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417a load line

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The CCS sets your plate current, the led sets your effective bias - there is nothing further to do. I'd use a 2V led as I am not sure you will not be driving the 417A into grid current for the required plate swing required by the 45 to achieve full output. (about 50Vpk) I did something similar running at about 12mA, works quite well and very linear.
 
I'm not sure you fully understand the requirements to drive the 45 with good linearity (and some headroom). You seem to be focusing on biasing the 417a as hot as you can and get agreement on it. Also, you started the thread by saying a friend is giving you a chance to rebuild his amp. So, what's wrong with it? How does it sound now? Are all of the tubes in good condition? Did he mention any specific changes and/or improvements he's looking for? Or is this just a chance to experiment and learn with no real expectations from the end result?

Regardless... here's a suggestion. If you want to get the driver stage right, start with what it has to drive, in this case, the 45 grid. First ensure the output stage is properly setup and working correctly. Once this is sorted, measure your voltages (assuming the 360V supply stated); grid bias voltage (cathode resistor drop) will likely be around 60 volts and should have a cathode current of around 34ma (again, 1.74K works very well with the higher voltage shown).

Next, understand the voltage swing requirements for the 45 (twice the 60v bias = 120V minimum). I still recommend having a 150V pk-to-pk output swing available (a bit of headroom and better linearity). The CCS will basically give you near mu factor voltage gain, which is about 43. Doing the math (ideal scenario) is 150/43=3.48. Divide this by 2 and you get the theoretical minimum grid bias for the 417a of 1.79 volts. I'd still suggest about 2.25 volts of bias on the 417a and no more than 10ma. There's not much of a point going higher, you just run the tube harder.

Also, you need to have enough voltage across the tube (cathode to plate) and across the CCS to allow a linear swing of 150 volts. Consider a 300 volt supply source and split it evenly between the 417a and the CCS. Looking at the plate curves, you can get a bias point with -2.2 volts of grid bias at 10ma with 150 volts on the plate. This would probably make a good starting point and then you can do some measurements and testing to verify. I'd also suggest using a cathode resistor to get the biasing right.

Regards, KM
 
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The 45 should not be operated with more than 300V on the plate, and IME sounds best at 250V and about 38mA - 40mA, under these specific conditions (measured) the 45 will need ~45Vpk to achieve full output. With a Vp of 300V it will take about 60Vpk to drive the 45 to full output at which point the 417A will become quite marginal unless an IT is used. (1:1.5) You do want some headroom of course, bearing in mind that once you hit 0 bias the grid current will increase rapidly and cause the driver to clip.
 
Well, his initial post with existing schematic shows a plate supply at 362 volts. Between the 1.5K cathode bias resistor showing 56 volts and the drop across the OPT primary at 8 volts, effective cathode/plate voltage is at 298 volts. I have a later updated 45 data sheet which shows 300 volts as maximum plate voltage and 10 watts as maximum dissipation, so he's maxed, hence my recommendation for a slightly higher cathode resistor value.

My current 45 SET amps have been in regular operation since end of 2006. I've had a few sets of tubes thru them over the first 6 months or so, but settled on NOS Sylvania ST with the heavier upper micas and riveted supports. With cathode bias, I run around 34ma and 300 volts on the plate, right at 10 watts dissipation and a 5K load. I've tried higher currents and a bit less voltage, but for me, this is what became more optimal both in measurements using a large sample base and listening as well. But I think you run fixed bias with your 45s... so perhaps that accounts for the difference.

Regards, KM
 
I'm not sure you fully understand the requirements to drive the 45 with good linearity (and some headroom). You seem to be focusing on biasing the 417a as hot as you can and get agreement on it. Also, you started the thread by saying a friend is giving you a chance to rebuild his amp. So, what's wrong with it? How does it sound now? Are all of the tubes in good condition? Did he mention any specific changes and/or improvements he's looking for? Or is this just a chance to experiment and learn with no real expectations from the end result?

Regardless... here's a suggestion. If you want to get the driver stage right, start with what it has to drive, in this case, the 45 grid. First ensure the output stage is properly setup and working correctly. Once this is sorted, measure your voltages (assuming the 360V supply stated); grid bias voltage (cathode resistor drop) will likely be around 60 volts and should have a cathode current of around 34ma (again, 1.74K works very well with the higher voltage shown).

Next, understand the voltage swing requirements for the 45 (twice the 60v bias = 120V minimum). I still recommend having a 150V pk-to-pk output swing available (a bit of headroom and better linearity). The CCS will basically give you near mu factor voltage gain, which is about 43. Doing the math (ideal scenario) is 150/43=3.48. Divide this by 2 and you get the theoretical minimum grid bias for the 417a of 1.79 volts. I'd still suggest about 2.25 volts of bias on the 417a and no more than 10ma. There's not much of a point going higher, you just run the tube harder.

Also, you need to have enough voltage across the tube (cathode to plate) and across the CCS to allow a linear swing of 150 volts. Consider a 300 volt supply source and split it evenly between the 417a and the CCS. Looking at the plate curves, you can get a bias point with -2.2 volts of grid bias at 10ma with 150 volts on the plate. This would probably make a good starting point and then you can do some measurements and testing to verify. I'd also suggest using a cathode resistor to get the biasing right.

Regards, KM


The amp is running fine, the weak link are the power supply and I was just going to rebuild it with new PTX and caps but then I though I should take the chance to see if I can learn about the circuit. After doing some measurement I though I would like to also change the operation points of the 45. I wanted to take it down a bit so the 45 doesn't have to run at max, 250 on the plate and about 34 ma on the cathode. Vintage 45 are rare.
As for the 417. I wanted to use CCS for loading the plate, something I never done before so I thought this is a chance for me to try it out. Anyway.. As you already know I have limited understanding of how all this work ( but I will ).

I try to accumulate as much info as I can on this subject but it is not easy to digest right away.

Kmaier,

Your explanation is much easier for me to understand and it's exactly what I need to know. I will try to work on this so I can get more familiar with it and will report back.
 
The amp is running fine, the weak link are the power supply and I was just going to rebuild it with new PTX and caps but then I though I should take the chance to see if I can learn about the circuit. After doing some measurement I though I would like to also change the operation points of the 45. I wanted to take it down a bit so the 45 doesn't have to run at max, 250 on the plate and about 34 ma on the cathode. Vintage 45 are rare.
As for the 417. I wanted to use CCS for loading the plate, something I never done before so I thought this is a chance for me to try it out. Anyway.. As you already know I have limited understanding of how all this work ( but I will ).

I try to accumulate as much info as I can on this subject but it is not easy to digest right away.

Kmaier,

Your explanation is much easier for me to understand and it's exactly what I need to know. I will try to work on this so I can get more familiar with it and will report back.
Using the CCS and setting it at 10ma at 150 volts giving you the swing need to drive the 45 also falls as less the wattage of the heater. This will give you longer life of the 417. It is a very good place to start . Please tell us of your listening results. Regards
 
Winter have been ruff, so not much progress with this project.
I did manage to sort out the power supply, so here's what I got.
Please let me know what you think
 

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316v b+, that should give me 253 on the plate of the 45 and 300v for the 5842 should be enough to swing 150v p-p. (? )
Hope this is right.

That's still way too much voltage on the 417a/5842, according to the data sheet. It's the wrong kind of tube anyway for this application. With 300V you could make a mu follower out of a twin triode instead that would probably provide enough voltage for the output tube.
 
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