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Thread: Upgrading Epiphone Valve Junior to a 7591 SE design?

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    bea
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    Upgrading Epiphone Valve Junior to a 7591 SE design?

    Hello,

    today my Valve Junior - heavily modded as a practice amp for bass - died. The EL84 got so hot that it got melt the socket and got stuck to it, making the Amp almost unrepairable; i have to completly build it up anyway.

    My idea is to set up a new amp based on an ECC83 and a 7591 in SE Class A.

    I did already check some prerequisites: the power transformer can actually handle two EL84 or a 6L6 when driven to its absolute limits - its output voltage is a bit high for the EL84 anyway, and my OT (a Wüstens ATRA0211) is specified to 60 mA. The impedance of the OT (5.2 kOhm) should also fit reasonably.

    So this might end up with something like 9 W instead of the 5-6 W coming from an EL84. Not a large difference, i know, but also not a large difference in price. A 7591 from JJ costs just a few EUROs more than an EL84.

    Any thoughts/suggestions?

    Thanks

    Beate

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    Is the OT specified as being suitable for single ended use? SE OTs usually need an 'air gap' in the laminations to avoid magnetic saturation and consequential loss of inductance. P-P OTs manage to avoid this using a winding 'trick' but must be used in P-P circuits for the fix to work. Pete.

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    bea
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    Yes, of course, it is.

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    Sorry, should have googled your OT.
    What B+ are you expecting?

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    bea
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    Well, unmodded Epis are able to deliver 315 V to an EL84 (345 V while heating, .i.e. without the load of the anode current). With the increased load probably somewhat less. Maybe some tweaking is possible in the power supply - setting the Voltage switch to 220 V... I am considering to adjust the anode current to about 60 mA which means that i will stay slightly below the current limit of the tube.

    Considering that a single ended EL 84 at 300 V needs 7 k load (and 5.2 k at 250 V), i expect the 5.2 k to fit fairly well to O(300) V with the 7591.

    BTW: the Amp is working again but unfortunately the EL84 sticks unremovable to the socket. I own a 2nd of these beasts which is not as thouroughly matched to the EL84's ideal of 250 V/5.2 in SE mode - this one has been the main target for my idea of the 7591
    Last edited by tboy; 08-03-2011 at 02:56 AM.

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