| Posted by Art Grannell | February 09, 2012 - 10:40 am |
| Lydia, I’m not quite sure of the actual physical layout that you are using for your modification. If you have constructed a small board, for the additional components, as I did, then you can mount the 4 470Kohm grid return resistors (2 for each channel) on that board, and run wires to each PC-13 board. Just be certain to first remove the two existing 470Kohm resistors, per channel, mounted between each output tube pair. Using this method, one lead of each of the two 470Kohm resistors (per channel) will be attached to the center, or wiper terminal of each 2K pot. A wire from the other end of one of these resistors will be run to, and soldered into eyelet 8 and a wire from the other resistor lead will be run and soldered into eyelet 9. Then do the same for the other channel, and if the 1k grid swamper resistors are in place from eyelets 8 and 9 to pin 2 of each output tube, then this part is complete. Below I’ve described another way to accomplish the same thing, if you wish to leave the grid return resistors in place on the PC-13 boards. Do not confuse these two different approaches; use one technique, or the other: If you will look at the bottom of the PC-13 board, so that the print is facing you, between the two EL-84 output tube sockets at the right end of the board, there is a “trace” with two connections, that are offset a little, directly between the two tubes. If you turn the board over, you will see that there are two identical 470K resistors (yellow-violet-yellow-silver [or gold]) that have one end attached to this trace on the other side. The other ends of the these grid return resistors are connected to eyelets 8 and 9, which connect to pin 2 of each output tube, either with a short wire, or a 1Kohm resistor if you have already installed the grid swamper resistors. Going back to the bottom side, note that the trace from center of the resistors goes upward to the left, picks up some other connections, and ends at eyelet 2 on the left edge of the board. This trace is the board “ground” connection. For this method, this ground connection trace between the grid return resistors must be isolated from the rest of the board ground, and attached to the adjustment (center-typically) pin of each one of the 2K pots, that is, one wire run from each adjustment pot to the common connection of the grid return resistor pair on each PC-13 board. There are a number of ways to do this. I think the easiest approach would be to break the trace to the ground connection, somewhere in the area just above the inner output tube. Just be sure you do this “break” on the trace somewhere between the connections of the 470 K resistors and the 33K (orange-orange-orange) resistor. I would use a razor blade or an Exacto knife, and cut away the trace so that it is no longer connected. The voltage here is very low so a small cut will work, but check it with your ohmmeter to be certain that it completely “open”. The board that I have in my hand, has the resistor leads protruding though far enough, so that the wire to the adjustment pot could be wrapped around one of them, and soldered directly on a resistor lead. As Steve pointed out, there is nearly no current carried at this point, so you can use small wire. If you cannot attach this way, then you will need to remove one resistor, and using a very small drill bit, enlarge the hole slightly, so that both the wire and the resistor will fit in the hole, then resolder the connection. Again, keep in mind that the parts numbering is different between the SCA35, and the ST-35. The R35 and R36 grid return resistors that Dave references are numbered R8, R9 on the ST-35. The printed circuit boards have a slightly different layout, also. Good luck, and remember to be careful! |
| Posted by Steve L. | February 09, 2012 - 07:59 am |
| Hi Lydia, Sorry for the delay but I got distracted and Art was going to answer. He is preparing a more thorough answer but here is a short one: Yes, the suggestion was to put the four 470K resistors on your EFB board. One way of fitting them in might be to stand them up, " vintage transistor-radio" style. If you want to keep them on the Dynaco board, cuts in the PCB tracks are needed. Art's answer will address that. |
| 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. Dave |
| 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?.....love 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 |