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Thread: Cathode V's and the great 6V6 dissipation question

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    Supporting Member Randall's Avatar
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    Cathode V's and the great 6V6 dissipation question

    I finished my first 5E3 today. Dim bulb tested and all seems good. O-scope tested the pre section without output tubes, looks good, and quiet, too. Put in the JJ 6V6's and went to work measuring. Here's what I got:

    PT secondary 335 VAC to grd each side
    B+ first cap 320v, 2nd cap 215v, third cap 156v
    6V6 plates 295v
    cathode 36.8 v

    By my calculations, and please correct me if I'm wrong, I am idling at 19 watts. This how I got to this conclusion:

    36.8v/250 ohms = 73.6 milliamps per tube, seems high?
    plate voltage - cathode voltage = dissipation voltage 258v
    73.6 mA x 258 v = 19 watts

    So..... am I doing this right, what do think of my voltages, and why am I drawing so much at the cathode? And isn't 19 watts too high? Thanks for your help.

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    Supporting Member Jazz P Bass's Avatar
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    My first question would be 'why is the 6V6 plate voltage low?'

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    Supporting Member loudthud's Avatar
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    Your math looks correct if the 250 ohms is right. Did you measure the resistor before you installed it? Try measuring it now in circuit with the cap across it (power off). Your meter may act strange but should not read higher than 250 ohms plus resistor tolerance. Check that pin 5 of each 6V6 measures about 220K to chassis ground.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    I have not measured the 250 ohm yet. Does 320v seem odd since it's a Mercury 380 -0- 380 PT? Why would that be?

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    Supporting Member loudthud's Avatar
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    320V seems low, but not out of line if the power tubes are drawing too much current. The original Fender transformer didn't have very good voltage regulation (meaning it would drop down alot under load), Mercury may have just copied that. It seems like something is not quite right in the amp. For some reason the power tubes are drawing too much current pulling down the B+. That's why I suggested checking pin 5 of the 6V6s.

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    Last edited by loudthud; 02-19-2013 at 01:18 AM. Reason: I need new glasses!
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Definitely check that cathode resistor - with 295V at the plates and 36.8V of cathode bias there is no way will you be getting 72mA per tube out of a 6V6 - UNLESS one or both grid1s are sitting positive, hence loudthud's recommended test above.

    19 Watts would be too high (aim for 12 watts anode + screen dissipation or even a little less) but then I don't believe you do have 19 watts.

    Make sure you measure the cathode voltage with no-signal.

    Cheers,
    Ian

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    Supporting Member Randall's Avatar
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    Right on loudthud, I had omitted the under board ground wire to the 220Ks. DOH! And I was so sure of my wiring! Now things look a lot better:

    B+ = 382v
    output plates = 371v
    cathode = 21v
    cathode resistor = 245 ohms

    This puts me at 15 watts, which is still a bit too high I think. I'm going to see where a 270 ohm cath res puts me.

    Another odd thing, with the B+ switched off (I put in a standby switch right before main filter cap) how is it I get some hum from the speaker? It goes away when B+ is applied. Weird.

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    I saw from a Weber layout where my error lies regarding hum with standby switched off. I had put the switch before the first filter cap per schematic, but neglected to realize that in the actual layout this still applied B+ to the OT from pin 8 of the 5Y3. That explains the banshee screams when standby was switched off!

    Moved the OT CT to the standby switch, moved the filament CT to pin 8 of the 5Y3 per loudthud's suggestion from another thread, and subbed a 270 ohm cathode resistor to get me down to 13.8 watts with JJ 6V6s. Now it is quiet, sounds GREAT, and I am very happy.

    Thank you everyone for you help. Great resource here. I look forward to being more help then questions someday with the help of the knowledge of the big brains here.

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    Last edited by Randall; 02-21-2013 at 09:39 PM.

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