Comments for page: EFB in a Dynaco ST-35

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Posted by Bill February 17, 2012 - 09:35 pm
Thanks Art. I have the cathode resistors on 2 lug terminals outside the amp on either cooling slot. Looks like it was meant for that.

Posted by Art Grannell February 17, 2012 - 09:16 pm
I see no reason why a "conventional" 3/4 or 1 turn "pot" should not work just as well as the "trimmer" style that I use. The adjustment really doesn't require the precision that many multi-turn trimmers provide. If you want to adjust the bias from outside the chassis the conventional style should be easier to mount, however keep in mind that you will still need to access a cathode resistor to measure the voltage drop.

Posted by Bill February 17, 2012 - 08:28 pm
I'm in the process of doing this mod and wanted some way of adjusting bias from outside the ST35. I have a 1/2W 5K linear pot that looks similar to the one Dave G. used in his amp. It only has one turn however. Does this allow enough flexibility to nail the bias? I also have a Bourns encased pot like Art uses, which has 24 turns, but is difficult to mount to provide outside access. Is one better to use than the other?

Posted by Lydia February 10, 2012 - 01:58 am
Thanks Art, steve, I understand this and I truly appreciate the info! Lydia

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.


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