Paleo-Electronics Blog

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.

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.