MOSFET source follower for output stage of AB763 blackface

[SixStringBender]

Martin, thanks for the pot info and resistor info.

I don't think there would be room for the electrolytic caps on top of the board if the board were mounted on top of the tranny. Plus the main PT wires go over the area this will all be fitted.

I'm mostly worried about how I would work a surface mount device on a board. I can't see the termination and I assume it is flat termination. I have found some Hammond PCB mount transformers.
I know I could make a template by punching the PCB leads through a sheet of freezer bag plastic (done it before with other PCB pin devices) then lay the plastic template on a blank Hoffman board. I could then drill the board on my drill press and fit the transformer directly on it and secure the board to the transformer.
I would leave some overlap of the board and mount turrets and wire the pins to the turrets. That way the pins will never be stressed. We can glue the wires to prevent them from being yanked. I might could fit the bridge rectifier on this board as you suggested.
Then wire everything else from the turrets to another small board that is mounted using standoffs.

Will these rectifier diodes need any heat sinking?

I might could even mount the tranny outside and put a dog house over it. However, it will go inside if I can find room.

Do these look like they will mount upside down like the Triad you posted? If so I think I can work with one of these since they have termination pins.

Also what do you think about the smaller tranny (229A230)?

PCB mount Hammond transformers.
Hammond 229A230 230V CT at 25mA
Hammond 229B230 230V CT at 50mA

Data sheet
http://www.hammondmfg.com/229.htm

Kagliostro, HA, , that is the same 229 series I found. Thanks for looking out for me. It would have helped me if I hadn't found them. These are the Hammond PCB trannies I spoke of in my last post.

John, I'll check out the link.

[martin manning]

Both the Triad and the Hammond flat-packs have pretty stout pins for terminals. I would stay with the 12VA rated transformer to be sure you don't run out of current driving the output into AB2.

1N4007 diodes won't need a heat sink.

[SixStringBender]

Alright, thanks! I'm about set to place the orders.

[SixStringBender]

I finally pulled the chassis instead of going on memory alone. There certainly isn't enough room to mount the tranny inside. So on a DRRI the only option is to turn the choke 90 degrees and mount a power tranny behind it. If I go through with this I'll just go with the 25VA Triad chassis mount tranny since an external mount is the only option. And it will be a close fit.

On the inside there is barely enough room between the PCB and the PT to put a small board for the filter caps. And they will have to be stacked on each other and held with hot melt glue to the board. The bolts that hold the main PT in would be good to mount the turret board to for the filter caps so no chassis drilling would be required. The caps are all that would fit here.

The bridge rectifier, mosfets and resistors would have to go on a turret board on the back panel on the inside of the amp. Right next to the fuse and down toward the rectifier tube end of the back panel. All wired to everything from there. This is right behind the output tube sockets (and down a little farther), so it isn't too much farther than the stock reach.

So if I go through with this what tonal change can I expect? From my understanding no more cross over distortion or harsh clipping. Does it add any second order distortion? Can I expect it to sound more like a Dumble SSS?

I'm gonna tame the reverb with a dwell control. The SSS has a send level and I'm guessing that is nothing more than a dwell control. I know the reverb on this thing is overkill. Even when it is dialed down to not enough reverb in the sound it is still too much reverb in the mix. I want the lush reverb after the note, but not so much of it. It isn't the decay I'm talking about either because I like long decay. It is like this thing is swimmin' in reverb.

[martin manning]

Can you post a picture of the inside of the chassis?

[SixStringBender]

Inside area around PT.

New idea. I would probably mount one cap in the below picture and the other on the other side of the PT.

[SixStringBender]

Here is where I would most likely mount one electrolytic cap on a turret board to the PT studs. I would mount the other electrolytic cap on a turret board to the PT studs on the other side of the PT.

[SixStringBender]

Outside where I could mount the new PT. near the hole where the old bias pot location hole is. I moved the bias pot for the new tremolo disconnect pot. I would have to twist the choke around and maybe move the OT over more too. I'm gonna probably mount a Super Reverb 4 bolt OT so it'll be easier to move closer to the cap pan. The speaker won't fit if I do this. I plan to do a head conversion anyway because combo amps just do not sound big enough to me. I don't like tube rattle either.

Believe it or not the speaker fits with this larger OT in the picture.

Do you think there would be any noise having two power trannies so close together? The Triad is a toroidal design.

[SixStringBender]

This is where I think I could mount a narrow long turret board with standoffs having the bridge rectifier, MOSFETs,resistors and zeners mounted on it.

[SixStringBender]

Here is the tremolo disconnect pot and bias pot so you'll understand why I had to move the bias pot. Kind of wish I hadn't moved it now because I'll have two holes in the chassis now. And I need the chassis to mount the new bias adjust board in that area. I plan to put two Bourns 25 turn bias pots on a small turret board with standoffs right where the bias pot is located now. I'll only need one turret to make a ground point for the 10K 1/2W resistor and 22uF 63V cap. The other connections I'll drill holes for each wire, run them up through the bottom of the board, glue the wires in and have her solder the wires directly to the pot lugs. I'll glue the pots to the board with hot melt. I just gotta find a good Bourns 25 turn top adjust pot with side exit lugs.

The new tremolo disconnect pot was so long the bias pot had to be moved.

[martin manning]

Pretty crowded in there already. What is the depth of the chassis, and what is the distance from the inside side wall to the PT cut-out?

[SixStringBender]

One inch from the cut out to the inside wall and about 1 and 11/16 from the chassis floor to the top.

So just enough room for the turret boards to sit on the nuts on the studs and have another nut snugged gently on the turret board. Then one electrolytic cap on each board. Two wouldn't fit stacked on one side like I thought they would. The PCB side is an even tighter fit, but the wires from the main PT will easily reach over a single cap there.

[martin manning]

Too bad the chassis is less than 2" high. Otherwise the flat-pack transformer would fit nicely on the end-wall by the PT.

[SixStringBender]

Yes it is. I hadn't thought of that. That would put them 90 degrees opposite of each other too.

I assume there won't be any problems with the toroidal design of the Triad I posted being mounted next to the power tranny. A piece of aluminum sheet could be mounted between the power transformers if there is increased hum, I suppose.

[SixStringBender]

Before jumping the gun I decided to study transformers more. I didn't like the idea of a power transformer right next to the PT. I don't know how well or not it will work. I also learned about the short coming of the toroidal transformers. Their inability to handle DC offset on the AC main voltage.

Since I plan to install a larger PT in the future (I was leaning toward the Allen TP35) I've decided to see if there is a PT with two high voltage center tapped secondaries out there. Or maybe see if Heyboer will wind me one.

I need to drop some B+ anyway because I'm at 476-484V with my SS rectifier and power supply caps. I was going to build R.G.'s voltage reducer using an 8V CT 3-5A tranny to drop my mains voltage. This would put my heater voltage at 6.34V instead of 6.78VAC as it is now, but if I can get a PT wound to drop my B+ and have the taps I need for the MOSFETs I think it will be the best plan. Plus I wouldn't have to fool with the voltage reducer and have to worry about someone not plugging it in between the amp and wall.

So, a stock DRRI is 115V primary and 330V-0-330V secondary. I'm sure about the primary, but not the secondary (sure about the primary due to my filament voltage readings). What should I get to drop the B+?
Here is my guess, based on some calculating. I plan on installing KT66s at a later time.

120V primary
315V-0-315V at 180mA
115V-0-115V at 50mA
50V bias supply
6.3VCT at 5A
5V at 3A.

2-1/2" by 3-1/8" mounting centers.

internal hum shield on a lead for grounding
paper stick-wound
M-6 grain oriented lamination steel
copper flux bands
Uni-strand pretinned 12 inch leads

Can you-all think of anything else I need to request when I have this built?
Will requesting the 120V primary make any difference in their decision to wind the secondary ratios? Instead of winding it as if the primary is 115V as it seems everyone does still to this day. Even my stock DRRI PT voltages would fall in place at 115V mains voltage.
Any idea what it will cost to have a single PT wound?