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Re: Cathode Bias Idling Current

12/27/1997 9:21 AM
Mike T	Re: Cathode Bias Idling Current I just fired it up and something is wrong. I get a fully distorted sound at all settings with barely audible volume. I'm getting about 350 V at the plates which is OK, and 25V at the cathodes with a 250rK, so that comes out to 50mA per tube, too low. I'll try a lower value rK as soon as I find the other problem. Is there a way to tweak the cathode voltage? In other words, will the voltage at the cathode remain the same if I change the resister, and if so, is there a component that regulates the voltage at the cathode? I'm using a Hammond OT with taps for 4, 8 and 16 ohms, and have hard wired it according to their spec sheet for 8 ohms. Could be a problem there. But I'm going to have to set this project down and come back to it, because my old shop is not easily accessable to me at this time, and I need the space, as this disassembled amp has been occupying my primary workspace for a few days now. After I think about it for awhile though, I'll probably come up with the solution for this distortion problem, as it is probably real dumb, as they usually are (for me, anyway). I have a couple other amps I can gig with (if I can get some gigs). I've read about that method for reading mA as voltage with a 1 ohm resister at the cathode, but never used it bacause all my amps, except for the Pro Jr are cathode bias. Wouldn't installing a 1 ohm resister from cathode to ground upset the bias if wired in parallel to the bias rK (path of least resistance)? Or are they wired is series with the 1 ohm first from the cathode and then temporarily jumped to ground from where they meet and then read accross the 1 ohm resister? Summertime. Man. It's sloppy, wet, and cold up here. A few months of winter to go, too, and January and February are usually the worst. But I'm used to it. Mike

12/27/1997 9:21 AM

Mike T

Re: Cathode Bias Idling Current

I just fired it up and something is wrong. I get
a fully distorted sound at all settings with barely audible volume. I'm getting about 350 V at the plates which is OK, and 25V at the cathodes with a 250rK, so that comes out to 50mA per tube, too low.
I'll try a lower value rK as soon as I find the other problem. Is there a way to tweak the cathode voltage? In other words, will the voltage at the cathode remain the same if I change the resister, and if so, is there a component that regulates the voltage at the cathode? I'm using a Hammond OT with taps for 4, 8 and 16 ohms, and have hard wired it according to their spec sheet for 8 ohms. Could be a problem there. But I'm going to have to set this project down and come back to it, because my old shop is not easily accessable to me at this time, and I need the space, as this disassembled amp has been occupying my primary workspace for a few days now. After I think about it for awhile though, I'll probably come up with the solution for this distortion problem, as it is probably real dumb, as they usually are (for me, anyway). I have a couple other amps I can gig with (if I can get some gigs).

I've read about that method for reading mA as voltage with a 1 ohm resister at the cathode, but never used it bacause all my amps, except for the Pro Jr are cathode bias. Wouldn't installing a 1 ohm resister from cathode to ground upset the bias if wired in parallel to the bias rK (path of least resistance)? Or are they wired is series with the 1 ohm first from the cathode and then temporarily jumped to ground from where they meet and then read accross the 1 ohm resister?

Summertime. Man. It's sloppy, wet, and cold up here. A few months of winter to go, too, and January and February are usually the worst. But I'm used to it.

Mike

Replies:

Stephen Mike,50mA per tube should be wo... -- 12/27/1997 4:28 PM