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2 different Bedini 150/150 Mk II Amps

I was comparing the insides of another Mk II with mine and they are really different. The first one is like my old one with no markings on the transistors. John Bedini was supposed to do some kinda voodoo inside them to work his magic. I remember this detail because my ex-girlfriends daughter cooked that amp 30 years ago by playing it all day at max volume when she was staying home from school one day. My repairman was never able to get any details on those trannys so the amp was never repaired. I guess we can't post pics so I'll just have to describe it.I purchased a different new to me B150MkII recently and the insides are really different. All the trannys are labeled Made in Mexico and there are 4 new ones on the circuit boards along with other different components. The other amp didn't have these 4 extra ones or slots to install them. Everything in my amp looks very fresh and new although it could be just well kept. It sounds fantastic.
I figure there are 3 possible scenarios with my new amp:1.I had read that John Bedini had modified these amps during the run so perhaps I have a later version.2.I had read that JB also would do mods for you when you sent it in for service.3. A really skilled tech modified this with a new design and new circuit boards which I doubt.
Is there anyone out there who could help me out.Also the only thing wrong with mine is the little light in the power switch doesn't work or will sometimes flicker. I can't find the direct replacement so if anyone knows a source could you please let me know. ThanksBobber


skeepowder
robertgalka's avatar
robertgalka
5 posts
 

my 250/250 mkii looks entirely different inside from the standard model. I upgraded the power supply to dual separate power supplies. All the transistors were upgraded to much better newer more linear transistors. And I mean all.  

The internal wiring is all Cardas now. The speaker fuses are gone. Power cord seriously upgraded. Threshold style heat sinks added. Bias increased. Slow start circuit added.


I just bought a 250/250 MKII this week and there isn't that much information out there. It has RCA MJ15024 transistors. You upgraded to the MJ21194? I read a post from John Bedini where he recommended MJ15025?

Mine has dual transformers and 19,000 MFD 75V Sangamd capacitors, hard to read, I'm assuming those are power supply caps?

My amp has 75 mv DC offset on one channel, and it reads up to 22 v DC when you turn it on. Hopefully my tech can repair it without a schematic.

If it can be repaired I will try to do some mods to it. Can you tell me anything else about your slow start circuit? I think they sell them at DIY audio?

minorl
1,016 posts
 
I believe the upgrade path on the transistors were:
1.  MJ15024 to,
2.  MJ21194.

Those were NPN transistors.  Verify on your amp if they were NPN or PNP.  I believe he used the same design on his amps with both rails using NPN output drivers in a nice circuit configuration.

The DC offset could be from a number of causes.  But, check the output/pre-drivers and bias transistors.  A standard inexpensive transistor checker and you are good to go.

You can upgrade the power supply caps to 42,000 microfarad caps with the soft start circuit implemented.  with bypass caps also attached.

my amp actually had enough internal room that I could separate the power supply and have one for each channel.  with added rectifiers, caps, etc.


I decided that I didn't want to spend the time designing a soft start circuit since there are many already out there, so I (ahem) borrowed a design from DIYAudio.

it is the DIYAudio soft start circuit  2012.  Very simple and inexpensive.

I got my own board and parts and it works very well.

If you can't find it on-line, message me and I'll send you the schematic

Most solid state amps have push/pull complementary NPN/PNP output drivers.  the Bedini amps I've encountered used NPN output drivers on both rails.  Easy to do if your pre-pre drivers are reversed.  I guess he didn't want to try to match NPN/PNP transistors.

enjoy
robertgalka's avatar
robertgalka
5 posts
 
Thanks, hopefully my tech can fix it first. I hooked it up to an old woofer and it has a pretty good hum also.

I did a quick read at DIYaudio and they sell a speaker protection circuit along with the soft start. I know I bought something like that on eBay a couple of years ago and never used it. It's powered on by 120v has a few second delay. It also senses any DC over 100 mv, then disconnects the speakers to protect them.
atmasphere's avatar
atmasphere
11,798 posts
 
We had one of the 150s come through our shop recently- desperately in need of new filter caps. Swapping them out was a bit tricky due to the construction, but once done the amp came right around. It proved to be one of the better amps we've had in our system.
robertgalka's avatar
robertgalka
5 posts
 
Are filter caps the same as power supply caps? I only see eight capacitors on this amp, four 2" and four little ones on the boards. It looks pretty simple to me.
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