Skylark Faceplate with plenty of rust (click for fullsize image)

Now it’s not going to matter one bit if I don’t do anything about the looks, but it would be nice to get her cleaned up. There is no way I am going to be able to clean around the silk-screened lettering or lines. So the whole thing is going to have to come off. I don’t even know how you clean these things.

Now, the chassis is actually just a single piece of metal, bent and punched. I’m not sure if it’s steel or aluminum but either way, it’s easily bendable.
Here is a picture of one out of the cabinet :

5T Chassis out of cabinet and looking from behind

So I suppose I could just have one remade. That would surely cost more than the amp, but I’d come out of it with the specs for new metal, which would be a good thing for anyone else trying to do this. Then I would need to get the art redone or redesign something. Again.. favors or money and no improvement in the sound.

I’m leaning towards leaving it, for obvious reasons, but very early in the process I’ll have to make the decision.. anyone have any thoughts?

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.

I’ve already done my 3 prong conversion, but now it’s time to make this thing more fun. Reminder for those reading along in sequence: I have two GA-5′s, one of them is the project that is not yet working and the other one is the one that worked after redoing the power caps. I use the working amp daily and that is the one I am modding.

The idea is to be able to play my working GA-5 at cranked levels in the conservatory (guest room) at night. This amp has great creamy cleans up to about 5 then it gets into a CCR crunchy sound followed by a very bluesy Claptony sound at 7 to 12 on the only dial it has. I want to be able to hear those sounds without waking the kids.

So, I bought a NOS Mallory T8 Attenuator on ebay not too long ago. The attenuator acts like potentiometer, but it shows a constant impedance (8 Ω) to the ‘source’ side. The thing is, just sticking a resistor in the output circuit is dangerous and even when done right, it changes the tone.

It’s dangerous because if you go too far from the expected impedance at the output transformer, you can blow the OT. It changes the tone because the relationship between the OT and the speaker is now changed. The ‘right’ way to do it is get an active circuit in there. The simple, cheap way to do it is get the right resistance in there.

My Lil’ Lanilei amp uses a combination of a power soak and a hybrid power section to give you the same sound at any volume. Less speaker interaction, I think it really does it, or at least damn close. You can pay THD a couple hundred or more for a ‘hot plate’ that seems to do the same thing. The THD may well be worth it to bring a ’59 Bassman into the studio or something like that, but I paid $15 for the T8 so lets see if it can work. [Note: this wasn't perfectly clear before, but the whole point of the T8 is maintain that constant impedance. It may color the sound, but it will not damage my OT]. There are lots of people who have built special resistor boxes for various amounts of dB cut and were perfectly happy with the results.

So here we go…

In order to work, attenuators need to get in between the output transformer and the speaker so on a simple old amp like the GA-5, that means I am going to have rewire the speaker connections. That’s fine, I’ve been meaning to put in/out jacks in the back anyway. The T8 is pretty much like your basic potentiometer, as you’ll see. Here is the usage :

So it’s a big wirewound potentiometer with 3 lugs. It dissipates 15 watts of audio, so for a Crestline Skylark, which is lucky to push 12 watts, we are in good shape.

The original plan was a shorting jack so that I could send to a cabinet with a speaker out and have the attenuator on board. Here is the diagram :

So that’s pretty simple, OT to attenuator to shorting jack. If there is no plug in the jack, the internal speaker gets the signal. If there is a plug, the internal speaker is cut off.

I wired it up with nice thick speaker wire, drilled out my back panel for the plug and then knob and we are good to go. Give yourself extra wire between the OT and the T8 and the speaker and the jack. When you open this thing up you’ll need the room.

How does it sound? Great! I guess I can hear some of the coloration because on the neck pickups the bass is very muddy at high gain. However, this amp ain’t that great with overdriven bass response on the neck pickup anyway.. so hardly a big loss.

I am really excited by this mod, it was pretty easy and it sounds great. I’ll get some samples up soon. However, I made a huge blunder and I’ll be rewiring this soon, more on that later.