This is a mental exercise at the moment, say I want to build a 300B SET on the cheap. I'm aware that 300B and cheap don't go well together.
Say I want to use Edcor OPT, and of course they come with only a single output. And let's say that despite that I insist on using them to drive speakers ranging from 4-8 ohms.
Is it better if I go with 3k:4 ohm OPT while using 8 ohm speakers?
Or go 5k:8 ohm OPT while using 4 ohm speakers?
In my research, someone hinted that one in theory is better than the other in terms of bandwidth, but I'm not informed enough to take the hint. 😀 So, someone please enlighten me.
Say I want to use Edcor OPT, and of course they come with only a single output. And let's say that despite that I insist on using them to drive speakers ranging from 4-8 ohms.
Is it better if I go with 3k:4 ohm OPT while using 8 ohm speakers?
Or go 5k:8 ohm OPT while using 4 ohm speakers?
In my research, someone hinted that one in theory is better than the other in terms of bandwidth, but I'm not informed enough to take the hint. 😀 So, someone please enlighten me.
Please remember that speaker impedances are nominal and fluctuate as frequency changes.
Buy your "iron" with a single 6 Ω secondary and have done with the matter.
Buy your "iron" with a single 6 Ω secondary and have done with the matter.
Thanks, I'm aware of that. I know I can go either way. I was going with 6 ohm, but I vaguely remember reading in theory one better than the other, and I am curious to know which.
Before you purchase any output transformers, measure the DCR of all your speaker systems (measure at the loudspeaker input terminals, not direct to the speaker driver).
And, check your Ohmmeter, get that 8 Ohm load resistor out and measure it first.
Now you have a reasonable estimate of the Minimum impedance of your “8” Ohm and your “4” Ohm speaker systems.
A two way speaker system, will typically have the following impedance characteristics:
The impedance at frequencies well below the woofer resonance; and the impedance well above the woofer resonance but which is also well before the crossover to the tweeter;
Will be near to or equal to the DCR you measured.
Your 8 Ohm speakers may have 6 or 4 Ohm DCR.
Your 4 Ohm speakers may have 3 or 2.5 Ohm DCR.
“A Tale of Two Output Transformers”
Not all things are equal. But for illustration, if you had an OPT that had a tapped primary of 3.5k and 5k, then the 5k primary would have 1.414 times more total turns, than the turns at the 3.5k tap.
And, the maximum DC current allowed in the 3.5k tap would be 1.414 times more than the DC current of the 5k tap (amp x turns amount, that does not saturate the laminations).
3.5k : 4
Primary DCR 175 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.5 dB (300B sees 3.5k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.25 dB (300B sees 7k)
5k : 8
Primary DCR 250 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 1.0 dB (300B sees 2.5k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.5 dB (300B sees 5k)
With a 300B driving 7k, the power out is the least, but the damping factor is highest.
With a 300B driving 2.5k, the power out might be the highest, but the damping factor is the lowest.
A 300B playing at very low to mid volumes, the 7k and 5k have lower distortion.
A 300B playing at very low to mid volumes, the 2.5k and 3.5k have higher distortion.
I picked 3.5k for these examples instead of 3k, but only because the math gives some
Very direct answers, just off the top of my head (turns ratio, current ratio, insertion loss, DCR, etc.)
Consider a 3k and 3.5k primary to be essentially the same.
3k : 6
Primary DCR 150 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.6 dB (300B sees 2k)
2k is quite low for a 300B (damping, distortion)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.3 dB (300B sees 4k)
5k : 6
Primary DCR 250 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.56 dB (300B sees 3.75k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.3 dB (300B sees 6.7k).
I would prefer the 5k : 6 OPT.
I really like the push pull Edcor OPT that I got in a trade. I may have to purchase a couple of single ended Edcor OPTs and try them
Have fun designing, building, and listening to your 300B amplifiers.
If you are prone to Hernias, build mono-blocks, and put a handle on each amp.
And, check your Ohmmeter, get that 8 Ohm load resistor out and measure it first.
Now you have a reasonable estimate of the Minimum impedance of your “8” Ohm and your “4” Ohm speaker systems.
A two way speaker system, will typically have the following impedance characteristics:
The impedance at frequencies well below the woofer resonance; and the impedance well above the woofer resonance but which is also well before the crossover to the tweeter;
Will be near to or equal to the DCR you measured.
Your 8 Ohm speakers may have 6 or 4 Ohm DCR.
Your 4 Ohm speakers may have 3 or 2.5 Ohm DCR.
“A Tale of Two Output Transformers”
Not all things are equal. But for illustration, if you had an OPT that had a tapped primary of 3.5k and 5k, then the 5k primary would have 1.414 times more total turns, than the turns at the 3.5k tap.
And, the maximum DC current allowed in the 3.5k tap would be 1.414 times more than the DC current of the 5k tap (amp x turns amount, that does not saturate the laminations).
3.5k : 4
Primary DCR 175 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.5 dB (300B sees 3.5k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.25 dB (300B sees 7k)
5k : 8
Primary DCR 250 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 1.0 dB (300B sees 2.5k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.5 dB (300B sees 5k)
With a 300B driving 7k, the power out is the least, but the damping factor is highest.
With a 300B driving 2.5k, the power out might be the highest, but the damping factor is the lowest.
A 300B playing at very low to mid volumes, the 7k and 5k have lower distortion.
A 300B playing at very low to mid volumes, the 2.5k and 3.5k have higher distortion.
I picked 3.5k for these examples instead of 3k, but only because the math gives some
Very direct answers, just off the top of my head (turns ratio, current ratio, insertion loss, DCR, etc.)
Consider a 3k and 3.5k primary to be essentially the same.
3k : 6
Primary DCR 150 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.6 dB (300B sees 2k)
2k is quite low for a 300B (damping, distortion)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.3 dB (300B sees 4k)
5k : 6
Primary DCR 250 Ohms
With a 4 Ohm load, the insertion loss of the primary DCR is 0.56 dB (300B sees 3.75k)
With an 8 Ohm load, the insertion loss of the primary DCR is 0.3 dB (300B sees 6.7k).
I would prefer the 5k : 6 OPT.
I really like the push pull Edcor OPT that I got in a trade. I may have to purchase a couple of single ended Edcor OPTs and try them
Have fun designing, building, and listening to your 300B amplifiers.
If you are prone to Hernias, build mono-blocks, and put a handle on each amp.
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"Not all things are equal. But for illustration, if you had an OPT that had a tapped primary of 3.5k and 5k, then the 5k primary would have 1.414 times more total turns, than the turns at the 3.5k tap".
Wrong.
Wrong.
daanve,
You are correct.
I had a senior brain moment.
Thanks for catching that.
But on a secondary that has an 8 Ohm and a 4 Ohm tap, the turns ratio is 1.414 : 1, respectively.
A multi tap 6k transformer and a 3k transformer primary also has a 1.414 : 1 turns ratio respectively.
Since the amp turns that causes saturation is constant, for a 1 : 2 impedance ratio, the current ratio is 1 : 1.414, respectively.
In the case of the 5k and 3.5k, the turns ratio is ~ 1.19 : 1.
And I still prefer the 5k : 6 Ohm OPT instead of the 3k : 6 Ohm OPT for a 300B to give more flexibility to be able to drive "4" Ohm or "8" Ohm loudspeakers.
You are correct.
I had a senior brain moment.
Thanks for catching that.
But on a secondary that has an 8 Ohm and a 4 Ohm tap, the turns ratio is 1.414 : 1, respectively.
A multi tap 6k transformer and a 3k transformer primary also has a 1.414 : 1 turns ratio respectively.
Since the amp turns that causes saturation is constant, for a 1 : 2 impedance ratio, the current ratio is 1 : 1.414, respectively.
In the case of the 5k and 3.5k, the turns ratio is ~ 1.19 : 1.
And I still prefer the 5k : 6 Ohm OPT instead of the 3k : 6 Ohm OPT for a 300B to give more flexibility to be able to drive "4" Ohm or "8" Ohm loudspeakers.
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Thanks.
No let me ask you this:
For a multi primary tap transformer, let's say 6k with a 3k tap, at which tap the transformer will saturate first? (the 6k tap will have twice the inductance....)
No let me ask you this:
For a multi primary tap transformer, let's say 6k with a 3k tap, at which tap the transformer will saturate first? (the 6k tap will have twice the inductance....)
daanve,
That is a great question.
I believe the factors are this:
Z is according to turns squared (turns x turns)
The turns ratio is 1.414 : 1 (6k versus 3k)
The 3k tap can have 1.414 times more current than the 6k tap.
70.7mA for the 3k tap, and 50mA for the 6k tap.
All according to Amp x Turns = magnetic field.
But, now as to the inductance.
The inductance also goes to turns squared.
1.414 x 1.414 = 2
The 6k tap will have 2 times more inductance than the 3k tap.
The 6k tap and the 3k tap will give the same low frequency bandwidth when driven by a current source, like a Pentode or Beam Power tube.
But only if the laminations do not saturate.
For Ultra Linear Mode, Triode Wired Mode, and true Triodes, the bandwidth should also be the same bandwidth in the 6k and 3k taps too,
but only if the laminations do not saturate.
I hope I got it right this time.
Interesting.
That is a great question.
I believe the factors are this:
Z is according to turns squared (turns x turns)
The turns ratio is 1.414 : 1 (6k versus 3k)
The 3k tap can have 1.414 times more current than the 6k tap.
70.7mA for the 3k tap, and 50mA for the 6k tap.
All according to Amp x Turns = magnetic field.
But, now as to the inductance.
The inductance also goes to turns squared.
1.414 x 1.414 = 2
The 6k tap will have 2 times more inductance than the 3k tap.
The 6k tap and the 3k tap will give the same low frequency bandwidth when driven by a current source, like a Pentode or Beam Power tube.
But only if the laminations do not saturate.
For Ultra Linear Mode, Triode Wired Mode, and true Triodes, the bandwidth should also be the same bandwidth in the 6k and 3k taps too,
but only if the laminations do not saturate.
I hope I got it right this time.
Interesting.
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Right about the inductance.
The higher inductance tap (6k) will give lower core excitation than applying the same voltage at the 3k tap, so at the 3k tap the transformer will saturate sooner.
The higher inductance tap (6k) will give lower core excitation than applying the same voltage at the 3k tap, so at the 3k tap the transformer will saturate sooner.
daanve,
I will think about the issue of voltage later.
Tired brain syndrome.
But, as to DC current, and as to DC current + AC current of a specific low frequency,
The saturation occurs according to Amp x Turns.
Yes?
I will think about the issue of voltage later.
Tired brain syndrome.
But, as to DC current, and as to DC current + AC current of a specific low frequency,
The saturation occurs according to Amp x Turns.
Yes?
Let's get primary DC current out of the equation for now, as we know that DC current will always lower the treshold of core saturation.
This is the equation to (roughly) calculate core excitation for power supply transformers (in this case for M6 quality c-cores):
B (flux density in T) = 50 / f (50 or 60 depending on line frequency) x (45 / Afe x N) x U.
Afe is magnetic core cross-section;
N is number of turns;
U is rms value of voltage.
You see that more primary turns lowers B, U remaining the same.
Applying primary DC current in case of a single ended output transformer does not change this but only shifts the onset of saturation at a lower level of primary AC voltage.
So the lower impedance primary tap will cause core saturation sooner with the same DC current.
This is the equation to (roughly) calculate core excitation for power supply transformers (in this case for M6 quality c-cores):
B (flux density in T) = 50 / f (50 or 60 depending on line frequency) x (45 / Afe x N) x U.
Afe is magnetic core cross-section;
N is number of turns;
U is rms value of voltage.
You see that more primary turns lowers B, U remaining the same.
Applying primary DC current in case of a single ended output transformer does not change this but only shifts the onset of saturation at a lower level of primary AC voltage.
So the lower impedance primary tap will cause core saturation sooner with the same DC current.
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I think if you check the current ratings for multi-tap SE transformers like James, Tango, and others, you will find that the 3.5k tap is rated for more current than the 5k tap rating.
And rated for more current on the 5k tap versus current on the 7k tap.
Right?
And rated for more current on the 5k tap versus current on the 7k tap.
Right?
Can you point to these current ratings for different primary impedance transformers?
I can't find them.
Current rating will also depend on wire diameter.
A high primary impedance transformer will have more turns, so wire diameter will be smaller which might limit DC current capability.
That however is something different.
I can't find them.
Current rating will also depend on wire diameter.
A high primary impedance transformer will have more turns, so wire diameter will be smaller which might limit DC current capability.
That however is something different.
We are talking Ampere Turns saturating the core, that is the BIG issue in SE transformers.
Wire diameter / resistance / heating / resistive losses is not the main problem worrying us today.
What 6A3 summer says is directly related to differet turns present at different primary turns.
Wire diameter / resistance / heating / resistive losses is not the main problem worrying us today.
What 6A3 summer says is directly related to differet turns present at different primary turns.
Assuming the 300B operating point is optimized for the transformer, triodes produce less distortion and a greater damping factor when operating into a higher impedance load. So using the 3K:4 ohm into an 8 ohm is better.
I can not seem to find an example transformer with the different current specs now.
Maybe later.
I do not think it is necessary unless someone dis-agrees with post # 15.
Maybe later.
I do not think it is necessary unless someone dis-agrees with post # 15.
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A higher impedance load also needs higher primary inductance.
When connecting an 8 ohm speaker to a 4 ohm tap, there will be LF loss as the output transformer has not enough inductance for a "real" 6k primary.
You would need some 50H of primary inductance; the 3k transformer would normally have no more than some 25H when it is wound correctly.
When connecting an 8 ohm speaker to a 4 ohm tap, there will be LF loss as the output transformer has not enough inductance for a "real" 6k primary.
You would need some 50H of primary inductance; the 3k transformer would normally have no more than some 25H when it is wound correctly.
I cannot either.
This does not tell me enough:
JS-6112HS 70mA 2.5K / 3.5K 10W 29Hz~50KHz-2dB 2A3 H 1.3 JS-6113HS 60mA 3.5K / 5 K 10W 27Hz~60KHz-2dB 6V6 H 1.3 JS-6115HS 50mA 5K / 7 K 10W 25Hz~50KHz-2dB 6BQ5 H 1.3
If you really need 50H for a 6k single ended transformer, you may need some very good speakers. The OPT will be -3dB at 19Hz, and -1 dB at 38Hz when driving a correct resistor termination.
Please check the impedance versus frequency of your loudspeakers.
That will tell you more about your actual OPT needs.
There are lots of single ended transformers out there with multiple tap secondaries.
I have a push pull Edcor output transformer, and it was a custom. It does have 4 and 8 Ohm secondary taps.
I have a few other brands, and they are single ended, they come standard with multiple impedance output taps too.
Please check the impedance versus frequency of your loudspeakers.
That will tell you more about your actual OPT needs.
There are lots of single ended transformers out there with multiple tap secondaries.
I have a push pull Edcor output transformer, and it was a custom. It does have 4 and 8 Ohm secondary taps.
I have a few other brands, and they are single ended, they come standard with multiple impedance output taps too.
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