy indeed. Sorry if I upset you, not sure how tho.

So Im looking for a reading across the 1.6r left as it is, of 32mV? dependent on whether I see this available on the sweep of the bias pot then tho.. as I mentioned I read only 11mA max available as it was with 2x 22k (thats 19mV not divided by the 1.6R figure).

[martin manning]

Leo was saying he was concerned tho about the bias voltage was it, being less than -30V mark.. and that the 6V6's were possibly damaged.

He meant higher than the -30V mark, and he might be right. For nominal 6V6's with 400V on the plates and screen, I would estimate -36V to be about 21 mA, but there is quite a bit of variation in production tubes.

[Sea Chief]

He meant higher than the -30V mark, and he might be right. For nominal 6V6's with 400V on the plates and screen, I would estimate -36V to be about 21 mA, but there is quite a bit of variation in production tubes.

Now I think that the pro tested both 6v6's after the red-plating affair, in his vintage french engineered tube tester (looked like an old upright typewriter with a VU meter on- a lovely thing).. and they were both fine.

Would this be enough to proceed with confidence if he confirms? he'll reply AM if this is what indeed he did.

So remind me would you: I'm increacing the pot sweep by adding the 10k (understood), and then ramping up the bias mV fig to 32mV (if that is, I have enough sweep now to do so). If so, is there a figure then I should be hoping for re. the -V ?.. at pin 5, the screen? I cant remember.

[Sea Chief]

Sorry Martin another Q.

If I do manage to increase the bias current to 32mA (/1.6 = 21mA).. doesn't the bias Voltage decrease as a result? isn't that what Leo was saying? if so, I might get -36V or so after all.

[martin manning]

So remind me would you: I'm increacing the pot sweep by adding the 10k (understood), and then ramping up the bias mV fig to 32mV (if that is, I have enough sweep now to do so). If so, is there a figure then I should be hoping for re. the -V ?.. at pin 5, the screen? I cant remember.

Yes correct, but shifting and increasing the pot sweep, really, and as I said below, I'd expect -36V on pin 5 for new, on-spec tubes.

[Sea Chief]

Ok thanks alot- I'll see what the new sweep is tmrw.

(Isnt cincinatti where Prince hails from- up north isn't it & v cold at this time?)

[martin manning]

Sorry Martin another Q.

If I do manage to increase the bias current to 32mA (/1.6 = 21mA).. doesn't the bias Voltage decrease as a result? isn't that what Leo was saying? if so, I might get -36V or so after all.

Careful, you want the CURRENT to be 21 mA, and in order to get that, the VOLTAGE on the cathode should be 32 mV. Again, increasing the grid bias voltage, going from -35V to -30V say, will cause the current through the tube to increase. The exact voltage required doesn't matter.

Prince is from Minnesota, and it's been very warm for December up there. Cincinnati is considerably further south, but also very warm just lately... 70F on Saturday.

[Sea Chief]

Ah right Im confused again. I thought that I was decreeacing the resistor, which gave more sweep on the bias pot, which meant an increaced 32mV reading (hope I can get this much on the pot now/ which equates to 21mA), which auotmatically meant a decreace in bias Voltage (measured at pin 5).

Is any of this wrong? am I incracing the voltage or something in order to decrease the bias current-?

70*F? thats not very fatherchrismassy.

[martin manning]

Ah right Im confused again. I thought that I was decreeacing the resistor, which gave more sweep on the bias pot, which meant an increaced 32mV reading (hope I can get this much on the pot now/ which equates to 21mA), which auotmatically meant a decreace in bias Voltage (measured at pin 5).

Is any of this wrong? am I incracing the voltage or something in order to decrease the bias current-?

You want the 32 mV on the cathodes, pin 8. That voltage on pin 8 is lower than that now, so you need more current flowing. The way to get there is to increase the bias voltage on pin 5, i.e. make it less negative, or closer to zero. Reducing R59 moves the bias range in that direction.

[g1]

Martin, I respectfully disagree on the semantics of what "increasing" negative voltage means.
But I don't want to add any confusion or begin a discussion that will just be another sidetrack. (we can discuss via PM if you want)
I think in order to avoid confusion it's best to only use the terminology "more negative" or "less negative" when speaking of the negative bias voltage (never "increase" or "decrease").

[martin manning]

I don't know, it seems pretty straightforward to me. Using the thermometer analogy, if it's -25 degrees outside and the temperature increases five degrees, then it's -20 no? Being algebraically correct is the only way to go.

[g1]

Unless stated otherwise, voltages are always with respect to zero. You can increase away from zero in two directions. More is increase. So how can "more negative" be a decrease?

[Justin Thomas]

G1,

I was gonna try to write a rational reply to that, but I think that question needs to be a thread of its own! Dagnabbit... :P

Justin

[big_teee]

We ran -48v in the phone offices.
They increased the voltage from -48v to -52v.
Like Greg said an increase is an increase from zero, whether it is negative or positive voltage.
What I haven't heard mentioned is the effect of the - minus, or negative bias volatage.
The minus bias voltage controls the plate current.
So when the bias voltage goes up more negative, the tube plate current goes down, and visa versa.
If you are measuring the cathode current in mv on pin 8, then you can ignore the bias voltage on pin 5 altogether.
I think one going up while the other is going down is the confusing factor.
Just stick with the mv on pin 8 and you will be fine.