DIY LA2A (P2P)

Posted by DibS in DIY      4 comments

Nov
28

I’ve started my DIY LA2A. I’ll be posting details as I go.

The faceplate was created using SSLTech (Keef)’s “Super 77 Method” The design was passed on to me by Nate Kahle which I modifed. I will not be using a 3 way switch as of now so I simplified that part of the design. I’m pretty happy with the results. There are some imperfections, but I’m chalking that up to “antiquing” the look.

Here is the turret board populated and wired. I’m currently using a 100uf cap for C4 in place of the vari-cap that normally goes there. All known sources of those vari-caps seem to have discontinued it. I’ll change that out if I need to when I’m all done. I’m using all metal film resistors and Orange Drop caps with a Nichicon on C10.

So when I fired her up, no love… No smoke and fire, but barely passing signal. I found a few mistakes and omissions in the wiring and tried again and whalah! It distorted unpleasantly at peaks and when I poked the wiring with a drumstick (output wire and cap) I was able to get it to go into full motorboat mode. I put some grid stoppers on the 12BH7 and Bobs yer uncle.

Man, does this thing sound good.

Edit:  I’ve since opened her back up to chase down what little hum there was coming out of this thing.  I was able to get it absolutely quiet by tying up the wires that are coming from the left side of the front panel and moving them away from the input transformer.

I replaced the fixed capacitor I put in C4 with an Arco 42.

Also, I replaced V1 with a 12AY7 and changed R9 and R13 to 100K in an attempt to lower the input gain a bit.  It helped a lot.  I’m now able to bring my knobs past 10.  I’m still thinking I might replace the 600/10k input transformer with a 600/5k to give me even more room to play.