Best cap for tube output!???

[quote author="pstamler"]
Only that a 100uF cap is almost certainly an electrolytic, which will sound worse than a good film cap. They're also leakier, and they eventually dry out and die.
Peace,
Paul[/quote]

Hi Poul,

One should add "to you" electrolytics sound worse than a good film cap. To me its all about what you want in a design..and I have a least used some pretty good sounding audio gear with electrolytis in the signal path .. Old Neve comes to mind..It all about sound and nothing else to me.. even now with years and years of use....still no problems with the original electrolytics...so most of them will have a good long lifespan ymmv

Kind regards

Peter

[quote author="Peter Simonsen"][quote author="pstamler"]
Only that a 100uF cap is almost certainly an electrolytic, which will sound worse than a good film cap. They're also leakier, and they eventually dry out and die.
Peace,
Paul[/quote]

Hi Poul,

One should add "to you" electrolytics sound worse than a good film cap. To me its all about what you want in a design..and I have a least used some pretty good sounding audio gear with electrolytis in the signal path .. Old Neve comes to mind..It all about sound and nothing else to me.. even now with years and years of use....still no problems with the original electrolytics...so most of them will have a good long lifespan ymmv[/quote]

Hi Peter:

Point taken. But I suspect high-voltage electrolytics, on a tube stage, will probably sound poor to a lot of people besides me.

Peace,
Paul

[quote author="pstamler"]
Hi Peter:

Point taken. But I suspect high-voltage electrolytics, on a tube stage, will probably sound poor to a lot of people besides me.

Peace,
Paul[/quote]

Hi Poul,

Sorry I meant not to say that you "where the only one" to say this ;-)

What I was trying to say was that it must be.. "or at lest to me".. personal liking or disliking that leads to the "design goals" in life... :grin:..I do agree with you.. I´m pretty sure you´ll find people who both like or dislike varius caps in high-voltage tube stages or elsewhere for that matter... *S*..as in audio as with so many things in life.. all black and white is a hard road to walk..to many interesting varibles on the way..

Kind regards

Peter

To be blunt, but not offensive, why don't you just try it instead of talking about it. If you want to spend the money for HV electrolytics to couple a high impedance stage and it sounds good to you, fine. If a .1 or so sounds just as good or better at 1/50th of the cost, the decision isn't hard to arrive at. So why the infatuation with electrolytics??

[quote author="BYacey"]To be blunt, but not offensive, why don't you just try it instead of talking about it. If you want to spend the money for HV electrolytics to couple a high impedance stage and it sounds good to you, fine. If a .1 or so sounds just as good or better at 1/50th of the cost, the decision isn't hard to arrive at. So why the infatuation with electrolytics??[/quote]

Hi B,
That's exactly what I have done, my goal is to look for "most" reliable and economical part instead of super performance/expensive part. The problem is that I would like to find out how long the part will last before I decide what to use but there is no way I can tell until wait for it to fail.....

Anyway, I wacked in a 400V 100uF cap there it works fine and no warm top bulgeing cap. However, the circuit is not working properly at the moment So I guess the problem was not the cap eventhough it was bit warm......

Now the IC that drives the grid gets burning hot!! I have had that before with my other filter circuits, I donno if its because I bought a dodgy batch of chips or may be I am shorting out the chip and killing it???

mmm......... strange problem??? Anyone ever had this???

[quote author="Learner"]Now the IC that drives the grid gets burning hot!! I have had that before with my other filter circuits, I donno if its because I bought a dodgy batch of chips or may be I am shorting out the chip and killing it???

mmm......... strange problem??? Anyone ever had this???[/quote]

It's possible your IC is oscillating. I think it'd be a good idea for you to post the circuit you're working with (including the earlier and later stages) so we'll have a better idea what's going on.

Peace,
Paul

[quote author="pstamler"]

It's possible your IC is oscillating. I think it'd be a good idea for you to post the circuit you're working with (including the earlier and later stages) so we'll have a better idea what's going on.

Peace,
Paul[/quote]

Got the circuit to work now but I am too embarassed to talk about the mistake..... :green:

My problem now is noise, even with full feedback I still get 0.5V of zszssszzsss.....tape noise like noise plus some hum.... tried using various(grid, plate, screen) biasing values, hours of soldering and unsoldering feel like gone around the world and ended back at square one. All the fiddly work for nothing.. sigh....:roll:

We need a schematic to help, but did you put a stopper R in series with the grid of a few hundred ohms?

[quote author="bcarso"]We need a schematic to help, but did you put a stopper R in series with the grid of a few hundred ohms?[/quote]

Hi bcarso,
The schem is here...

http://www.groupdiy.com/index.php?topic=7906

Stopper R with the grid??? Not quite sure where you meant....

:?

I see a couple of problems. First, the pentode is connected as an "anode follower" with 100% feedback, via the 100uF capacitor. The schematic shows the cap with its negative terminal hooked to the tube plate, which should cause overheating and possibly eventual explosion.

An anode follower is approximately equivalent in function to an inverting opamp; in this case you have zero ohms for a feedback resistor as far as AC is concerned. Unfortunately, an inverting amp of this sort creates a virtual ground at the grid, so in effect your opamp is driving into a short. (Edit: So is the tube.) That could account for its running hot. Something else that might: as shown, the opamp's + input has no DC path to ground. When you plug something into the input, if there's still no DC path to ground, the opamp's input bias current will have noplace to go, and the opamp's output will probably fly to full + or - rail voltage. That could also make it hot.

Finally, do the tiny arrows next to a couple of the resistor signify that they are really variable resistors? Because if now, the cathode resistors on the pentode are way, way high. If the 50k resistor is really variable, I'd suggest that 5k might be a more appropriate value.

[Oh, as for stopper resistors...they're small resistors mounted directly at the grid of the tube, designed to stop oscillations. They wouldn't solve the problems here, but once you get them cleared up it's probably a good idea to also install a stopper. 100 ohms is a good number.]

One possible solution to the shorting-out problem is to mount a 47k resistor between the output of the second opamp's coupling capacitor and the unconnected side of the tube's grid stopper resistor you're going to install. Then mount another 47k resistor between that junction (first resistor and stopper resistor) and the output side of the 100uF cap. You'll still have approximately unity gain from the tube stage.

While you're changing things, I'd suggest changing out the dual opamp to a FET-input unit. The offset problems will be much less (you might be able to get rid of the 1uF coupling cap) and you'll be able to tweak the gain control (if that's what it is) without it getting all scratchy. Start with a TL072 and work your way up to an OPA2604.

By the way, will it matter to your application that the overall circuit inverts signal polarity?

Peace,
Paul