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    Thermistor for Mk4?

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Thermistor for Mk4?

    Post by Pooch on Thu Dec 03, 2009 5:34 pm

    Any advantage to this? Seems I read somewhere it wasn't really necessary with the Mk4 tubes?
    Gord

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Thermistor for Mk4?

    Post by Bob Latino on Thu Dec 03, 2009 5:42 pm

    Hi,

    If you use a GZ34/5AR4 tube rectifier in your Mark 4 you have a built in "high voltage delay" already. A GZ34 is indirectly heated and won't apply the B+ DC high voltage to pin 8 for at least 15 seconds or so. You don't really need a thermistor for any high voltage delay if you use a GZ34/5AR4 rectifier tube.

    Bob

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Thermistor for Mk4?

    Post by Pooch on Fri Dec 04, 2009 5:23 pm

    Thanks, Bob. How would a 5u4 rectifier work with the VTA board?
    Gord

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Thermistor for Mk4?

    Post by Bob Latino on Fri Dec 04, 2009 5:41 pm

    Pooch wrote:Thanks, Bob. How would a 5u4 rectifier work with the VTA board?
    Gord

    Gord,

    A 5U4 WILL WORK in an ST-70 amp but a 5U4 has a directly heated filament and warms up much quicker than the normal GZ34/5AR4 that you would normally use with an ST-70. It also has a higher voltage drop than a GZ34/5AR4 rectifier tube and will give you a lower B+ DC voltage in your ST-70. The result will be that you will probably lose a few watts of power with your ST-70 with a 5U4 in there as the rectifier. You will also have to reset the bias somewhat if you use a 5U4. 5U4's are relatively inexpensive as used tubes because many old tube color TV's of the 1960's used a 5U4 rectifier tube. IMHO a 5U4 makes a good backup rectifier tube that you can pull out to keep the music going if your GZ34 rectifier suddenly bites the dust.

    Bob

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