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Old 15th March 2013, 12:20 AM   #1
myfi is offline myfi  United States
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Default Input Stage of the TSE

I have started my TSE build and wish to start a discussion on the operating conditions for the input stage of the TSE. Iím trying to understand the fundament theory behind the design. I am new to tubes and my electronics knowledge is very rusty to say the least.

As I understand it the Constant Current Source (CCS) does just that. It delivers a constant source of current which is set via the valve of R16. But should I add in any other resistance to ground i.e. R10? The CCS datasheet shows about 8ma out for an Rk of 330 ohms. This seems low as the 5842 datasheet lists an average plate current (Ia) at 25ma. R17 is to help stabilize the CCS. Iíd like to be able to draw a load line for the operating parameters of the 5842 tube. Which I think will be a horizontal line.

Drawing
TSE Input Stage0001.PDF

The 5842 datasheet lists an average plate voltage of 150V. On the diagram it says that the R9 Trim Pot is used to adjust the Plate voltage. How does that work?

Any help getting me going in the right direction is very much appreciated.

Steve
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Old 15th March 2013, 12:45 AM   #2
tomchr is offline tomchr  United States
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My suggestion: Get your paws on Morgan Jones, "Valve Amplifiers", 4th edition. Actually, any edition will probably do, but 4th is the latest. It'll teach you everything you need to know to understand a CCS loaded input stage.

~Tom
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Old 15th March 2013, 01:33 AM   #3
rknize is offline rknize  United States
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If you randomly click around enough, you'll find George's page on what he calls "power drive". It's the combination of a CCS loaded tube voltage gain stage driving a source-follower FET current gain stage. It is the primary unique aspect of the TSE design.

PowerDrive

He even wrote a cookbook on how to setup the circuit in your own amp design:

PDcookbook
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Old 15th March 2013, 01:40 AM   #4
myfi is offline myfi  United States
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Tom, thanks for the reply. I have purchased Morgan Jonesís 4th edition of ďValve AmplifiersĒ and IĎm wading through it. Starting out, I havenít found the book all that clear, at least for me. It will probably get better once I get my mind around some of the basic concepts of valves as they pertain to audio amplifiers. My electronics classes in college were 32 years ago, and I forgot most of that a long time ago.

Iím better at learning by example, especially if I I can hold it in my hand. Thatís why Iím building the TSE.

Do you have a feel for the Ia current value? and how it's set up?


Steve
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Old 15th March 2013, 02:08 AM   #5
myfi is offline myfi  United States
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Hi Russ,

I have poked and clicked around. A lot. Some people (think spouse) might think too much.

From the Tube Lab Website
"1) The ideal load for a triode is a constant current. The constant current source IC (10M45) provides this at DC. The 91 ohm resistor sets the current (30 mA) "


So 91 ohms gives me a 30ma current (Ia) which more or less matches the too small graph on the datasheet I have for the CCS. OK I get that. Now if I use the 332 ohms(R16 ) from the TSE schematic and the too small graph for the CCS, I come up with 8ma - 9ma. Is that right? Seems small.

Steve
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Old 15th March 2013, 02:52 AM   #6
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A triode works best with an infinite load resistance. The usual plate load resistor is a compromise and results in the triode producing about half the gain that it is capable of with increased distortion. A typical triode would produce more gain and very low distortion if the load was increased. The CCS circuit will allow a constant current to flow through it, yet exhibit a very high AC impedance. The 10M45 chip is about 1 megohm at all audio frequencies.

R16 sets the 10M45 current at about 8 mA. R17 keeps the 10M45 from oscillating in the RF region. We need a lot of drive voltage to push a 300B tube to full output, 80 to 100 volts peak to peak.

The data sheet value for the 5842 tube does indicate a typical tube current of 18 to 33mA, if the tube is used in its intended application, a wide bandwidth high frequency or IF amplifier. We want to use it for a high voltage gain audio amplifier that produces 100 volts P-P of output. This means we must figure out what conditions it needs for this application ourselves. I tested a box full of tubes and found that about 8 -10 ma worked the best for this application. We dont need, or want 10's of MHz of bandwidth.

Just simply stuffing 8 mA of current into the plate of a tube will not make an amplifier. The tube must have a bias voltage applied between the grid and cathode to set its operating conditions. We forced the tube current to be 10 mA with the CCS, but we must apply a bias voltage to set the plate voltage. Testing revealed that the best plate voltage for 100 volts P-P of output was about 175 volts. To get this we should need the grid to be about 5 volts more negative than the cathode. It is desirable to operate the grid at zero volts so that a capacitor is not needed in series with the input, so the cathode must be about 5 volts positive. We can do this by putting a resistor in series with the cathode.

I found no published curves for the 5842, so I tested my box full of tubes and found that they are all over the place. To allow for the wide range of bias voltages required for typical 5842 tubes, I added an adjustment pot in series with the cathode resistor. It can be adjusted for 175 volts on the 5842 plate, or tweaked for minimum distortion if measuring equipment is available.
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Old 15th March 2013, 03:31 AM   #7
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Hi George and thank you for the reply and sharing your wisdom. I love the methods you have used to derive your amps.
Iím a sucker for doing things out of the box. I starting looking into tubes about 2 months ago and my Edcors showed up today! But I canít just build it, no I have to understand it, completely.

I have a datasheet for a 5842 and as you must know doesnít even show a -5vdc grid voltage on the plate chart!

So your testing showed for the desired 100v p-p output that Ia of 8-10ma and a Vgc of -5vdc is needed.
The R16 sets the 10M45 to source 8-10ma (Ia). And R10 (adj with R9) with 8-10ma is approx 5.0vdc ( .010a * 500ohms = 5vdc)

Our tube is on and running were you want it.

Tomorrows post will be the FET stage. Stay tuned.

Thanks

Steve
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Old 15th March 2013, 03:52 AM   #8
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Default Thanks George!

Quote:
Originally Posted by tubelab.com View Post
.....or tweaked for minimum distortion if measuring equipment is available.
Well, it looks like I'll be hoisting both of my TSE's back on the bench, and firing up Rightmark and the sound card........I'll use my attenuator box thingy and proceed carefully to preserve the remaining functional sound card channel.

Should be interesting......
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Old 15th March 2013, 09:33 AM   #9
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Quote:
Originally Posted by boywonder View Post
Well, it looks like I'll be hoisting both of my TSE's back on the bench, and firing up Rightmark and the sound card........I'll use my attenuator box thingy and proceed carefully to preserve the remaining functional sound card channel.

Should be interesting......
Apologies to the OP as this is a bit off-topic.

I've been meaning to ask you about your TSEs Boywonder. My next project is a TSE of one flavor another. I have the board already and will be bringing some James transformers back to the US with me when I leave Korea this summer. The plan was originally to build a 300B, but as I'm also planning a large house-shaking PP octal amp after that (how's that Universal Driver board coming George ), I've been toying with the idea of a 45 TSE and some smaller efficient speakers. Anything you can share on the merits of the two? (If I remember you have a 300B and a 45?)
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Old 15th March 2013, 12:20 PM   #10
myfi is offline myfi  United States
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Looking over what has been posted so far I still have a few questions.

Iíd like to know the Amplification (Av) of the input stage.

Amplification (Av) = ∆Va/∆Vg

Quote:
We need a lot of drive voltage to push a 300B tube to full output, 80 to 100 volts peak to peak.
Do we need the 80 - 100v PP after the input stage (Va) or after the ďcompleteĒ power drive stage i.e. at the source of the MOSFET and the grid of the Output tube?

Anybody know what a typical signal voltage peek to peek that is riding on Vg is?

Thanks Steve
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