Valve (tube) amplifier schematic help

[CDRIVE]

Kris, if that supply gives him noise issues I gave him this earlier. It is probably a better choice than the switch mode. Yes, it's going to supply a B+ of about 17V but I don't think that will greatly change the characteristics he looking for. I would have used a 6V transformer or 12VCT but I thought solidus had a 12V 450mA transformer already. A 6V transformer would give him ~ 8.5VDC B+.

https://www.electronicspoint.com/at...ve-tube-amplifier-schematic-help-12au7psu.jpg

It has far plenty of ripple filtering. With the very low current demands on the B+ there should be only a mV or two of ripple.

Chris

 

[CDRIVE]

solidis, I forgot to mention that a good brad point drill works well on acrylic. As always the work should be backed with a flush waste block.

Chris

 

[KrisBlueNZ]

Kris, if that supply gives him noise issues I gave him this earlier. It is probably a better choice than the switch mode.

Yes, I saw that earlier. That's what I would recommend as well.

Solidus, please have a look at CDRIVE's schematic. This shows everything I was trying to describe. He's even included a pi filter with a 470 ohm resistor. This should give you a very good result - almost NO ripple on the B+ rail.

It runs the heaters at 12.6V instead of 9V, and the B+ rail is about 17V instead of 9V, so it won't be exactly the same as the original. I would expect it would be better, if anything, but don't take it from me; I'm just a heathen who has no interest in "the valve sound'

 

[Solidus]

Okay, I'll give it a shot at constructing it next month. And by that, I mean in 4 days or so - project budget is pulled from college allowance.

And yes, I had it backed by scrap craftwood during drilling. I found that after making the cut making 2-3 dry passes under rotation on the press helped to do 95% of the deburring for me.

 

[Solidus]

I figured in the interest of being flexible I'll wire jacks on the power supply and on the filament / plate lines on the valve circuit.

That way, I can test it with the switching supply and if it generates bad results, I can move up to the supply Chris drafted for me.

I will eventually build and incorporate that supply anyway, as the higher-gain series (12AT7, 12AX7) valves will demand more power off the line. It's probably better to make the device capable to run higher power and use low-drain valves than to design it to run limited and only be able to use certain valves.

 

[CDRIVE]

I figured in the interest of being flexible I'll wire jacks on the power supply and on the filament / plate lines on the valve circuit.

That way, I can test it with the switching supply and if it generates bad results, I can move up to the supply Chris drafted for me.

Sounds logical to me. You have nothing to loose. Heck, you may find it works fine with no perceptible noise.

Chris

 

[Solidus]

Sounds logical to me. You have nothing to loose. Heck, you may find it works fine with no perceptible noise.

Chris

That's true.

I'm going to email the schematic author and ask him what the intended supply is. While I have seen this design billed as able to run off a 9V battery, it would murder that pretty quickly.

I have a hunch it may be intended to run off a wallwart, because to the best of my knowledge I haven't seen a separate schematic for a supply build.

 

[CDRIVE]

I don't believe it was ever intended to be powered off a 9V battery. They're feeble!

Regarding the 12AX7 being used in this circuit... My simulations indicate that it would plug right in with no change in components. It produced a voltage gain 10 times higher than the 12AU7. This doesn't make it preferable to the 12AU7 in this application though.

 

[Solidus]

I don't believe it was ever intended to be powered off a 9V battery. They're feeble!

That's what I think.

When I first happened across this circuit, a bunch of guys took the approach of "you could if you wanted to" but I doubt the battery would last more than minutes at the demand the tube would put on it.

Regarding the 12AX7 being used in this circuit... My simulations indicate that it would plug right in with no change in components. It produced a voltage gain 10 times higher than the 12AU7. This doesn't make it preferable to the 12AU7 in this application though.

I would use the AX variety if I wanted a more overdriven, stronger distortion effect than the AU would provide. Now, I may assemble this and deem that the 12AU7 is perfectly fine for my needs and never need the increased power of the 12AX7.

As per the universal saying that you can't make something out of nothing, justifiably a valve making 10 times the gain is drawing more power off the line. I wouldn't be running the AX on the 9V supply; I'd make a 12V supply for that.

 

[CDRIVE]

As per the universal saying that you can't make something out of nothing, justifiably a valve making 10 times the gain is drawing more power off the line. I wouldn't be running the AX on the 9V supply; I'd make a 12V supply for that.

Higher voltage "Gain" does not translate to higher power output or PSU current drain. The only thing that draws significant current (Power = E * I) in any of these tubes is the filaments. A little 9V battery could power the plate circuits for months.

 

[KrisBlueNZ]

I don't believe it was ever intended to be powered off a 9V battery. They're feeble!

Under 2 hours with an Alkaline, or around 6 hours with a Lithium battery. I thought it would be even less than that, but I think most people would say even that's impractical.

Higher voltage "Gain" does not translate to higher power output or PSU current drain. The only thing that draws significant current (Power = E * I) in any of these tubes is the filaments.

Exactly.

 

[Solidus]

Well, where does the extra gain come from, without delving too much into the rabbit-hole that is valve mechanics?

Because obviously the gain can't come from nowhere.
If it draws about the same as the 12AU7, there must be some parameter increased that allows the 12AX7.

The only way it could have identical energy parameters and have a higher gain is if it was more efficient, and I doubt that the AU is 10 times more inefficient than the AX.

I believe you, but I'm just wondering what is different about the circuitry that allows an increased gain.

 

[KrisBlueNZ]

It's true that if device X produces ten times as much energy output as device Y, for the same energy input, that device X is ten times as efficient as device Y, and it's true that energy can't come from nowhere. But in amplifiers, gain does not correspond to efficiency.

Gain is the RATIO OF AMPLITUDE of the AC signals at the output and input of an amplifier. It depends on the characteristics of the amplifying element and other components in the circuit.

In the amplifier you're talking about, the actual amount of ENERGY in the signal (both the input and the output signal) would be measured in microwatts. The filament alone consumes about 2 watts, and that's just to get the valve to "get out of bed in the morning", if I may use the supermodel analogy. The current that flows in the anode (plate) resistor is what allows the valve to actually amplify the signal, and this is measured in milliwatts.

So in terms of energy, the signal is minuscule at the input, and not much bigger at the output; the amplification itself requires a small amount, and the valve filaments require quite a lot. This might seem counterintuitive, but I'm afraid it's the truth.

 

[CDRIVE]

.

Gain is the RATIO OF AMPLITUDE of the AC signals at the output and input of an amplifier. It depends on the characteristics of the amplifying element and other components in the circuit.

.

Solidus, got that? It's not related to efficiency. Power efficiency is the ratio of DC input power to the pate vs audio (signal) output power picked off at the plate. The heater is not relevant to this parameter.

Amateur radio operators should or did know this well. This is because the FCC grants them 1KW Input power to the RF final. This is a DC measurement of plate supply voltage * Plate supply current. An RF Wattmeter connected to the antenna jack MUST read lower than 1KW or we would be entering the wacky world of the Over Unity screw balls.

Chris

 

[Solidus]

Oh okay.

The miniscule amount of input power means that even though the gain factor is large, the output value may not be.

That's why there isn't a noticeable change in valve characteristics on the datasheets even though there may be a 10x gain increase.

Did I get it right?

 

[KrisBlueNZ]

The miniscule amount of input power means that even though the gain factor is large, the output value may not be.

I wouldn't put it like that.

BTW CDRIVE's comment, while true, relates to power amplifiers, where the output power is an important factor. You are working with a small-signal voltage amplifier and the gain you're talking about is voltage gain; the amount of power in the signal (at both the input and the output) is very small and is not directly of interest.

That's why there isn't a noticeable change in valve characteristics on the datasheets even though there may be a 10x gain increase.

The difference relates to the "mutual transconductance" of the valve. It's there if you look for it. But most of the characteristics relate to setting the conditions for the valve to be able to amplify in the first place. That's the one that tells you how much gain it will have in a particular circuit.

 

[CDRIVE]

I wouldn't put it like that.

BTW CDRIVE's comment, while true, relates to power amplifiers, where the output power is an important factor. You are working with a small-signal voltage amplifier and the gain you're talking about is voltage gain; the amount of power in the signal (at both the input and the output) is very small and is not directly of interest.

Kris, initially my reaction to this was "Exactly!". In fact I drifted off to another thread but the more I thought about it the more that little voice kept saying "Well, not exactly". While it may seem true that the only time DC input power Vs Signal output power plays a role is in power amplifiers, that explanation is too simplistic and not entirely true. Consider the following...

In their heyday vacuum tubes were found in portable radios. They were equipped with an "A" battery for the heaters and a "B" battery for the B+. DC input power to voltage amplifiers was certainly important to the designer, as it would be to any battery operated device. Actually it's a valid and important spec in any design because it usually specifies the no signal quiescent power consumption of a device.

I know what your thinking.... Sounds like I'm nitpicking. ... This my plan to combat old timer's disease!

Just for the hell of it I separated the input amp but kept the grid resistor in this spice. Power dissipation is minuscule but it's fun to see the input power vs the output power . DC Input power and Signal output power were measured with and without signal input.

Chris

 

[KrisBlueNZ]

I'm talking about the power levels of the input and output SIGNALS, which are tiny in a small-signal voltage amplifier. You're using "input power" to mean the DC power consumption of the amplifier, from its B+ rail, which is a different thing. I agree it is relevant.

In post #93 I listed the three power figures that relate to the valve-based voltage amplifier:

- Input and output signal power levels - microwatts
- Anode/plate resistor power - milliwatts (this is the DC power consumption you're referring to)
- Filament power - watts.

 

[CDRIVE]

I'm talking about the power levels of the input and output SIGNALS, which are tiny in a small-signal voltage amplifier. You're using "input power" to mean the DC power consumption of the amplifier, from its B+ rail, which is a different thing. I agree it is relevant.

Yes, my post was exemplary of anal retentive discourse.

Chris

 

[Solidus]

Will one of you put that into the n00b terms for me?

Conductance, if I remember the basics correctly, is the reciprocal of resistance, correct? (Conductance is how readily a current will flow, whereas resistance is how much the medium impedes the flow?)

I just finished hardwiring the circuit board, will solder and make bona fide the connections tomorrow.

Last pot is wired, waiting to take down the length of the wiper shaft before mounting however.

As soon as the wallwart and valve socket arrive I'll wire those and theoretically be done aside from purchasing the 50W amp circuit itself and wiring the speakers.

Will post photos tomorrow when it isn't 2AM my time.