What am I Seeing Here?

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WntrMute2
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Re: What am I Seeing Here?

Post by WntrMute2 »

Two volts seems possible without distortion. Works fine in most situations but I would like more "headroom" for certain situations.

There are two tubes in each channel. A #76 as the first gain stage and a 2C22 as the second.

Both tubes have a leak resistor the # 76 has a 1M and the 2C22 has a 249K.

What is weird is there is less gain at the output tube than at the output of the first stage.

Thanks for your help!
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Tables_and_Tubes
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Re: What am I Seeing Here?

Post by Tables_and_Tubes »

OK, I have taken a look at the 2C22 tube specifications and characteristic curves graphs, and I found your schematic for the preamplifier over at Hi-Fi Haven. Forty eight pages! Yikes, not even going to dig into that thread. My interest has been peaked though, these look like nice, linear triodes. Looking at the specs for the 76 tubes, they should be (and measure) happy, but the 2C22 is another matter.

Your schematic shows the B+ at 210V and that must drop quite a bit across the plate resistor. So, what is that leaving you, 100-120V? Reading the 2C22 specs. and looking at the graphs, it seems to me that tube is running really "cold". What is you plate current? Center values for the tube in A1 amplifier configuration are plate voltage of 300V and plate current of 11ma. I have a feeling if you took some readings at different operating points and plotted them of the graph of characteristic curves for plate current vs plate voltage, you will find your load line squeezed into one small corner of the graph. Not much room for the tube to "breathe" as it were.

So, you can try increasing the value of the grid leak resistor to the 2C22s. Try just tacking on another 250K in series to double it to 500K. If there is no improvement, or, if the cut-off distortion gets even worse, I think you are looking at getting more B+ to that tube. If they want 300V on the plate, you may need 400V B+ so you can get a high enough value plate resistor in there for good voltage drop and high enough voltage swing on the output.

Kind of a shame if you need to do that as those voltage regulator tubes sure are pretty!

Dan
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WntrMute2
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Re: What am I Seeing Here?

Post by WntrMute2 »

Thanks for all that!
I could try the OD3, that would give me regulated 300V I believe.
This is going to sound really stupid but why do I need a plate resistor at all? If I have 214V B+, can't I just eliminate the resistor? Obviously, I need better understanding of tube circuits!
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WntrMute2
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Re: What am I Seeing Here?

Post by WntrMute2 »

The OD3s give me 295 B+ and about 150V on the plate. Will scope the waveforms a bit later. Someone recommended chokes instead of plate resistors. Hammond 156C was the recommendation.
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Tables_and_Tubes
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Re: What am I Seeing Here?

Post by Tables_and_Tubes »

Changing the B+ is going to require you to redesign the circuit around the tube as your bias and plate dissipation will change. You will also need to change part of the circuit (plate resistor to start) on the 76 tube.

To answer your question in a very basic manner (keep in mind, I am still learning this tube stuff myself), you need the plate resistor for multiple reasons, it helps set your plate voltage, helps set the plate dissipation, but, most importantly, it forms a voltage divider circuit along with the dynamic plate resistance and that is what generates your amplified output voltage. Remember, tubes are not voltage amplifying devices. It really helps, when looking at a tubed circuit, to use the English term: Valve. Those triodes vary the current flow through them by the input voltage acting upon the control grid. Tiny little change in input voltage, the grid controls a bigger current flow change, change in current flow through the plate resistor (and the plate, hence "dynamic plate resistance"), causing a bigger voltage drop across the plate resistor, and "magic" amplified output voltage. Then your output cap "blocks" the DC and you are left with your, amplified, audio/AC output.

That is a very general, super simplified explanation and the tube gurus, I'm sure, could shoot it full of holes. The thing with tube circuits, is everything is interrelated. Change one thing, everything changes. You end up chasing your tail! That is why, most of the experienced designers know about where to start, breadboard the circuit up, and then start testing and listening while changing and fine tuning.

Me, I will stick to speakers and crossovers. That stuff I like to design!

Dan
Jeffrey B
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Re: What am I Seeing Here?

Post by Jeffrey B »

What is a typical input impedence of a solid state amplifier and is it typically different from a tube amplifier?
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Tables_and_Tubes
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Re: What am I Seeing Here?

Post by Tables_and_Tubes »

Speaking in generalities is always dangerous, but the "typical" input impedance of many SS amplifiers will run in the range of 22-47K ohms. 50K ohms would be a fairly low input impedance for a tube amplifier. 100-250K ohms would be the expected range on most modern tube amplifiers.

Of course, there are always outliers. Some older tube amplifiers had very high input impedance values of 500K or more. I think part of the trend away from the high input impedances is, in part, due to the extended bandwidth required in newer tube amplifiers. A very high input impedance interacts with the Miller capacitance of the input tube causing a low-pass filter that cuts high frequency response. Not so much of an issue when your 1920's-1940's amplifier only needs to have a "flat" response to 8-12K Hz! I think this is one reason why input pentodes became popular in tube amps. Their Miller capacitance is less of an issue than with the typical triode input tube.

Some solid state amplifiers can have very low input impedance, especially pro. oriented gear. 10K ohms is not at all unusual.

So, when using a tube preamplifier, one needs to be careful in load matching with the power amplifier. Especially if your circuit is designed around an older (pre. 1950's) circuit. Some of those configurations had very high output impedance. Not an issue if the next stage has an input impedance of 250K ohms or more, but don't expect to drive a solid state amplifier with 10-47K ohms input properly. This is why, in the late 1970's-1980's the ubiquitous cathode follower started showing up in nearly all commercial tube preamplifiers. The popular setup of the time, was to drive a solid state power amplifier with a tube preamplifier (my current configuration). Now, what the ever popular cathode follower does to the sound/imaging of said preamplifier, I will let you all argue that amongst yourselves!

Dan
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