I’ll save the final bit for last, because I want to do a quick walk through all the steps it took.

1. Received Amp, turned it on, loud hum with volume at 0 and nothing in the jack.
2. Opened back and found missing 6X4 rectifier with two shitty diodes instead.
3. Ripped out diodes, compensating resistor and death cap, plugged in 6X4. Same hum.
4. Replace all of the filter caps with 500v rated caps. Lost hum at zero volume. Hum now follows volume knob with or without input.
5. Scope traced a reference signal and isolated signal issues to output pin of power tubes.
6. Tried different power tubes. All is well.

The very first step I took was to swap out tubes but that didn’t help. It didn’t occur to me to try the tubes again after step 4 because I had already tried that. It didn’t occur to me to start from the beginning … but that was the right call. I could have fixed this without the scope, but that’s ok, because it was definitely fun to trace the signal.

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.

Let’s have a look shall we? If you’ll recall from my previous post, the rectifying circuit in the GA-5 looks like this :

The death cap, a .022 mF is there to help suppress hum. Have that removed!! It is actually very dangerous, and is no longer allowed in circuit designs (in fact, I think it was outlawed in the mid sixties). I found my first reference to the death cap here and a subsequent conversation with somebody with a lot more experience than I do made the problem clear.

When that cap goes, and it will, it will do one of two things. It will blow open, or it will short. Open is no problem, but shorted means that there is now a direct path from power to ground. Just touching your metal cord shield should be enough to put you in the circuit (I hate metal plug assemblies on cords). If you are touching your strings or metal on the guitar, your odds of being electrocuted are much better.

Remove the death cap and ground your amp with a three prong plug first. More details on the way.

So I took off the back to have a first look. And sure enough.. there was something I had missed. There was a piece of silver tape over one of the tube sockets. WTF? The missing tube is the 6X4, which is a rectifier (converts AC into DC, sort of). There are two diodes soldered onto the tube socket. So I go googlin’ for a reason and find out that some people replace the tube rectifier with a diode network.

I email the seller, explain what I found and ask if the amp’s been modded. He says he doesn’t know anything about amps, but it was blowing fuses and he took it into the shop and now it sounds great, honest!

Modern rectification has come a long way from the old tube style rectification. With a little thought, you can replace the old tube with a solid state circuit. There are good ways to do it, but this wasn’t one of them. Lets look at the circuit :

V5 is the rectifier tube. When you power up the amp, two things happen : 1. About 260 VAC starts hitting the plates of the rectifier (on the left and right sides) and 2. 5 VAC starts hitting the heaters of the 6X4 tube (the ‘V’ with x’s above at the top of the tube).

Once the tube has warmed up, current will start flowing up the cathode (shown leaving straight up from the tube). However, until it warms up little if any current will flow. This gives the other tubes in the circuit that are being powered a chance to warm up before they get hit with 240 V on their plates.

Diodes, of course, don’t warm up. For reasons that I don’t want to go into, you really want the tubes to warm up before you hit them with power. Mind you, I’m not talking about playing the guitar, just powering up.

So now all my tubes are suspect. Yay!

Finally, I crawled out of the shell and started looking for an amp I could play at home at night and also in a studio/practice situation. I settle on the “Lil’ Lanilei” by Tris at Songworks. However, that took some eight weeks to build and I was getting antsy …

Then I read about Clapton using the Gibson Skylarks in the studio. A little research turned up some interesting points: They sound great, are damn cheap and have very simple circuits.

Turns out the real ‘Class A’ single ended Skylarks are not so cheap. This one went for $255 + 40 shipping and that is as cheap as I’ve seen one go in a long time. 400+ is the usual. They look like this :

After a couple of weeks on ebay, I found two 60′s Crestline GA-5′s (well, one GA-5T (Tremolo) and one GA-5). These are both Gibson ‘Skylark’ amps, but not likely the kind used by Clapton They each cost 200 with shipping and both arrived with significant hum. New, they looked like this :

As the GA-5 arrived first, I worked on it first. I removed the old speaker wire and added a longer run of real speaker wire, and I replaced the power filter capacitors. That amp rocks. Simply fantastic clean at low volumes and great 60′s crunch as you turn the (only) dial. The other ‘dial’ is the on off switch.

The other one still has a nasty hum.. I’ll start telling that story soon.