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johnheath

johnheath

Well-known member
Joined
Jul 31, 2014
Messages
890
Location
Sweden
Hi all...

I am building a HiFi tube amp as a wedding gift for a friend.

It seems that modern HiFi speakers are 6ohm but still the output transformers have taps for 4, 8 and 16ohm.

My question is if I am supposed to use the 4 ohm tap or the 8 ohm tap for these 6 ohm speakers. I have plowed through some HiFi forums in the hunt for a good explanation about it all. There seems to be answers that span from "it is ok" to "I have weird distortion problems". I figure that I might get a good answer here though? :)

So what are the ups and downs between the two options... 4 or 8 ohm?


Best regards

/John
 
I very much doubt it matters but I don't know for sure. Speakers are not a flat resistance anyway. Speaker impedance is low at low frequencies but with a peak at LF resonance and then gradually rises. So if I get pedantic about it, if you use the 4 ohm tap, 6 isn't completely loading the output as much as it should in which case you might get peaking in the response or when overdriven it might not distort in exactly the same way (which is maybe important for a tube hi-fi amp if you really believe in that sort of thing). If you use the 8 ohm tap, again the damping and distortion might not be perfect which is kind of what hi-fi is. The most obvious difference will be the difference in power. With 8 ohms, you double the power. It will just be louder. Add a switch for 4 ohms / 8 ohms. That way, if it's a problem, it's the operator's fault.

Or just get them a bread maker.
 
The real question is what is the reflected load impedance that the tube sees. You would need to draw load lines or consult tube manuals to find the optimal point. There is no  real correct answer without this.

But as a rule of thumb it is generally better to go higher than lower,  for example with an optimal 8 ohm output,  a 16 ohm speaker would be safe to use at the expense of some loss of power and headroom,  maybe increased distortion . Using a 4 ohm speaker could potentially damage the amp.
 
I agree with the general rule of thumb, put the higher 6 Ohm rated speaker load on the amp's "lower" 4 Ohm tap. 

If you get the "match" the wrong way around, there may be a risk of faster wear on the output tubes or damage to an output transformer if it is under-rated. It may also be very naughty from a hi-fidelity perspective.

Not having a perfect match in a wedding present tube amp could be a huge faux pas.
The bread maker idea does look good from here.




 
The real danger with impedance mismatch is the problem of punch through on the output transformer insulation caused by excessive voltage spikes. They are made worse by having speaker impedance higher than the taps. Always go lower.
 
Either way you wont do major damage as long as you have a load connected at all times ,an open load is dangerous for a tube amp ,a badly missmatched load might not sound great ,but it will sorta work .Even a  dead short at the output terminals wont damage it actually.
Lower output impedence into higher speaker impedence is generally a good thing , you most likley will get a slight change in tone between four and eight ohms ,see which tap sounds best with your speaker . In my experience once you have any load from short circuit upto 16 ohms connected across a portion of  the transformer secondary ,high voltage primary spikes wont be a problem.

Always always reduce the volume to zero before you change taps or dissconnect the load ,like was said in previous posts  ,even a momentary disconection of the output can can cause arching and possible damage to the output transformer itself. If your amp happens to osscillate  for any reason with the output dissconnected  it could more or less instantly destroy the transformer ,so really to be safe its best to power down your amp before even changing the taps until your sure its working correctly .
 
squarewave said:
I very much doubt it matters but I don't know for sure. Speakers are not a flat resistance anyway. Speaker impedance is low at low frequencies but with a peak at LF resonance and then gradually rises. So if I get pedantic about it, if you use the 4 ohm tap, 6 isn't completely loading the output as much as it should in which case you might get peaking in the response or when overdriven it might not distort in exactly the same way (which is maybe important for a tube hi-fi amp if you really believe in that sort of thing). If you use the 8 ohm tap, again the damping and distortion might not be perfect which is kind of what hi-fi is. The most obvious difference will be the difference in power. With 8 ohms, you double the power. It will just be louder. Add a switch for 4 ohms / 8 ohms. That way, if it's a problem, it's the operator's fault.

Or just get them a bread maker.
Click to expand...

Believe or not believe has little to do with it in fact. I just want it as good as possible :)

The reason I chose to build a quite expensive wedding gift is that my friend saved my life during a long sailing tour last summer and he has nothing to listen to music with... I guess a bread maker is a bit cheap in this sense?

But back to the subject... You say 4 ohm is the way to go?

Best regards

/John
 
john12ax7 said:
The real question is what is the reflected load impedance that the tube sees. You would need to draw load lines or consult tube manuals to find the optimal point. There is no  real correct answer without this.

But as a rule of thumb it is generally better to go higher than lower,  for example with an optimal 8 ohm output,  a 16 ohm speaker would be safe to use at the expense of some loss of power and headroom,  maybe increased distortion . Using a 4 ohm speaker could potentially damage the amp.
Click to expand...

Thank you sir...

Yes, I am aware of this and it has been calculated for in the design process... the OT are 6k6 on the primary used with a pair of EL34's. You say go higher than the speaker 6 ohm and that using the 4 ohm tap could potentially damage the amp?

Best regards

/John
 
tony hunt said:
I agree with the general rule of thumb, put the higher 6 Ohm rated speaker load on the amp's "lower" 4 Ohm tap. 

If you get the "match" the wrong way around, there may be a risk of faster wear on the output tubes or damage to an output transformer if it is under-rated. It may also be very naughty from a hi-fidelity perspective.

Not having a perfect match in a wedding present tube amp could be a huge faux pas.
The bread maker idea does look good from here.
Click to expand...

Thank you sir

You say go for the 4 ohm choice... so what are the risks when using the 8 ohm tap?

best regards

/John
 
waxscum said:
The real danger with impedance mismatch is the problem of punch through on the output transformer insulation caused by excessive voltage spikes. They are made worse by having speaker impedance higher than the taps. Always go lower.
Click to expand...

Thank you sir

You seem pretty sure about your opinion and the outcome of using the "wrong" tap seems to be a potential hazard?

Best regards

/John
 
Tubetec said:
Either way you wont do major damage as long as you have a load connected at all times ,an open load is dangerous for a tube amp ,a badly missmatched load might not sound great ,but it will sorta work .Even a  dead short at the output terminals wont damage it actually.
Lower output impedence into higher speaker impedence is generally a good thing , you most likley will get a slight change in tone between four and eight ohms ,see which tap sounds best with your speaker . In my experience once you have any load from short circuit upto 16 ohms connected across a portion of  the transformer secondary ,high voltage primary spikes wont be a problem.

Always always reduce the volume to zero before you change taps or dissconnect the load ,like was said in previous posts  ,even a momentary disconection of the output can can cause arching and possible damage to the output transformer itself. If your amp happens to osscillate  for any reason with the output dissconnected  it could more or less instantly destroy the transformer ,so really to be safe its best to power down your amp before even changing the taps until your sure its working correctly .
Click to expand...

Thank you sir

Yes, I am aware of the risks when using a OT without load, but what you are saying is... listen for the different frequency response when using 4 or 8 ohm?

Best regards

/John
 
Listen to tubetec. Just wanted to ensure you didn't think going higher was safer, which could lead to believing open circuit is ok.
Fender heads often have shorting jacks to ground the secondary in the event of forgetting to plug in the speaker.
 
johnheath said:
You say go for the 4 ohm choice... so what are the risks when using the 8 ohm tap?
Click to expand...
None really. Indeed you must make sure you don't exceed the tubes rating. It depends on the actual B+ voltage.
Using the 8 ohms tap with a 6 ohms load provides better linearity. Also, in most cases, the 8 ohms tap uses a larger proportion of the secondary, which improves the coupling coefficient of the xfmr - decreases leakage impedance thus improves HF response.

If using the 4 ohms tap you have to make sure the amp is stable with the lighter load. You need to check the frequency response for possible ultrasonic humps or the square wave response for underdamping. This can be easily solved with an adequate Zobel network.
 
I would say use your ears in the final decission on which tap to use ,it will certainly affect the bass responce in combination with the speaker your using. Lower output impedence into higher speaker will dampen the bass at the expence of some volume ,higher output impedance will have the opposite effect and exhibit less control of the lows, if the mismatch is too bad things will start to sound a bit fuzzy .
 
abbey road d enfer said:
None really. Indeed you must make sure you don't exceed the tubes rating. It depends on the actual B+ voltage.
Using the 8 ohms tap with a 6 ohms load provides better linearity. Also, in most cases, the 8 ohms tap uses a larger proportion of the secondary, which improves the coupling coefficient of the xfmr - decreases leakage impedance thus improves HF response.

If using the 4 ohms tap you have to make sure the amp is stable with the lighter load. You need to check the frequency response for possible ultrasonic humps or the square wave response for underdamping. This can be easily solved with an adequate Zobel network.
Click to expand...

Thank you sir

Concerning the B+ it is a bit high... roughly 510V according to my math and rectifier. The power transformer is a Sowter 9079 with 360V C.T.

I would like to get it down towards 420 - 440V but I don't see how to do that at the moment. I will have to wait to see how the power supply acts with the amp fully loaded. (I will get the last OT this week and the I can fire it up)

Accordning to the EL34 data sheet Va is limited to 800V which seems a bit high... I do not know for sure?

About the zobel network? Would that be placed on the secondary of the OT? (I have only seen it on input transformers before)

Best regards

/John
 
Tubetec said:
I would say use your ears in the final decission on which tap to use ,it will certainly affect the bass responce in combination with the speaker your using. Lower output impedence into higher speaker will dampen the bass at the expence of some volume ,higher output impedance will have the opposite effect and exhibit less control of the lows, if the mismatch is too bad things will start to sound a bit fuzzy .
Click to expand...

Thank you sir

I-ll have that in mind when I finally fire it up... hopefully at the end of this week

Best regards

/John
 
johnheath said:
About the zobel network? Would that be placed on the secondary of the OT? (I have only seen it on input transformers before)
Click to expand...
Putting it on the primary makes more sense; just make sure the capacitor voltage is adequate and the resistors are of adequate wattage.
 
abbey road d enfer said:
Putting it on the primary makes more sense; just make sure the capacitor voltage is adequate and the resistors are of adequate wattage.
Click to expand...

Thank you sir

I'll have that in mind when checking.


Best regards

/John
 

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