Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on July 13, 2011, 04:48:50 pm
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My plate voltage is just too high. I'm running a SE 6V6GT at 370V plate to cathode, I like the 6V6 sound so a 6L6 is out. What do you cats think about the Zener diode voltage drop trick? Any issues?
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The zeners work..If I might ask, what voltage are you looking for on the 6V6. Mostly they sound pretty good from 350 up to 380. (According to which amp they are running in.)
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I see a lot of schems here have them at lower voltage & the talk seems to be that the sweet spot is closer to 300.
I need a 720R cathode resistor just to stop the redplating. That doesn't seem right. I was thinking of a reverse polarity zener on the power CT to drop the entire B+ @50-75V; currently I have a 220R 10W after the recto, dropping @10V, but that feels like poor design, plus I don't want a hot power resistor right next to my filters. There's no room in there.
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A zener on the center tap will work, but there's a much less expensive alternative. There was a detailed discussion either here or on AMPAGE within the past month or two of the various methods. I can't figure out what the right SEARCH terms would be right now but I did save the attached schematic.
Search the net for "MOSFET Follies" and you'll find a discussion of this circuit (I think).
BTW I found that I liked the OT (plate) voltage on a 6V6 at around 300-320 much better in a Tweed Princeton. Blackface Princeton Reverb voltages much higher. It all depends on the sound you want.
HTH
Chip
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I already ordered a 50V Zener. I'm hesitant to do much messing w/ the B+ itself. Tho I did a VVR on the TOS which I like.
Oh well.
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That 50 volt zener is probably going to get pretty hot.
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(http://www.geofex.com/Article_Folders/mosfet_folly/mosfol4.gif)
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A big ole power resistor right after the rectifier will work just fine in a SE 6V6 amp too, if you want simplicity.
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That 50 volt zener is probably going to get pretty hot.
Assuming 2 x 6V6s running about 140mA max and another 10mA for preamp tubes, .15A x 50V = 7.5W, so a 20W zener will be fine.
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Just one 6V6. I also want less voltage on the 5879. And the CF grid has 230v, probably exceeding the 12AU7 grid-to-heater max.
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Just one 6V6.
SE 6V6 Class A uses about 36-40mA, so your 20W zener is ample
I also want less voltage on the 5879. And the CF grid has 230v, probably exceeding the 12AU7 grid-to-heater max.
increase the pre-amp supply resistor size in the power rail