Ok between 4 and 3,5 itś 3,05v and over the cathode resistor it is 3,7v.
I changed the tube that made the noise and now its quiet.
And itś playing fine.
I changed the tube that made the noise and now its quiet.
And itś playing fine.
3A5 is an excellent tube, but it needs a little bit of "attention".
I use it as an input tube in my GM-70 monoblocks. Bias is fixed. 5mA 100V per section, or 10mA 100V triodes in parallel is the best operating point and it is on the limit for this tube. 0.5W dissipation per triode.
Heating is via modified 5V SMPS with additional RC filtering and inductor in series with heating (right on the tube socket). They still can be microphonics, but only when I tap on them, otherwise they are very quiet.
Also, I have some Philips DCC90, RCA 3A5, Raytheon 3A5.
For linestage, gyrator version has too much gain ...
I use it as an input tube in my GM-70 monoblocks. Bias is fixed. 5mA 100V per section, or 10mA 100V triodes in parallel is the best operating point and it is on the limit for this tube. 0.5W dissipation per triode.
Heating is via modified 5V SMPS with additional RC filtering and inductor in series with heating (right on the tube socket). They still can be microphonics, but only when I tap on them, otherwise they are very quiet.
Also, I have some Philips DCC90, RCA 3A5, Raytheon 3A5.
For linestage, gyrator version has too much gain ...
No that is with the 380 ohm resistor...strange but it is so.
If I use 10 ohm I cannot go higher than about 55v on the anode..
Thats really odd. I've built 3 of these, and all hit in or around the target voltage every time. One valve might be 94V and another might be 104V but not a million miles from each other. Is there any chance that one half of the filament is burnt in either valve (I'm not even sure if that is possible)? You'd have to pull the valve from the socket and measure continuity from pin 4 to pin1, and pin 4 to pin 7. You have the HV supply going to both triode plates (I think its pins 2 and 6)?
Grasping at straws here to see why it would be!!
Fran
Grasping at straws here to see why it would be!!
Fran
Yes theres anodvoltage at 2 and 6.
And I have tried several tubes and they all behaved the same,except for one that had higher
Filament voltage ,2.8 v or higher.
It is strange that it is the same in both channels.
And I have tried several tubes and they all behaved the same,except for one that had higher
Filament voltage ,2.8 v or higher.
It is strange that it is the same in both channels.
Have you connected everything up as per the bartola circuit and how do all your voltages compare?
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I'm no electronics engineer but to me we're talking apples and pears. As has been said before, the Coleman circuit in post 7 isn't the same as the Bartola one I posted above, the Coleman is cathode bias and the Bartola is filament bias. Try configuring your build as per the Bartola and see how it performs, by reputation it might sound better.
I've read sommewhere than coleman stated that the one in post 7 sounded better.
Well I dont know.
Well I dont know.
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It doesn't really matter which sounds better, the point is that the confusion around the 10R/380R resistors is simply because they're relevant to two different circuits (if you look at the schematics you'll see that the difference isn't simply about a ground connection).
Ryssen, as I wrote earlier this schematic in #7 post is pure cathode bias, not filament biased preamp.
If you use fixed bias and ground the centre pin of 3A5, it will be better. I don't like filament bias, this is only my opinion.
Ryssen - I somehow missed this. Yes, they are all completely different, so you can't compare set points between them. You'll have to pick one or the other and go with that.
You'll get opinions advocating for filament bias or conventional bias, and the only way you'll really find out which you prefer is to try both. I can tell you that using the gyrator from Ale and the Coleman regs, filament bias, with that circuit in post 37, it sounds fantastic as a DAC output stage. Pretty much a set and forget setup unless you are changing tubes. If you have a search in the tubes forum, you'll see a thread on the 3a5 that shows the good advice I got from others here. Worth a read if you haven't come across it yet.
About using filament bias or cathode bias:
So long as you have a Raw DC supply that gives ca. 7-10V output (For the V8 version of my filament regulator) you can try filament bias or cathode bias, without too many changes.
I would try cathode bias first, since it is easier to set up. But filament bias can sound better, depending on other factors.
So long as you have a Raw DC supply that gives ca. 7-10V output (For the V8 version of my filament regulator) you can try filament bias or cathode bias, without too many changes.
I would try cathode bias first, since it is easier to set up. But filament bias can sound better, depending on other factors.
Which SMPS do you use and how is it modified? And what specific parts do you use for the additional filtering?
If you could supply some links and a schematic that would be great.