IRF TO-3 devices for A-75

I have apparently blown 48 matched TO-3 devices for my A-75!

Good grief. After much help from forum members, leading to a successful diagnosis of main board bias problems, which I fixed, I "fired up" my output stages (slowly with a variac) and found excessive voltage across the source resistors. Its impossible to believe, but I found I had the polarity of the unregulated supply leads reversed--ie, the output devices saw reverse supply voltage. They appear to be toast.

Options: 1. secure a source for these discontinued products Does anybody know where to look? I have found no leads.

2. use only 6 devices per bank. I still have 12 extra matched IRF 231 and 12 extra matched IRF 9231 (one bank of six has a range of .12 volts, the other three banks are very close). Will this work? Do I have to compensate for the change somehow?

3. convert to different output devices (problem: my output stages are wired point to point with huge bus wires and TO-3 sockets. This means re-engineering the whole heat sink module) What devices would work?

4. convert to another topology. Aleph 5?? Are the heat sink requirements similar to the A-75? Needless to say my chassis and power supply should be put to some use, if that's the only option.

Any thoughts would be appreciated. I still CANNOT BELIEVE I reversed the leads (Its like: "Oh, N-device, it must need a negative supply lead!" BEEEP!! Thanks for playing.)

:bawling:

Larry Wright
Seattle area
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Six per bank is just fine. I used 12 per in the original
because that's how many sets of holes I had in the
heat sink....

Also, there is a wide variety of IR devices which will
do the job, and if you can convert to plastics physically,
Digikey will sell you IRFP240's and 9240's, and since
they are twice as robust as the 230's, you can get away
with as few as 4 per bank, for a total of 16.

:bigeyes:
 
Thank you Mr. Pass! I have so much invested in this project in dollars, time, and committment, that I really want it to work. I will try the six per side and be more careful this time.

Also I have a BLS and Zen V4 in the wings, and I've got to finish this before starting those.

Thanks again.
:D
 
1/137 said:
I have discovered 100 EA of IRF9231 and IRF230 TO-3's (obviously bought a long time ago)...are these no longer considered as appropriate for the outputs on the A-75?

Or...if they are no longer available should I be using current parts? I am planning on 4 Mono A-75's

thanx
rt

Mr. Pass has pointed out that some of the new devices are more robust. They are definitely easier to mount on a sink. But I like the shiny metal on the T0-3 devices. (Pretty dumb reason, eh?). And I spent about a year drilling the 192 holes it took to mount 48 devices on the two pieces of 1/4 thick U channel which bolt to my heat sinks, so I am biased. Plus, I wanted to look down inside my amp and see two long rows of those things, like (I believe) one of NP's old Threshold amps. I believe psychiatrists refer to this kind of compulsion as "Idee Fixe" (with apologies to our French-speaking brethern for lack of the appropriate accent mark.)

If you decide to go with plastic, I might be interested in purchasing your TO-3 devices for future use or spares. Just let me know.

Larry Wright
Seattle area
 
Zapped said:


And I spent about a year drilling the 192 holes it took to mount 48 devices on the two pieces of 1/4 thick U channel which bolt to my heat sinks, so I am biased.

That U channel is made out of titanium or what?;)

I used Irf230 and IRF9130 in my A75 and recently am using them in my AX amp. They work fine.
 
Nope. I just work slow. Since I was working with transistor sockets, I wanted to be accurate on the drilling, and it took me forever. (OK, OK, so it was not a year, or forever, but it SEEMED like forever and a year on the drill press, even using an accurate predrilled template).

Anyway. News flash: I pulled off 16 of the 48 output Q's at random from the 4 banks, and set them one at a time into the test setup specified in the A-75 article and on the PassDIY site, and to my surprise got a Vgs voltage reading that was within 10 millivolts of my original matching measurements for each device :xeye:

Question: Is is possible for one of these TO-3 devices to be damaged and still read correctly here?

If not, perhaps I just toasted a few of the driver or input devices on the main board, which are no problem to replace. I looked at this yesterday and didn't see anything working right there. Maybe my reversed + and - connnections problem caused problems upstream rather than downstream. The neverending saga continues.

Larry Wright
Seattle area
 

Melon Head

Banned
2006-11-10 10:14 am
Question: Is is possible for one of these TO-3 devices to be damaged and still read correctly here?

If not, perhaps I just toasted a few of the driver or input devices on the main board, which are no problem to replace. I looked at this yesterday and didn't see anything working right there. Maybe my reversed + and - connnections problem caused problems upstream rather than downstream. The neverending saga continues.

Larry Wright
Seattle area

I wold not think so. If they have been toasted then I would think they would be rendered useless.

I am not that familiar with the circuit but maybe you have burnt out resistors.

A picture would help significantly.
 
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You can get the power supply board here:

Old Colony Sound Lab -

Don't know if the A75 amp board is available anywhere.



:)

Yeah, you'd think so ... but, then again he said he worked slow.
thanks johnloudb i asked to old colony but they have the power supply board and i will use another one, sorry for asking for something old for everybody but i decide to build the amp in this time.
Regards.