ST-35 Build Using PC13 Boards Questions and Thoughts

[atwageman]

I've got some PC13 boards laying around just asking to be used. I'm gonna build this amp on a steel hammond chassis 17x10x3. Yeah I know thats overkill in terms of size but I've got 5 unused chassis that size, so I figured I'd save a few dollars and use one rather than order a smaller one.

I'm gonna make some changes from the original schematic, mainly the power supply section.

Instead of using a 4 section electrolytic can, I'm gonna go with individual caps. The original values of the can are C8A 60uf, C8B 40uf, C8C 20uf @ 450v and C8D 100uf @ 25v.

I've got 3 large caps each 100uf @ 450v that I want to use inplace of A,B,and C. These will be mounted on top like the transformers. C8D will be mounted inside the chassis since what I have on hand at 100uf @ 25v is relatively small. The large 3 large caps measure 1.375"Dx2.125"L

Inplace of D1 and D2 on the B+, I'm gonna use a full wave bridge rectifier. Also in series on the B+ I'm gonna install a inrush current limiter rated at 2 amps.

I'm thinking of also running filtered/regulated 6.3vdc out to the heaters instead of ac.

The audio inputs will be on the backside instead of the front. Only thing on the front will be a power switch and indicator lamp.

I haven't drilled the first hole in the chassis yet, so I would like some opinions on what I'm gonna do, If you would do it differently ie...different cap values etc, I would appreciate your input.....pros and cons.

[Eli Duttman]

Bridge rectifying the B+ is FINE. Use a pair of 600 PIV "GI" diodes (see Jim McShane's site) in parallel with 10 nF. snubbing caps. on the ground side and 600 PIV SiC Schottky diodes on the "hot" side.

Use separate EL84 RC bias networks in each channel. It's MUCH easier to get well matched pairs than it is to get quads. The cost is less too.

If you want to run DC on the 12DW7 heaters, construction of a regulated 12 VDC supply is very easy, even if the AC winding is 6.3 V.

[Ty_Bower]

Go read what these guys have done with the diyTube Stereo 35. It'll probably give you some ideas.
diytube.com :: View Forum - stereo 35

I'd prefer not to use the 12DW7 for availability reasons, and go with half a 12AX7 and half a 12AU7 instead. Of course, if you are building this on Stereo 35 circuit boards that may not be an option. It might also be nice to split up the common cathode resistor on the finals, and go with individual cathode resistors for each output tube.

[atwageman]

Thanks for the replies. I've been spending some time over at diytube.com too.