Weak left channel?

[Ty_Bower]

I'm making myself a dummy load with two 10ohm/25W resistors I've mounted on a heatsink, I couldn't find any 8ohm ones but this should be close enough.

I'd think so.

So when I hook up my oscilloscope to check the two channels will it work if I put I 1Khz sine wave into the amp with my computer and hook up the scope across the two leads going to the dummy load?

That's how I do it.

Does it matter what the volume of my sound card is set at when I do the test?

If you set the volume of the sound card too high, I guess you might end up clipping the first stage of the SCA35 before you hit the limit of the 6BQ5 output tubes. I'd probably set the volume of the sound card somewhere about mid-way, and adjust later if needed.

Here's some other 'scope related discussion for your perusal:
http://www.diytube.com/phpBB2/viewtopic.php?t=1680

[K-MAN]

Thanks for the nifty test Bower, I never heard of testing the tubes current draw that way but it makes allot of sense once I thought about it. I found three output tubes were around 35mA but one on the left channel was only 30mA. So I knew for sure the problem was on that channel and my tubes were recently tested, so I carefully looked over all the traces and found a hair line break in the trace going to the 1mf coupling cap. I can't believe after all these weeks of trying everything it ends up being a simple break in the trace, I only found it because of my continuity tester.

[K-MAN]

Been listening to it all day and it sounds great :) I went nuts finding the problem but I sure learned allot it the process. Only thing left is to replace the filter caps with some JJ-electronic ones and it's done for good.


[/img]

[K-MAN]

I've noticed working on this thing that the twist lock caps seem to be insulated from the chassis with a phenolic spacer. So am I right in figuring the ground lugs on them are isolated from the chassis then a wire connects all of them to the ground lug by the diodes? Does ground usually get routed to the area by the power transformer?

[EWBrown]

IIRC, there should be a single ground bus to chassis connection, which eliminate any possibility of creating ground current loops. Look for conections to any of the center "ground" terminals on the terminal strips. I don't recall if my SCA-35 was grounded by the diodes or by one of the can caps.

As most of these were built from kits, there could be some odd bits of "builder's creativity" or other strange mods done during the build, so there is no guarantee of any standardization.

The factory built units just look a lot nicer inside, and are better built than myst kits were (I have seen some scarey building "disasters" in my day), and sometimes have pop-rivets instead of screws in some locations.

/ed B in NH