Davis 201A PA Conversion

[Dave H]

Is this something I might more easily track down with my scope? I don't have a function/signal generator. Could I just ring out an E note and see the static in the signal? That way I might at least track it down to where in the signal chain it's taking place.

Yes, you could do that. From the schematic the B+ is less than 300V so with a x10 probe it will be safe to probe anywhere in the signal chain except for the plates of the power tubes.

[Dave H]

I did a quick load-line chart yesterday and discovered today that I was off by a decimal point. When I thought I was looking at 220R for the operating point, I was really looking at 2200R. I recant my confession (that the confessor already recanted upon), and stand by my claim that 2.2k is probably what you want in there for a 470k anode resistor. The distortion you're hearing may well be coming from that way-too-hot stage.

It is biased way hot with 220R and probably distorting on large signals. What plate voltage did you get? I have it at about 50V. 50V is centre bias for the cathodyne but way too hot for the driving stage. A better compromise would be to use a 470R or 560R cathode resistor. LTSpice tells me that 500R gives a clean 30V peak output from the cathodyne which is more than enough to drive the EL84s to full power.

[Pdavis68]

ARGHHH... So I got it all set up to check it with the scope, turned it on, and it made a really nasty noise. Attached is a recording of it. Is this oscillation?

It sounded worse at full power. This was going through a 40W bulb. The tubes warm up enough so that I start getting sound and 2 seconds later, it does this. I don't have a suitable load resistor to test this with (ordering one as soon as I'm done writing this). I don't want to break anything. Any suggestions on where to look?

I haven't made any changes. I went and mowed the lawn, went to see the chiropractor, came back and started setting it up for the scope and this is what I'm getting.

[olddawg]

There are a number of free signal generator Apps if you have a smart phone or a computer that does apps. There are also online signal generaters and downloadable programsif you only have an old computer.

[Pdavis68]

Oh cool. It never occurred to me that there might be phone apps. That's awesome. Thanks! I'll get one for my android phone.

[Pdavis68]

Well, silly me. When things stop working, I sometimes get frustrated. After it started making that noise, I got frustrated and didn't want to deal with it during the week (didn't have a lot of free time anyway). I just assumed it was going to take hours of my time and didn't want to mess with it until the weekend. So I fire up the soldering iron with the intention of re-cooking all the connections I made. While it was warming up I started pushing and pulling on wires and sure enough, the cathode bias resistor and bypass caps were disconnected from the ground. Well, not entirely, but effectively so. Popped a bit of solder on them and not only was the loud buzz gone, but so was the staticy buzz as the sound was fading. At least so far. I'm letting it warm up a bit and I'll play it some more in a bit, but so far it's sounding AWESOME!!!

I'm going to build a cable to hook up my Android phone headphone jack to a 1/4" plug so I can use it as a signal generator and then I'll take a look at the PI. I also want to rewire the phono volume to be the master volume (a phase cancellation type like I did on the Hammond, with terminal 3 going between C12 and V5 grid and terminal 2 going between C11 and V4 grid (or vice-versa)). And I also need to remove 2 of the input jacks and add the switches so I can do the channel mixing. Psyched that it's working and sounds great!!! Woohoo!

[Chuck H]

Here's what happens if I am interpreting it correctly. The positive half of the wave is passed as is and the negative half is now positive but the peak value is not changed so the rectified voltage is not increased. It stays the same as the zero to whatever voltage, not the voltage to voltage as in 0-250 not 250-250. So 250 X 1.4, not 500 X 1.4 is the output voltage. I think we are saying the same thing, only a different way or needed to clarify what the situation is.

[ATTACH=CONFIG]34908[/ATTACH]

I just wanted to address this because it was sort of left floating. And in case I'm misinterpreting. I think the diagram is misleading. Though it is often said that the negative wave gets "flipped" I don't think so. There's a reason it's called a "full wave" rectifier. Because it's actually two "half wave" rectifiers joined at the end. At least that's how I've perceived it. Corrections accepted.

[DRH1958]

It's just re-wording things now. However you want to word it, the voltage produced was the whole point of the topic and by your diagram, the voltage is still the same as the way I described it. The pulsating DC is now 120Hz and not 60Hz as in a half wave. The voltage is still the same. If you want to call it flipped or however you want to word it, makes no difference. It's just semantics now. The end result is still the same. The transformer was 250-0-250 and even by your diagram the voltage produced is 250 X 1.4. That was the context of the discussion. Look at how Merlin draws his waveforms in this link: http://www.valvewizard.co.uk/bridge.html Looks flipped to me. And the voltage is as I described.

[Chuck H]

Easy friend I posted for my own education. If I have this wrong I hope to be corrected. No one taught me this stuff and my understanding is limited to my fairly basic mechanical perceptions as they apply to most passive electronics.

I agree that the voltage is the point, that the voltage is the same and that for the purposes of the discussion the semantics are unimportant. I only try to be vigilant because "I", as much as anyone, peruse threads for the education.

[DRH1958]

I pursue this for the same reasons as you. I didn't mean to be offensive. Emotions don't translate well in the world of ones and zeroes. I regret if offense was taken, none was intended. I think we are saying the same thing from two viewpoints. Mine is from one phase(winding) only. In this viewpoint, the negative portion appears to have been "flipped". But I believe you are viewing it from BOTH windings, so the way we describe it will be different. The result is the same. I hope this clears things up. I respect your contributions to the forum and always look forward to your insight to the various threads.

[Justin Thomas]

I don't look at it as anything being flipped... I just see two out of phase AC sine waves, and depending on how the diodes are placed, the negative half gets chopped off of each. Not negative getting flipped, negative getting "gone."

I suppose if one of those diodes were reversed, we'd end up with a null waveform? . This is the kind of stuff that makes me wanna go to the local comm college & muck about in their lab - better to blow up THEIR gear than mine. I can cobble allthis stuff together and get the expected results, and I kinda get it, but not really... ;p. But apparently, my 3 amp builds sound pretty darn good for a semiclueless noob.

Justin

[Chuck H]

Well... With the diagram from Merlin's site there is a bridge rectifier. Which draws current from the whole winding all the time. Where a full wave rectifier operating with a CT draws current from half the winding all the time (Or is it half the current from the full winding!?! Anyhoo...). A bridge does seem to invert half the wave. Or, probably more correct to say it's direction is reversed and added in series. I know the mechanics of operation, but just how this happens still confuses me. At any rate the figure is still 1.4X for a single winding with a bridge rectifier or 1.4X for half of a CT winding with a full wave rectifier. BUT only .7 for the full CT winding with a full wave rectifier.

Sorry to be so mind bendingly specific, but since there are example of bridge rectifiers used on CT windings with the CT employed to improve filtering I thought it was important to be clear and avoid flames.

[DRH1958]

I agree with you. The devil is indeed in the details! This wasn't an example of what our specific situation is here, I was just pointing out the depiction of the waveforms showing the negative pulses "appear" to have been "flipped" to positive if aligning them vertically. Obviously nothing physically gets flipped, only the depiction of the waveforms as drawn out. I suppose I should have made that distinction at the time. I should have found a different example but I have Merlin's site in bookmarks, so it was easier to find. There are many good depictions in his book, but none that are like our amp in this thread on his site.
BTW, I have his book on Power Supplies and he says he doesn't like it because of a few typos. I have learned a lot from it and found the few typos to not hinder learning anything. If you can find a copy, it would be worth your money.

[Chuck H]

I've seen the 'dash wave bottom/solid wave top' diagram used for both bridge and full wave rectification a few times. And read the term "flip" or "flipped" for both as well. It just doesn't matter for the purposes of this discussion. My bad for the side track. I wanted to see if I had it wrong is all.

[Chuck H]

Pdavis68...

Kudos! Glad it's on track. Sorry for side stepping right on your thunder. Run it through all the paces. Like cranking it up, dialing it in for max clean, running effects into it, etc. Make basic tonal adjustments before other changes. It might save trouble shooting later changes.