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OT primary impedance

9/28/2005 7:54 AM
OT primary impedance
I want design a new tube amp with pre close to fender stile tweed.  
In output PP stage I'd like use differently type tubes: EL34, EL37, KT66, 6V6, 6L6, 5881, 6550 or 7581A (obviously with bias pot).  
I'm unsure on OT primary impedance for tone:  
4k, 4.3k, 6.6k,....??? I don't now!!!  
Please help me.  
9/28/2005 11:42 AM
Wild Bill

Iky, unfortunately you are going to have to make some compromises!  
Different tubes like to see different OT primary impedances. Each tube has an optimimum value. You are suggesting using one OT with a variety of tubes that want to see impedances ranging from 3400 to over 8K ohms. So there is no one impedance value.  
This doesn't mean that you can't just pick an impedance in the middle of the range like something 4-5k and live with it. You will not have the optimum value for many different tubes but it will still work. You might actually prefer the mismatch with some tubes! It's all personal taste anyway. A lower than optimum value gives some "bite" to the tone and higher values more "chimey".  
ALL the values you mentioned will work and not likely harm any tube, as long as you are not trying to run the absolute maximum voltages and current levels. If the tubes are being run in a comfortable range then there's no harm done.  
Hope this helps!  
---Wild Bill
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9/29/2005 12:14 AM

Thanks Bill!!!  
I see on different schematics that primary impedance change with the power of amp:  
1) For amps of 5/15W primary impedance change between 6.6k and 8k (champ have 7k).  
2) For amps of 40/50W primary impedance change between 3.8k and 4.3k (all fender have 4k).  
3) For amps of 100W and more primary impedance is close to 2k (marshall 1.9k).  
In these circumstance I think a good compromise is 4.3K.  
What do you think?  
9/29/2005 7:38 AM
Wild Bill

Luca, the impedance is NOT a function of power level! It is a function of the specific tubes involved.  
EL34's need ~3400 ohms. 6L6's need 6600 ohms.  
Drastic changes in power level will indeed change the optimum impedance, such as using 6L6's in a 20 watt application. If the power is similar then the tube is absolutely the important factor.  
More simply, a pair of EL 34's running about 50 watts will need an OT of 3400 ohms. A pair of 6L6's at 50 watts will need 6600 ohms. If you use 4 of these tubes in P-P parallel for 100 watts then you cut these values in half, NOT because of the change in power level but because the plate loads of the tubes are in parallel. Adding a tube in parallel drops the plate load in half, just like putting two resistors in parallel.  
If you really want to understand how this works I suggest you look at the tube data sheets. There is usually a table or some graphs that will show you the optimum plate load, power level and distortion ratings. You will then easily be able to see what effect running a tube at a different OT impedance will have on these parameters.  
---Wild Bill
9/29/2005 5:33 PM
Output transformer impedance is not determined directly by the tube you use. Instead it is determined by the combination of plate voltage and plate current that you run your tube at. Refer to the RCA Radiotron Handbook for a discussion on this.  
I'd give you the exact formula, but I'm sick of doing this. Find a Radiotron book and look it up. It's on page 561.
9/29/2005 6:40 PM
Wild Bill

Geez, why do I bother answering an anonymous post? Especially one so arrogant...  
---"I'm sick of doing this."  
So why bother? We'll all live without your correction! Might be a little happier too...  
Anyhow, it seemed strange to me that despite your formula for years I've looked at data sheets that showed different plate loads for different tubes at similar voltages. What's more, the sheets show curves of optimum plate load vs. power output and distortion, which don't exactly track. Since we're talking guitar amps here, if I follow your formula as an absolute which factor am I designing around? What if I want a cleaner amp rather than max power out?  
For that matter, since the Golden Years mfgrs like Fender and Marshall used different tubes and different loads in similar circuits of similar plate voltages and currents. Were they all fools for not following your formula? Would their amps sound the same if they had?  
So I went to the RDH4 book to page 561. It does indeed give your formula...for IDEAL pentodes! Section 3 goes on to discuss real world examples and modifies the formula by throwing in a factor of .9, and furthermore goes on to say that although the equation Rl is approx equal to 0.9 x Eb/Ib you may have to experiment to "secure the best compromise between distortion and power output."  
Page 566 has a lovely graph of optimum plate load of a pentode in cathode bias. Doesn't look to me like your formula would be an absolute here either. If your not too sick perhaps you could explain.  
Just for kicks I scrolled back a few pages and page 546 caught my eye. Interesting stuff about how a speaker is not an optimum but a varying load. Since an OT by definition is an impedance transformer and has no inherent impedance of its own I can see how this would modify things even more. It directed me to Chapter 21, to read more about optimum plate resistance and maybe even loudspeaker damping.  
So it would appear that your reference is not the absolute gospel you cited.  
But hey, what do I know? Since I didn't already know that you were an authority and hadn't already accepted YOUR words as the literal gospel of God then I must be an ignorant sinner who will burn in hell anyway...  
I swore I wouldn't respond again to an anonymous poster but the irony and arrogance of your post was just too attractive to resist. Imagine, someone anonymous who complains he's sick of saying the same thing! It's like a man who wears a mask complaining no one recognises him!  
---Wild Bill
9/30/2005 8:54 AM
Why bother?  
Because I gave the guy some very good information, that's why bother!  
I gave him the name of a reference book which will explain to him exactly what he needs to know. Furthermore, I gave him the exact page in the book he needs to go to so that he can get right down to the task at hand.  
As the old saying goes, "Give a man a fish and you feed him for one day. Teach a man how to fish and you feed him for life."  
Seems like you are saying I'm suppossed to give him a fish rather than teach him how to fish. Can't agree with that approach.
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