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    Voltage chart in original Stereo 35 manual...

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    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Voltage chart in original Stereo 35 manual...

    Post by PeterCapo on Tue Aug 02, 2011 12:49 pm


    Looking at the voltage chart in the original Stereo 35 manual, V1 and V4 are the two 7247 driver tubes. The voltage chart seems to indicate that pin # 1 of V1 should be 215 VDC while pin # 1 of V4 should be 230 VDC. How can this be? It's the same pin of the same tube for each respective channel. Is this an error in the manual or am I missing something?

    Thanks
    Peter

    tubes4hifi
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    Join date : 2008-11-30

    Re: Voltage chart in original Stereo 35 manual...

    Post by tubes4hifi on Tue Aug 02, 2011 1:59 pm

    Peter,
    you are correct! The two halfs of the circuit are identical and thus the voltages should be identical.
    My guess is the tech who wrote the manual or took the voltages had tubes that were conducting slightly differently (internal resistance)
    and so he just wrote down the actual voltages he had. Let's call it 225v +/- 10v, huh?!
    Also, BTW, your other email question about bias, the 13.5v across 100 ohms common cathode resistor is 33.75ma per tube
    (assuming they are all drawing exactly equal current).

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Voltage chart in original Stereo 35 manual...

    Post by PeterCapo on Tue Aug 02, 2011 5:06 pm

    Thanks, Roy.

    At the present time, I have a single 100-ohm cathode resistor, and with a 120 VAC line I am measuring about 14.8 VDC, which I believe means a current of about 37mA through each tube, which might be a bit high (?)

    14.8 VDC is also at the upper limit for this node, according to the 10% tolerance indicated in the manual. If I go to a bigger resistor, the voltage on the node will go even higher.

    Any suggestions?

    Thank you.
    Peter

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Voltage chart in original Stereo 35 manual...

    Post by tubes4hifi on Tue Aug 02, 2011 7:58 pm

    try putting a 1K resistor (1/2w or 1w) in parallel with the 100 ohm resistor, that will knock it down to 90.9 ohms and based on your current voltage reading that should bring the bias down to 13.5 volts. Hopefully your tubes are well matched as in stock Dynaco amps if the tubes aren't matched or don't wear out evenly one tube can pull alot more current than the other and you never really know unless you mod the circuit so each one (or at least each pair) has it's own cathode resistor.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Voltage chart in original Stereo 35 manual...

    Post by PeterCapo on Tue Aug 02, 2011 10:31 pm


    I purchased the tubes as a matched quad from a reputable seller.

    If I lower the resistor value to reduce the cathode voltage, then the current will go even higher than it is now, 37mA, correct? And if I increase the resistor's value beyond what it is now in order to reduce the current, then the cathode voltage will increase above the maximum indicated in the manual. A bit of a quandary.

    I do not know what the consequences would be of running the cathodes at a higher voltage than Dynaco originally recommended. I know running too much current through the tubes will burn them out sooner...

    Any additional thoughts appreciated.

    Thanks.
    Peter

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