From cign–(at)–elios.phy.OhioU.Edu Tue May 21 07:40:21 CDT 1996
Article: 9985 of rec.audio.tubes
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: zener diode for dropping B+
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Date: Mon, 20 May 1996 17:23:19 GMT
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JER
>I would be interested in the details of the cathode bias. Can you give us
>a full discription of the *full* mod?
For cathode bias, I’m using a 590 ohm, 25 watt resistor shared by both
tubes. I chose this value because I happened to have the resistor. 🙂
You could probably go as low as 500 ohms, especially if you’re reducing
the supply voltage, but 590 seems to work pretty well. The resistor is
bypassed with a 100uF/100V electrolytic cap.
To install: remove the ground straps that connect the cathode pins on
the tube socket to the chassis. You can remove them from just the tube
socket if you want to be able to reverse the mod easily, but you *must*
make sure that the loose ends are well insulated. If one of them touches
the cathode pin on the tube socket your precious 6V6s could be ruined in
an instant. A piece of shrink tubing over the strap might do the job.
Connect both of the cathodes together with hookup wire and connect the
resistor between one of them and ground. Be sure the resistor has a high
enough power rating — *at least* 10 watts. I like the aluminum cased jobs
that you can buy from Mouser. I found an existing hole in the chassis that
worked well to bolt to resistor down. I connected the ground end of the
resistor to the same ground point as the filter caps and soldered the
bypass cap right across the resistor with as much space between the two
as possible. The heat from the resistor is not good for the cap, but it’s
a little cramped inside the chassis with heat sources everywhere. After
some consideration I decided this was the best option in this case.
Remember to put the ‘-‘ end towards ground.
Now you need to remove the grids from the fixed bias supply and provide a
path to ground: there is a wire going from the junction of a pair of 220k
resistors to the tremelo ‘intensity’ control. Remove this wire from the
intensity control and solder it to a good ground connection. The trem will
no longer work (sorry).
That should do it. Connect a voltmeter across the new cathode resistor
and turn the amp on while watching the meter. The voltage should rise
to 25-30 volts as the heaters warm up. If it doesn’t, or if your output
tubes get red, shut the power off and check your work.
I described the zener mod in a previous post. If you need more details
you can email me.
— Dave Cigna
