Comments for page: EFB in a Dynaco ST-35

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Posted by Lydia January 30, 2012 - 04:27 pm
Hi Dave,
Thank you so much for spending the time to answer my questions and after reading them over and over I am beginning to understand what I need to do in order to implement these desires, into the ST-35 and get the most out of it! I will be looking up some old books of the fundamentals of tube circuit design and get some more foundation.. Thank you and will talk
to you later, Sincerely, Lydia

Posted by Dave January 30, 2012 - 11:21 am
Lydia -- In regards to your other questions, I have been where you are, with more questions than answers. While I can answer the questions you've presented, I think it would be best for you to seek out some of the old books as reprinted and available through Antique Electronics, and other sources. They can help you with the fundamentals of vacuum tube circuit design, so that as your questions arise, you will have a solid foundation from which to gain understanding.

As to your particular questions:

1. The EFB(tm) modification does not use any traditional feedback within itself, whereby a portion of an output signal is fed back into the input signal to modify the ultimate output signal obtained. However, it does use a sampling connection, whereby the ADJ voltage is derived from the available B+ voltage, so the regulator can then always keep the bias voltage presented to the output tubes proportional to the B+ voltage presented to them. In the strictest sense then, the regulator is really operating as a high current buffer circuit.

2. The 4.7 uF cap is sized to properly filter and decouple the B+ connection to the ADJ circuit, without being so large as to not allow the regulator to quickly respond to any voltage changes the power supply may produce. As such, it is not directly in the audio path per say, and therefore, really any quality component can be used at this location.

3. Triode "strapping" is slang for connecting pentode type output tubes for triode mode operation. The term comes from electrically producing "triode" operation from a pentode tube, by "strapping" the screen grid to the plate, through an appropriate screen resistor. The results of such a connection are exactly those produced by the same tube if no screen or supressor elements had been installed.

It is common with many modifications to install a "triode/UL" switch to allow the screen element to either be connected to the plate (for triode operation) or to the OPT UL tap for UL operation. When the switch is in the triode mode, triode strapping has been achieved. The EFB modification works equally well when either of these modes are used in the small Dynaco amplifiers.

4. I'm not sure I understand your question regarding auto transformers and heating elements (presumably, the heaters), but the question takes on significantly more importance when SE DHT designs are considered. In classic modern designs where indirectly heated tubes are used, the question of what type of heater power is best is usually confined to the phono preamp portion of the design only.


Posted by Dave January 30, 2012 - 10:12 am
Hi Lydia -- This post will deal with the comments you received regarding auto-bias versus fixed bias operation.

From the outset let me state that I believe the comments you received were likely made by someone who has no experience with the EFB modification, doesn't really understand how it functions, and also likely does not understand the theory involved as to why the EFB modification makes such a significant improvement to the small Dynaco amplifiers. I mean this as no disrespect to that person, but based on the (now) world wide installation record, and universally enthusiastic acceptance the modification has received to date, his comments are simply not in keeping with the facts.

Of the users that have commented, all have stated in fact how remarkably stable the quiescent current is once set. Yes, any new set of tubes will requiring rebiasing a few times during the first (say) 10-20 hours of use when operating with fixed bias. But once the tubes have settled in, the setting remains quite stable. On my SCA-35 that I used for the initial development of the EFB modification, the quiescent current remains at the same level today that it did when first initially set -- and this with near daily use. Also, because the EFB modification operates the tubes at a notably reduced dissipation level, they run cooler, last longer, and hold a given quiescent current level setting far longer than if operated at the current level of the original design. The idea that the bias adjustment would need to be reset almost every time you turned the unit on is simply a gross misrepresentation of reality.

In the end, the choice regarding which type of bias to use really comes down to which is more important to you: convenience, or performance and tube life? Fixed bias operation will require an "occasional" check to maintain optimum performance, but this is a small price to pay for the significant tube life and performance gained over the original auto bias design.

Finally, let me emphasize that the regulator device in the EFB modification is not operating as a CC device -- therefore, it is not working to "hold" the tubes to a 27ma quiescent current level. Rather, it is functioning as a full blown voltage regulator -- and this is important -- keeping the bias voltage fixed in spite of the actual current flow through the output tubes. This is very much unlike auto bias, where the bias voltage at any given time is very dependent on current flowing through the tubes. Once set, the EFB modification does allow the bias voltage to the output tubes to change slightly -- but only based on any changes that occur to the B+ voltage supplying the output stage. This is a highly desirable condition to achieve, as it forces the tubes to operate at their most linear point, under all power supply voltage conditions. This feature comprises the "Enhanced" element of the modification, such that besides the simplicity of converting to fixed bias operation, it adds a degree of performance that can only be matched by the use of regulated power supplies.

I hope this helps!


Posted by Lydia January 30, 2012 - 08:33 am
Hi Dave,
I didn't know this, but you had mentioned a thing or two about NFB. Negative Feed Back I presume.. How would you implement that into the EFB mod also? What is your thoughts on placing autoformers in line for the heating elements or is this only for SE amps? What is meant by Triode Strapping? would this help in The UL/Triode Toggle switch I want to implement on the front amplifier face plate? I love the detailed of the HF range and wanted to know If I put a really good quality 4.7mf 35Volt Capacitor in place on the EFB board would that make any sonic improvement, headroom? Depth and transparency? do you have to use an electrolytic here? with your EFB, how would you hook up the UL/Triode toggle?
Lots of questions all of a sudden....Hm ......
Thanks Dave, Lydia

P.S. As I am building this amplifier I WILL ONLY USE a multimeter to check continuity only so as to make sure soldered connection is approved. I will be taking the finished amplifier to a qualified professional near me in town that will plug it up, run all simulations, etc. and bias the amp.

Posted by Lydia January 30, 2012 - 03:35 am
Hi Dave,
Another question is , how often do you have to bias one's new 6BQ5's? I had spoke to a technician that owns and operates a repair shop for dynaco and other hi fi components for customers, and he said quote: "Why would you
want fixed bias in any form!" He said I would have to adjust the bias trimmer pots almost every time I turned it on.... I totally understand the reasoning behind EFB, but he said that it's more hastle than it's worth. He also said that even if you get N.O.S. output tubes one doesn't know the tubes capable output on various conditions and the possible variation in component construction of the tubes themselves. He also implied that you would have to monitor your settings constantly even if you get matched tubes....He said auto-biasing is the way to go!. I don't agree with him because that is what the adjustible regulator's job is to keep current 27mA , going to the tubes the same no matter what the voltage swing is right? anyway I would love to hear your comments on this one! thanks Dave, Lydia

Posted by Dave January 29, 2012 - 11:25 pm
Hi Lydia -- I see that Art and Steve have done a great job of helping you with your questions in my absence. I hope their answers resolved the questions you had. If I can be of any assistance with your project, please don't hesitate to ask! Thanks for your interest in the EFB modification.


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