Strange Voltages

[andrew7667]

I'm still having issues with my Cali Dreamer build. I've narrowed it down to a problem in the preamp and I know of at least one problem with the PI.

I'm getting some sound through, it's just very, very quiet. I actually have to put my ear to the speaker in order to hear it. The voltages in my power amp are checking out, so I'm pretty sure it's not that.

I checked the voltages on my PI and they're insanely high. Around double the value they should be. Now, I'd say this is a problem with my tube not getting good connection, but if I'm getting some sound, then the PI must be partially working, correct?

http://home.comcast.net/~daniel.souder/ ... tic_V3.pdf

At the point where the PI is being fed the B+ and it's split with the 82k and 100k resistor, I'm getting higher voltage by about two volts AFTER the resistors than I had before. The B+ has the correct voltage, but the point after the resistor is way too high. Any idea what could be causing this?

Also, the point before the PI that has the 22k, 470, and two 1M resistors in a box-like formation is seeing about double the voltage it should be as well. I've already replaced the socket entirely to rule out the possibility of having bad connections, but still no luck.

Any input would be greatly appreciated. Thanks in advance!

Also, here's a schematic of a prior version with a pre-PI master volume that has the voltages.

http://home.comcast.net/~daniel.souder/ ... em_2_5.pdf

I'd assume the the voltages wouldn't be too different, even if the master volume was put in a different location?

[andrew7667]

I just checked the plate voltages for the rest of my preamp and they're all too low by about 40volts. Again, the B+ levels all check out but once it hits a resistor and heads to the tube, it gets screwy.

[andrew7667]

Anyone have an idea?

[andrew7667]

Would you be able to get any sound from the amp even if the PI wasn't loading?

[Manoteal]

Actually you can get sound even if things are screwy. It looks like your PI is not drawing enough current to drop the voltages at the plate correctly. I may be stating the obvious, but have you tried another tube in the PI section? What are you getting on the cathodes ot the PI tube? Are the cathode resistors correct (measure them and make sure that they are connected to the parts that they belong to). I think that the 100R resistor where the negative feedback is connected might not be making a good ground connection (check ALL your grounds). Check that and see what happens.
Good luck! You'll find the problem and the toughest ones are the easiest to fix, usually.
Manoteal

[morcey2]

Do you have any pics? That would help.

Also, when measuring around the PI, things can get really wierd. The plate measurements that you made are probably accurate. Not what you wanted to see, but accurate. Measuring the grid voltage is next to impossible because of how high the impedance is there. Most voltmeters have a low impedence relative to this and will actually change the voltage because of the act of measurement. Kind of a Hiesenburg thing.

I was able to get sound out of one of my pedals without the input being hooked up at all. The input wire was sitting next to the input jack and was picking up the signal through capacitance. I would get a little output, but it was very thin and weak. Very similar to what you are seeing. A mis-wired input wouldn't explain the wierd voltages though.

Matt.

[andrew7667]

I'm positive the input is wired correctly. I've checked it more than once.

Would the PI not drawing enough current explain the goofy voltages on the other tubes? I'd assume that those tubes are drawing too much current while the PI isn't drawing enough. What would cause this?

The grounds check out. I'll recheck them in a minute though. My grounds are all good.

I've yet to check the cathode voltages, but if I remember correctly, they were high on the first three preamp tubes and low on the PI.

Here's my photobucket album.
http://s34.photobucket.com/albums/d125/Andrew7667/

Don't mind the output jack wiring. I've already fixed that.

[andrew7667]

I've found something even stranger! When I first put my probe on the cathodes of my PI, it shows ~100v then slowly starts to climb until it hits about 270v. Now, I realize that's WAY too high a voltage, but I'm wondering what could be causing it? The tube must be at least partially loading, because I get different values when I pull out the tube. Could this be wear everything is screwing up? I'd imagine if the cathodes were drawing so much current it'd explain why the voltages are too low on the other tubes.

The other tube cathodes check out. They're a wee bit high (around 2volts), but nothing like they are on the PI.

Any suggestions?

Edit: I checked the resistance on my filter caps and the two that feed the preamp are only measuring around 10k. Would my B+ voltage still be okay even if these went bad? Should I replace them since they're showing such low resistance?

[andrew7667]

Morcey? Manoteal? Anyone?

[morcey2]

Morcey? Manoteal? Anyone?

Be with you shortly. Fixing dinner. ( corned beef and cabbage ).

Matt.

[morcey2]

There's a wire in the center of:
http://s34.photobucket.com/albums/d125/ ... 5922_0.jpg

that looks like it's not connected to anything. I'm not sure which pot it's supposed to be on.

More later....

Matt

[andrew7667]

It's hard to see, but there's actually a cap right there.

[morcey2]

Dangit. Sorry I'm not more helpful. My next suggestion, which you've already mentioned, is getting the cathode voltages. From what you've said earlier, the PI looks to be pulling just under 5 ma. Measure the cathode voltages on the rest of the preamp and reverb stages.

I'm going to bed now. I have 4 teenagers and as a result my brain isn't working very well. But I might have that causal relationship backwards.

Matt

[Manoteal]

I can't see what is wrong either. It might be time to just get away from it for awhile and then when you come back you might see or measure something that you overlooked. A fresh pair of eyes so to speak. Just take your time and start to put all your voltages on a printout of your schematic and measure resistors for the proper value. You might just have a resistor swapped for another and that is all it is.
I wanted to ask- do you have star grounding for this build? if you do, take all the wires off that connection and sand the chassis a bit there before putting it back together. It really looks to me like a bad ground and not enough current is flowing through the tubes.
Walk away and do something else for awhile (you know, eat, have sex, and watch the telly- not necessarily in that order)! I have had things take a long time to fix, but you have to let your subconcious mind work a bit on the problem.
Good luck!
M

[andrew7667]

I do have a star ground and will look into that tomorrow. I kinda figured it was a ground somewhere.

The cathodes on the rest of the preamp are all sitting at about 2volts. Is that strange given the rest of the voltages on the amp?

I tried swapping out the 5Y3 and putting a GZ34 in its place just to see what it'd do to the voltages. Well...after a quick re-bias, I'm getting more sound. Does this affirm what you guys think is not enough current?

[Manoteal]

You can have gounding problems anywhere, and they are easily overlooked. When you said that the cathodes started low and went way up above 200V, that made me think that there is a problem in that cathode circuit. The tube would warm up and pass a little current through the tube, but most of the voltage was at the cathode since there might be a high resistance there to ground. It might be making some contact, but you have to make sure that you are getting a real solid 0 ohms there. I suspect a cathode resistor that may be out of spec (or incorrect value) or the grounding thing. Measure, measure, and perhaps touch up solder connections to your ground buss. Keep at it. You'll find it!
M

[morcey2]

Another quick check. With the nfb switch off, Pull the PI tube and measure the resistance to ground from the PI cathodes to the chassis somewhere. Does the volume change with the feedback off? Here's where I'm going with this one: When you measured the cathode voltage on the PI, you said that the voltage increased.

If you're PI cathode isn't grounded and your output tranny is, you'll be getting a huge amount of negative feedback as the PI is now grounding through the tranny. With the switch off, there won't be any ground, or a high resistance ground that forces the PI into a very, very small operating area.

It looks like manoteal is saying pretty much the same thing.

Matt.

[andrew7667]

I actually didn't build my amp with the NFB switch. I hard-wired it to ALWAYS have the NFB on, but that could easily be changed. My buss wire is also fairly thin (about 22awg) for most of the preamp. Should I fatten that up?

I"ll go ahead and throw the NFB on a switch as I have a few extras lying around and try touching up my grounds. My DMM is reading about .3ohms on most of my ground points but didn't think much of it as it even does this in places connected DIRECTLY to ground that are working just fine.

I'll try the sanding thing as well and redo my entire ground scheme.

[andrew7667]

You can have gounding problems anywhere, and they are easily overlooked. When you said that the cathodes started low and went way up above 200V, that made me think that there is a problem in that cathode circuit.

I should note that this whole process would start over once if I took my DMM probe off and put it back on. This doesn't happen every time I switch on my amp; it happens every time I switch on my DMM . That would still make since though, right? As the voltage is now finding a new way to get to ground through my meter, albeit a very lousy one.

[Schorsch]

O.K. Your PI voltages are way too high at the anode and the cathode - which means there's no current floating through your PI.

First of all, check if you have the socket wired correctly. Pin for pin!
Heater is working ? Check the heater-voltage (AC)
With the Amp shut down and discharged:
Measure the resistance from Pin 3/8 to ground. There should be approx. 22-23k

If everything is checked and nothing strange:

Take a voltage-chart with all the voltages at all the tube sockets and post it here. That would be helpful in analyzing your problem.

What layout are you using? Dan Souder's Cal.-Dreamer 3.0 ?
Have you changed any other things than the NFB?