Putting aside the heater question for the moment, your cathode bias resistor looks awfully small. At 2mA, you've only got a quarter of a volt bias. And the plate voltage is much lower than that tube is happy with; you want 150V plate to cathode or a bit more to stay out of grid current trouble.
anatech said:
Hi Chris, very strange things happening.
I've measured the AC component of the heater supply with my DMM (and I don't trust on it) and I get 15.9V with the heater supply set at 7.5V (more on this voltage later). But whitout a scope it's useless...
Let's summarize what's happening on there. Of all my Siemens used tubes:
I have a bad tube: it has 20ohm heater resistance. As SY said, it's out of spec. I've put aside it for the moment.
Other tubes have 12ohm heater: they are happy to conduct about 3mA in my circuit (attached, but now 1M resistors are 100k) with 7.5V on the heaters. This leads me to think that those tubes are PCC81 instead of ECC81, but they are marked ECC81!!! 😕
That's not a big problem, I've got an adjustable heater supply, so I'm ready for strange heater requirements. But I'd like to know what the h*ll is going on! 😱
Btw, SY, what cathode resistor are you refering? the 120ohm one or the 10K one?
edit: now the voltages are obviously different than ones in the attached schematics, now I've got about 30V on the cathodes (32 and 28) and 143V on the plates.
Attachments
Hi Giaime,
I don't think the tubes are incorrectly marked. I do think that the circuit can not be worked on until the cathode temperature is resolved. In other words, you must ensure the heaters are running properly before you can look any further. Do you have a 12V lead acid battery you can sub. for the time being (motorcycle battery)?
I agree with Stuart that the 120 ohm resistor between the cathode and tap for your grid seems far too low. This sets your grid bias.
-Chris
I don't think the tubes are incorrectly marked. I do think that the circuit can not be worked on until the cathode temperature is resolved. In other words, you must ensure the heaters are running properly before you can look any further. Do you have a 12V lead acid battery you can sub. for the time being (motorcycle battery)?
I agree with Stuart that the 120 ohm resistor between the cathode and tap for your grid seems far too low. This sets your grid bias.
-Chris
Thanks Chris, could you suggest a value for that resistor? Or better yet, explain me how to calculate it 
You know I'm not too expert 
I'll try now with a CB supply (13.8V) with a 12V regulator, to see what's going on.
Attached there is the schematic of my heater supply (straight out of the LM317 datasheet):
You know I'm not too expert I'll try now with a CB supply (13.8V) with a 12V regulator, to see what's going on.
Attached there is the schematic of my heater supply (straight out of the LM317 datasheet):
Attachments
Just for a quick test, disconnect the regulator and rectifiers/caps, then run the AC via a twisted pair of wires right to pin 4 and 5 (i.e., normal 12V connection). If the tube acts normally, I think I may know what the problem is...
Hello people, I've tried heating my tubes with a CB 13.8V PSU regulated down to 12V with a 7812. The results are... that obviously the same tube heaten with the same voltage (as my psy) glows the same. 😀
But I think that the problems here are solved, I'm now running my heaters at 12.6V (series connected) on the good tubes (the bad one has been set aside) and I've got 25V on a cathode and 16V on the other. Do these values seems fine or are the heaters
starving?
And an important question: now I hear some high frequency buzz. I think it's related to the fact that my heater psu isn't referenced to anything in respect of the other hi-volt psu. So, what is the best way to reference them togheter? Should I reference the heaters to audio ground or to an elevated voltage? Same as cathodes? Directly to cathodes? Higher?
After that, if we set down the heater problem, maybe we can start to talk about the biasing problems...
But I think that the problems here are solved, I'm now running my heaters at 12.6V (series connected) on the good tubes (the bad one has been set aside) and I've got 25V on a cathode and 16V on the other. Do these values seems fine or are the heaters
starving?
And an important question: now I hear some high frequency buzz. I think it's related to the fact that my heater psu isn't referenced to anything in respect of the other hi-volt psu. So, what is the best way to reference them togheter? Should I reference the heaters to audio ground or to an elevated voltage? Same as cathodes? Directly to cathodes? Higher?
After that, if we set down the heater problem, maybe we can start to talk about the biasing problems...
Giamime,
Do you have the plate curves for your tube?
http://www.pentalaboratories.com/pdfs/ECC81.pdf
Mark your Plate to Cathode voltage (about 70) and your Plate current (about 0.005). Your "Q" point , Q = quiet or steady (DC) is very close to the zero grid voltage curve.
You want to move your Q point to the right: more plate to cathode voltage. Operating so close to 0 grid voltage is a problem...
Try 330 Ohm for R4. 9100 Ohm for R6. Your volatge at Cathode should 28 volts. Your voltage at the grid should 27 volts (1 volt negative grid bias) but this can be a false reading. You must have high impedance meter. Otherwise If the cathode voltage is right... you are good!
It would help to raise your B+ (like the great SY
says)
😀
Do you have the plate curves for your tube?
http://www.pentalaboratories.com/pdfs/ECC81.pdf
Mark your Plate to Cathode voltage (about 70) and your Plate current (about 0.005). Your "Q" point , Q = quiet or steady (DC) is very close to the zero grid voltage curve.
You want to move your Q point to the right: more plate to cathode voltage. Operating so close to 0 grid voltage is a problem...
Try 330 Ohm for R4. 9100 Ohm for R6. Your volatge at Cathode should 28 volts. Your voltage at the grid should 27 volts (1 volt negative grid bias) but this can be a false reading. You must have high impedance meter. Otherwise If the cathode voltage is right... you are good!
It would help to raise your B+ (like the great SY
says)😀
Hello Poobah, I think your values aren't right. Now these are the voltages on the CF:
Triode section 1 2
Va-k 118.5V 127.2V
Vk-ground 26.4V 17.3V
Va-ground 145.6V
Iq 2.61mA 1.71mA
Apart from the big differences between the 2 sections, I can see that my q points are these in the image attached (blu = triode 1, red = triode 2). They are sufficiently far from the Vg=0V line, but I understand they're a bit on the low side of current. I think a point like this, Va = 200V Iq = 7mA would be better... So I have to raise the B+ or could I go with a more friendly (for me) point of maybe Va = 150V Iq = 5mA?
Triode section 1 2
Va-k 118.5V 127.2V
Vk-ground 26.4V 17.3V
Va-ground 145.6V
Iq 2.61mA 1.71mA
Apart from the big differences between the 2 sections, I can see that my q points are these in the image attached (blu = triode 1, red = triode 2). They are sufficiently far from the Vg=0V line, but I understand they're a bit on the low side of current. I think a point like this, Va = 200V Iq = 7mA would be better... So I have to raise the B+ or could I go with a more friendly (for me) point of maybe Va = 150V Iq = 5mA?
Attachments
Giaime,
You should really have 175 to 200 for B+. This would allow 150 to 175 for Plate to Cathode voltage. This would allow the Q points to move UP (resistors would change too).
This is a move in the good direction... getting away form zero grid bias... how does it sound?
You should really have 175 to 200 for B+. This would allow 150 to 175 for Plate to Cathode voltage. This would allow the Q points to move UP (resistors would change too).
This is a move in the good direction... getting away form zero grid bias... how does it sound?
Dear Poobah,
I've been quite amazed at how the preamp still sounds the same, even tweaking the bias points. Maybe my ears aren't too "golden", or it's my test system that isn't "audiophile" enought...
But now I'm looking toward another direction: since it is almost impossible to me to obtain higher B+ voltages (how I would? I've got a 117VAC tranny, I can only go like this or voltage double, which leads to 320V... a bit too much) I've connected some trimpots in place of the 10K and 120ohm resistors. I've tweaked them to get at least Iq = 2.5mA, and I'm at Va-k = about 110V. I think that's a prettier bias point: farther from the Vg = 0V line. Btw resistor values for this are 220ohm and, still, 10k.
I've been quite amazed at how the preamp still sounds the same, even tweaking the bias points. Maybe my ears aren't too "golden", or it's my test system that isn't "audiophile" enought...
But now I'm looking toward another direction: since it is almost impossible to me to obtain higher B+ voltages (how I would? I've got a 117VAC tranny, I can only go like this or voltage double, which leads to 320V... a bit too much) I've connected some trimpots in place of the 10K and 120ohm resistors. I've tweaked them to get at least Iq = 2.5mA, and I'm at Va-k = about 110V. I think that's a prettier bias point: farther from the Vg = 0V line. Btw resistor values for this are 220ohm and, still, 10k.
I was going to suggest a 220 ohm for R4. that moves your Q point up. You are the right track now... just be sure to stay away from low grid bias and low Va-k. They cause grid problems.
If you can get more B+... you can see now this is the center of the problem. Get a knife and take turns from the tranny primary! It is crazy... but it is DIY! 😱
😀 😀
If you can get more B+... you can see now this is the center of the problem. Get a knife and take turns from the tranny primary! It is crazy... but it is DIY! 😱
😀 😀
Hi Giaime,
Use a voltage doubler. Then use C-R-C-R-C to drop voltage and increase isolation. Better yet, use a regulator circuit. This way you can tailor make the supply voltage. No need to mess with transformer windings.
-Chris
Use a voltage doubler. Then use C-R-C-R-C to drop voltage and increase isolation. Better yet, use a regulator circuit. This way you can tailor make the supply voltage. No need to mess with transformer windings.
-Chris
Hello!
I've just finished a long day of prototyping and diy. 😉
So the results. I tested all the tubes I've got, to select the better one, since I've changed my circuit. It now has 10k, 220ohm and 100k, like poobah suggested. I wrote down the cathode voltages respect to ground, after 10 minutes of pre-heating:
tube 1: first section 30.9V, second one 11.5V
tube 2: first section 29.5V, second one 35.3V
tube 3: first section 3.3V, second one 1.46V
tube 4: first section 24.5V, second one 15.6V
All those voltages have a plate voltage of about 145V.
You can clearly see that tube 3 is probably defective: it's that famous tube that needed 15 and more volts on the heater to work properly. I discarded it.
I choosed tube 2 since it was the most consistent between the two sections. This make an Iq of 3/3.5mA per section, at 145V.
I'm now happy with this for now 😀, but I have big time hum. No, not hum, but buzz. Big buzz, especially on startup. Then it fades away, but it's still there. Things it could be:
- the heater supply is still separate from B+ supply, no common ground reference.
- the heater supply needs a bypass capacitor? I used one before the regulator, but do I have to add one maybe on tube pins, after the long leads?
I'm also looking forward that voltage doubler + regulator. I'll have to rebuild my previous supply (which did use the voltage doubler) and find a suitable schematic for the regulator. Some ideas: I have many LM317 (but don't know if they can regulate such an high voltage: Vout-Vin will be high!), many 2n3055 and 2955, BD***, IRF840, even some strange hi-voltage NPN power transistors (something rated at 450V 7A... probably some sort of switching transistor). Some suggestions?
I will also think about a relay muting circuit for startup...
I've just finished a long day of prototyping and diy. 😉
So the results. I tested all the tubes I've got, to select the better one, since I've changed my circuit. It now has 10k, 220ohm and 100k, like poobah suggested. I wrote down the cathode voltages respect to ground, after 10 minutes of pre-heating:
tube 1: first section 30.9V, second one 11.5V
tube 2: first section 29.5V, second one 35.3V
tube 3: first section 3.3V, second one 1.46V
tube 4: first section 24.5V, second one 15.6V
All those voltages have a plate voltage of about 145V.
You can clearly see that tube 3 is probably defective: it's that famous tube that needed 15 and more volts on the heater to work properly. I discarded it.
I choosed tube 2 since it was the most consistent between the two sections. This make an Iq of 3/3.5mA per section, at 145V.
I'm now happy with this for now 😀, but I have big time hum. No, not hum, but buzz. Big buzz, especially on startup. Then it fades away, but it's still there. Things it could be:
- the heater supply is still separate from B+ supply, no common ground reference.
- the heater supply needs a bypass capacitor? I used one before the regulator, but do I have to add one maybe on tube pins, after the long leads?
I'm also looking forward that voltage doubler + regulator. I'll have to rebuild my previous supply (which did use the voltage doubler) and find a suitable schematic for the regulator. Some ideas: I have many LM317 (but don't know if they can regulate such an high voltage: Vout-Vin will be high!), many 2n3055 and 2955, BD***, IRF840, even some strange hi-voltage NPN power transistors (something rated at 450V 7A... probably some sort of switching transistor). Some suggestions?
I will also think about a relay muting circuit for startup...
If someone send it to me for free... 😀
Seriously, those "not-so-typical" tubes are HARD to find in Italy. You "could" find them at ham radio meets, but in my place there is one once a year... I'll have to wait 'til april!
Mmmm methinks the way to go is sand 🙄
Seriously, those "not-so-typical" tubes are HARD to find in Italy. You "could" find them at ham radio meets, but in my place there is one once a year... I'll have to wait 'til april!
Mmmm methinks the way to go is sand 🙄
How about we send you a new shiny red Christmas transformer... for a hard working student of the electron?
😉
😉
Ehehehe why not?
I have to say that one thing on which I have choice is the PT. Aside from those 220V/117V ones that I've got (4 of them, one is powering the preamp of this thread), I also got a 300V CT, 5V & 6.3V one, very big, that once was part of a stereo PP EL84 amp. You say, why didn't you build a stereo EL84 tube amp? Because I only have one OT, the other one is fried...
Seriously, here in Italy there are so few vintage amps for cheap (eBay, garage sales etc etc) to obtain transformers. And new ones DO cost, even if I've found some local manufacturers that do decent stuff.
I have to say that one thing on which I have choice is the PT. Aside from those 220V/117V ones that I've got (4 of them, one is powering the preamp of this thread), I also got a 300V CT, 5V & 6.3V one, very big, that once was part of a stereo PP EL84 amp. You say, why didn't you build a stereo EL84 tube amp? Because I only have one OT, the other one is fried...
Seriously, here in Italy there are so few vintage amps for cheap (eBay, garage sales etc etc) to obtain transformers. And new ones DO cost, even if I've found some local manufacturers that do decent stuff.
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