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PostPosted: 30 Jul 2009, 22:56 
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I have spent hours and hours on this preamp (here is the Precision Fidelity C-8 Preamp Schematic), when I first got it from my uncle it was barely holding itself together. It had major thermal issues and a ton of noise oscillation! After a week of waiting for a dozen new resistors I spend hours more fixing solder traces from the horrible dual layer PCB!! :x After all this I get heatsinks that are 3 times larger than what are originally used on the HV Tip50 regulator and filament supply and they still burn waaaaaaaaay to hot to touch! I also go through the trouble of rewiring input and output pots, rotary switch, and the whole other lot of inputs, THE PROPER WAY!!!!

I must say if that didn't make a difference in performance, there is something truly wrong with this entire preamp design, because it still runs just as bad as it did, just 50% cooler and with one non working channel and some strange noisy tube vibration that does not change with each tube replace!! :mad:

I have spent so much time on this project that I am starting to question if I should build the darn thing from scratch, using the same parts and Phono stage, but with an entirely new power supply and maybe the use of 6SL7 phono output, and maybe 6AS7 Line stage for the fun of it, that would certainly make for a better preamp!

And what about those Plate transistors? I would really like to know thier purpose in wasting a perfectly good tube circuit?

So what are my options? Is it worth wrecking two weeks of work and starting over? The main reason I am asking this is because I am hoping to find someone who actually understands how this preamp works, I have the schematic so now pleeeeeeese help me so I don't have to pull every thing out again, and disappoint my uncle, who may just put this jewl back up on the shelf to collect dust for the next hundred years!

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PostPosted: 31 Jul 2009, 12:22 
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Hi Alan, any chance you could post some photos to show us what you are working with?

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PostPosted: 31 Jul 2009, 23:49 
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Yes I new I should have posted some pictures, I just didn't have any scaled down and it was all ready like 1 at night.

I should also point out the main issues: #1 one channel in the phono section seems to be dead, #2 strange ringing noise from the output tube in the phono stage (look at last stage in schematic), #3 horrible oscilating nois like a ground loop or bad connection, changes with dual pot rotation... #4 extreme HEAT! :hot: I have no idea how to cool the TIP50 regulator(the one with the large heatsink 3-4 times larger than original and still too hot to touch!)

Now on the positive side I have made great progress in terms of stabalizing heat dissipation and reliability, I have dosed all the old 75K's 15K's to 2 watts and replaced a bunch of other burnt resistors, through the Tuner input I can use direct DAC output from my JVC cd player and the preamp performs and sounds allot smoother than it did but not up to spec, but here are the pics, probably the only pics you will ever see of the C8 preamp (searched every were for information).


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PostPosted: 02 Aug 2009, 12:29 
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Hey, that looks like a pretty decent unit. Is that stock or has it been modified?

I would start at the power supply. Are those filter caps doing their job? Did you check the current draw of the heaters? That could be the source of the hot regulator (which I am assuming is regulation for DC heaters).

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PostPosted: 04 Aug 2009, 19:02 
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Well Gio, this is one of the newer Precision Fidelity preamps, but it is (was) in very bad shape, the main problem being the some how noisy hot power supply that I have decided needs to be scraped, and redesigned. This time I can use a tube rectified power supply and a nice Hammond 8 Henry 100ma choke, that should solve any noise in the power supply.

But FIRST! I have to solve the dead output in the Left channel of the phono stage, I built a simple signal probe and found out that the audio ends at the last tube stage where the MPSA42 is on the output? I really would like to know the purpose of these transistors, are they there for more gain? Tube bias? CCS? could I change them to FET's? IRF630/640? And what if these output transistors go bad? will the tube resonate to the point where I could hear it like I am?

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PostPosted: 07 Aug 2009, 09:08 
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Hi Alan, do you have a schematic? If so can you either post it or PM it to me? I have run into some similar situations with other amps.

Good listening
Bruce

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PostPosted: 07 Aug 2009, 11:12 
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Well, the Precision Fidelity C-8 Preamp Schematic has all ready been posted on the forum but I guess it doesn't hurt to post it again, anyways, I found the problem with the left channel of the phono stage..... it is indeed the MPSA42, I think it probably got too hot and died because it measures with shorts and the other one does not. :worried:

I checked the IRF630/640 and it only withstands 200V but I found two identical IRF740A's and they are rated for 400V! :P So I think the conversion should do just fine as long as they except the values of course.

But here is the schematic Bruce, since I all ready have them scaled down, but after I rebuild the power supply and get everything working I plan on making a construction page for this preamp. :)
Attachment:
Precision Fidelity C-8 Preamp(small).jpg
Attachment:
Precision Fidelity C-8 Second Stage.jpg


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PostPosted: 07 Aug 2009, 12:19 
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Hi Alan, I see anything really odd in the circuits, that would cause excessive power consumption (thus heat in the PS). I must conclude you are on the right track in reworking the PS. I would look carefully at the filter caps. If any are getting warm it could be an indication of excessive leakage. I didn't see the PS schematic, but if it uses SS rectifiers, check them. Lots of times the PS will generate B+ with a bad diode in one leg of a bridge (even one bad in a conventional supply). The filters then pass a large amont of AC to ground and it has effects on the whole circuit. One give away on that is lower than expected B+ and parts getting hot. It can easily overload the filters and regulators (if used). A scope of the output from the rectifiers will spot the problem easily.

Hope this helps
Bruce

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PostPosted: 07 Aug 2009, 13:31 
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Well, at least your on the right track for finding problems in terms of "heat" but from what I've heard the designer purposely built this preamp to run hot, or something to that affect, and I do have a power supply schematic that my uncle printed off from some where, but I won't be needing it because I have decided the only way to make this thing work at its best is to gut the thing and start from scratch, It's just waaaaaay to hard to modify and replace components on these annoying PCB's when the entire circuit has sutle flaws. :bawling:

And I am saying this to anyone with one of these preamps! Every time I fix something..... another part breaks!!! :x I replaced the old MPSA42's with IRF740A fets and it was working great!.... for a couple of minutes until the left channel in the second stage died :mad:

Now I can't even run it in mono! because all the switches are before the second stage!!!! :hot:

I can't help but rant on about the problems I am having because I know I'm not the only one, so I give up on keeping things original, I just want it to work without the insane heat and noisy flaws, so now I build it my way! :twisted:

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PostPosted: 07 Aug 2009, 20:08 
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So one question I would like to confirm.... can I use an EL84 and IRF740A for the last stage of the preamp? If so will a 640VCT 90mA transformer with 100mA 8H choke be suitable for such an output stage? If so, would there be enough room to spare for a regulated TIP50 250V supply for the phono stage?

I should take a picture of the parts that I just to give an idea as not to be misunderstood, because I would like to rebuild this thing as fast as is safely possible.

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