5W Class A2 Concept

andrewRneumann

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Ok @Jerry garrcia @printer2 and anyone else looking for a good time. Here's my concept for your bucket of 6**7 tubes.
It only uses 3 tubes and puts out about 5W. Maybe a simple volume and tone control ala Princeton 5F2A.

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What's interesting about this design?
- The power tubes idle at Vgk = 0. There is no cathode bias resistor or cap or bias supply.
- Parallel cathode followers as drivers. Direct coupled for now, but maybe will AC couple them if they can add into the overdrive mix. It will depend where the bias point falls on these.
- It's simple! Look at drivers and power tubes--only 2 resistors and 1 capacitor in this section.

Seems like a lot of valves for only 5W. It is, but it's only 3 tubes if a solid state rectifier is used. Could be built in a Princeton chassis I assume.
 

andrewRneumann

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Fleshing it out a little bit. This is just a skeleton for now. Feel free to point out any errors.
Updates:
- Realized the 6SC7 has a single cathode for both valves. Looks like fully bypassed for both stages becomes required. Also this seems to remove the possibility of NFB. It won't be missed. Even less parts now!
- Put together a power supply scheme. Power tubes and drivers will run off of the same node. Because the drivers have to source a lot of current in Class 2, they need a low impedance source of plate voltage. Thus they are on the power tube node. Choke always seems advisable for single-ended.
- Pre-amp power supply doesn't come out of B+1. Because B+1 is not drawing a constant current, and we don't want the B+2 going up and down as B+1 does, it gets its current from the reservoir cap with two stages of smoothing.

This is all very preliminary. I haven't drawn any load lines except for the 6N7 which will idle at Vak = 300V and Vgk = 0V for a both section total of 34mA. I could, and probably will, run into a road block at any time.

This amp is Class A, but it won't draw constant current like the typical Class A, so it requires a little more thought for the power supply. There may be some opportunity here to introduce tasty typical Class AB sag into a class A amp... will have to explore this more.

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printer2

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C2 makes it capacitor coupled, it will create blocking distortion once the signal goes above 0V. You can only use direct coupled. Losing the capacitor you need the cathode of the 6SN7 to be at 0V. Have to run so I can not give much more in the way of suggestions. Mosfets would work here but the power supply will be more complicated.

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andrewRneumann

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C2 makes it capacitor coupled, it will create blocking distortion once the signal goes above 0V. You can only use direct coupled.

I've got to think about this. My intuition tells me you're incorrect, but I can't explain it in an simple way. I think you would be right if this were driven by a high impedance source, but since grid clipping doesn't occur at Vgk = 0, I'm not sure it creates blocking distortion. The grid sucks down a lot of current, the cathode follower supplies the current, the capacitor is the middle man. As long as the impedance is low on both sides of the circuit, it seems to me that capacitor won't change its average charge. ??? I will investigate more. Thanks for raising the issue.

I'm also concerned about the maximum voltage on the 6N7. The data sheets are hard to interpret. If I idle the plates at 290V, but the voltages swings up to 520V peak, am I going to arc the tubes?
 

andrewRneumann

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@printer2 I'm starting to agree with you. What then is the effect of driving it with a CF? I know that with a low impedance source, the grid can be driven positive. I know this is possible, but why does the coupling cap make it impossible?
 

printer2

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@printer2 I'm starting to agree with you. What then is the effect of driving it with a CF? I know that with a low impedance source, the grid can be driven positive. I know this is possible, but why does the coupling cap make it impossible?
I wanted to find a good visual way of explaining blocking distortion, got mired around in a number of web pages, got sidetracked also (which I will look at later). Then I thought someone might have made a video, and I found one! But I watched it, and it is about a circuit that mitigates blocking distortion, which is interesting in its own right. But it reminded me of my college days and I had to fight to not fall asleep. Watched the whole thing, be warned if you do. Then I found someone developing a hifi amp from what it looks like. And (I want his test gear) while not explaining the voltage generated that causes blocking distortion (the previous one does) it shows what blocking distortion does with different levels. Not an output stage (the first video is) but I hope you can envision what happens in the output stage. The third video (also about blocking distortion) I have started it at an interesting time in terms of noise. Wasted enough time tonight on it, might find a good explanation of the blocking distortion yet.

While looking at web pages on Class AB2 amps I thought, might be a good time to use a transformer to drive the outputs rather than needing a complicated PS. I am going to think if there is a reasonably cheap transformer to do it with.

See, I'm feeling sleepy. Almost forgot the links.







And capacitor distortion.

 

printer2

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I need to go through the thread more but a quick snippet,

"Beyond the heating, the positive grid also attracts ions, typically residual gas which emerged from the metal structures, mica, and cathode coating, either because of natural diffusion or being baked out of oxides. That makes the grid even more attractive. Most tubes are not designed for positive grids (see my above comments about transmitting tubes and heat sinks), and the tube's characteristics become non-linear.

So a positive grid and current both alter the grid's behavior and cause distortion. This is why one of the rules of HiFi is: Thou shalt not drive thine grids positive, lest the Tube Gods destroy thine clean output signal and visit upon thee the plague of distortion and also potentially destroy thine tubes before their normally appointed hours, for the Tube Gods are mighty and fickle, and may do that to spite thee for not honoring the Holy Dictates of the Datasheet, hallowed be its name, and, by the way, thou shalt also honor the Holy Dictates of Screen Dissipation, also in the Datasheet, just in case thou felt like blaspheming there as well, thou hast been warned.

So that's a different issue than the coupling capacitor charging.

When the coupling capacitor charges it temporarily maintains the grid at a constant DC level until that charge dissipates. That prevents the AC signal from the previous stage from controlling the plate. Instead we have the plate at a fixed DC voltage until that capacitor discharges. The only way that can happen is back to the previous stage during the AC reversal (it is at the plate voltage so this is unlikely) and through the grid leak resistor (primary means). That's why the blocking distortion has a bias excursion time controlled by the RC constant. Bigger capacitors extend longer, bigger resistors slow the discharge rate.

So we have both (a) positive grid and (b) DC bias with an RC decay time.

While a larger coupling capacitor better passes lower frequencies and compresses them less, when the coupling capacitor is too large (this depends on both the capacitor and the grid leak resistor) the bass response suffers, becoming flabby and muddy. A smaller capacitor better passes the highs at the expense of bass, which is less compressed but is attenuated. So that's why the first response to increase capacitance (from, say, 0.22 µF to 0.47 µF) can cause significant bass issues if the grid leak resistor is not accordingly adjusted. This is the reason why we don't see 100 µF capacitors being used for interstage coupling, and why it is common to see 0.1 µF and rare to see anything larger than 0.22 µF or 0.47 µF. Hence the values I chose for simulation."

 

andrewRneumann

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I appreciate it. I have fooled around with designing amps long enough that I have a good grasp of what blocking distortion is and what it sounds like. I even have a home brewed Class AB2 guitar amp in my shop at this moment. It works great--but it is direct coupled (and I did have to use one MOSFET in the power supply). I should have remembered that capacitor coupling isn't going to work. For some reason I was under the misconception that as long as the source impedance was low enough, the signal wouldn't clip at Vgk=0 and there wouldn't be any bias shift. I thought, wrongly, that the bias shift was caused by the clipping alone.

I fooled around on Falstad trying to understand how a cathode follower driving a capacitively coupled stage into grid current would affect things. My current hypothesis is that the cathode follower actually drives the bias shift even farther negative! So negative, that the positive side clipping disappears (but may cause cut-off clipping on the negative side). So the CF does reduce grid current clipping, but the trade-off is a huge negative bias shift. A capacitor anywhere in the grid circuit is just not going to work for Class 2.

So back to the drawing board. I am always on the look out for a design that doesn't require solid state solutions. A coupling transformer may be the only option if transistors are off the table. I am also going to investigate a balanced power supply with say +170V and -170V available.
 

printer2

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I appreciate it. I have fooled around with designing amps long enough that I have a good grasp of what blocking distortion is and what it sounds like. I even have a home brewed Class AB2 guitar amp in my shop at this moment. It works great--but it is direct coupled (and I did have to use one MOSFET in the power supply). I should have remembered that capacitor coupling isn't going to work. For some reason I was under the misconception that as long as the source impedance was low enough, the signal wouldn't clip at Vgk=0 and there wouldn't be any bias shift. I thought, wrongly, that the bias shift was caused by the clipping alone.

I fooled around on Falstad trying to understand how a cathode follower driving a capacitively coupled stage into grid current would affect things. My current hypothesis is that the cathode follower actually drives the bias shift even farther negative! So negative, that the positive side clipping disappears (but may cause cut-off clipping on the negative side). So the CF does reduce grid current clipping, but the trade-off is a huge negative bias shift. A capacitor anywhere in the grid circuit is just not going to work for Class 2.

So back to the drawing board. I am always on the look out for a design that doesn't require solid state solutions. A coupling transformer may be the only option if transistors are off the table. I am also going to investigate a balanced power supply with say +170V and -170V available.
I am thinking a line transformer as long as it has two output taps, say 8 and 16 ohms. Found I have one, wonder how it will like dc current through the primary? Could go P-P into it but might even push a watt or two through it and not much of a gain getting 5W out when you could get 2W in the first place. I have some 25V line transformers somewhere, think they might only have one secondary impedance though.

Found them. Seems the cut of the middle secondary tap, will have to splice a wire to it. The downside is that it has a 600 to 8 or 4 ohm impedance. Mind you it is not like it will be loaded with that impedance. How much dc you can put through the core? Could put an air gap into it but that would reduce the inductance also. No time for fun and games though.

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2L man

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6N7 on parallel single ended power amp needs about 25Vpp grid drive (-5V...+20V, bias about +7V) for full power. Hammond 1760C has also 5k primary and 16 ohm output so winding ratio comes about 18:1. Primary sweep is 18x25Vpp=450Vpp which easily comes from 300VDC B+1 power supply. SE OT should not mind at all if there flow bias current on secondary. Not creap option and definitely not smart because it is good for 5W but 6N7 is interesting tube :)

Is there a need to use load resistor on SE OT output which is used as interstage transformer? Perhaps if 6V6 or EL84 is used but if weaker tube is used to power the SE OT obviously the need for load is lower?

Obviously about +7V bias supply needs to deliver some current? Rectifying and filtering filament and using adjustable voltage regulator is easiest but 6,3VAC after bridge rectifier isn't high enough for regulator (which waste about 2V) for 7VDC. 12,6VAC would be!
 
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andrewRneumann

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6N7 is interesting tube :)

I have become obsessed with figuring out how to use it.

Obviously about +7V bias supply needs to deliver some current? Rectifying and filtering filament and using adjustable voltage regulator is easiest but 6,3VAC after bridge rectifier isn't high enough for regulator (which waste about 2V) for 7VDC. 12,6VAC would be!

This might just be genius. A full wave rectifier at the end of the heater chain to generate about 7.5VDC might be perfect for biasing the 6N7. I haven't figured out how to do it yet though.

I am looking at a bipolar supply that gives +200V and -200V. I incorporated something like this on my Class AB2 Push-Pull using the bias winding. It's a balancing act, but it can be done. I probably definitely need to adjust some values. And there is the problem of only +200V for the preamp.

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NTC

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A little history (See attachments). Part 3 shows the curves - and the prescribed method of interstage coupling for a 6N7 - which is just a type 53 with a different base and housing. The interstage transformer has a ratio of 5 from the primary to each half of the secondary, or 2.5:1 with a center tap. I suppose the intrepid sorts could wind their own. This is very similar to a tube I have one of and would like to try, a 19. These types are from an earlier age when winding transformers must have been cheaper than having additional tubes or coupling capacitors. They REALLY want that interstage transformer. But if you can get that follower drive working, THAT would be something.

EDIT: The datasheets are from RCA RC-13. The RC-14 version of the data sheet is not as detailed - these had already fallen out of favor, I think.
 

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2L man

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Looks good Andy!

I like this 6N7 thread because I have few GT and building amp using them has haunted me too :) So I wrore my ponderings here if there are ideas to breed for others:

For a push pull PI interstage transformer a PP OT, which has 4 and 16 ohm outputs, produce symmetrical 180 decree opposite drives when bias is fed to 4 ohm wire and drive signals are took from 0 and 16 ohm wires. This is because of winding ratio from 4 ohm to 16 ohm doubles.

However getting high enough ~25Vpp output is not easy because cheaper, smaller low power OTs are often high winding ratio/impedance transformers. So if you know such OT bring it here!

Printer idea to use line transformer has potential because theit winding ratios are lower. There are many standards and 100V seems most common. Obviously transformer which has 30V and 120V inputs it can be used as Push Pull beeding B+1 to 30V and driving anodes to 0V and 120V?

Also if primary has 4 and 16 ohm inputs feeding B+1 to 4 ohm and connecting power tube anodes to 0 and 16 ohm inputs. This idea I recall Printer recently wrote somewhere?

Using line transformer as Push Pull Interstage transformer should work without saturating core when bias current flow thru secondary which wound count is low?
 

andrewRneumann

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Here's tonight's update.
Revisions:
-Spec'd out PT, OT, and Choke from Hammond
-Higher voltage PT moves bias on power tube a little cold of center. Expecting a little sag as Class 2 current increases during drive, so may be center after all.
-Changed power supply to give a little more voltage to the preamp. Up to 236V now. I think it will work.
-Simulated power supply in LTSpice to get more accurate voltages.
-Annotated voltages and currents.
-Added Princeton tone control.
-Added some grid stoppers.
-Investigated choke. Using a fairly high value 14H check. It needs a large capacitance to damp things out.
-Added current monitoring 1Ω resistors to cathodes of 6N7. We need to know if this thing is working as designed!
-Changed CF bias resistor to a pot. This controls the bias of the CF and the 6N7.

Still to be done:
-Signal analysis through entire chain. Determine what kind of drive is actually to be expected. (Should have done this earlier... but my intuition tells me there's enough drive for the 6N7's.)
-Size coupling caps.
-Size grid stoppers.
-Verify resistors are correct wattage.

As usual, comments welcome and appreciated. Anyone interested in doing a layout?

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andrewRneumann

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Spent some time with the numbers... filled in values for all the caps and stoppers. Examined the gain structure. Should have no trouble overdriving the power tubes if my math is right.

@Jerry garrcia there's only one way to find out if this works. But I'm pretty sure it will work. ;) Have to stay positive!

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Jerry garrcia

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Spent some time with the numbers... filled in values for all the caps and stoppers. Examined the gain structure. Should have no trouble overdriving the power tubes if my math is right.

@Jerry garrcia there's only one way to find out if this works. But I'm pretty sure it will work. ;) Have to stay positive!

View attachment 1010753
Thanks for ruining my marriage and work career 😀. Will have to read up on this. Been carried away with my own simple design.
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Well I have to build it to be certain I guess…
A lot of different components and the layout will probably ruin the last two weeks of my vacation. What could one expect from such an amp?

Edit: @andrewRneumann I didn’t realised that you partly did this for me to use my 6N7 tubes! That’s why I didn’t respond previously. I thought it was a rampage to solve a theoretical problem. My lack of knowledge in tube circuit design makes this a true challenge to understand your design. Like R12, 4.7K MF (Metal film?) and U2 5KL (5K L pot?). You see, a true noob.
It’s totally different from other circuits I’ve seen.
Found this page on Class 2A https://tubecad.com/2018/01/blog0410.htm Will have to read more.
DC heating?

Will have to read more
 
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