MAC Autoformers?

Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur.  

Lets get clear about matching plate impedance to tranformer impedance. We don't do that in amplifier design. The primary impedance reqired is determined by the voltage and current of the output tubes. For instance a EL84 amp with a 385V B+ uses a 8,000 ohm transformer while my EL84 RM-10 has a 13,500 ohm primary because it runs at higher voltage and lower current. 

While the plate impedance will provide some damping this is not a large effect on the damping when the loop feedback is applied. 

I have doubts about the plate impedance being 3,000 ohms in pentode. It should be much higher and I will measure it at my first opportunity. In triode it looks like 1 to 2 Kohms.

@ramtubes,

Roger, take a look at the New Sensor specs for the KT-150.  I believe it says the plate resistance is 3000 ohms.

https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdf

I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life.  Ergo, use the 4 ohm tap. You called it light loading.  

So if the speaker impedance is higher than the nominal tab impedance, resulting in a primary impedance presented to the output tubes that is higher than the plate impedance, it will result is less distortion and longer tube life. Ergo, you said to use the 4 ohm tap.

Of course, the 4 ohm tap has lower output impedance and therefore a higher DF, which is a good thing. 

What did I not understand??

BIF

BIF,

I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life. Ergo, use the 4 ohm tap. You called it light loading.

What I have been saying is that the primary impedance chosen has nothing to do with the plate impedance of the Pentode. Ideal plate resistance of a pentode is very high as demonstrated by the flatness of the plate curves above the knee. If the plate impedance of a KT150 were 3000 ohms it would produce something close to a 45 degree line on the curves. I just think the 3000 ohms is a mistake thought is it also shown on the KT-120 data sheet, which is curious it would be the same for both tubes.. Modern spec sheets are not done with the care of the good old days and there are few out there to check them.

RCA, GE, Tungsol and Sylvania all had identical plate curves or at least very close and such a mistake would have been found right away. The printed specs for most tubes are IDENTICAL from one maker to the other. There is no other maker for the KT 120 or 150 to monitor the situation.

Thus is the current state of tubes.

Here is a much better data sheet, made not by the manufacturer but someone else..

http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdf

On the first page are the pentode curves and the plate resistance is the slope of the line above the knee. The top curve has a slope of about 20 ma/200 volts = 10K ohms, the lower curves, where the amplifier typically operates is flatter making the impedance even higher.

Also note on the last page the the the load impedance is given as 3,000 ohms. So that is likely the source of the mistake in the Tungsol data sheet, which was made by the Russians I would think. Perhaps something got lost in the translation.

The simplest way to explain light loading is to look at what heavy loading does. If the load is two heavy (4 ohms on the 8 ohm tap) the tube will have excessive current and voltage across it (ie it wont get to the knee but be to the right of it). That product of voltage and current does two things. It reduces the output of the amplifier and it heats the tube excessively.

Rodger,

Take a looks at the specs for a Tung Sol 6550.  

https://www.newsensor.com/pdf/tungsol/6550-tungsol.pdf

Interestingly, the 6550  "load resistance" is 3000 ohms. But the plate resistance is 15K ohms.  

The KT-150 spec sheet you showed us says that "Tube impedance is    [f]rom 10.0KOhm to 12.5KOhm.

I think the importance point here is that whatever the relevant resistance spec is, I think your point is that if one has a speaker with a flat impedance function of 4 ohms, using the 4 ohm tap will present an optimal load to the output tubes off the primary winding of the output tranny.  

By contrast, if one's speaker has a rock and rock impedance function, ranging from 4 ohms to 20+ ohms, using the 4 ohm tap is still the way to go even if the back impedance presented to the output tubes is higher than the tube's impedance spec, whatever it is. In other word, opt for light loading.  It will result in less distortion, longer tube life, and a higher DF.

BIF  

The KT-150 spec sheet you showed us says that "Tube impedance is   [f]rom 10.0KOhm to 12.5KOhm.

And is even higher at low currents where we are often listening. It really helps for someone get a scope or some kind of monitoring device to see the voltage the speaker is using. Some listeners are at very low levels and some much higher. Anyone who is going through tubes is up at the high endor has a terrible mismatch.This may not be obvious, but once one goes over the knee at 0 grid volts the extra current comes at the expense of large voltage drops across the tube and the amplifier is surely clipping at this point. . 

There is no optimal load for a power tube. The load belongs to the circuit in which it is used and the load that is expected. That is why an RM-9 can do equally well with the KT 88 family of tubes (beam tubes) or EL34 (true pentodes). Since these tubes have very different curves they will certainly sound different. One must realize that installing different tubes, either the same type with slightly different specs, or another type entirely will mostly change the feedback of the amp, thus the damping and distortion. 

In most cases the largest difference will be in the bass and overall frequency response due to the higher damping (more feedback) with the beam tubes.