Comments for page: EFB in a Dynaco ST-35

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Posted by Lydia February 09, 2012 - 12:03 am
Hi Dave maybe you can answer my previous
question about the grid resistors 470k ohm?
Since I will be using your 5k-2k-2k bias setup per
your fig.10. I was informed that I could place the four (4) grid resistors on my EFB board? it is going to get crowded..... IF I can keep them on the driver boards pc-13, then how am I going to wire this up?
Thanks, Lydia

Posted by Lydia February 06, 2012 - 10:18 am
Hi Steve, as per our conversation about the "grid" resistors, if I put two (2) ends soldered to the adj.
on the 2k Pot then the other two loose ends go to eyelets #8 and #9 on the PC-13 Board? can you elaborate a little more on physically hooking this up? most appreciated Lydia

Posted by Dave February 04, 2012 - 06:59 am
Lydia -- The fact that the older transformers have an 8 and 16 ohm winding merely means it would require operating a 16 ohm speaker on the 8 ohm tap to extend the class of operation. But the effect is exactly the same as with the newer transformers, where it takes operating an 8 ohm speaker on the 4 ohm tap to extend the class A portion of operation.

The two primary factors that determine the class of operation for a given output stage are quiescent current level and the load impedance the stage operates into. Since these factors are largely fixed elements for the case at hand, moving from UL operation to triode operation has little impact on how long the stage remains in class A operation, relative to the total power output the stage is capable of developing. There will be a greater amount of class A power developed in UL mode, but only because UL develops a greater amount of total power output. However, the relative percent of class A operation between the two modes will remain the same.

The different classes of operation have their useful purposes for different applications, depending on the goals established for the design. Class A operation favors low distortion, low power output, and low efficiency, while class B favors high power, greater potential for distortion, and high efficiency. In the past, class A was the only way to go, as class B designs were always characterized by producing a "notch" in the waveform as the production of power was handed off from one tube to the other. The notch was ultimately found to be the result of inadequate coupling between the halves of the primary winding in the output transformer. With modern transformers, this concern has been largely eliminated, so that with class AB operation, it is now quite possible to obtain virtually all the low distortion class A is capable of, while still maintaining the high power output and efficiency of class B operation.

Once again, it is a personal preference as to which class of operation you prefer. I have numerous class A and class AB amplifiers in my line up for reference purposes, and I enjoy them all. But it is all but impossible to hear the difference between a well designed class A amplifier, and a well designed class AB amplifier if quality components are used in both.


Posted by Lydia February 02, 2012 - 12:58 am
Hi Dave,
I fully agree, and The new dynakit clone Z565
taps are 8ohm and 4 ohm. however one may ask, If in triode mode or ultra-linear mode what impact will that have on class A before Class B commenses? 16 and 8ohm taps are on the original transformers. I guess the difference is the clones have given this added expansion of Class A at half power output but keeps the amplifier in Class A longer! speaking of the precision of the EFB, does this play a part in Keeping the little dynaco more in Class A longer? I think you know where I am going with this!.....More questions than answers but I feel like I am catching on! there is a Class AB2 Class B, B1, B2 etc. seems different classes has their advantages and disadvantages. Which one do you prefer in a PP Tube amplifier? to hear from you on this! maybe I am just re-iterating what youv'e already told me, sorry if that's the case. Lydia

Posted by Dave February 01, 2012 - 03:26 pm
Hi Lydia -- The short and easy answer to your question is yes: just keep the volume turned down. The ST-35 is a Class AB1 type amplifier, where class A operation is had at low power levels (on the order of say 1or 2 watts), but then quickly changes to class B operation at higher power levels.

If you are questioning if the amplifier can be held to class A operation through out its power range, the answer is no. Amplifiers that operate entirely class A though out their available power range are characterized by tremendous power consumption, poor efficiency, and low power output -- but they do sound wonderful. In any event, that is why any class A vacuum tube design that produces any significant level of power is always quite heavy, large, and hot. With the design restrictions placed on the original ST-35 amplifier, it would simply not be possible to produce its full power output while operating entirely class A.

One thing you can do to extend class A operation before class B commences is to connect your speakers to the next lower tap -- i.e. (for example) connect 8 ohm speakers to the 4 ohm output tap. Such a connection will reduce total power output by one half, but allow the amplifier to produce more power in class A operation before class B commences.

I hope this helps!


Posted by Lydia February 01, 2012 - 10:23 am
Hi All,
Just wondered, Is their a way to run the dynaco st-35 in Class A all the time? would it sound better?
just some thoughts that needs appreciated attention....... Lydia

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