A75 Power mosfet replaces

A-75 Outputs

Hey all,
Wow! what a great site for us to share
our experiences with others.

Special thanks to the man himself NP.

First time on, so my questions are
I`m currently building the A-75 and would like to know if
anyone has usedthe IRF130, 9130 ever?
I`m planning to use these because
they were available within a reasonable time.

I also would like to know what is the max. size tranny VA
without going overboard, and can 2 smaller sizes be recomended? I have 4 [email protected] 50v caps set aside for this.
Would I need to look at different size bridges, than what the
original plan calls for?

My heatsink plan is to use 2"X4"X1/8" extruded angle cut to
12" lengths using 34pcs.mounted vertically on a 13"X 34"X 1/4" Sheetplate, and mount 4-2"X2" angles horizontally across the inside drilled for the TO-3`s to be mounted to.
I will touch base as things progress.

I`m located in western NC and have a dedicated lintening room, and work bench that meets, and/or exceed the wife factor.

Anyone nearby is welcome to hangout and listen.
Many thanks,
Joe
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
130 and 9130 should be perfect.

Big trannies and caps are just fine, but you
might consider a power Thermistor in the
circuit to help keep the turn-on thump under control,
and with that much capacitance, I would consider
making a pi filter in the supply using a couple mH
coils like the Erse from Zalytron, with a cap on
either side.

Smaller transformers are also fine, particularly if you can
give each channel their own....
 
Thank you Mr. Pass:

The project calls for a 6A thermistor, what will my substitute be?

I still need alittle more help regarding the Pi filter.
How can I find the value cap, choke needed for my application?
Does the filter need to be located at the line in voltage?

Can the 25A bridges suffice?

Thanks again for the valuable info
J
 
Ooops I already have 640's

I hate to throw another number into the fray, but I purchased
a large quantity of IRF640 / IRF 9640 mosfets for my A75.
Does anyone have an opinion on this series? (Manufacturer is
International Rectifier). Seemed like a good idea at the time.

Also, I have a large number of 8200 mfd. 60volt caps. is there a downside to parallel wiring multiple caps instead of a few large caps? I thought I could distribute the energy storage closer to the output mosfets by using multiple caps.

Any thoughts, any help, THANK YOU!
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Some quick answers:

You can use Digikey KC004L-ND thru KC006L-ND
without fear.

For pi filter, something 1 mH or larger; I usually use
2 to 4 mH, and the value is not at all critical. On the
caps, I use lots of capacitance, usually distributed
equally on either side of the inductor. Again, this is
not critical.

25 A bridge is OK, particularly if you are using
Thermistors on the circuit, but be sure to heat
sink the bridge to the chassis or something.

640's and 9640's are OK, although they do have
slightly more distortion than the lower voltage
parts. This is not really a problem.

Stringing multiple caps in parallel is fine by me.
Some argue that this creates some sort of
problem with current sloshing around, while
others point to the probably lower inductance
of parallel caps. I don't believe in the former, and
the latter is certainly curable with film cap bypassing.
 
Thank you again for your reply.

In regards to the Pi filter (sorry for the ignorance), Are we looking
for nonpolorized caps over 250vac? Do the caps go parallel with
the coil in series (butterworth filter)? What amount of capacitance is going to be needed? Doe`s the network need to
be located between transformer, and TR1, or between A/C line-in
and transformer, presumebly the hot leg.)

Thank you for your continued support,
Joe
 
Joe,
You're up near my old stomping grounds.
The power supply filter (or storage bank--either view is correct, it simply depends on your point of view) always comes after the rectifier:

AC->power transformer->rectifier->filter->circuit

A PI filter is constructed by running a capacitor to ground from the rail (in parallel with the load), an inductor in series, and another capacitor to ground:

_|___inductor___|___
cap.................cap
gnd.................gnd

Ignore the periods. They're just there to make things space out properly, as the site scans of excess white space characters. Jason told me once how to turn that off, but I never remember. Must be my mind going.
I suppose you could use non-polarized caps, but most people use computer grade electrolytics (which are polarized) for their power supplies.

Grey
 

UrSv

Member
2002-01-31 5:48 pm
Sweden
A75 Output replaces

Hi,

I am also trying to build a pair of A75 amps. I have got a large number (50+ matched pairs) of closely matched 2SJ201/2SK1530 combinations which should be a decent couple of transistors. Does anybody know wether these could be possible substitutes for the speficied output devices? I am little worried that they require more drive current the original IRFs and that this may cause a slightly lower bandwidth/slew-rate. Anybody know (or asked Mr Pass)?

(If Mr Pass reads this - thanks for the help with my Zen amps a few years back, they are great)

/UrSv