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Danieln

La Scala impedance curve vs Tube Amp

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Hello, my name is Daniel and this is my first post here.

 

Couple of weeks ago, I bought a pair of black 1988 LaScala for a very good price.

All drivers are original, cabinet in fair condition that I plan to re-veneer in the next few months.

CrossOver is the infamous AL type.

 

I read a lot of very good information here, and quickly get interested in the crossover.

I have a Tube Amp based on the RH-807 design.

It is basically a class A pentode based single ended output stage.

The Amp have very little feedback, around 6db I figured.

One very important characteristic is the output impedance that I measured at a huge 20 Ohms.

In fact, any pentode based amplifier, including push-pull without a lot of feedback will exhibit a high output impedance.

In theory, the output impedance is the tube plate resistance reflected on the secondary of the output transformer.

A typical plate resistance value for a 6L6 based tube is around 30 kOhms.

With a 5k to 8 ohms transfo, the plate resistance (output impedance) is 30000/625= 48 Ohms.

Considering the 6db feedback, it fall to 24 Ohms, not very far from the value I measured at 20 Ohms.

 

Now, why bother???

 

This 20 Ohms resistance is in series between the load (Speaker) and the source (The Amp)

Consider the lower frequency where the speaker presents itself as a 6 Ohms load.

For this case, the voltage at the speaker input terminal will be: Vin = Vsource (6/26) = .23Vsource

 

Consider the mid frequency where the speaker presents itself as a 30 Ohms load.

For this case, the voltage at the speaker input terminal will be: Vin = Vsource (30/50) = .6Vsource

 

It can be concluded that the midrange will be 2.6 time more driven than the bass (.6/.23). a 8.3db bump.

At first, I was skeptical about this. Then I measure the input signal to the La Scala from the tube Amp and effectively proven right.

It also explains very well why anything midrange was screaming at me :-)

 

I then used a more modern transistor Amp with a <1 Ohms output impedance, and everything was normal (speaker Input Voltage and sound).

The 1 Ohms output impedance does not absorb any significant voltage from the source, whatever the speaker impedance.

 

Conclusion is, output impedance matters a lot when using speakers presenting a non constant impedance with frequency.

Any pentode based design without feedback needs to pay attention.

Triode based design are much less affected due to a lower plate resistance, but will still exhibit the bump if no feedback is used.

Solid state get a free pass :-)

 

The next step was to fix the problem, which will be detailed in a next post

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There's no question that synergy becomes especially important when dealing with a tube amp's generally high output impedance.  It would be interesting to do an actual measurement as opposed to speculating via calculations.  A local ham operator offered a nice way to do the measurement here:

 

https://www.youtube.com/watch?v=ieAhBejHe2M

 

If you have time, I'd like to know what you come up with.  Also, it would be helpful if you can post the schematic of the RH-807 so others can see the way Alex set up the local fb arrangement.

 

 

Maynard

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Thanks for your reply Maynard,

 

As i stated, i actually measured to confirm my calculations.

 

First the amp output impedance was measured by driving a 10 Ohms, followed by a 20 ohms resistance.

10 Ohms yielded an output of 4.0 Vpp while 20 Ohms yielded 6.0 Vpp.

Little maths give you a 20 Ohms output impedance.

 

Second,  i drove a 200 Hz tone from the tube amp to the LaScala.

Measuring  input voltage to the speaker gave 1Vpp.

The same volume setting at 2000 Hz gave 3.5 Vpp, which is even more than what I predicted by calculation.

 

Third, I ran some sweep scan from 100 to 15000 Hz and observe the midrange input voltage jumping .

 

The same experiment using a solid state amp gave an almost completely flat response, as expected from a low output impedance source.

 

All of this is consistent with the the theory, which is a simple Ohms law in fact: The voltage source from the Amp is divided between output impedance and the speaker according to the speaker impedance curve.

 

I will come back with more detail about the Amp

Dan

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This is my home made mono block. I built two of them.

I have been playing with tubes since I was 13 years old, and now 57.

I have a big collection of tubes, and one big criteria of this built was how the tubes looks.

I love how the mid 20's to early 30's globe tube looks.

So I choose the 224A tetrode (Used as a triode) as the input tube, and the 227 as the driver.

Output are the famous WW2 807, which is a derivative of the 6L6.

I used two in parallel single ended class A, for a total of about 14 watts.

I will post the schematic a bit later.

IMG_3178.JPG

IMG_3173.JPG

IMG_3155.JPG

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Very nice built wdecho.

I can see you also use Edcor transformer which are bargains.

I will consider your advise concerning painting end caps. I am not a big fan of this blue ....

Do you run any feedback?

By the way, my 807 are Russian :-)

I turn on the Amp in the morning and close it at night.

12 to 15 hours per day for the last 10 months. No sign of deterioration yet.

I run them at 75% of max plate dissipation, and feel they will last few years @ 3000 hours/year.

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Nice and simple.

Since you are operating in triode mode, you will not experience the impedance shift I described above.

 

Back to the main subject...

 

Anyone skeptical can do this simple experiment:

I just did connect a 20 Ohms resistor in series to the left channel of my solid state amp.

Not to the right channel. I used me Heresy which have the original impedance shift crossover.

Listening in alternance to left and right, it is very obvious that the bass/mid mix is very shifted to the midrange using the resistor.

This simulate a tube Amp high output impedance.

A bit later, i will measure the AC voltage using 200 and 2500 Hz tone.

Dan

 

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Thanks for that.

I admire people doing such a clean job.

Mine is ok, but this one so good looking.

I noticed he is using global feedback.

My design uses schade feedback (Plate to grid 1)

I also use the 15 Watts GXSE Edcor transfo, 2.5k:8 in my case since I use two tubes in parallel.

I think the CXSE is a bit an overkill for my application.

The GXSE can go down to 40 Hz @1dBu which is perfect match to LaScala .

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Thanks for your nice words, I also appreciate DIY very much for everything.

I must admit I am a bit perfectionist for the top look of my built, including the screws.

For the bottom, I do my best, but often modifications just ruin the perfect looks.

 

I was inspired by your previous post, and connected my 807 in triode mode.

I mapped the load from my existing transformer over the plate voltage/current graph in triode mode.

It provides a very good operating point.

I computed 1.8 Triode watts per tube, giving me a total of about say 3.5 Watts.

Since i use autopolarization (Current source on the cathode), this retains the DC operating point at 300Volts/60ma per tube.

At 18 Watts plate dissipation, this runs at 70% of the CCS spec at 25 Watts.

Conservative.

 

Wow I am impressed how much volumes I can get from the LaSaclas!!!!

What a punch on the basses.

I'm listening at Leonard Cohen right now.

 I feel these 3.5 watts are solid as an hammer.

This comparison makes sense. What is preferable....15 watts  but high output impedance, or 3.5 watts lower output impedance?

A big rubber hammer or a smaller steel one? :-)

It is possible to get the pentode amp to be low impedance, but it would requires 25 dB of feedback to equal the triode mode without feedback

 

i am not an audiophile (no golden hear anymore at 58 yrs old), but I really feel the refinement and details.

I will measure the output impedance for comparison.

 

Dan

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