Hello,
Was a long time ago i build some tube amp.
I have a 300b SE amp with plate volt at 370v.
I have set mA to 74. Is that ok?
It has a LL1664 output transformer with 3K/8 Ohm.
Would the tubes be dammage if a connect a 6 or 4 Ohm speaker?
BR/ Daniel
Was a long time ago i build some tube amp.
I have a 300b SE amp with plate volt at 370v.
I have set mA to 74. Is that ok?
It has a LL1664 output transformer with 3K/8 Ohm.
Would the tubes be dammage if a connect a 6 or 4 Ohm speaker?
BR/ Daniel
I don't see any different between 55 to 75 so I stay at 55. However, I got advise to adjust Vk instead for my PP where the Vk are the same in 4 tubes. I set at 325v so the the bias of 4 are the not the same but pretty close, between 55-56
Fixed Bias:
370V x 0.074A = 27.4 Watts.
That is OK.
If you use Self Bias, then then recalculate:
(370V - Bias Volts) x 0.074A = less than 27 Watts
That is OK.
Putting a 6 or 4 Ohm speaker on the amp will cause the amp to:
Put out less power,
Or put out less undistorted power.
The speakers will not be as well damped by the amp output impedance.
If it sounds distorted, then turn the volume down.
It should not damage the amp.
If there are no other impedance settings and taps on the Lundahl output transformer, then you might want to purchase a different output transformer.
You might investigate one that has 4 and 8 Ohm output taps.
There are 3 versions of the LL1664 transformer:
The 100mA version is better for your SE 300B amp, it is good at 74mA (it is marked 100mA).
The 50mA version will not work properly for your SE 300B amp. It is good for 50mA, not good for 74mA. If your transformer is marked 50mA, you need to re-bias the 300B for 50mA, then it will work better, but at lower power and lower damping factor, (or purchase another output transformer).
The push pull version will not work properly for a SE 300B.
370V x 0.074A = 27.4 Watts.
That is OK.
If you use Self Bias, then then recalculate:
(370V - Bias Volts) x 0.074A = less than 27 Watts
That is OK.
Putting a 6 or 4 Ohm speaker on the amp will cause the amp to:
Put out less power,
Or put out less undistorted power.
The speakers will not be as well damped by the amp output impedance.
If it sounds distorted, then turn the volume down.
It should not damage the amp.
If there are no other impedance settings and taps on the Lundahl output transformer, then you might want to purchase a different output transformer.
You might investigate one that has 4 and 8 Ohm output taps.
There are 3 versions of the LL1664 transformer:
The 100mA version is better for your SE 300B amp, it is good at 74mA (it is marked 100mA).
The 50mA version will not work properly for your SE 300B amp. It is good for 50mA, not good for 74mA. If your transformer is marked 50mA, you need to re-bias the 300B for 50mA, then it will work better, but at lower power and lower damping factor, (or purchase another output transformer).
The push pull version will not work properly for a SE 300B.
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Thanks for the answers.
I don’t remember if i have a 50 or 100mA transformer.
I can check that tonight.
BR // Daniel
I don’t remember if i have a 50 or 100mA transformer.
I can check that tonight.
BR // Daniel
I have both the LL1664/70 and the LL1682/50. The LL1682 is 5K into 5 ohms. It sounds quite a lot better to me. Cleaner, just a better all-round sound with more life. I'm running both at a low HT of 300mA at the moment, with self-bias. The LL1664 is the 70mA version and is running at 60mA. At this operating point it sounds rather boring and lifeless, so a different operating point looks to be needed.
I've seen several recommendations to run a 300b at 5K, and just on the basis of this comparison of the 2 OPTs I'd go with that. For the LL1664 the operating point would probably need to have at least 70mA, maybe more.
I have two O-Netics OPTs at 3.5K and an amorphous NP Acoustic at 3.5K. I've compared all five output stages with the same low operating point, and these three sound as good as or better than the LL1682. The LL1664 at this operating point is a level below the other four in sound.
I've seen several recommendations to run a 300b at 5K, and just on the basis of this comparison of the 2 OPTs I'd go with that. For the LL1664 the operating point would probably need to have at least 70mA, maybe more.
I have two O-Netics OPTs at 3.5K and an amorphous NP Acoustic at 3.5K. I've compared all five output stages with the same low operating point, and these three sound as good as or better than the LL1682. The LL1664 at this operating point is a level below the other four in sound.
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I used to have a link to a page of distortion figures for 300b at 3K and 5K, where the 5K data was better. Anyone remember whose site that was? There was a lot of data for different operating points. It was really useful. Haven't seen it for a few years and not getting any success trying to Google it. Anyone know what it was?
Andy, is this chart of 300B operating conditions what you had in mind?
STC is the "moniker" Western Electric used in the UK.
STC is the "moniker" Western Electric used in the UK.
I think this is more illustrative.
Higher impedance load -> lower harmonics, but also lower output power (p.s.: with higher Uak).
Higher anode current -> lower harmonics (under max. Panode).
I prefer higher impedance loading (5k), with moderated current (70-80mA) at -relative- high anode-cathode voltage (about 400V).
Higher impedance load -> lower harmonics, but also lower output power (p.s.: with higher Uak).
Higher anode current -> lower harmonics (under max. Panode).
I prefer higher impedance loading (5k), with moderated current (70-80mA) at -relative- high anode-cathode voltage (about 400V).
Attachments
No - it was somebody's test figures, but I can't remember who it was. He ran a lot of tests at different operating points and measured the distortion figures.
But that link is very useful. Thanks!!
Three 300B amplifier parameters; versus the plate voltage and plate current combinations:
1. 2nd and 3rd harmonic distortions percentage
(And 2nd and 3rd Intermodulation distortion percentage).
2. Power output
And, the often forgotten parameter . . .
3. Damping Factor
Each of these 3 factors will affect how the 300B amplifier sounds on your loudspeakers.
Generalizations:
2.5k primary, lower damping factor.
But usually the 300B plate voltage is set relatively low, and the plate current is set relatively high. So rp is relatively low.
Low rp helps the damping factor to be better, versus high rp.
5k primary, higher damping factor.
But usually the 300B plate voltage is set relatively high, and the plate current is set relatively low. So rp is relatively high.
High primary impedance helps the damping factor to be higher;
But the higher rp takes away part of that.
3.5k primary . . .
Plate voltage medium, plate current medium, rp medium, distortion medium, power out medium, damping factor medium.
Sort of a compromise.
For No negative feedback 300B single ended amplifiers:
The damping factor is controlled by the output transformer's primary impedance, the correct load impedance on the transformer's output tap (but speaker impedance varies with frequency, above and below the manufacturer's Nominal Impedance Rating), and the rp of the 300B at its operating plate voltage and plate current.
However, the output transformer can also have a large part in the damping factor of the amplifier.
The Primary DCR versus the primary rated impedance, and the Secondary DCR versus the output taps rated impedance.
A 3.5k primary that has 350 Ohms DCR, and an 8 Ohm output tap that has 0.8 Ohms DCR, will have a poorer damping factor;
Versus a 3.5k primary that has 175 Ohms DCR, and an 8 Ohm output tap that has 0.4 Ohms DCR.
By the way, the insertion loss of the first example is 2dB, and the insertion loss of the second example is 1 dB.
2 dB is lots of power loss.
Your 300B SE amp may vary.
1. 2nd and 3rd harmonic distortions percentage
(And 2nd and 3rd Intermodulation distortion percentage).
2. Power output
And, the often forgotten parameter . . .
3. Damping Factor
Each of these 3 factors will affect how the 300B amplifier sounds on your loudspeakers.
Generalizations:
2.5k primary, lower damping factor.
But usually the 300B plate voltage is set relatively low, and the plate current is set relatively high. So rp is relatively low.
Low rp helps the damping factor to be better, versus high rp.
5k primary, higher damping factor.
But usually the 300B plate voltage is set relatively high, and the plate current is set relatively low. So rp is relatively high.
High primary impedance helps the damping factor to be higher;
But the higher rp takes away part of that.
3.5k primary . . .
Plate voltage medium, plate current medium, rp medium, distortion medium, power out medium, damping factor medium.
Sort of a compromise.
For No negative feedback 300B single ended amplifiers:
The damping factor is controlled by the output transformer's primary impedance, the correct load impedance on the transformer's output tap (but speaker impedance varies with frequency, above and below the manufacturer's Nominal Impedance Rating), and the rp of the 300B at its operating plate voltage and plate current.
However, the output transformer can also have a large part in the damping factor of the amplifier.
The Primary DCR versus the primary rated impedance, and the Secondary DCR versus the output taps rated impedance.
A 3.5k primary that has 350 Ohms DCR, and an 8 Ohm output tap that has 0.8 Ohms DCR, will have a poorer damping factor;
Versus a 3.5k primary that has 175 Ohms DCR, and an 8 Ohm output tap that has 0.4 Ohms DCR.
By the way, the insertion loss of the first example is 2dB, and the insertion loss of the second example is 1 dB.
2 dB is lots of power loss.
Your 300B SE amp may vary.
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Green77,
I never did find out if your amplifier used fixed bias, or if it used self bias.
If it used self bias, what is the self bias voltage, and the voltage on the plate?
Just my curiosity.
I never did find out if your amplifier used fixed bias, or if it used self bias.
If it used self bias, what is the self bias voltage, and the voltage on the plate?
Just my curiosity.
Another generalization:
A 300B amp with a 2.5k primary has lower 300B grid bias voltage,
versus a 300B amp with a 5k primary which has a higher 300B grid bias voltage.
That makes the driver's peak voltage swing that is required less challenging for the 2.5k amp, versus the driver swing that is required for the 5k amp.
A 300B amp with a 2.5k primary has lower 300B grid bias voltage,
versus a 300B amp with a 5k primary which has a higher 300B grid bias voltage.
That makes the driver's peak voltage swing that is required less challenging for the 2.5k amp, versus the driver swing that is required for the 5k amp.
I'd just like to thank 6A3 for his extremely useful tutorials. I certainly learn a lot from his posts. Thank you!!
The difference is minimal, because a tube with low output transformer is run with more current to bring the same voltage swing on the output. So the differenc in 300B is 3 Volts when used with a 5K transformer vs. a 2.5K transformer.
Your theoretical thinking about it is therefore wrong.
Schmitz77,
From the STC charts in Post #7,
(by the way, those are lifted/copied right off of the 1950s WE charts).
Vp, RL, g1 bias, Ip, 2nd Harmonic distortion:
250V, 2.5k, -50V, 50mA, -26dBc
300V, 2.5k, -65V, 40mA, -20dBc
400V, 5k, -91V, 40mA, -26dBc
450V, 5k, -102V, 50mA, -27dBc
I hope I did not have a typographical error in my post, when I copied the STC chart here.
It is true. As you go down in the STC chart (higher Vp), the output power tends to increase too.
Which makes sense, since as you go down in the STC chart, the plate dissipation of the 300B is going up too (makes for a hotter tube).
All generalized charts (including the STC chart) have exceptions.
But . . . According to the chart, the 5k load operating conditions . . . Do require more volts from the driver to the 300B grid.
Your mileage may vary, especially if you depart widely from the STC chart.
I have to make note of the fact that WE and STC gave more operating conditions and suggestions for the 300B, versus any other manufacturer gave for
any tube that they designed themselves.
WE and STC gave so many operating choices, you almost do not have to be a real engineer to pick one choice, and get pretty close results, to the choice you made.
From the STC charts in Post #7,
(by the way, those are lifted/copied right off of the 1950s WE charts).
Vp, RL, g1 bias, Ip, 2nd Harmonic distortion:
250V, 2.5k, -50V, 50mA, -26dBc
300V, 2.5k, -65V, 40mA, -20dBc
400V, 5k, -91V, 40mA, -26dBc
450V, 5k, -102V, 50mA, -27dBc
I hope I did not have a typographical error in my post, when I copied the STC chart here.
It is true. As you go down in the STC chart (higher Vp), the output power tends to increase too.
Which makes sense, since as you go down in the STC chart, the plate dissipation of the 300B is going up too (makes for a hotter tube).
All generalized charts (including the STC chart) have exceptions.
But . . . According to the chart, the 5k load operating conditions . . . Do require more volts from the driver to the 300B grid.
Your mileage may vary, especially if you depart widely from the STC chart.
I have to make note of the fact that WE and STC gave more operating conditions and suggestions for the 300B, versus any other manufacturer gave for
any tube that they designed themselves.
WE and STC gave so many operating choices, you almost do not have to be a real engineer to pick one choice, and get pretty close results, to the choice you made.
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Btw, I'm unsure myself what to choose with a 300B: higher output impedance with lower current of lower impedance with higher current.
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Schmitz77,
I was not very kind in my last post, sorry.
Start with your needs and wants.
How efficient are your loudspeakers?
How loud do you like to listen?
What kind of music?
How big is the listening area?
Or is this for a pair of speakers on either side of your computer? (near-field listening).
Quiet time, or a loud party?
What tube amps have you heard?
Our vacuum tube club used to have audio crawls (before Covid 19).
3 hosts would play their CDs, and guests would bring CDs to play too.
3 groups of 5 would go (serially "round-robin") to the 3 hosts.
Each group had listened at all 3 sites, then we would all gather together at a restaurant. What a fun day!
Part of what 300B single ended design you use, may vary depending on what parts you can get in your country at a price and shipping price you are willing to pay.
In my opinion, there are many single ended 300B amplifiers out there that are very good (commercial and DIY), which you could build a copy of.
Is it possible for you to find a friend (or someone at a local tube club), to bring a 300B amplifier to your home for a listening session (I know, Covid 19 may be the thing that stops that idea).
I think it is very hard to tell someone you will like this 300B amplifier, without knowing anything about their needs, what other equipment the own,
and what they have heard elsewhere.
I am also of the opinion that there are many that build and are pleased. That can also cause them to try something more powerful, different, etc.
I was not very kind in my last post, sorry.
Start with your needs and wants.
How efficient are your loudspeakers?
How loud do you like to listen?
What kind of music?
How big is the listening area?
Or is this for a pair of speakers on either side of your computer? (near-field listening).
Quiet time, or a loud party?
What tube amps have you heard?
Our vacuum tube club used to have audio crawls (before Covid 19).
3 hosts would play their CDs, and guests would bring CDs to play too.
3 groups of 5 would go (serially "round-robin") to the 3 hosts.
Each group had listened at all 3 sites, then we would all gather together at a restaurant. What a fun day!
Part of what 300B single ended design you use, may vary depending on what parts you can get in your country at a price and shipping price you are willing to pay.
In my opinion, there are many single ended 300B amplifiers out there that are very good (commercial and DIY), which you could build a copy of.
Is it possible for you to find a friend (or someone at a local tube club), to bring a 300B amplifier to your home for a listening session (I know, Covid 19 may be the thing that stops that idea).
I think it is very hard to tell someone you will like this 300B amplifier, without knowing anything about their needs, what other equipment the own,
and what they have heard elsewhere.
I am also of the opinion that there are many that build and are pleased. That can also cause them to try something more powerful, different, etc.
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Sorry, my fault.
My listening room has 380 sq feet but that will expand in the future. Speakers are a horn system with 100dB. Music is pop/classical and all kinds of music. I heard 300B with 8W and they suited me well. Its just the tube has so many options to choose that I'm quite unsure what impedance to go for. I have my approx. 6W F2A mono's but in the long run want to go for a 300B to compare. Preamp is all tube with a Garrard 301/Ortofon/Shindo system.
My listening room has 380 sq feet but that will expand in the future. Speakers are a horn system with 100dB. Music is pop/classical and all kinds of music. I heard 300B with 8W and they suited me well. Its just the tube has so many options to choose that I'm quite unsure what impedance to go for. I have my approx. 6W F2A mono's but in the long run want to go for a 300B to compare. Preamp is all tube with a Garrard 301/Ortofon/Shindo system.
Attachments
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Choosing power tube's operating point (i.e. current) in SE amplifier strongly depends of OPT parameters.
When designing an OPT, the one important starting point is the core saturation.
At low (maybe 20..30 Hz) frequencies the DC excitation (depends of current) + AC excitation (signal) must be below core's saturation.
Each OPT manufacturer gives recommended DC current and maximum power, which are granted, that core will below the flux saturation.
If you increase DC current, at the same "volume" the cumulative excitation is approach to the limit, so THD will be increasing.
BTW each transformer's THD will be increasing when frequency decrease to the designed lower border.
So, OPT's core's saturation limits the maximum power at specified frequency.
If you increase the current, the estimated power (at specified THD) will be decreasing.
When designing an OPT, the one important starting point is the core saturation.
At low (maybe 20..30 Hz) frequencies the DC excitation (depends of current) + AC excitation (signal) must be below core's saturation.
Each OPT manufacturer gives recommended DC current and maximum power, which are granted, that core will below the flux saturation.
If you increase DC current, at the same "volume" the cumulative excitation is approach to the limit, so THD will be increasing.
BTW each transformer's THD will be increasing when frequency decrease to the designed lower border.
So, OPT's core's saturation limits the maximum power at specified frequency.
If you increase the current, the estimated power (at specified THD) will be decreasing.