Importance of impedance matching with tube amplifiers?

[coylum]

I have the possibility to purchase a 1960's vintage stereo tube amplifier rated at 10 watts per channel with a single output impedance tap of 15 ohm.

It is a long time since I have worked with tube amplifiers and I can't recall how important it was to match output impedance with that of the speaker. I would plan to use high efficiency Celestion Ditton 15's which have an impedance between 4 and 8 ohm depending on frequency.

I realise I will get sound but what will this impedance mismatch do to the power output and fidelity of the sound?

The output transformers are good ones with ultra linear screen taps so I don't want to be changing them!

 

[dcgillespie]

Coy -- You will always get maximum power output and minimum distortion when the load the amplifier is driving is connected to an equivalent tap on the output transformer of the amplifier.

Today however, there is often the response given of just use which ever tap sounds best, but this approach then becomes a personal preference, rather than an effort to strive for the most accurate sound reproduction. It is always best to use tone control adjustments to modify response, rather than allowing an impedance mismatch to generate a preferred sound.

In general, if you do operate with a mismatch, operating your speaker on a lower than rated impedance tap will favor lower distortion, but typically cut available power output by at least half. Operating your speaker on a higher than rated impedance tap will maintain mid-band power output, but reduce the power bandwidth of the amplifier overall.

One thing you might do if possible is to get an additional decent quality 15 watt output transformer with 4-8 & 16 ohm output taps available, and use it as an external impedance matching auto-transformer. Tape off the primary leads, and let the 15 ohm output impedance of the amplifier drive the 16 ohm secondary winding of the new transformer. Then connect your speaker between the Common and 4 or 8 ohm tap of the new transformer. With the new transformer now operating as an auto-transformer, there will be a much closer impedance match between the 4-8 ohm impedance of your speaker, and the 15 ohm output impedance of your amplifier.

Good luck with it, and if you get it, welcome back to tubes!

Dave

 

[charles 1973]

I too believe it is best to use a speaker with a reasonably close impedance to the amps taps. You couldtry installing a good 5w 6 - 8 ohm resistor in series with your speakers to correct the mismatch.

 

[kward]

Since I have my amp on the bench right now, I thought I would do an experiment that might mirror your question, more or less.

On my amp:

1) Using a 1Khz test signal, an 8 ohm dummy load on the 8 ohm transformer tap produces 56 watts output at 0.35% Total Harmonic Distortion.

2) Keeping everything the same but changing the dummy load to 4 ohms (on the 8 ohm transformer tap) produced 84 watts output at 1.85% THD.

So for the second test (which might kinda be similar to your situation), power went up by a factor of 1.5, but distortion went up also by over 5x. Visible distortion on the sine wave started at 9KHz! (terrible).

Power bandwidth went down a bit too in the second test (not by much), but I have "beefy" transformers (Dynaco A431S clones) that are capable of handling way more than 56 watts output.

 

[coylum]

One thing you might do if possible is to get an additional decent quality 15 watt output transformer with 4-8 & 16 ohm output taps available, and use it as an external impedance matching auto-transformer. Tape off the primary leads, and let the 15 ohm output impedance of the amplifier drive the 16 ohm secondary winding of the new transformer. Then connect your speaker between the Common and 4 or 8 ohm tap of the new transformer. With the new transformer now operating as an auto-transformer, there will be a much closer impedance match between the 4-8 ohm impedance of your speaker, and the 15 ohm output impedance of your amplifier.

Good luck with it, and if you get it, welcome back to tubes!

Dave

Dave,
Thanks for the response. It seems the current owner has been running 8 ohm speakers routinely and says that it sounds fine.... but then I would prefer to have it matched. I will keep my eye open for a couple of audio transformers that could be used as auto transformers. I may well also watch out for some 16 ohm speakers! - or even try some 8 ohm units in series - lots of options.
Thanks again

 

[coylum]

Since I have my amp on the bench right now, I thought I would do an experiment that might mirror your question, more or less.

On my amp:

1) Using a 1Khz test signal, an 8 ohm dummy load on the 8 ohm transformer tap produces 56 watts output at 0.35% Total Harmonic Distortion.

2) Keeping everything the same but changing the dummy load to 4 ohms (on the 8 ohm transformer tap) produced 84 watts output at 1.85% THD.

So for the second test (which might kinda be similar to your situation), power went up by a factor of 1.5, but distortion went up also by over 5x. Visible distortion on the sine wave started at 9KHz! (terrible).

Power bandwidth went down a bit too in the second test (not by much), but I have "beefy" transformers (Dynaco A431S clones) that are capable of handling way more than 56 watts output.

Wow - now that's what I call service!

Just one question comes to mind - in test 2) do you have any idea what the THD was at an output of 56 watts?

Again, I really appreciate the effort you have put into answering my query...

 

[Dandy]

If the solution is adding another output transformer, wouldn't it be simpler to replace the originals with this one? Assuming similar size of course.

The originals could then be sold on to cover some or all of the cost of the replacements.

 

[coylum]

If the solution is adding another output transformer, wouldn't it be simpler to replace the originals with this one? Assuming similar size of course.

The originals could then be sold on to cover some or all of the cost of the replacements.

The new price in Australia of two good quality 9000/8 ohm push pull 10 watt transformers with 20% ultra linear taps is considerably greater than the price being asked for the complete amplifier.....Yes, I could probably sell the old transformers but then the single output impedance of 15 ohm will reduce their value considerably.

I may just keep looking for a tube amplifier that more closely matches what I'm after which is basically the equivalent of a Leak Stereo 20 but at a much reduced price.... How I wish I had kept my old Mullard 5-10's from the 1960's!!

 

[Dandy]

There is a vendor on the 'bay from Indonesia, willycom1 who sells OPTs. He might have something suitable for Dave's suggestion. See item 190915640925.

I have no link at all with the seller.

 

[jaymanaa]

Have you tried the originals? I have some Fisher EL84 OPTs rated at 2.5 ohm output, and they sound find on a 4 ohm load to me.

 

[kward]

Wow - now that's what I call service!

Just one question comes to mind - in test 2) do you have any idea what the THD was at an output of 56 watts?

Again, I really appreciate the effort you have put into answering my query...

It was really no trouble...I had the whole rig setup already, as I was in the middle of measuring the performance characteristics of my amp for another purpose.

THD will certainly go down at less power in the conditions of test 2) in the midband area, but I expect distortion at the fridges of the audio spectrum (20 Hz and 20 KHz) would still be significantly higher than they would have been if using a matched load.

Thus, if one is going to use "mismatched" speakers on an amp, the better approach is to use higher impedance speakers than for which the tap is rated. That will produce less power, but other conditions (distortion, power bandwidth) won't suffer nearly as much.

For example, on my amp, here's a little table of some THD measurements I made:

8 ohm load on 8 ohm tap (@max power of 56 watts at 1 KHz):
20Hz - 1.4%
1KHz - 0.35%
20KHz - 3.0%

16 ohm load on 8 ohm tap (@same conditions as above test, which produced 30 watts output at 1KHz)
20Hz - 1.75%
1KHz - 0.37%
20KHz - 1.65%

4 ohm load on 8 ohm tap (@same conditions as first test, which produced 84 watts output at 1KHz)
20Hz - 2.3%
1KHz - 1.85%
20KHz - 4.6%

And...to answer your specific question:
4 ohm load on 8 ohm tap (input signal reduced to output 56 watts on a 4 ohm load @1KHz, thus matching max power of 8 ohm load on 8 ohm tap from above test)
20Hz - 1.35%
1KHz - 1.0%
20KHz - 4.5%

Distortion numbers don't tell the whole story for this last test, but a clue is the 1KHz distortion number being so high at 1%. Distortion starts to be visible on the scope at about 6KHz! It becomes painfully obvious at 10KHz, and excruciating at 20KHz. So I would call power bandwidth in this case 20Hz - 6KHz.

For comparison, consider that the matched case (8 ohm load on 8 ohm tap) has power bandwidth flat and clean up to about 30KHz.

 

[gadget73]

no chance the existing output transformers have extra secondary leads for 4 and/or 8 ohm that are just hidden under the chassis is there? Might be worth a quick look. If they do exist, this problem becomes a non-problem

 

[jaymanaa]

no chance the existing output transformers have extra secondary leads for 4 and/or 8 ohm that are just hidden under the chassis is there? Might be worth a quick look. If they do exist, this problem becomes a non-problem

Yep, seen it plenty of times now that you mention it. They will be taped off.

 

[Windup]

It was really no trouble...I had the whole rig setup already, as I was in the middle of measuring the performance characteristics of my amp for ....

Wow, that's why I LOVE this forum!!!
Where else can you get a response like that? And the ones from dcgillespie too!!

Not as frightening when you have to jump into the deep end of things.

Thanks for being there guys. :yes::zoom:

 

[coylum]

There is a vendor on the 'bay from Indonesia, willycom1 who sells OPTs. He might have something suitable for Dave's suggestion. See item 190915640925.

I have no link at all with the seller.

I may well contact this seller to see what other transformers he can source as he seems to sell quite a few different models and may have a local source who can make to order at a reasonable cost.

 

[coylum]

Have you tried the originals? I have some Fisher EL84 OPTs rated at 2.5 ohm output, and they sound find on a 4 ohm load to me.

No - I haven't yet purchased the amplifier, pricing is being negotiated! The current owner reckons it sounds fine with 8 ohm speakers

 

[coylum]

It was really no trouble...I had the whole rig setup already, as I was in the middle of measuring the performance characteristics of my amp for another purpose.................

This response calls for a double WOW...!

Thanks for all this info - certainly seems that matching impedance is the way to go even if a mismatch "sounds OK".

Thanks again

 

[coylum]

no chance the existing output transformers have extra secondary leads for 4 and/or 8 ohm that are just hidden under the chassis is there? Might be worth a quick look. If they do exist, this problem becomes a non-problem

I don't yet own the amplifier but a quick visual inspection did not reveal additional fly leads although they may be well tucked in. I think this particular output transformer (A&R OT4005) was made to order in terms of any particular output impedance requirement

 

[coylum]

I may just keep looking for a tube amplifier that more closely matches what I'm after which is basically the equivalent of a Leak Stereo 20 but at a much reduced price.... How I wish I had kept my old Mullard 5-10's from the 1960's!!

Someone must have been listening.... today I purchased two Mullard 5-10 amplifiers (Australian Aegis brand) complete with passive pre-amps, complete but in unknown condition for $260 - I'm rapt...Obviously the start of a new project and back into the tube (valve in my terminology) world...