Bias cap value?

[Chuck H]

I'll check again, but if i missed something it was a major case of space.

EDIT: rechecked and i just cannot find anything. I'll have to just stare at it endlessly i guess, but i'll get back when theres just no use in looking further or i find it.

Take heart. If I had a nickel for every time I was sure "Nothing is wrong, therefor it works. Yet it's broken". I'd have enough $$$ for a decent lunch (no free lunches you know). Patience is hard to come by, but it's key here. Remember you have other amps. This is for fun and tonal advancement. Nothing to lose. You invest your time because it's a worthy investment, etc., etc... Don't pop a vein or lose it. The answer is coming

[daz]

Patience is hard to come by, but it's key here. Remember you have other amps. This is for fun and tonal advancement. Nothing to lose. You invest your time because it's a worthy investment, etc., etc... Don't pop a vein or lose it. The answer is coming

Yes, and no veins popping. This should be an easy one actually, and i know i'll figure it out. Just didn't have time to mess with it tonite or i'd probably already have it done. Too simplema circuit to be too much of a puzzle. I'll be on it tomorrow and post back. Thanks...

[Dave H]

EDIT: rechecked and i just cannot find anything.

Check the actual bias voltage. If there's no bias current it could be that the bias voltage is far too high (negative).

[daz]

Check the actual bias voltage. If there's no bias current it could be that the bias voltage is far too high (negative).

I thought about that and was actually going to do that but couldn't get to it last nite. I will in a few. I don''t know if it would cause that, but maybe the 27k in my drawing was actually a 270k because it was a 3w oxide and i don't recall having one in 27k. Anyways, just woke after a tough night (a few too many) and when i get to it i'll see if that was a 27k or 270k and check the voltage.

[Chuck H]

I'm actually thinking the 15k may be a 150k. That would be the more likely scenario since a 270k as it WAS wired would produce a higher voltage (and therefor a colder bias) than how it should be wired now. This isn't the case because it WAS working. However, if the 15k is actually a 150k, in it's new position it would raise bias voltage in the same way as a 270k scenario in the old design. Further, the 150k would be a more typical value than 15k for tapping off the HV and so it's a more likely value to have been spec'd for that circuit than the 15k indicated. In fact, when I consider it, the circuit as it was indicated using a 15k would have almost certainly produced too much bias voltage. I'm still betting that 15k is actually a 150k.

If this turns out to be the case, or, no matter what resistor may be a different value than the original indications, the ratios and values will need to be reworked. I based the circuit I posted on the values you had indicated. It's not a generic, works in all cases circuit. It's designed to achieve the same ratios your original circuit had. So if the original posted values were wrong, so will my circuit be wrong.

P.S. Enjoy your coffee But also HYDRATE! Tying one on dehydrates, so does coffee. Juice is best because it replaces lost vitamins and electrolytes as well as rehydrating you. Oh... And a couple of aspirin.

[daz]

That was it, tho i can't imagine how i possibly could have read that resistor wrong because i swear it was NOT a 220k, and thats all i have in 3w oxides thats anywhere near that high. The rest are mostly 8.2k's, 2.2k's, and 10k's and 15k's. Nothing higher than that but the 220's and i cannot imagine missing that color band. Unless maybe i DO have a 270k but i can't find it. But no matter, as i put a 220k and it biased right up to almost exactly where it was before, 17.5mA. And i read the voltage between the 220ks at the grids and it was more than double the typical. 80v vs what i know it should have been, around 30-40 where it is now. (35 to be exact)

So it's not working fine Chuck, but should i change things as you suggested or just leave it as is?

[daz]

Oh, and i beat ya to it Chuck.....whenever i drink i chug a big glass of water and a couple aspirin ! usually one b4 bed too. Thins the blood which helps to counter the fact that drinking does the opposite via dehydration. I may not be very good at understanding electronics, but i'm pretty darn experienced at drinking.

[Chuck H]

That was it, tho i can't imagine how i possibly could have read that resistor wrong because i swear it was NOT a 220k, and thats all i have in 3w oxides thats anywhere near that high. The rest are mostly 8.2k's, 2.2k's, and 10k's and 15k's. Nothing higher than that but the 220's and i cannot imagine missing that color band. Unless maybe i DO have a 270k but i can't find it. But no matter, as i put a 220k and it biased right up to almost exactly where it was before, 17.5mA. And i read the voltage between the 220ks at the grids and it was more than double the typical. 80v vs what i know it should have been, around 30-40 where it is now. (35 to be exact)

Wow?!? I'm super unclear about much of what you're saying. How about this...

What is your bias voltage at the tube pins?

and

Please re post one of the diagrams that represents the circuit as it is now with the present and correct component values.

[daz]

Wow?!? I'm super unclear about much of what you're saying. How about this...

What is your bias voltage at the tube pins?

and

Please re post one of the diagrams that represents the circuit as it is now with the present and correct component values.

around 17mV at cathodes. I was wrong about the pot too by the way, it's 50k. So look at your drawing and change the 22k to 22k and the pot to 50k and there you have the current state. I'm wondering tho if the cap might be too small because the bias seems to change a lot. Cold, warm, and after a few minutes of playing all showed different values. Not huge changes but it doesn't seem to stabilize at one reading and varies between 16 and 19mV.

[daz]

here ya go

[Chuck H]

The 22uf value should be enough "capacity" for the circuit. That's a decent circuit. It would be better if you had a pair of 10uf with a 10k in between them. Or if you used a 25k pot and a 100k off the HV. Or better yet, both those things. Not sure it alter your drift issue though.

There are a few things that can cause the bias to drift. Usually it's the tubes themselves. Not uncommon. Especially with new tubes. I've had a few brand new tubes that drift a little. That's a bad thing indicative of other issues and a tube like that should be returned. But they get sold all the time.

Since you're tapped off the HV, and changes in AC wall voltage are multiplied four to six times at the HV winding and that will show up on your bias voltage. Where an actual bias tap of, say, 60V will actually exhibit about half the drift at the wall. Then there's your large-ish pot. And it's probably a panel pot too. Like an Alpha? Not a good choice for bias controls because of the reliability issue, but I've done it. NBD really.

But the pot value should be brought down simply because the range is too great with a 50k, increasing the potential for drift there (though I haven't seen that), making it possible to bias the amp dangerously hot and increasing adjustment sensitivity making fine adjustment more difficult. Obviously a 25k pot in a static position would have half the potential for drift? That's just an out loud thought.

The bias voltage will also drift more than usual with operation because the source is subjected to any voltage sag at the HV winding due to current through the amp. But it's not uncommon to tap off the HV for bias and it works fine. Just not quite as fine as a bias tap.

You can check to see if it's your tubes or your supply that is causing the drift by monitoring bias voltage. If the voltage doesn't drift then the change is due to the tubes and not the bias supply. You may as well also monitor your wall AC and see if it's drifting.

[daz]

It would be better if you had a pair of 10uf with a 10k in between them. Or if you used a 25k pot and a 100k off the HV. Or better yet, both those things. Not sure it alter your drift issue though.

I happen to have several brand new 10UF caps at 50v. Can you explain hot i would go about wiring it up with those using your diagram to explain? You don't have to draw it up, just explain it.... "put one cap where the 22uf is and the other...." etc etc.

[daz]

Like this?





Oops....forgot to note some changes on the diagram, but it's now a 25k pot and the 15k was changed to 47k to set the pot's range correctly.

[Chuck H]

here ya go:

[daz]

here ya go:

Thanks. Seems to work good now and not drifting so not sure if i should change it. Waddayathink? I'll do it if you think it's beneficial but all i can deternmine is it works and doesn't seem to drift like before. Tho it DOES drift when i play going up from 17 to about 19 and back down as soon as i stop. Isn't that normal tho?