Negative AC voltage? - Page 1
Forum section: electronics / beginners

author: iamnothim
author_rank: Regular Contributor
timestamp: April 12, 2013, 08:42:00 pm
content: This question relates to my valve testing project.In all the 9-pin valves schematics the grid voltage is shown as negative in the graphs plotting Grid voltage against Plate current.The more negative the grid voltage is the lower the plate current.I understand AC sine waves, and I think I understand that the how the Grid controls the flow of electrons to the plate.What I don't understand is the implementation using an AC transformer to achieve "negative voltage". All the valve reference diagrams show negative grid voltage.The circuit does not have any diodes. The mains plug is 2 prong and there is not a ground symbol in the schematic.The schematic has the 6BN8 cathode, pin 9, tied to the grid, pin8, via 0.01 µF cap C4. The grid voltage for the valve under test is provided by the 6BN8 grid via SW-4BSo grid voltage is coming from the 6BN8 cathode. The C-4 is a disc cap and does not have polarity. The cathode is supplied 30 VAC from a trans secondary. So here I am in the Beginners Forum. Is there a forum lower than Beginners.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: April 12, 2013, 10:25:47 pm
content: I've seen term negative voltage used in AC case for phase offset. If we remember that instantaneous voltage would be for example Acos(wt+f), we can tune f so that one instantaneous AC voltage is always negative compared to another instantaneous AC voltage. For example, if we have voltage A*cos(wt+0), compared to it voltage A*cos(wt+pi) would always be negative.So where do transformers come into play? Well on transformers, you have "phase dots", which show the transformer electrodes that have output which is in phase. For example if we take the simplest transformer with just two coils and connect both dotted electrodes to positive leads of a pair some theoretical voltmeters that can take DC voltage readings extremely quickly, they would both show same voltage polarity at both sides of the transformer, meaning that the two AC voltages are in phase. If you connected positive lead of one voltmeter to phase dotted side of one coild and the positive lead of the second voltmeter to the non-phase dotted side of the other coil, the voltage polarity would always be opposite.Here's a Wikipedia linkhttps://en.wikipedia.org/wiki/Dot_conventionand a All About Circuits link:http://www.allaboutcircuits.com/vol_2/chpt_9/4.htmlas some further reading.Also note that in my example I used term "theoretical voltmeter" since regular voltmeters on DC won't be able to take readings quickly enough to show instantaneous voltage and could have some sort of low-pass filter on DC setting. On AC setting, you wouldn't be able to compare the phase, since they'd just show RMS voltage. To really see what's happening, you'd need an oscilloscope.I'm also attaching two images where you can see on an oscilloscope how this looks like. The red channel is a primary side of a transformer and yellow is secondary in this case. The out of phase image was made by simply reversing the connection on the secondary side of the transformer. If we take the red channel to represent the positive voltage, then the voltage of yellow channel will always be negative.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 01:48:49 am
content: Excellent material and explanation.Thanks for taking the time to take the scope images too.BTW: It's ok if you want to move to another thread and ignore my question.I've been to a handful of transformer sites including All About Circuits.I've gone through most of DC and skipping around AC. I'm not the greatest at complex algebra.I "get" the Fundamentals of transformers, turns, taps, isolation.The DOT reference helps. some. I have a handful of simple AC transformers to pro to-board. I do not have an "Output Transformer". I was thinking I needed tone until I, moments ago, realized, it's output from the valves to the speakers. The plate is receiving it's source elsewhere...Let me see if I can embarrass myself and restate the wiki DOT article.DOTs are aligned (i.e.. Leads tied for that application) Current increases (Load) "from DOT, to Inductor"…positive voltage.DOTs are reversed (i.e.. Leads tied for that application) Current increases (Load) "from Inductor to DOT"…negative voltage.Is this the same as connecting the secondaries for parallel vs Series? Is this a "Push Pull" transformer? Seems like it's no longer a (I'm going to call it a source, but I know it isn't) with current flow on both sides of the inductor / trans. (2 x inductors = transformer)I understand in and out of phase and lagging and leading current. One of the members had an excellent youtube video on caps vs inductors. … (Wait ! is that "it" ?)The capacitor in the circuit shifts the phase when combined with an inductor?So...So in this case the secondaries have polarity? Would I scope one secondary lead against ground and then scope the complimentary secondary lead against ground?I can look for the phase shift of the sine wave. Would this represent "negative voltage?I see that in reference to time, one lead will be negative when the other lead is positive.My confusion. The load wants negative voltage, it's not like it's sampling.Is this also "DC Bias"?I still have that single ended issue. The schematic, I've stared at it for over a month, has no loop to make it a circuit.My circuit has no ground in the schematic and the mains is 2 prong. All the transformers have 2 primary leads. If I pull in the mains earth ground I had best be real careful probing because I no longer have the transformer isolation. I think that's correct.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 02:21:21 am
content: It think this is something....Came from Wikipedia "Operating Point"Sounds like I can apply DC biased voltage "to the same point on a circuit as AC"Most often, bias simply refers to a fixed DC voltage applied to the same point in a circuit as an alternating current (AC) signal, frequently to select the desired operating response of a semiconductor or other electronic component (forward or reverse bias). For example, a bias voltage is applied to a transistor in an electronic amplifier to allow the transistor to operate in a particular region of its transconductance curve. For vacuum tubes, a (much higher) grid bias voltage is also often applied to the grid electrodes for precisely the same reason.A hot bias can lower the tube life span, but a "cool" bias can induce crossover distortion.Bias is also the term used for a high-frequency signal added to the audio signal recorded on magnetic tape. See tape bias.Bias is used in direct broadcast satellites such as DirecTV and Dish Network, the integrated receiver/decoder (IRD) box actually powers the feedhorn or low-noise block converter (LNB) receiver mounted on the dish arm. This bias is changed from a lower voltage to a higher voltage to select the polarization of the LNB, so that it receives signals that are polarized either clockwise or counterclockwise, thereby allowing it to receive twice as many channels.We still need to determine the optimal values for the DC biasing in order to choose resistors, etc. This bias point is called the quiescent or Q-point as it gives the values of the voltages when no input signal is applied. To determine the Q-point we need to look at the range of values for which the transistor is in the active region.

author: IanB
author_rank: Super Contributor
timestamp: April 13, 2013, 02:24:02 am
content: Quote from: iamnothim on April 12, 2013, 08:42:00 pmIt's important to understand that valve circuits, like transistor circuits, operate with DC bias voltages to set the various operating parameters of the circuit.So you are not dealing with AC, you are dealing with DC.The 6BN8 is a diode (actually it is two diodes and a triode, but pins 2 & 1 are the cathode and anode of a diode). So C-2 gets charged up with a DC voltage via rectified current flowing through that particular diode.Therefore, what the circuit generates is a negative DC bias voltage. Negative voltage in this context means that the grid voltage is lower (more negative) than the cathode voltage. (But DC, not AC.)

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 02:27:57 am
content: Ian !I was just reading that. I'd thought about that (DC) but was confused because I didn't see it in the schematic or on the old trans.This Article sates exactly what you said.http://en.wikipedia.org/wiki/Biasing#cite_note-1Sure, I can put in a rectifier (I have a couple) and dial in the appropriate bias to the grid. yes?

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 02:30:19 am
content: It also links to the Superposition Principal.Which to my pea brain, looks like overlaying voltage.(I've only skimmed it)http://en.wikipedia.org/wiki/Superposition_principle

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 02:33:07 am
content: Ian.Man that's a great explanation.

author: IanB
author_rank: Super Contributor
timestamp: April 13, 2013, 02:34:30 am
content: In the days before silicon, valves (vacuum tubes) were used as diodes for rectification and used as triodes or pentodes for amplification. Today, of course, you can use a silicon diode where a valve diode might have been used in times past. There have been plenty of hybrid circuits with a mix of silicon and vacuum tube technology.

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2013, 02:47:27 am
content: Quote from: IanB on April 13, 2013, 02:34:30 amOne point to make about that - though I haven't had a good look at this circuit, it's pretty important to remember in general. Vacuum diodes arespectacularlybad, and the circuits were designed to compensate for that. Stick a silicon diode in there and you might end up with a bit of a high voltage surprise. I'm repairing a circuit right now that loses over 60V in the diode - replace with a silicon diode and suddenly you're massively over the voltage rating of several caps.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 03:00:35 am
content: Once again.....I'm reading your last reply and saying..... "I don't have a diode.... for DC"Then I re-read your first reply.... "The 6BN8 is a diode"So... I pull the Sylvania 6BN8 spec.... In the pin schematic, I could never figure out why the grid pin-8 never went all the way between all the cathodes and plates.Looked at you reply again. "actually 2 diodes and a triode" I have that cleared up.Next, I'm not equating a "component diode" to a valve diode..... Same thing....I'm looking at the valve as a thing I need to feed rather than a functioning component...Reading it again.... Cap get's fired up...I've been looking at this for a long long time and now I have a feeling I can get it.ThanksWhen I get the above concept down. Do I try to recreate the voltage sources as drawn or considering parceling them out?I am blown away by the schematic showing the 6BN8 cathode pin 2, tied to heater pin 4. The the circuit is tied into mains.I think I can feed the cathode the necessary voltage.I've decided to use separate 6.3V 600mA heater trans for the valve under test and the 6BN8.I've tested with the heater loads from all the different valves I want to test.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 03:07:32 am
content: Quote from: c4757p on April 13, 2013, 02:47:27 amI was thinking in that direction, rectifier, until I reread Ians post. The 6BN8 valve is the necessary diode for the grid voltage bias.I agree with you, and I'm very glad you brought that up. I do not want to change the fundamental operation of the device. It's retro tech testing old valves.Other than taps for DMM's and scopes and some digital meters.Rev 1 of the Panel:

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 06:20:54 am
content: Ian,Did i get your explanation right?

author: G7PSK
author_rank: Super Contributor
timestamp: April 13, 2013, 09:00:33 am
content: That junction you have labeled in blue as confusing you, what is shown is an indirectly heated cathode, usually a tube covered in thorium with a filament inside it the connection is inside the valve (tube). What you need is a valve data book or the data sheet for that valve for the correct pins.

author: IanB
author_rank: Super Contributor
timestamp: April 13, 2013, 09:08:46 am
content: Quote from: iamnothim on April 13, 2013, 06:20:54 amNot quite. I think it is more like this:The positive side of C-2 is your ground reference and V-1B rectifies the 120 V mains to produce a voltage of about -170 V on the negative side of C-2 (which is a reservoir capacitor to remove the ripple).This voltage is applied through SW-5 and R-4 to the grid of V-1A.R-3 is a potential divider that permits varying voltages to be applied to the cathode of the valve. However, without a circuit description I am not clear about the underlying purpose or function of this part of the circuit.There is nothing odd about having pins 2 and 4 of V-1B electrically connected. The cathode and filament of a valve can be treated electrically as the same component.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 13, 2013, 10:16:06 pm
content: G7PSK,Thanks for replying, please don't misinterpret my response, as you can't express intonation when writing. I appreciate anyone that takes the time to read my post and respond.I do understand heater concepts and applying 6.3V to a filament, which in turn, heats the cathode, which in turn, emits electrons. I've set that upon a proto-board many times to test the heaters on several different valve types. (6922, E188CC, ECC88, PCC88, etc) I have the specs for many dual triode valves as they range from 300mA to 600mA. When testing with a 6.3V 600mA trans, naturally the secondary voltage varies with the valve. For the tester I want to get as close to the 6.3V spec as I can. I have 120V on my mains. The transformers I've tried want 115V on the primary. Hence I inserted a 150 ohm thick film resister in series. Between that and a 5 ohm POT, I can dial in the heater voltage for any dual triode valve I wan to test.Now I'm off to tackle the other, important have of the circuit. I knew that the testing circuit had a 6BN8 in it (see schematic), and that the pin assignments were different. 4 and 5 heaters seem to be standard. What Ian pointed out that I did not pick up, was the 6BN8 is a dual Diode and Single Triode. That tidbit went a long way in helping me understand negative grid bias. Key word… "helping me understand" This is the negative DC bias I was reading about but I couldn't understand where it was coming from. I kept thinking. "Don't I need a diode for that?"Ian,Wow. thanks for annotating the schematic and for pointing me in a direction where I can understand the circuit. I've included the passage from the manual that describes the function of SW-5. It is the Grid Emissions Test. I've been rereading it for some time to connect the dots. It ties the 6BN8 and Valve under test grids together. I am repeating whats in the manual to help the concept sink in. Gris emission are caused by residual cathode particles that land on the grid that intern turn the grid into an emitter. I get the concept. What I have to learn is how the tests detects the emissions via the circuitry.As noted above, I am going to decouple the 6BN8 heaters from the test circuit. The valve under test already was isolated via the heater selection switch that selects one of the taps to supply the appropriate voltage.That leaves the 30V bugger and the 120V mains. I know from the valve spec sheet that the normal operating plate voltage is 90V - 120V and they can max at around twice that.The 30V supply goes to SW-4B of the 3PDT "Quality" test. SW-4B disconnects the 6BN8 Grid from the tube under test grid and gives it 30V. I'll figure out why when I get the other part down.My thought is, rather than direct mains, inserting an isolation trans. Otherwise I'll look at other possibilities. I'd like to keep the transformers to 2 rather than 3. I know I could use one heater trans for both the 6BN8 and the tube under test. But they are small so I'm splitting them.Here's a cleaned up schematic based upon your suggestions. The 30V part is probably incorrect. I'm hoping the 120V mains into pin 2, V-1B is ok w/o being tied into the heater circuit.So let's call that my question for the day. Can (should) I pull those 2 apart? I've included the Emissions test description that applies to the left side of the schematic.You've sent too much time on this. Please do not feel obligated to read the test info. I provided it in case you were curious.Can't thank you enough.Couldn't help but smile when reading the earth ground thread. Not just saying this, but I think I understand you perspective. I can retain tidbits and waves and phase mostly make sense without going into geometric equations.

author: iamnothim
author_rank: Regular Contributor
timestamp: April 14, 2013, 03:36:18 am
content: This should be a safe bet to cover my power issues.