Spare parts amp

[Ken Moon]

I still have some heavy items I don't want to pay to ship to Colorado, so I decided to make another amp

With this 370EX PT (275-0-275/150mA) and the Deluxe-style PT (6K6/4,8,16), I decided to make something like a 5G9, with fixed bias and no NFB this time.

And yes, those are cap cans - If the JJ 40/20/20/20 is used for the PI and preamp B+ like I willl here, I've never had ground loop issues.

I'm using an F&T 50/50A@500V dual cap for the reservoir and screen supply cap, with a Mercury JTM45 choke in between (7H/125mA/125R). I'll use a SS Stealth 8A/1200V rectifier. With the SS rectifier, I could go to a much larger reservoir, like many amps do (100uF or more), but I'm not convinced that is needed, so I want to play with that a bit.

The preamp is wide open to whatever strikes my fancy with 4 triodes, so I'll probably start with a single-input version of the 5F6A/JTM45 preamp to sort out the PSU and power amp, then make changes to match my mood at the time

I need to add a couple more small holes for bias test points, star and safety grounds, and a hole for an HT fuse, then I'll start wiring the PS. I'll take pics at each stage and make a little build gallery this time, since it's so easy to take and upload pics with a phone nowadays

My son from Toronto is here in Florida helping us pack and doing some house fix-ups for us, and he brought some mics and his ProTools laptop, so I can finally add sound clips when it's done

more to come...
[img768]http://i514.photobucket.com/albums/t346 ... 083947.jpg[/img]
[img768]http://i514.photobucket.com/albums/t346 ... 083904.jpg[/img]

[David Root]

Looking good!

I have used the JJ multi-can on plate-screen-PI with no ground loop issues, and it can be used in a TW Express where all PS caps go to a single ground point.

I also use that SGS Stealth 2x8A 1200V dual diode a lot. Cheap and low footprint.

+1 on lower reservoir. Even 50+50 is higher than I'd use in a 5G9 or other low power tweed. With that 7H choke you don't really need 40 on the PI either. Unless of course you're looking to really tighten up the entire PS.

Fixed bias with no GNFB wouldn't hurt with a tighter PS though, I think.

[Ken Moon]

Thanks for the nice comments, David

I do like a tighter PSU than on the tweeds - I sold a variety of 5e3 variations when I first started out 25 years ago (a couple of folks told me they saw old Moon 5e3's on stage in the last few years).

When I tightened up the 5e3 PSU, the notes didn't smear together as much, and the amp started to sound more like a Marshall. I've made several PSU designs, from CLCLC to cap mutiplier SS designs, but the simple design of the Marshall 2204 PSU sounds great to my ears. I'm just lowering the filtering a bit, and will adjust the dropping resistors accordingly.

If I need some sag, I'll put a 25W/100R resistor in series right after the rectifier.