Thanks Allenscanon..
Hmm, interesting.
As I see it according to the graph, at 265 plate voltage with the tube in, and assuming 0.035mA current draw I'm well above A and B but still under the curve. From this can i assume that although I'm on the boundary of the curve, I'm just within spec?
Now, assuming R1 @ 510R this would take the plate voltage down closer to line B. I don't have the math to work out the (new) plate voltage with the tube in, based on the likely voltage on the plate with the tube out, which will be in the order of 315v.
Currently I'm showing 338v on the plate with the tube out, and 265v on the plate with the tube in, and 510R at R1 gives a voltage drop of 22.95 assuming a current draw of 0.045mA (which is probably high)
If the current draw is indeed 0.035mA then the voltage drop is reduced to 17.85v with R1 at 510R.
You said ****What is the voltage on the negative side of C5 connected to D4 and R4. That should be closer to your real bias.****
That is entirely logical to look for the real bias value there. Next time I've got the bottom panel off I'll check.
Many thanks both Allenscanon and Carlp for taking the time to explain things so well...
Hmm, interesting.
As I see it according to the graph, at 265 plate voltage with the tube in, and assuming 0.035mA current draw I'm well above A and B but still under the curve. From this can i assume that although I'm on the boundary of the curve, I'm just within spec?
Now, assuming R1 @ 510R this would take the plate voltage down closer to line B. I don't have the math to work out the (new) plate voltage with the tube in, based on the likely voltage on the plate with the tube out, which will be in the order of 315v.
Currently I'm showing 338v on the plate with the tube out, and 265v on the plate with the tube in, and 510R at R1 gives a voltage drop of 22.95 assuming a current draw of 0.045mA (which is probably high)
If the current draw is indeed 0.035mA then the voltage drop is reduced to 17.85v with R1 at 510R.
You said ****What is the voltage on the negative side of C5 connected to D4 and R4. That should be closer to your real bias.****
That is entirely logical to look for the real bias value there. Next time I've got the bottom panel off I'll check.
Many thanks both Allenscanon and Carlp for taking the time to explain things so well...
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Yes, 510R was a deliberate choice. Note that current draw in a tube is related to the chosen operating point, so 0.045A (45mA) isn't the only current option, but it may be considered optimal by the manufacturer. I know of one design (one I plan to build soon) that runs the 13EM7 at 0.050A (50mA). Fred was being conservative at 35mA but it's part of the whole picture of his design. You might consider googling "how to choose a tube operating point." The first link from diyparadise is an amusing read and an easy to follow primer on working with tubes. There are better approaches, e.g. looking for the most linear portion of the tube curves and deciding on B+ and other factors from that. But I digress.
You can try the 100 ohm R1 and see what happens. Key thing is to keep an eye on the tube's plates. If you start seeing red glow in the metal, you're burning too much power in the tube (it'd be the output section glowing in your amp). A nice even red glow may not kill the tube quickly but it will reduce the life of the tube. There are people here who run tubes over their stated dissipation limits, but they've typically played with tubes a lot before doing so.
You can try the 100 ohm R1 and see what happens. Key thing is to keep an eye on the tube's plates. If you start seeing red glow in the metal, you're burning too much power in the tube (it'd be the output section glowing in your amp). A nice even red glow may not kill the tube quickly but it will reduce the life of the tube. There are people here who run tubes over their stated dissipation limits, but they've typically played with tubes a lot before doing so.
I only belatedly saw the reference to 35mA on the schematic and realised its significance. I wondered how you arrived at 35mA, and should have seen it-it was plain enough.
I didn't know about being able to choose the tube operating point, in fact I had no view at all, just assuming that tubes had their specs and you designed around them. All part of learning.
As for the tubes, the heaters do rather seem to glow hot, very yellowy, but I assumed that was the heater doing its job, being incentivised by the 12.6v or so up its clacker, as it was designed to, and paid it no more heed. Interestingly my other tube amp with triode strapped EL34's seem to glow rather dull by comparison, but that might be just the way it is with EL34.
I've ordered a couple of 5w 510R resistors, so after we come back from hols I'll swap them in. Don't know why 5w, but that's what Allenscanon suggested so...
As I said, I wont be doing much now for a few weeks and get back to it the new year..
I didn't know about being able to choose the tube operating point, in fact I had no view at all, just assuming that tubes had their specs and you designed around them. All part of learning.
As for the tubes, the heaters do rather seem to glow hot, very yellowy, but I assumed that was the heater doing its job, being incentivised by the 12.6v or so up its clacker, as it was designed to, and paid it no more heed. Interestingly my other tube amp with triode strapped EL34's seem to glow rather dull by comparison, but that might be just the way it is with EL34.
I've ordered a couple of 5w 510R resistors, so after we come back from hols I'll swap them in. Don't know why 5w, but that's what Allenscanon suggested so...
As I said, I wont be doing much now for a few weeks and get back to it the new year..
See the two dots in the chart on post 78. The one in line A is for no signal idle per Fred N's design. Since the two transformers that you have installed provide higher B+ at 265V, if you want to go back to near what Fred N's amp characteristic, you will need to drop the B+ voltage. With a 225V B+, I think it is close enough given the amp's bias voltage. See the 2nd dot on line B. The 510 ohm R1 is aiming at that idling point.
Not to confuse you too much, In Fred N's design, he used a voltage doubler on the 12.6VAC transformer tap. When that voltage is higher, the bias voltage also go more negative. This is why I think your bias voltage is more like -40V. The dot in line B adjust for that.
The Voltage drop when R1 = 100R is (70ma + 4ma (frond end)) * 100 ohms = 7.4. Power is 7.4 * 74mA = 0.55W so Fred selected 1W.
Not to confuse you too much, In Fred N's design, he used a voltage doubler on the 12.6VAC transformer tap. When that voltage is higher, the bias voltage also go more negative. This is why I think your bias voltage is more like -40V. The dot in line B adjust for that.
The Voltage drop when R1 = 100R is (70ma + 4ma (frond end)) * 100 ohms = 7.4. Power is 7.4 * 74mA = 0.55W so Fred selected 1W.
Logo, the glow of the heater is normal and will always be there (OK, I have some 3S4 battery tubes that don't glow visibly, but that's a different story). What I was talking about was the metal surrounding the heater. It's the gray stamped metal boxes around the heaters. If those metal boxes start to glow red, that is usually a sign of over-dissipation. Red plates mean too much power.
And yes, Allensoncanon was aiming for 225v, so 40v less than your current 265v. Here's the thorough description:
To drop 40v over a resistor, use Ohms law. 40v = 0.079mA * x. X is the resistance you are looking for (I've added a few mA for the current for the driver stage). X = 506R, and 510R is the closest normally available resistor value. Now, to find the power, the equation is W = V * I (power in watts = voltage * current). Multiply the voltage drop over the resistor by the current (0.079mA) to get the power. W = 40 * 0.079. W = 3.2. The general rule is to double the power rating on the resistor, so 6.4W would be ideal under my assumptions of current. 5W is the easiest to find and should be just fine.
Keep in mind we don't know exactly what your PS current is. You could measure it by inserting the 100R in place of the choke, then measuring the voltage drop over that resistor, then using ohm's law to find the current. That current may change some if you change R1 (as it shifts the B+ at the plate and thus the operating point). But probably not enough to worry about. The idea, though, is that the process is sometimes iterative, so you try a 510 resistor and it drops too much or too little. Try a different value until you reach where you want it to be. But in practice I find it's usually close enough to just use your assumptions.
And yes, Allensoncanon was aiming for 225v, so 40v less than your current 265v. Here's the thorough description:
To drop 40v over a resistor, use Ohms law. 40v = 0.079mA * x. X is the resistance you are looking for (I've added a few mA for the current for the driver stage). X = 506R, and 510R is the closest normally available resistor value. Now, to find the power, the equation is W = V * I (power in watts = voltage * current). Multiply the voltage drop over the resistor by the current (0.079mA) to get the power. W = 40 * 0.079. W = 3.2. The general rule is to double the power rating on the resistor, so 6.4W would be ideal under my assumptions of current. 5W is the easiest to find and should be just fine.
Keep in mind we don't know exactly what your PS current is. You could measure it by inserting the 100R in place of the choke, then measuring the voltage drop over that resistor, then using ohm's law to find the current. That current may change some if you change R1 (as it shifts the B+ at the plate and thus the operating point). But probably not enough to worry about. The idea, though, is that the process is sometimes iterative, so you try a 510 resistor and it drops too much or too little. Try a different value until you reach where you want it to be. But in practice I find it's usually close enough to just use your assumptions.
I think the driver stage is about 1.8mA so the extra you added in is fine. What I don't get is the reason for doubling the current ie. 70 or so mA instead of the 35mA. That doubling + a little for the driver stage doesn't square with me.
About the metal boxes overheating and glowing red, I was aware that the heater filaments will glow, how much depending on the tube, ie 300B which I'd love to use at some future time, are a bit of a damp squib and hardly glow at all. My 13EM7 glows nicely, and I've seen a Youtube video of the 'metal box' glowing blue in the dark when over driven, and pulsing in and out in response to hefty loads on bass lines. Don't want to overdrive my 1watters, and so far listening to the one completed amp in my listening room, 10 o'clock on the dial is sufficient, therefore with twice the volume with both amps running it should be more than enough. I have a pair of TQWT Fostex-driven speakers which are fairly efficient and have served me well for many years.
I'm always dabbling with speaker ideas but these work very well in the absence of anything with which to compare them. There may well be a whole world of undiscovered delights with various speakers, but I think that is part of the attraction of our hobby, the eternal quest for the unattainable but fun along the way...
I'm 70 by the way, and it seems odd to be thought of as the new kid, but it's true nevertheless, I have so much to learn about tubes, but what fun it is...
I appreciate replies from yourself and Allensoncanon, ( and others along the way) and I now have a constant reference via your replies...
About the metal boxes overheating and glowing red, I was aware that the heater filaments will glow, how much depending on the tube, ie 300B which I'd love to use at some future time, are a bit of a damp squib and hardly glow at all. My 13EM7 glows nicely, and I've seen a Youtube video of the 'metal box' glowing blue in the dark when over driven, and pulsing in and out in response to hefty loads on bass lines. Don't want to overdrive my 1watters, and so far listening to the one completed amp in my listening room, 10 o'clock on the dial is sufficient, therefore with twice the volume with both amps running it should be more than enough. I have a pair of TQWT Fostex-driven speakers which are fairly efficient and have served me well for many years.
I'm always dabbling with speaker ideas but these work very well in the absence of anything with which to compare them. There may well be a whole world of undiscovered delights with various speakers, but I think that is part of the attraction of our hobby, the eternal quest for the unattainable but fun along the way...
I'm 70 by the way, and it seems odd to be thought of as the new kid, but it's true nevertheless, I have so much to learn about tubes, but what fun it is...
I appreciate replies from yourself and Allensoncanon, ( and others along the way) and I now have a constant reference via your replies...
OOps, I was thinking you are building a stereo amp and not a mono, block. Sorry,
R1 should be (40V/37ma) or 1K is needed. It could be 3W.
From an earlier post I understood the clever voltage doubler that Fred used, utilising the heater lines. You suggest nearer -40v if the 12.6VAC happens to be higher. I tested for this and got around 13v on the heaters.
Does this voltage change under load?
**Australian Standards AS60038."...and will therefore endeavour to maintain voltage to the point of supply at 230 +10%, -2% for 95% of the time....")**
I can see that it can change depending on our grid voltage load which can vary depending on time of day, load on the system if air-cons in the summer months are placing extra demands on the system etc, but wouldn't my 12.6vac have to increase substantially to reflect -40v? Or as usual am I missing something?
Glowing pink or red are signs of the plate being too much over driven. Blue color glow are plasma/gas in the tube, not a healthy sign for the tube but not necessary from over driven. BTW: you might end up liking the sound of your moderately over driven 13EM7 better.
If the heater is 13V "AC". After the rectifier, it is 13 * 1.414 = -18.4V "DC". Double that it is = -36.8V.
I see, easier than I thought to get close to -40v
R1=1k x 3W coming up.
Thanks for the clarification...I thought there was some as-yet unexplained voodoo going on when I saw the doubling of the current to 70mA
Haha. Me too. Oops! And 79mA was too much anyway. I must have added the driver current twice. 75mA is probably right.
Well, deep, rich clear blue dancing on the glass is supposedly fluorescence or something like that and is, indeed, not a problem. Like this: YouTube. Some tubes just do it (like the 6L6 in the video or the 6AQ5s in my push pull amp). But the 13EM7 doesn't have that glow as far as I've seen.
But a hazier blue closer to the plate or between the plate and cathode is a sign of gas in the tube and is BAD. Like this: YouTube. When you see that the tube is failing and will probably fail spectacularly soon.
One more thing. When the entire plate glows red instead of in a spot or line, things are generally more stable. You are still over-driving but the tube is likely to last longer. (At least as far as I can tell). As Alllensoncanon said, slightly or even moderately overdriven tubes can sound terrific and may still last a long time. I don't know if your 265v/-40v operating point would be considered seriously overdriven or slightly. Experience is your friend here. Since the tubes are cheap, you might consider just running it as you originally built it (with the choke and 265v on the plate) then watch the plates and run the original tubes for a few years and test them. They may just last a long time that way. And you'll know more about operating them under those conditions and be able to reply to other DIYAudio-ers who have questions.
Cheers, mate!
"Blue glow on the glass" (harmless) does not depend on the tube type.
Except that some small tubes will have fewer stray electrons at lower voltages so will be less exciting.
It is "benign impurities" in the glass. Which is a bulk commodity in a tube factory.
Glass is made in many grades and types. Tube makers were not real fussy about their glass. There's cheap window glass, bottle glass, fine drinking-glass stuff. Lamps used about the cheapest glass that could be formed reliably. Tube makers used essentially the same stuff, at least for Receiving Tubes. (Big hot transmitting tubes got a harder glass.)
Glass is sand, lime, and some other raw commodity stuff which come from natural deposits in many places (or recycled from various sources). They sift out the sticks, caps, rocks but don't really question the minor impurities. The lime tends to burn-out the crap which will make problems in melting and blowing.
Several stray minerals will fluoresce when hit by electrons. It is a surface effect (tube-volt electrons do not penetrate) so a very thin scum can be a visible glow.
Fluorescence is not a problem for windows bottles glasses. It is certainly not a problem for lamps! It is detectable on tubes but has no effect on tube use. Nobody ever bothered to explore the non-problem.
Tube factories ordered glass tubing by the ton in several sizes, to cover all the receiving tubes they planned to make this month/year. The way glass is made in batches, some batches may glow, or not. Once put into and taken out of inventory, a run of tubes may all glow, or just a few each day.
Interesting explanation about tube glass. I imagined that 'glass is glass' for general purpose applications, tubes included. As far as the tube makers were concerned, so long as the tube performed within its design parameters and lasted a reasonable amount of time, a tube would be considered a 'consumable'.
Nowadays with the resurgence of interest in tubes, is it possible that the main tube makers in China and Russia, and to a lesser extent in USA, now concentrate more effort into the purity and suitability for high-end expectations than previously, or is it still same old same old?
I don't really have a view on the functionality of the tube and its glass construction, as long as I get a decent sound out of the things. My 13EM7's are purportedly GE units in late-model clean crisp boxes and although they've been re-stamped with "13EM7" they are in fact 15EA7. The green 13EM7 stamp rubs off with finger pressure, and "15EA7 USA" is firmly etched into the glass. They're US-made, and that's good enough for me. When they were made I can't know, although they would have to be at least 40 years old NOS
So, they're 15EA7's, which are functionally identical to 13EM7 anyway, and I'm as happy as Larry with them. Lovely little tubes are they...
Nowadays with the resurgence of interest in tubes, is it possible that the main tube makers in China and Russia, and to a lesser extent in USA, now concentrate more effort into the purity and suitability for high-end expectations than previously, or is it still same old same old?
I don't really have a view on the functionality of the tube and its glass construction, as long as I get a decent sound out of the things. My 13EM7's are purportedly GE units in late-model clean crisp boxes and although they've been re-stamped with "13EM7" they are in fact 15EA7. The green 13EM7 stamp rubs off with finger pressure, and "15EA7 USA" is firmly etched into the glass. They're US-made, and that's good enough for me. When they were made I can't know, although they would have to be at least 40 years old NOS
So, they're 15EA7's, which are functionally identical to 13EM7 anyway, and I'm as happy as Larry with them. Lovely little tubes are they...
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If it holds vacuum, sticks to pins, doesn't break in shipping, and isn't ugly (recycled mixed green/brown beer bottles).... what more could a tube-maker want?
He's up to his knees in impurities in plate-metal, cathode oxide, mica, "vacuum". Stuff that keeps the tube from WORKing. Shaping and sealing glass for electric lamps was well developed long before tubes. Glass-making was old long before that.
I think one modern maker favors blue-tint glass. Maybe they drink beer in blue bottles? Maybe their glass-maker does perfume or eye-wash bottles and can't quite get the blue stain out of the vat? Or maybe the blue just looks good, sets that brand apart.
I think the glass is only slightly more important than the cardboard of the box. There was some pretty crummy cardboard in some old-time brands' boxes. As we know as we collect chipped brittle scraps to preserve history not made to last forever.
He's up to his knees in impurities in plate-metal, cathode oxide, mica, "vacuum". Stuff that keeps the tube from WORKing. Shaping and sealing glass for electric lamps was well developed long before tubes. Glass-making was old long before that.
I think one modern maker favors blue-tint glass. Maybe they drink beer in blue bottles? Maybe their glass-maker does perfume or eye-wash bottles and can't quite get the blue stain out of the vat? Or maybe the blue just looks good, sets that brand apart.
I think the glass is only slightly more important than the cardboard of the box. There was some pretty crummy cardboard in some old-time brands' boxes. As we know as we collect chipped brittle scraps to preserve history not made to last forever.
Thanks for that background, PRR. I didn't know most of that. But I wonder, then, why certain tubes (e.g. the 6AQ5 and tubes like the 6550) virtually always have the blue glow while others (the 13EM7/15EA7) virtually never show it (it may be there but generally not visible)? If it were just glass quality I'd expect it to vary within tube types. I've read that it may be related to a couple of things including current draw and the presence of holes in the plate. Anyway, I love blue glow!
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