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    PCB Question

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    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    PCB Question

    Post by denny9167 on Tue Dec 06, 2011 9:33 pm

    I am considering a rebuild of the 120W amplifier found in one of the many catalogs Dynaco put out years ago,and I am wanting to put everything on one board,and was wanting to know if there are any drawbacks,in terms of stress,or other considerations ex: heater traces,etc. would love the input,I have a pic of the board below.






    [img][/img]

    Bob Latino
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    Location : Massachusetts

    Re: PCB Question

    Post by Bob Latino on Tue Dec 06, 2011 10:46 pm

    Can you post a schematic of the circuit for this board ?

    Bob

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PCB Question

    Post by denny9167 on Wed Dec 07, 2011 10:40 am

    Bob Latino wrote:Can you post a schematic of the circuit for this board ?

    Bob

    Hi Bob,

    Here is the schematic for the board,let me know of any changes,your input would be greatly appreciated.Thanks

    Denny

    [img][/img]

    Bob Latino
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    Re: PCB Question

    Post by Bob Latino on Wed Dec 07, 2011 11:31 am

    The driver portion of that circuit is almost the same as an original Dynaco Mark III circuit. Not just the use of a 6AN8 driver tube, but the resistor and capacitor values are almost the same at various points in the circuit. A 6AN8 tube is a pentode/triode design and (IMHO) there are better driver circuits out there. The use of a pentode/triode driver front end driver circuit allows the use of just ONE driver tube. If you use an all triode design front end you need a tube and a half or three triode sections to do the job. Whoever designed this circuit is basically using a Mark III front end mated to an output stage of their own design.

    Also, the power supply caps C7 - C12 according to the circuit are all 47 uF @ 400 volt. Since C7 and C8 are in series and C9 and C10 are also in series then this part of the circuit is good since you have an 800 volt rated power supply up to this point. BUT - C11 and C12 are only 400 volt rated. The problem is that running the Hammond secondaries of 400-0-400 you are going to get a B+ in the range of 480 to 500 VDC which is well beyond the range of these two (C11 and C12) 400 volt rated caps. This to me is the worst part of this circuit and IMHO those 400 volt rated caps won't last long looking at 480 - 500 VDC. Also with a 23.5, 23.5, 47, 47 uF you have just 141 uF of DC power storage. This is not enough IMHO to run a 120 watt mono amp and trying to feed 4 X 6550 tubes. IMHO before the amp ever reaches 120 watts it will run out of DC storage. In comparison, the VTA M-125 monoblock, which is a similar amp running a single channel parallel push- pull circuit, has 502 uF of DC power storage (with 550 volt or higher rated caps) which is more than 3 1/2 times what this circuit provides.

    Another thing is that the designer attempts to bias all four output tubes with just ONE bias control. If you do this, all you need is just one bad or mismatched output tube to throw the circuit out of balance.

    The one good thing I see is the use of the Weber WZ68 solid state rectifier. This WZ68 can flow 450 milliamps which is really more than this amp needs.

    Below is the VTA M-125 amp circuit which I think is a better 120/125 watt parallel push-pull circuit ...

    Bob



    tubes4hifi
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    Re: PCB Question

    Post by tubes4hifi on Thu Dec 08, 2011 2:55 pm

    gasp . . . !!! you want to run 4 6550 tubes with an 6AN8? I'll 2nd and 3rd everything Bob mentioned,
    but on the other hand, if you want to do that from scratch, go for it, that's what DIY is all about!

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PCB Question

    Post by denny9167 on Thu Dec 08, 2011 8:30 pm

    I really appreciate the input,I was out of electronics for several years only to get interested again just within the last few,so I'm still on a learning curve here! If I'm not mistaken doesn't Audio Research use a pair of 6H30's in the driver stages of some of their amps? I know it is a very hardy tube from what I understand.

    I will take these suggestions and re-work the curcuit. Thanks again!!

    tubes4hifi
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    Re: PCB Question

    Post by tubes4hifi on Thu Dec 08, 2011 8:42 pm

    just as a reminder, if you want to save a ton of time and work and KNOW that you're gonna wind up with a really great amp,
    consider buying one or two of our M125 (or M70) monoblock kits.

    denny9167

    Posts : 152
    Join date : 2011-05-09
    Age : 49
    Location : Texas

    Re: PCB Question

    Post by denny9167 on Sat Dec 10, 2011 6:33 pm

    Could this be used as a good driver stage for the quad's?[img][/img]

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