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    Applications for A-441 OPT/amp schematics

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    stromberg67

    Posts : 6
    Join date : 2014-03-08

    Applications for A-441 OPT/amp schematics Empty Applications for A-441 OPT/amp schematics

    Post by stromberg67 on Mon Feb 04, 2019 6:55 pm

    Hi All: I have a pair of A-441 output transformers, and would like to build a couple of amps using them. I have the Dyna transformer catalogs with the schematics examples for other xfmrs, but just the schematic for the A-441 terminations. Many years ago I did build an amp using one of these OPTs using a P-782 from a Mark 2, and a beefed up filter section, by tying the tertiary windings together and connecting it up as a "normal" ultra-linear circuit. It's OK, but I want to do something different now. Any comments on the A-441 will be appreciated. I have read some comments that they are less than "great".
    Thanks! Kevin (stromberg67)
    Peter W.
    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

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    Post by Peter W. on Tue Feb 05, 2019 9:20 am

    My understanding - limited - of the 441 is that it was, as you suggest, problematic, mostly with internal oscillation and, it was also prone to self-induced feedback loops. It was shortly replaced with the 451. Massive power handling capacity, though.

    Much as I hate to rain on a DIY project, you might consider flogging them for the considerable sum they should bring, and investing in one of the VTA products instead. At least you have a pretty good idea they will work.

    Or, make an instrument amp - all that distortion will not matter in that case.
    WLT
    WLT

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    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT on Tue Feb 05, 2019 9:58 am

    A few things can be tried that are different than your previous effort.

    One is to use the KT 120 tubes available now that may not have been then. Use the same circuit and power transformer as before. More power out but still works well with the 4300 ohm winding resistance. Still this not much different sounding than with KT 88 or 6550s.

    A second way would be to use that third high voltage winding with a different power source. Both plate tube windings would then be connected to the primary B+ supply. Then build a (vacuum tube?) voltage regulator (450 VDC?) to supply the Screen B+ winding. The output tubes would then be configured in pentode but with a voltage regulator the screen grids will hold their voltage when the plate circuit voltage sags. Not as much power as an ultralinear circuit but very nice sounding. Using the KT 120 tubes would bring back the higher power ratings.

    The last item is only if you try to get the most power out of the amp. The P782 is OK for a Mark II or II but may be short on VA potential at higher output. Dynaco made the PC706 that was meant for use with the A451. Other manufacturer’s transformers can be used that were designed for use with 4 - 6550s as well.

    Using two separate power transformers and keeping the screen grid circuit on its own transformer may be the best of all.

    Using low value resistors where the screen grid and plate connections attach to the transformer can help reduce oscillations. Look at the A 451 amp schematic in the transformer catalog you mentioned. Up to 250 ohms on each of the screen grids. Not sure how effective this will be but worth trying.

    Lots of things to try.
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    stromberg67

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    Join date : 2014-03-08

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    Post by stromberg67 on Sun Feb 10, 2019 6:35 pm

    Thank you Peter and WLT for your replies. I have been considering selling them for a while, and would use the money to construct two more Mark 3 amps from parts, since I already have two chrome re-plated chassis I bought a while ago. Perhaps another person can use them in a better way than I.
    Thanks again!! Very Happy
    Kevin

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