designed for a 6.6K Ohms impedance source.

[Oddvar R]

When an OT is designed for a 6.6K Ohms impedance source, what would the difference between 6.6k and 5.8k mean in an 60w amp be?

[martin manning]

It's more about the impedance presented to the output tubes, 5k8 vs. 6k6. What type are the tubes? What are the plate and screen voltages?

[Stevem]

If this is in terms of your Highwatt build that you have been having problems with, then you are way off base!
A two output tube EL34/ 6CA7 60 watt model wants a OT of about 3200 and a four output tube model wants to see about 1/2 that impedance.

Also note that when you go up higher in impedance you get less wattage , less distortion and longer tube life, if you go below the recommended spec without lowering the plate voltage ,the screen voltage and reset the bias, then the opposite takes place.

You need to learn to read load lines if your going to be messing with these details if you do not want to burn stuff up!

[Oddvar R]

I have tried to come by a transformer for the Komet 60, but the Heyboer people doesn't seem to be of any help.
So I was wondering if I could use another transformer, Edcore has one, but it has 6.6k and I was wondering what this would mean, generally speaking, in a build with two EL34. What would have to be changed to accomadate this 6.6k transformer?
(The Hiwatt is in perfect order now. Loud and clear.)

[sluckey]

...in a build with two EL34. What would have to be changed to accomadate this 6.6k transformer?

Just connect a speaker that is half the impedance of the OT secondary. IOW, if you have a secondary tap that is labelled 8Ω, connect a 4Ω speaker to that tap. This will refelect an impedance of 3.3k to the primary and the tube plates. Your EL34s will be happier.

[martin manning]

Komet 60 plate voltage is in the high 400's, so 3k3 will be too low for 2x EL34 IMO. This 50W 5k would be a better choice: https://edcorusa.com/collections/tube-a ... ransformer

[Oddvar R]

Great tip, thanks.

[R.G.]

On an historical note: it was common in the 1950s and 1960s for makers of power tubes to test their tubes over a wide range of plate and plate-to-plate loads and generate charts of power out, second and third harmonic distortion versus load for the same power supply voltage and other conditions.
In general, there was a broad, flattish peak in the power curve, and a broad, flattish minimum in the distortion curve for each tube type, depending on the tube's construction and materials. The peak power load was not the same as the minimum distortion point. Peak power loading was a lower impedance than lowest distortion for the same tube type.
The one I remember best is the 6L6. 6L6s - or at least the ones they tested 50 years ago - tended to have a distortion minimum around 6.6k plate to plate, and a power maximum at around 4.4k. The hifi folks were competing not only on power, but also hugely on distortion, so they tended to wind transformers that converted nominal 8/16/etc speakers to 6.6k. The guitar amp makers tended to use the higher power point down at 4.4k.

Practically speaking, there is hardly any difference at all between 5.8K and 6.(something)K in power or distortion for a given tube set. Likewise, hardly any difference between 4K, 4.4K, etc.
There's not much to be said about one impedance wearing out tubes faster, and other stuff >> as long as the power dissipation limits of the tube are observed << by changing bias and power supply voltage.

[Colossal]

Oddvar, the Komet K60 is not running at 6k6. It is 5k8.

[martin manning]

Oddvar, the Komet K60 is not running at 6k6. It is 5k8.

I believe he was looking for 5k8, but only found 6k6. Going lower (to 5k) will be better than going higher from the screen grid's point of view, better than 5k8, IMO.

[Colossal]

Oddvar, the Komet K60 is not running at 6k6. It is 5k8.

I believe he was looking for 5k8, but only found 6k6. Going lower (to 5k) will be better than going higher from the screen grid's point of view, better than 5k8, IMO.

I agree. The K60 runs fairly high plate and screen voltages, IIRC.

[pdf64]

…
In general, there was a broad, flattish peak in the power curve, and a broad, flattish minimum in the distortion curve for each tube type, depending on the tube's construction and materials. The peak power load was not the same as the minimum distortion point. Peak power loading was a lower impedance than lowest distortion for the same tube type.
…

It may be worth noting that the non linearities meant by ‘distortion’ above are a different kettle of fish to the clipping overdrive that guitarists might typically think of when distortion is mentioned

[R.G.]

It may be worth noting that the non linearities meant by ‘distortion’ above are a different kettle of fish to the clipping overdrive that guitarists might typically think of when distortion is mentioned

It is. That's why I mentioned that the plots of distortion were second and third harmonics in that first sentence.

[Oddvar R]

Outputtransformer for Dumble style projects with 6L6GC tubes

Pri.: 5000 Ohm C.T.
Sec.: 4-8-16 Ohm
Power: 50 W (20Hz -3dB)

Outputtransformer for Dumble style projects with EL34 tubes

Pri.: 3800 Ohm C.T.
Sec.: 4-8-16 Ohm
Power: 50 W (18Hz -3dB)

So what would be the difference here I wonder. The Komet60 could be using different tubes, bot El34 and 6l6, so where is the difference? Could any of the two transformers above be used? And could the tubes be used in either one?

[pdf64]

Outputtransformer for Dumble style projects with 6L6GC tubes

Pri.: 5000 Ohm C.T.
Sec.: 4-8-16 Ohm
Power: 50 W (20Hz -3dB)

Outputtransformer for Dumble style projects with EL34 tubes

Pri.: 3800 Ohm C.T.
Sec.: 4-8-16 Ohm
Power: 50 W (18Hz -3dB)

So what would be the difference here I wonder.

The different OT primary impedances may be optimised for the applicable valve type.

The Komet60 could be using different tubes, bot El34 and 6l6, so where is the difference? Could any of the two transformers above be used? And could the tubes be used in either one?

Its OT may be optimised for one valve type or may be intermediate, optimised for neither, fine for either.
Provided that the HT isn’t super high, eg above 500V, there shouldn’t be an issue.