Paleo-Electronics

As you’ll have probably guessed by now, I don’t really know what I am doing. The GA-5T had a nasty buzzing that was independent of volume. So I re-did the power filter caps and now I have a hiss/buzz that is dependent on volume. I have run through with a DMM and every voltage is within 10% and the cathodes are all exact.

So, I was out of options essentially, other than replacing everything. I actually probably had a ton of options, but I just don’t know them. As luck would have it, I got a little bonus from a project I worked on two years ago and a sweet little Oscilloscope came up on Craigslist. 75 bucks for a decent HP 1725a! Excellent timing.

Then I started shopping for a signal generator on CL and Ebay… well it turns out that this is the 21st century! Who knew? I found a mac software product called Signal Suite 2 and it is a perfectly good signal generator. Sine, square, triangle and sawtooth.. bunch of other options I haven’t tried yet. It’s control interface is shown here (click any image for the big picture)

Here is a shot of the amp on the bench. Notice that the chassis has been cleverly designed so that it can rest on the cabinet while you are working.

Now the scope. Yes that is a glass of scotch.. amp debugging is thirsty work.

[Image coming back as soon as I explain to my mom about the drinking.]

Very cool! Scope and generator, done. So I’m off to trace the signal. First up.. the reference signal

Nice and clean, standard 1khz signal 0.5 V p-p. Out of the 1st preamp stage we have

Still plenty clean, now at about 18V p-p. Second stage is after the volume knob (which I have at about 1)

Now this is back down to about 0.12V p-p and has picked up some noise (not easily seen in the thumbnail, but pronounced on the screen) and a ‘ghost’ signal or something. I don’t mean the very faint much larger sinusiod, which I assume to be a scope artifact, I mean the double lines in the signal. Not what I expected to see. That volume pot is noisy as hell so maybe it is the culprit.

Next up is the output from the splitter (half a 6EU7 on the GA-5T and just a 6C4 on the GA-5)

The noise is still there and it looks like the whole signal ‘fattened’ but the ‘ghost’ is still there. Finally, we get really weird :

That is the signal from the output tubes (Pin 5 of the 6AQ5). I’ll be ordering / swapping tubes asap to see if this is just a tube problem. Also, some contact cleaner in the pots will be happening soon.

Furthermore the scope will hide DC ‘lift’ from me in AC mode and it’s not made for the DC I see. (it can’t do more than 50 VDC)

So I have some more clues now.. I guess. If anyone knows what output signal that means, I’d love to hear it.