Hoffman Style AA764 Layout

[ncusack]

Hey guys just wondering if the following layout makes sense following normal Hoffman style layout design. This is for a blackface champ no vib. It's just a quick hand drawing and im sure i forgot to label a few things but let me know if it makes sense and if you would change anything following the hoffman style layout

http://www.glowfoto.com/static_image.../img6/glowfoto

[loudthud]

Photo won't download for me. I get a page that looks like it should contain the photo with a red X in the box. If I left click on the box and select show picture, the red X comes back after a few seconds. Eventually the page goes to : Glowfoto :: Photo Sharing .

[ncusack]

Thats strange it loads fine for me. Ill try another hosting site tonight.

[fyl]

let me know if it makes sense and if you would change anything following the hoffman style layout

The TS section should be redrawn (check Doug's AB763 layo, replace the mid cap with the 15K to ground), the 220K should connect to the clean ground. I would also modify the NFB section. If I get a few minutes I'll draw an alternative layo and post it tomorrow.

[ncusack]

Yeah if you could provide an example of how this is supposed to work that would be sweet. Only thing im not sure about is that 220K resistor I thought since its tied to the power tube it was part of that ground section but I guess since its feeding the grid its still part of the clean side.

[fyl]

Here's a quick and dirty layo jotted on a 5 x 5 mm notepad and photographed. I'll scan it tomorrow morning and post a higher quality version.

[ncusack]

That looks pretty much spot on with my revision my only question is why you grounded the 3rd filter cap which would be for the preamp section to the rest of the power ground. Hoffman normally has that in with the clean side grounds attached under the board.

[fyl]

Good point. The third cap should go to the clean side.

[fyl]

Here's a clean scan of a revised layo:




nJoy!

[MWJB]

Without knowing the spec of your PT, I'd be inclined to use 50V or better cathode bypass cap at V2, pin 8...probably a 5W 470ohm resistor too.

[ncusack]

Yeah i generally use 50V rated on the bypass anyway cause they are easier to find local and the cathode resistor will be a 5W and the power section dropping resistors will be 3W. Better safe than sorry as far as im concerned especially when it's pennies in the difference.

[fyl]

Ck sees max. 20 V, so a 25 V cap is OK. Rk dissipates only 850 mW, a 1 W resistor is OK for bean counters, a 2 W is fine by all engineering standards and will outlast most other parts in the build, a 3 W gives a huge safety margin and looks nicer.

The first PS R, a 1K, sees 10 V across it and dissipates an enormous 100 mW, the second one, a 10K, sees 20 V and dissipates a glorious full 40 mW. Any flame proof R will be fine here, I routinely use 2 W models as they can do double duty by somewhat acting as heat sinks for the connected caps - quite useful with challenged tech wanabees who replace perfectly good parts with grossly inferior cork-sniffer-approved overpriced crap with absurdly high ESR.

You may of course use higher rated parts...

[MWJB]

"Ck sees max. 20 V, so a 25 V cap is OK." You don't know that without knowing PT spec (I mean experience with the PT in question) & rectifier used (NOS 5Y3 or Russian, so called 5Y3 which will add 20-30v to the B+). It is quite normal to see well over 20v+ on a SF Champ cathode resistor, sometimes even in excess of 25v. Besides, the thread is titled "Hoffman AA764"...it's what Doug would do.

[fyl]

"Ck sees max. 20 V, so a 25 V cap is OK."

You don't know that without knowing PT spec

If you don't use the right PT and recto, then it's not an AA764 anymore...

Besides, the thread is titled "Hoffman AA764"...it's what Doug would do.

That's what Doug does. That's what I do. No problemo.

[MWJB]

The "right PT", as in the one fitted to a tolex Champ (or replacement made to same spec), will run the 6V6 hard, even with an NOS 5Y3, usually over 400vdc to ground at the plate.