Paleo-Electronics Blog

You may recall that I ended this post : Mods to the working GA-5 with a problem.

The problem was that like a bonehead, I didn’t try to dry fit any of it together and once the T8 was in place, it was hitting the PT.

So I had to rip the damn thing out. Just as well. I went researching T8 prices and found that I had gotten a steal! I paid 15 bucks for an NOS Mallory T8 T-Pad Attenuator. I can’t find them for under $80. I didn’t know.. in fact, I had just lucked into the idea and bought the first one I found.

Since I wouldn’t be putting a it into every amp, I needed to build a separate box for it.

Here is the final (with the original box for show) :

The wiring is just the application guide drawing with jacks instead of Amp and Speaker :

And thats all she wrote.   Works great but as I play with it more I  definitely hear the loss of tone.

So yes, I have removed the hum, and to a large degree the amp appears to be working well, but I noticed that bass notes got kind of farty sometimes and some specific notes would warble all over.

Weird…  I’ll be putting the output section up on the scope again soon to see if I can figure out whats up..  I’m betting on a blown speaker, but I suppose the OT could be gone too.

One of the problems with an amp this old is that I don’t know what the primary impedance is  meant to be.  I can look for a short easy enough, but some shorts only appear at high power.

The speaker ought to be easy to test, I’ll plug in the good amp to that speaker.

Film at 11.

But I have done it! The amp sounds great. Some contact cleaner, a 3 prong mod and then I’ll button up the cab and give it a final run through, but I think I finally got it.

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

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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.

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This just went for $361 on ebay. I plan on building one from scratch and I’ll be lucky to do it for that.

What’s the difference between my amp and that one? Well, I’m not actually sure sonically. Mine (a Crestline Skylark) has push pull 6QA5’s. That one is a classic single ended 6V6 like the champ it took its basic design from.

Odds are.. you missed out. That one looked pretty good too.

Of course I’m gonna mod it, this ain’t a museum!

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.

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So.. those of you following closely (both of you :-), will recall that I had last determined that an idiot had put in a couple of diodes in place of the 6X4 rectifier tube. Amazingly, this person may have owned a multimeter, because he seems to have recognized that the diodes increased the voltage out of the power supply circuit. He actually replaced the 1k dropping resistor with a 2k to bring the B+ voltage near spec (242V) while leaving the power to the center tap of the output transformer raw. This had something like 265V .. which is hot, but I think the iron can take alot of abuse on these things.

So I snipped off the offending diodes, and replaced the dropping resistor and voila! we are no longer blowing fuses and voltages are back in spec. We don’t know what damage may have been caused by those damn things… I stuck in a real 6X4 rectifier and I still get the loud hum.

To add to the fun, a buddy of mine, the guy who came up with ‘Paleo-Electronics’, found the Weber Copper Cap rectifiers. These are the RIGHT way to do solid state rectification. They are simply tubes with solid state components, but done right, they protect your other tubes in a way that diodes can’t.

In a tube rectifier, high voltage is not passed until the tube warms up. As the tube gets closer to temperature (various widely, but well under a minute) it produces more voltage. It’s not until the tube reaches operating temp that it puts out full power. This is a good thing because the other tubes are also warming up. If they get full voltage before they are warm, they can damage the tube immediately or at least have their lifetimes seriously curtailed.

The Copper Cap rectifiers mimic this warm up with what I assume is a capacitor network inside the bottle. They even mimic ’sag’ (the failure to produce full power when driven hard and quickly) although that is a conversation for another time.

You can get them here

I tried swapping the Weber out vs. the Original 6X4 I had in my other GA-5 and while I could hear a slight difference in the reaction to hard fast strumming, that was it.

This is actually why I took the Paleoelectronics.com domain in the first place. I got tired of hearing how an amp tech had cut out the old 2 wire cord from a classic amp to make it safer.

You certainly need to get an earth ground hookup in these old two wire amps, but you don’t have to ruin the resale value of the amp to do it.

Here is how I do it, I take a small plastic project box and stick a 2 prong receptacle on one end, a three prong (IEC) receptacle on the other end and I pull out the earth ground wire and solder it to the PT chassis ground.

Here is a dumb diagram :

Doing this means you can leave the old plug in place, assuming it is not frayed and dangerous. You still get the benefits of 3 Prong safety, assuming you do a good job of soldering the earth ground (the half round part of the 3 prong plug) to the chassis. I did this at the ground wire of the PT. Don’t go alligator clipping this thing on or anything stupid like that. If it gets loose, you have a good chance of blowing something.

Now you can use a standard 3 prong computer power cable, and just plug the two prong plug into the other end.

This and removing the death cap will make your amp much safer, but it does have a drawback. You will now discover how well your amp is grounded. My GA-5 Crestline is pretty well star grounded but I still get some popping when I touch the strings.

Many of you are likely wondering what is that tantalizing thing called a “death cap”.

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.

As noted previously, the amp had a loud hum with no inputs connected and the volume and tremolo turned all the way down. Nothing I could do for it, I was going to have to open the case.

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!