Bedini 250W ?

[AVWERK]

What bias setting is your 250 set at? ( measure across any emitter)
If the mounting hardware, pads, thermal compound, etc. Is good I can,t see that kind of heat difference front to back?
This is not a high bias Krell design. Something is wrong here..

You did Emial me earlier this week on your broken 150 and the last email you mentioned yours didn,t have the driver TO3's installed!

I hope your not comparing this one to the 250 this way, as John Bedini has responded to the same issue earlier on another site, that this is not a factory mod but someone elses.,.
He does give info on correcting this and getting it back to square one.

Regards
David

 

[WireToWire]

Hi and thanks for following up with this. My amp is not the same as the one in post #11. Mine is an earlier model, almost identical in the circuitry and power, but slightly different in the construction and a different model number. Sorry for giving the temperatures in F; in C temperatures are more like 20 degrees apart. I may post an interior picture when I take the amp off my currently active system again. No schematics are available for most Bedini amps, but they are all scaled up versions of the same idea. While they were all advertised as Class A, most except the early ones (like 25/25) are considered A/B by most.

 

[WireToWire]

I found this pic online, it is the same as my 250. As you see there is a lot more cooling area afforded to the rear outputs than there is to the front. Which is one way to explain the 20 degree C (35F) difference between the front and the rear of the heat-sinks. Will check bias when I get it open again and write what I find out.

 

[WireToWire]

Thanks David,

I found that advice, and for anyone with a 150/150 Mk ii without the drivers, who is following this thread (it appears roughly half of them are missing them) re-posting here: Simple, but he does not say where the resistors are supposed to go.

JS,
Why not put the drivers in the board? You could do this yourself 4 Mj15024's and 4 36 ohm resistors 4 small heat sinks T03's. Then re-set the bias to 35Ma. That amp would really sing. Could you take a better picture of the boards so I can see what it is. My only interest is to make it sound the best for you.
Mod Parts
4 MJ15025's
4 PC Mount Heat Sinks TO3
4 36 Ohm 3 watt resistors
Re-Set Bias to 35 Ma across emitter resistor Positive Side
John Bedini

 

[LineSource]

Sorry I failed to notice that your temperature measurements were in F and not C. 18C temperature variation between outputs from the heatsink mechanical design is mediocre, but adequate for proper operation. The emitter resistor attached to each output transistor should be large enough to balance current among the outputs, even with individual temperature generated current variations. Personally, I would not invest time and money in changing the heatsink on your amp. From your picture, the construction looks good for its age. The advice of removing each output transistor and replacing the thermal grease and mica insulator is a good value project.

Cheap sonic experiment…. Replace the small value feedback capacitor with a silver Mica, and make sure the feedback resistor is a high quality metal film construction. A schematic is necessary for any circuit update option discussions.

The attached figure shows some of the better modern heatsinks used for plastic TO-3P package which would likely have 3x better cooling than your amp’s ,multi-piece heatsink. The “Heatsink USA” has good value and is sold here and on eBay. If you get the urge to construct a modern DIY circuit amplifier these are cool.

 

[WireToWire]

Thanks, I was checking those heatsinks out earlier, as I was pondering the temperature differential in that amp.

Has anyone checked why the Bedini outputs from the earlier amps had their tops cut and epoxy filled? The common assumption was that it is to dampen mechanical resonances, but there may be more to it like an internal mod. All those cut and filled outputs come with hand written ratings of VL, L, M, H, VH and each board is always populated with the same letter. Yet the two channels can sometimes be different from one another. Obviously sorting the outputs into matching sets based on how they tested (or how they were modified.) So replacing them could degrade the quality. Reminds me of the chunks of epoxy I found out that constitute the innards of the BASE units, when I tried to figure out what the secret was. Mr. Bedini says psycho-acoustics, sure, but how? Whatever is buried in those blocks, would have to be x-rayed to figure out.

 

[WireToWire]

OK, so I popped the top of the 250/250 Mk II open, and checked the actual temperatures of the outputs, and the tops of the to3's all feel the same to the touch except for the center one on both channels, which is a bit less warm at idle. Obviously the heatsink design and the obvious variations of temperature between the two ends of the sink had little impact on the actual heat distribution between the outputs. As you predicted.

When I listened to it again, something did not sound right. Checking things over, I discovered that one of my speaker cables was plugged in in reverse phase. But that was the speaker that sounded better! So I reversed the other one as well. And what followed was truly breathtaking. Listening to Roger Waters at 4AM, playing my 89dB speakers at levels that would not bring complaints from my upstairs neighbors, I'd say a relaxed volume, rather than quiet, everyone in that record were in the room with me, in fact one was sitting right by my side and whispering in my ear. And the crickets were everywhere. With just two speakers. The amazing 3-dimensionality of that recording has always amazed me, but never at this level.

I read that some Bedinis were not corrected for phase at the terminals. How can I check that? Other than by ear. You'd say "listen to it the way you like it better" and I am certainly taking that advice. But would like to know nonetheless.