In the process of getting a quote for custom PP OPT's, the winder asked if I had considered a single output impedance tap. I have no paticular speakers in mind, and want many options available to me. A quick research showed that most i'm interested in are 4 or 8 ohm. The winder suggested going with a 6 ohm tap to split the difference.
The reasoning was that this would allow a wider bandwidth, and a lower price point. (well that sounds ok)
What are your thoughts on going with a single 6 ohm tap rather than separate taps for 4 and 8? And, should a 16 ohm tap even be in consideration?
The reasoning was that this would allow a wider bandwidth, and a lower price point. (well that sounds ok)
What are your thoughts on going with a single 6 ohm tap rather than separate taps for 4 and 8? And, should a 16 ohm tap even be in consideration?
Your speaker load will change the primary Z. If you connected a 4 ohm load to a 5K/8 ohm tap, your primary Z would half to 2.5K.
So if you got a 5K/6 ohm OT, connecting 8 ohm load would give you 6.6K and a 4 ohm load would give you 3.3K primary Z.
tell him to get on his bike he is winding you up if he think's 6 ohms would be ok for both 4 and 8 ohm speakers.
not only will it sound BAD it may well take out your amp.
not only will it sound BAD it may well take out your amp.
Would the winder be named Jack? Jack at Electra Print convinced me to go with a single secondary OPT when I built the very first Tubelab SE. It has a 5K primary and an 8 ohm secondary. It is also the best sounding amp that I have! I have connected that amp up to all sorts of speaker systems up and never heard anything that I could attribute to a mismatch situation. I use the amp with 45's and 300B's. It works with 2A3's too but only for about 10 minutes. The power transformer really doesn't like it.
I don't see a requirement for a 16 ohm tap. I suspect this is just a hold over from the 50's/60's. I believe they exist now for maintaining a semblance of old-school authenticity (and versatility). I remember seeing 16 ohm speakers back then, not now. They don't really offer an advantage unless equipment demands them.
And I don't think anyone NEEDS to daisychain a bunch of speakers in series to run them all on a single channel nowadays.
Boy, I feel old now.🙁
I do use 4 and 8.
..Todd
And I don't think anyone NEEDS to daisychain a bunch of speakers in series to run them all on a single channel nowadays.
Boy, I feel old now.🙁
I do use 4 and 8.
..Todd
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Bear in mind that the speaker load presented to the amp is also dependent on the frequency of the signal.
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Note that the impedance can fluctuate quite a bit. Most good speaker manufactures will publish their impedance curves and you will see that these impedances can vacillate quite a bit.
There may be some logic to setting a single winding to 6 Ohms when you consider that no true speaker cabinet has a flat impedance response.
So what are suggestions for which tap to use?
I have the choice of 4R or 8R taps and rated 6R speakers. From the above it seems that if i connect to the 4R tap, I would increase the average primary impedance overall and that would reduce power and lower distortion. Or that is my understanding, correct me if I am wrong.
since i have enough power generally, with 20W or 40W amps, I guess 4R is the way to go.
When i build an 8W 300b, maybe I should have a 6R tap put in since I want to optimize the power better without loss.
I have the choice of 4R or 8R taps and rated 6R speakers. From the above it seems that if i connect to the 4R tap, I would increase the average primary impedance overall and that would reduce power and lower distortion. Or that is my understanding, correct me if I am wrong.
since i have enough power generally, with 20W or 40W amps, I guess 4R is the way to go.
When i build an 8W 300b, maybe I should have a 6R tap put in since I want to optimize the power better without loss.
uhh... NO! There's no danger to the amp from a 2:1 or even 4:1 impedance mismatch - though it will certainly have a cost in power output or distortion (maybe less than you think...). But a single untapped secondary will make a better TRANSFORMER since every bit of the winding space can be filled with copper that will have the best possible coupling primary to secondary. Whether it will make a better AMPLIFIER is another question - if you already have a speaker in mind, it may.
Ideally, you need to consider the speaker you intend to employ and look at the impedance curves.
Bear in mind, 50% of the amp's power is required to drive 500 Hz and lower on the audio spectrum. That may impact your selection point.
There are other factors that probably come into play here, too, and I'll let some of the brighter minds make their case.
That SHOULD be the case in a well designed amplifier. It probably IS the case for most HiFi amplifiers, but I have seen an arc over in a guitar amp with 16 ohm speakers wired on a 4 ohm OPT tap.
Most of us know that the 807 and 6BG6 are really 6L6GA's in a different bottle with a plate cap. Why do they have much higher plate voltage ratings when the "guts" are all the same. It turns out that the weak link in the 6L6, and all of the other audio tubes that share the same pinout, is the pin arangement. The plate is pin 3, the heater is pin 2. In normal operation the peak plate voltage can rise to twice the B+ voltage. This will approach 1KV in a 6L6 type amp before the amp hits clipping. When Johnny Metalhead sets his effects box on kill, and his volume control on 11, the voltage at the plate of the output tube has spikes touching 2KV (observed with a scope), when the output is connected to a 16 ohm reactive load, maybe more. If everything isn't built right you will have an arc. When you have 2KV on pin 3 and ground on pin 2 and a little dirt on the tube socket, an arc will often jump right at the bottom of the socket. Other places for the arc are inside the tube base, and inside the OPT (repeated arcs will kill the OPT). What if the heaters are not grounded? Then the arc will jump from pin 3 to pin 2, and then arc to ground inside the power transformer (usually fatal).
I made a few P-P guitar amp heads about 20 years ago and sold them to local bands. They all had a 4-8-16 ohm switch on the back. One found its way back to me after the owner had killed it. Guitar players often experiment to get a unique "tone". This guy had discovered that he could get this wicked sound when he played his amp through his "small" cabinet. He described a scenario where he got a typical metal distortion with everything cranked, but the guitars volume set on 7. At the end of a particular song he would start an acoustical feedback and turn the guitars volume up as it died out. He said that this most unusual tone would erupt that seemed to contain many notes. After doing this many times the sound just wasn't right and the amp started blowing fuses and smelling funny. Then it didn't work any more.
Autopsy:
The "small" cabinet contained 4 X 10 inch 4 ohm speakers wired in series. It was intended to be paired with another similar cabinet for an 8 X 10 "stack". A single cabinet presented a 16 ohm load to the amp. The user was running the impedance switch on 4 ohms because the amp would clip at a lower volume. The "tone of many notes" was an arc from pin 3 to pin 2 on the bottom side of one of the output tube sockets. Pin 7 was grounded which provided a low impedance path for the arc to ground. This arc proved to be non fatal in the short term and created a rather unusual modulation on the output signal. The arc died out as the note died out (also not usual). Eventually the black phenolic tube socket had enough of this and a carbon path developed on the socket creating the funny smell and blowing fuses. In this case the tube socket was the only casualty. This is usually NOT the case. Some how the other compnents including the tubes all survived.
...I just blew coffee out of my nostrils!
OK
So what's the difference between a 5K:6R OPT and a 6K6:8R OPT? They have the same turns ratio... Does the 6R winding have lower DCR than the 8R winding? Do you get fewer primary turns and less Lpri? Why not just specify a speaker winding that matches the nominal impedance of your speaker with the proper DCR and increase the primary Z to the same end?
Windings don't have an "impedance" by themselves. there is only impedance ratio, DCR, and inductance. As previously stated, you need to work out the load on the primary side for a given secondary load based on the turns or impedance ratio.
Will a compromise winding work well with both 4 ohm and 8 ohm speakers? It can, depending on the speakers and the the actual output impedance of the amp. But I wouldn't try it in most cases. I usually hear a big difference between two taps at a 2:1 output impedance ratio.
So what's the difference between a 5K:6R OPT and a 6K6:8R OPT? They have the same turns ratio... Does the 6R winding have lower DCR than the 8R winding? Do you get fewer primary turns and less Lpri? Why not just specify a speaker winding that matches the nominal impedance of your speaker with the proper DCR and increase the primary Z to the same end?
Windings don't have an "impedance" by themselves. there is only impedance ratio, DCR, and inductance. As previously stated, you need to work out the load on the primary side for a given secondary load based on the turns or impedance ratio.
Will a compromise winding work well with both 4 ohm and 8 ohm speakers? It can, depending on the speakers and the the actual output impedance of the amp. But I wouldn't try it in most cases. I usually hear a big difference between two taps at a 2:1 output impedance ratio.
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