by dcgillespie » Fri Oct 08, 2010 11:13 am
Hi Shannon --
No panic intended or wanted on this end either! In my mind, the value of your efforts to this hobby are inestimable. It is simply mind boggling to me that directly because of your efforts, folks can easily build a clone of the original ST-35 with improved physical layout and input stage configuration, from a great set of instructions provided to make it all happen with certainty. It is a testament to you, and Dynaco for the original product they produced. I am quite certain that those who want to install the EFB mod with your boards will find it rather easy to do. As always, where there's a will, there's a way.
I would welcome very much any further documentation you can provide regarding your performance evaluation for both any type of resistive cathode bias concept, and that of EFB operation if you chose to try it.
For the Dynaco 6BQ5 amplifiers, the primary issue at hand is that the loading of the Z-565 transformers is simply too low for any type of traditional resistive cathode bias operation. The Z-565 is an 8K primary transformer, and is an ideal load for these tubes with traditional cathode bias when operated in pentode mode. But these amplifiers use the 575's screen taps which effectively raise the required primary impedance for low distortion operation when traditional resistive cathode bias is used. Since the impedance offered has not been raised, it is therefore too low for proper resistive bias operation. As a result, when resistive bias is used anyway, the excessive current draw developed as a result of the "lowered" impedance causes too much bias deviation across the cathode bias resistor with the application of power. This produces the large shift in operating point, and the raised levels of distortion as a result. Dynaco attempted to resolve this somewhat by connecting all the cathodes together, so that one channel might help hold back on the bias shift in the other channel, as I explained in the article.
The net result of this is that really any type of stand alone resistive bias system -- applied either on a per channel or per tube basis -- will suffer similar results as that which happens when both channels are driven in a stock SCA/ST-35 amplifier. The shift in operating point is excessive, which results in the raised distortion levels. Also, since any type of resistive bias system limits peak current capability, power output is reduced as well. While I did not know who provided the data published on the Triode website when I wrote the article, I do believe it still illustrates valid single channel performance when separate tube or channel cathode bias resistors are used. That it mirrors my data for the stock SCA-35 when both channels are driven is very telling. But again, I would most welcome your confirmation of the results I provided. For the record, my test conditions included:
1. 120 VAC maintained under all operating conditions, as specified for the SCA-35.
2. Tests made from the full 16 ohm winding of the Z-565 transformers, into a 16.0 ohm, 100 watt resistive load for each channel.
3. Maximum power output was measured at the onset of visual clipping in all cases.
4. Distortion readings were taken at a power level 1 db down from 17.5 watts RMS in accordance with the original specifications. Note that this power level is very close to the maximum power your distortion readings were taken at in the data posted at Triode.
5. Equipment included an HP 339A THD distortion analyzer for test signal, power output voltage readings (true RMS), and THD distortion readings, and a Heath IM 5248 distortion analyzer for IM distortion readings.
Understand too that a 2-3 volt drop in B+ is nowhere near the same as a 2-3 volt drop across a cathode bias resistor. The presence of the resistor itself limits peak current draw as mentioned earlier, and also causes the tube to limit it's peak current handling ability as well. Since the dynamic resistance of the EFB cathode regulator is effectively zero, it neither limits the peak current capabilities either due to itself, or through the tube due to a changing operating point. Therefore, power output increases with it's use, and distortion falls.
Finally, as explained, by tapping the bias for the 3TR from the B+ supply for the output tubes, it further causes the bias (once set) to always maintain the same relative relationship with the B+ level, regardless of how or why it might vary -- be it due to changes in AC line voltage, current draw, or otherwise. It is this feature that basically allows the same low distortion figures to be maintained when both channels are driven, or just one is. The use of fixed bias produces a significant drop in distortion to begin with, due to the improved operating conditions it provides for the load offered. But the floating feature of the EFB cathode regulator allows those reduced distortion figures to be maintained under all conditions of use, whether there is a significant drop in B+, or not. It really is a 1-2 punch for improved performance in this case.
Thanks for your comments. EFB is pretty unique in the benefits it offers to these Dynaco amplifiers. I hope you can try it out and report back. I would look forward to it!
Dave