Output devices off the pcb - will this cause oscillation?

[VictoriaGuy]

I'm planning an Aleph J build and my heatsinks will be 'split' - i.e. in two pieces for each side (each channel). It might be easier (not sure as I don't have the pcbs here yet) if I could place the output transistors on the heatsinks 'where I want' vs at the spacing on the pcb. Lead length would be a few inches - not long.

Will this likely cause a problem? I have only seen one build picture with the outputs off the pcb.

I really don't want to smoke a bunch of transistors in this build, as they are expensive (either semi-unobtanium, or need matching, etc).

 

[jan.didden]

You really should try to keep leads as short as possible. What's so important about that physical placement?
In any case, if you use longish leads, and the design has gate stoppers (I guess with MOSFET designs which are most Pass designs), be sure to place the gate stopper as close to the gate as possible. Using an SMD directly on the MOSFET gate lead would be great.

Jan

 

[VictoriaGuy]

You really should try to keep leads as short as possible. What's so important about that physical placement?

Jan-
Thanks for the reply.
I'll be using 2 pieces of heatsink for each channel - see attached pic for the profile of each piece.
I'd prefer to put each output device on the part of the sink with the longer fins.

 

[Zen Mod]

put devices in middle (by width) and ditto on lower third* (by height)

there is proper base thickness to spread heat evenly on fins

*that would be on 33% height , going from bottom

 

[Dennis Hui]

Hi VictoriaGuy,

I think I have very similar (if not the same) heatsinks. Here's a picture
of a pair with an Aleph-J board to give you an idea of the placement
of the mosfets.

Cheers,
Dennis

 

[VictoriaGuy]

Hi VictoriaGuy,

I think I have very similar (if not the same) heatsinks. Here's a picture
of a pair with an Aleph-J board to give you an idea of the placement
of the mosfets.

Cheers,
Dennis

Thanks, Dennis-
Certainly pertinent to my question - great picture which shows the board and sinks.
Yes they are the same heatsinks - from a govt surplus VHF amplifier?
Did you use 4 of them - two per channel?
How did they work out?
And what's the temperature of your amp when operating?
I'm thinking I might be able to cut down the height a bit, but I'd feel very foolish if that resulted in overheating....

 

[Dennis Hui]

Hi VictoriaGuy,

Mine came from some rack mounted power supply (at least that's what it said
on the front). I haven't actually used those heatsinks so I can't say
how they work in practice. (My Aleph-J was built with a store case.)

Cheers,
Dennis