I am wondering about making the bias adjustable on a PV C30. Can I just replace R64 with a 30k-50k pot? Maybe with a 15k in series with the pot. Will that affect the -14 volt setting? Will the -14 volts go up and down with the bias trim pot?
Thank You
http://www.schematicheaven.com/newamps/ ... ssic30.pdf
Peavey C-30
Moderators: pompeiisneaks, Colossal
Re: Peavey C-30
That is essentially what I am proposing, although Steve presents it in a very professional and clean manner.
Maybe I missed it, but I did not read where he discusses the -14 volts. Will the bias mod screw with that?
Thank You
Re: Peavey C-30
Yes that bias voltage will be raised and lowered with the trim pot. But the bias voltage is not the critical thing to measure - the critical thing is the tube current, which is why you need to have the 1R resistor between the cathode(s) and the ground in order to be able to read the mV drop across that resistor (with your meter plugged into the test points, which you also should install), which will gives you the equivalent in mA (E = R x I, so mV = 1 x mA). When you change EL84s the bias will alter just because not all tubes area the same, and therefore relying on the grids being at a pre-determined -ve voltage is riskier, unless you make that voltage conservatively high (which will bias the tube conservatively cold)
Re: Peavey C-30
Yeah....I understand all of that. I do not even want to get into the stupid mistake I made reading this schematictubeswell wrote:Yes that bias voltage will be raised and lowered with the trim pot. But the bias voltage is not the critical thing to measure - the critical thing is the tube current, which is why you need to have the 1R resistor between the cathode(s) and the ground in order to be able to read the mV drop across that resistor (with your meter plugged into the test points, which you also should install), which will gives you the equivalent in mA (E = R x I, so mV = 1 x mA). When you change EL84s the bias will alter just because not all tubes area the same, and therefore relying on the grids being at a pre-determined -ve voltage is riskier, unless you make that voltage conservatively high (which will bias the tube conservatively cold)
......it involves the the -36 volts. Just take my word for it, I am definitely an imperfect human. It is all crystal clear now. Sorry to have made you go through the basics for the MILLIONTH time. Good Grief!!!Anyway, now that I have both feet on the ground.....
Is Steve's method for installing the 1 ohm resistors pretty much the way to go? I guess if you want the ease of those 1 ohm's, the traces will have to be cut.
Thanks Again
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iknowjohnny
- Posts: 1070
- Joined: Thu Apr 24, 2008 2:10 am
- Location: los angeles
Re: Peavey C-30
yes, the 1 ohm resistor is a great bias method. No voltages to worry about, and you can even mount bias points on the chassis. On my homebrew i have 2 test points that just go to the 1 ohm resistors and a 3rd ground point, all 3 being "jack" for lack of the correct term that accep meter probes. it's great to be able to bias safely and quickly any time i want. Be sure to compensate for screen voltage when you check bias.
Re: Peavey C-30
Yeah, I always check the screens, just to make sure there is not more than a 2mA loss without taking it into consideration.iknowjohnny wrote:yes, the 1 ohm resistor is a great bias method. No voltages to worry about, and you can even mount bias points on the chassis. On my homebrew i have 2 test points that just go to the 1 ohm resistors and a 3rd ground point, all 3 being "jack" for lack of the correct term that accep meter probes. it's great to be able to bias safely and quickly any time i want. Be sure to compensate for screen voltage when you check bias.
What I am really wondering about is the method of getting the 1 ohms in there. I do not have a whole lot of experience with PCB amps, and the PV C30 is kind of a tough example. Steve is a pretty sharp guy, I was just wondering if anybody else had tackled 1 ohm bias resistors on the C30?
Thank You
Re: Peavey C-30
Yeah you have to break the trace (thoroughly) in the way he describes, by scraping the copper entirely off the board there, then you bare back a little more of the trace on either side of the break - just to expose the bare copper this time, and then you can pretty much tack you little 1R resistor to those bare trace ends with solder. The best resistors for this are those little 0.6WMF resistors that are rated for about 360V breakdown voltage (not that you need all of that capacity), but they are sized for a good fit'n'all in the tight space
Re: Peavey C-30
Great, thanks again.tubeswell wrote:Yeah you have to break the trace (thoroughly) in the way he describes, by scraping the copper entirely off the board there, then you bare back a little more of the trace on either side of the break - just to expose the bare copper this time, and then you can pretty much tack you little 1R resistor to those bare trace ends with solder. The best resistors for this are those little 0.6WMF resistors that are rated for about 360V breakdown voltage (not that you need all of that capacity), but they are sized for a good fit'n'all in the tight space
Looks like I have a rope around all of this now. Sorry I did not start by asking the right questions to begin with.....
Thank You