Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

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PeterGreenFan
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Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by PeterGreenFan »

Hello All,

I know the topic of VVR's has been beaten to death on this forum and others, but I built a prototype 5F6-A style amp this summer and I wanted to add a fixed bias VVR to it. I couldn't find the PCB for sale anywhere, so I found an old instruction manual (music-electronics-forum.com/attachments/28470d1397674879-vvr2.pdf) and tried to recreate the circuit. I have looked at that PCB a million times now, and cannot seem to see where I have gone wrong, but clearly I have. Understand that I have no training in electronics, so errors which seem obvious to you are not going to be noticed by the likes of me. I can read schematics, make layouts, and wire up amps, but unfortunately I don't have the background to truly understand what is going on in there.

Here is my schematic interpretation of the PCB:
VVR Circuit (Original Power FET).png

Here is my problem:

When I have the dual ganged pot set to max B+ (VVR "off"), my idle values using a pair of standard 6L6 tubes are the following: (Plate Voltage: 445 VDC, Bias Voltage: -58 VDC, Plate Current [per tube]: 28 mA --> Plate Dissipation [per tube] ~ 12.5 W or ~ 60% max plate dissipation of 20 W). So at idle, my amp is working perfectly and sounds wonderful.

From reading discussions about the thermal requirements of the power MOSFET (http://ampgarage.com/forum/viewtopic.php?f=6&t=14261), I know that at a 1/2 turn of the dual ganged pot I should be at ~ 1/2 max B+ and 1/2 max plate current resulting in 1/4 power output. This reduction in the power output is the whole point of the circuit, as you well know. Unfortunately, as I turn the pot almost to 1/2 way I get: (Plate Voltage: 405 VDC, Bias Voltage: - 32 VDC, Plate Current [per tube] 80 mA!!! --> Plate Dissipation [per tube]: 32 W!!!). If I turn the pot to exactly 1/2 my rectifier tube will begin to arc, so needless to say I don't do that anymore. Notice how at nearly a half turn, my bias voltage has been cut in half, but my plate voltage is only reduced by a 1/10th or so.

Has anyone seen this before? Looking at my schematic can anyone see a glaring error that I am not?

Also, for anyone who has used this type of circuit, when you adjust your dual ganged pot, is your bias voltage getting less negative or more negative?

Logically, I would think it should be getting more negative (to reduce the current), but looking at the PCB and my schematic I simply cannot see how that would be possible.

I have come here in desperation because I simply have no idea what the problem is and I am hoping that PRR or UR12 will be willing to help me get my dream amp up and running.

Thanks,
Kyle

FYI: I have a mica insulator between the power FET and the chassis. The thermal compound I am using is Thermalcote and has a dielectric strength of ~ 1,200 V/mm, so it should not be shorting to ground. I have checked my FETs and diodes and none of them are showing continuity where they should not be.

Also, a question out of curiosity. When I test the circuit with the power tubes removed, I can see adjustment of the bias from max to ~ 10% or (-58 VDC to -6 VDC); however, I am not noticing any change in my B+ out. Do the power tubes have to be in to see any change, or if the circuit is working correctly, should I be able to see the change in the B+ whether the power tubes are in or not? The reason I ask is that while I'm tinkering with the problem I've created, it would be nice to not have keep giving my power tubes the business.

Here are a few pics of the prototype in case anyone was curious:
Front View (6L6).jpg
Actual Power Layout (2).jpg
Actual Preamp Layout (2).jpg
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xtian
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by xtian »

Lou and Matt have gone to significant effort to create fixed bias VVR PCBs. I can't help you with the maths either, but perhaps Lou can point you to his schematic.
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by martin manning »

Try it like this. The bias regulator is the same circuit as the B+ regulator will the all polarities reversed.
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by JazzGuitarGimp »

It looks like I'm a bit late to the party. :-)

Martin has given you good advice; rewiring the bias vvr per his redline should give better performance from the bias supply.

However, there appears to be an issue with the B+ vvr, as your B+ voltage should be closer to 1/2 B+ with the vvr pot at 50% rotation. At 100%, you've got 445V, but near 50% rotation, you're at 405V where it should be closser to 222V. The dual gang pot is limear taper, yes? If so, there is something else going on that is causing the B+ vvr circuit to malfunction. Is the B+ mosfet well heatsinked? Has it been allowed to overheat to the point where it could be damaged?

Also, I notice in your build (which is really clean, BTW) that you have the 100K Date stopper resistor located far away from the MOSFET gate. This resistor is essential for keeping the MOSFET from oscillating, and it needs to be as close (electrically) to the gate as possible. This applies to both the B+ MOSFET as well as the Bias MOSFET (which I do not see in the pics).

Cheers,
Lou
Last edited by JazzGuitarGimp on Sat Sep 23, 2017 6:32 pm, edited 1 time in total.
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by Ten Over »

What does the gate voltage on the NTE2973 do when you adjust the dual-ganged pot?
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by PeterGreenFan »

Hey All,

I was out fishing this weekend so I didn't have a chance to thank everyone for their replies. I probably won't get to tinker with the amp today, but I'll definitely be doing some experimentation tomorrow.

To Martin:
Just to make sure I'm correctly interpreting your edits, I've made a new schematic. How does it look?
VVR Circuit (New).png
To Lou:
I agree with you about the B+. Whether or not the P-channel MOSFET is actually doing anything in my current circuit, I don't know, but the bias voltage is scaling from 100% to 10% as described in the VVR instruction manual (music-electronics-forum.com/attachments/28470d1397674879-vvr2.pdf), while the B+ is hardly scaling at all. This makes me think my problem is somewhere on the B+ side of things, but it could also be the result of some kind of odd reaction to improper scaling on the bias side. How these two interact is beyond my understanding?

The pot is a dual ganged B1M rated at 500VDC.

The power MOSFET is attached to the chassis via a Thermalcote/mica/Thermalcote interface. According to what I've read, such an arrangement should lead to a case-heat sink thermal resistance of 0.1 C/W (https://www.fairchildsemi.com/applicati ... N-4166.pdf). Given that the NTE2973 has a junction-case resistance of 0.45 C/W (http://www.nteinc.com/specs/2900to2999/pdf/nte2973.pdf), and that the face where the FET is attached to my chassis is a 23" x 7" x 0.125" piece of aluminum (which I've considered to be an "infinite heat sink" >140in^2 with a thermal resistance of 1.4 C/W--http://www.designworldonline.com/how-to ... heat-sink/), my total thermal resistance is about 2.0 C/W. I've made a derating curve for my power MOSFET and calculated that if my ambient temp is about 61 C and I'm constantly dissipating the maximum 22.5 W through the FET, my chassis temp would rise to 106 C. However, even in that maximal dissipation scenario that FET can effectively dissipate 100W (if my calculations are correct...), so I don't think heat is my problem. Also, I've never had the VVR activated for more then about 10 minutes because the increasing current tells me right off that something is definitely not right:
Derating Curve.xlsx
FYI: Ignore sheet 2, I was just fiddling with numbers.

Much appreciated, I'm pretty OCD so I like to have things organized. It's interesting that you bring up the gate stopper resistor. I've seen that mentioned a few times throughout the threads about VVRs. While that could certainly be my problem, and I will definitely check for parasitic oscillations on an oscilloscope tomorrow, I'm not sure that is what is causing this phenomenon. I did some research last night and while much of the information I read was WAY over my level of electronics understanding, I did glean a few tidbits which make me think my circuit isn't falling victim to this problem.

First, parasitic oscillations seem to primarily be a problem that occurs during power MOSFET switching. If I understand this VVR circuit correctly (which I may not), the MOSFET is not really working as a switch at all, but rather as some kind of unorthodox voltage mediated variable resistor which is constantly in the "on" state. While parasitic oscillations can continue after the initial switch on state, they seem to dampen out and disappear as the voltage, resistance, and current stabilize. Therefore, given that when the VVR is "off," the MOSFET is in its maximal "on" state I would expect parasitic oscillations continuing from the initial switch on would be causing serious problems even before I start to turn the voltage down, but I'm not. There is no ringing, noise, or any instability that I have observed yet.

Second, parasitic oscillations in power MOSFETs seem to be most common when at least two power MOSFETs are in parallel with the signals to their gates being 180 deg out of phase. As I only have one, I don't have to worry about this problem.

Last, parasitic oscillations appear to be most prevalent when the input resistance to the gate is low. In the article I read, they were experimenting with switching applications that had a gate resistance of 1 to 4.3 ohms. As the resistance is increased rapid changes in voltage per unit time appear to decrease in a exponential decay type manner. So having a 100K resistor makes me feel like I should have more than enough resistance to dampen any high frequency signals.

Although the distance of the 100k from the gate is longer than most, the article wasn't particularly against long lead lengths, and even referenced a couple of instances where long lead lengths reduced parasitic oscillations ( https://www.microsemi.com/document-port ... el-mosfets.).

Either way, I'll still test for it, and fortunately if this is my problem, the addition of a simple ferrite bead to my gate lead seems like it would completely eliminate the problem.

The P-channel MOSFET is on the power layout picture. It is in the middle of the board with a white wire colored with a purple marker attached to the drain. The 100K resistor is connected directly to the gate and a jumper under the board connects the gate to the zener attached to the source.

To Ten Over:
I cannot remember what the gate voltage was doing specifically. I think it was decreasing the same paltry amount as the plate voltage, but I can't say for certain. It definitely was not going down like it should have been--of that I am certain. I'll check it tomorrow and let you know.



Here is my game plan for tomorrow:

First, I'm going to check for parasitic oscillations at the gate. If so, I'll add a ferrite bead.

Second, I'm going to try an experiment. I'm going to completely bypass the bias scaling circuit to see if holding the bias voltage constant will allow the B+ side of the circuit to function correctly.
--This experiment comes from a question rattling around in my mind since I first encountered this problem: How am I ever going to be able to reduce the plate current if I am making the bias voltage less negative? As I've stated several times, I have no formal training in electronics, so my interpretation of the KT66 operating curve below may be totally off base.
KT66 Curve.png
However, it appears to me that even if my B+ circuit was working correctly and I were able to cut my plate voltage in half (from say 400 to 200), if I simultaneously cut my bias voltage in half as well (from say -40 to -20) my plate current is going to go from 20 mA all the way up to 90 mA per tube!!! Which is eerily similar to what I'm seeing in my 6L6s. So how can a circuit which makes the bias voltage less negative result in reduced plate current? If I'm reading the chart correctly, and my B+ was working correctly, leaving the bias voltage fixed at -30 would yield a plate current per tube of 50 mA at 400 V and a total dissipation of 40W which is about 74% max for KT66s. Turning the B+ down to 100 V, keeping the bias voltage constant, would produce a plate current of 30 mA per tube resulting in a total dissipation of 6 W. So it seems to me that one can produce the desired power reduction effect without ever touching the bias. Maybe there is something about the way the power MOSFET clamps the current that I would need to make the bias voltage less negative to compensate, but I'm a curious critter so I'm going to try bypassing the bias scaling this before I make any changes to the circuit.

Third, I am going to re-institute the bias scaling pot but change the R54 value to 820K which should leave me a range from max to 82% of max or (-58 to -48). If I do need to compensate for some kind of power MOSFET current clamping by making my bias voltage less negative, this reduced range should make the current enhancing effect less pronounced.

Fourth, using the same value resistor as above, I'm going to switch the bias in and R54 so that my bias voltage actually becomes more negative (-48 to -58) as I decrease the B+. If the power MOSFET isn't limiting the plate current, this progression to more negative bias voltage should do the trick. That way I should be able to get to lower power outputs without dropping my voltage to a point where the curve starts to fall out of linearity.

Fifth, I'm going to rewire the bias circuit as per Martin's suggestions (if my interpretation is correct). I saved this until fifth because I figure I'll try some easy modifications before I really dig in and start moving things around.

Finally, I'm going to rewire the bias circuit as per #12s suggestions on the allaboutcircuits.com thread about the same topic (https://forum.allaboutcircuits.com/thre ... st-1181029). A circuit which is quite similar to Martin's

Well everyone, thanks again for all of your suggestions. I'm going to test em' all tomorrow and let you know what I find.

Take care,
Kyle
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by JazzGuitarGimp »

Hi Kyle,

I think you are overlooking the screen voltage in your idle plate current analysis. As the plate voltage decreases, so does the screen voltage. When the screen voltage goes down, all of the grid curves on the datasheet chart move down as well, thus making the grid less sensitive to bias voltage.

Also, if the Gate voltage of the B+ MOSFET is scaling as it should, but the Source voltage (output of VVR) is not scaling down with it, then you more than likely stressed the MOSFET by exceeding the its Maximum Gate to Source parameter - and it would take much less than 10 minutes to sustain the damage. Like, Ten Over, I suspect the Gate voltage may not be scaling as expected.

Certainly, you have enough neat sink area! :-)

Lou
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by Ten Over »

Pretty funny what you said over on Allaboutcitcuits. But if you really want to be called an idiot, you have to go to TDPRI.

That MOSFET is wired-up as a garden variety source follower, nothing mysterious. Wired as such the Source will be within a few volts of the Gate. Up until the zener diode breaks over, the voltage on each end of the 100K resistor from the Gate to the Wiper should be the same. Don't measure the voltage to ground on the 100K resistor because the resistance of your meter will give you erroneous results -- measure the voltage from one end of the resistor to the other. Leave the bias alone for these measurements so you don't fry the tubes. The results of these voltage measurements should give a good indication of what is wrong so that you don't tear-up and replace circuit unnecessarily.
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by martin manning »

PeterGreenFan wrote: Mon Sep 25, 2017 6:12 pm To Martin:
Just to make sure I'm correctly interpreting your edits, I've made a new schematic. How does it look?
Looks ok now, but as Lou suggests I think you may have damaged something. Try isolating the VVR's and testing them: Disconnect the outputs of the B+ and bias from the rest of the amp's circuit and put a light load on them, say a 270k 2W to ground on the B+, and a 1M 0.5W to ground on the bias. Power up and rotate the dual pot while monitoring the voltages at the outputs and see what you get.
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by PeterGreenFan »

Hello again,

I just wanted to thank you guys once again for the help with this mystery. Today I found the problem!!!

After bypassing the bias scaling circuit, I began to turn the pot to the power MOSFET expecting that I would see some change in my B+, but again I was seeing very little change. However, my plate current remained unchanged. At that moment I knew for certain the problem was in the B+ circuit.

Just to double check, I turned off the amp, made sure the caps were drained, and tested the resistance on the pot--it checked out. Working just fine.

In preparation to perform Ten Over's experiment to measure the voltage from the wiper to either side of the 100K gate stopper, I had a "holy crap" moment. Despite painstakingly drawing out the layout for my amp, by hand...to scale...correctly. As you can see from this zoomed in shot of the VVR section.
VVR section of the layout.png
When I actually wired up the pot, I had wired up the 47uf cap, drain, and WIPER in parallel. So my circuit looked like this:
How not to wire up VVR....png
Instead of this:
What it should look like....png
Seeing as how I was turning that straight B+ infused wiper closer and closer to that 100K to ground, no wonder things were acting like they were going to ground...the were! More or less. Good grief!
Now who's the fool.png
Once I got the pot wired up as it was supposed to be, with the bias scaling still bypassed, I was finally able to turn down the plate voltage; however, as Lou predicted my plate current had basically vanished by a 1/2 turn of the pot due to the decreasing screen grid voltage in conjunction with the plate voltage leading to idle tube response not readily observed from the charts (which appear to hold the screen voltage constant): something an electronics rube like me would have never thought of. Now feeling confident that the circuit should work properly (as it was once commercially available and used effectively in at least a couple different boutique amps that I know of) I reconnected my bias leads to the pot and low and behold, scaling the bias to become less negative in conjunction with the decreasing B+ led to a 1/2 turn plate voltage of ~ 1/2 max B+ and a plate current of ~ 1/2 max yielding the ~ 1/4 output.

So, now the amp is in it's Death Star II like state--doesn't exactly look finished but is fully armed and operational. And it is thanks to all of your help!

Now that it is at least working in a satisfactory manner, I'm gong to hold off on altering the location of the MOSFET in the bias circuit until I complete all the actual life tasks I've been putting off trying to figure this thing out. I may not get back to finish off and report on the rest of my experiments for a while, but I'll definitely pop back on when I have the chance and once again when I put the finishing touches (back & top control panel plates as well as cabinet face plate) on this thing and post some pics.

Take care and once again much appreciated!!!
Kyle
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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by frankdrebin »

You see? Problem is always in front of the Amp, it happens to me all the time too.

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Re: Mysterious 5F6-A Variable Voltage Regulator (VVR) Problem

Post by JazzGuitarGimp »

Good news! And good work, Mr. Fan :-)
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