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Old 2nd February 2015, 06:03 AM   #1
thehoj is offline thehoj  Canada
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Default Need help with driver idea for 845 amp - have a few ideas

So.. I know there are a ton of different ideas out there for 845 drivers, but I have a bit of a different situation.

As I have my 845 stage configured, I will need to provide voltage gain of at least 105, but I'd like to have a little bit of extra umph there, so probably somewhere in the neighborhood of 120.

So here's my dilemma. I have a preamp that I really really like, it's set up perfectly for my equipment, so ideally I'd like to keep it in my setup. I think it's safe to assume that my signal sources will provide at least 1V. So let's just assume that that's what we've got at the input of the preamp in this discussion.

In it's current configuration it has a voltage gain of 6.3, it can be configured to provide a gain of 11. So either 6.3 or 11V out of the preamp.

So that leaves me with ideally using a single stage driver that can take that gain of 6.3 or 11 and bumping it up to 120.

So if I leave the preamp as is I'd need a stage that can handle an input of 6.3V and provide a gain amount of 120/6.3 = ~20.

Or if I make a simple change to the preamp giving it a gain amount of 11 I would need a stage that can handle an input of 11V and provide a gain amount of 120/11 = ~11.

I've been playing around with this triode loadline simulator Triode Loadline Simulator v.2.5 (20141229 www.trioda.com) and this seems to be a bit harder than I had expected.

I've come up with a few scenarios though, with 450V available from the power supply, and just wanted to get some feedback on my best option, or maybe some better options. I don't plan to drive the 845 into class A2 territory, so that should keep things fairly simple. I also don't want to use an interstage transformer.

So here's one scenario if I keep the preamp as it is, providing a gain amount of 6.3. A 6N1P stage biased at -6.5V providing a gain of 27.4, which would mean my preamp would likely not have to provide more than 4.38 gain. Here's the screenshot.
Click the image to open in full size.

Here's another scenario if I were to modify the preamp to provide a gain amount of 11. A 5687 stage biased at -11.6V providing a gain of 13.8, which would mean my preamp would likely not have to provide more than 8.7 gain. Here's the screenshot.
Click the image to open in full size.

I'd love to get some feedback on either of these ideas, or maybe some better suggestions given my scenario. I guess I'd say I'm leaning towards the 5687 stage. I know I'm probably not using the best terminology, but hopefully I'm making sense here..

Last edited by thehoj; 2nd February 2015 at 02:06 PM.
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Old 2nd February 2015, 07:26 PM   #2
Ketje is offline Ketje  Belgium
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The 6N1 sees not only the Ra=~25k but also the gridleak from the 845.
You have a static loadline in blue and a dynamic one in white.
With a working point at 240V 7mA there is just space for +-120V.
It's always best to stay on the high current side of the loadline.On the low side the -Vg lines getting closer and closer = less linear.
Mona
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Old 2nd February 2015, 08:27 PM   #3
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by Ketje View Post
The 6N1 sees not only the Ra=~25k but also the gridleak from the 845.
You have a static loadline in blue and a dynamic one in white.
With a working point at 240V 7mA there is just space for +-120V.
It's always best to stay on the high current side of the loadline.On the low side the -Vg lines getting closer and closer = less linear.
Mona
I need to be biased negative enough for the amount of swing at the grid though..

I guess I could probably bias for -4.5V at the grid though and be okay if I have gain of ~26 - ~27 or so..

Maybe this would be more suitable for my scenario with the low gain preamp I'm using?
Click the image to open in full size.

Or what about that 5687 loadline, if I make that change to the preamp giving it a gain amount of 11.. ?

Thoughts ?
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Old 2nd February 2015, 08:33 PM   #4
Ketje is offline Ketje  Belgium
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You still don't take into account the grid resistor of the following stage (845) !
Mona
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Old 2nd February 2015, 10:04 PM   #5
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Why not 6P15P in triode mode? With anode chocke it looks good.
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File Type: jpg 6p15p driver.jpg (28.0 KB, 151 views)
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Old 3rd February 2015, 12:27 AM   #6
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by matejsirk View Post
Why not 6P15P in triode mode? With anode chocke it looks good.
I'm not sure how to draw a loadline for a choke load.. Have you done one?
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Old 3rd February 2015, 04:13 AM   #7
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You might want to explore Rod Coleman's Shunt Cascode Power Valve Driver thread.
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Old 3rd February 2015, 04:16 PM   #8
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by Ketje View Post
The 6N1 sees not only the Ra=~25k but also the gridleak from the 845.
You have a static loadline in blue and a dynamic one in white.
With a working point at 240V 7mA there is just space for +-120V.
It's always best to stay on the high current side of the loadline.On the low side the -Vg lines getting closer and closer = less linear.
Mona
Just reading up on the AC load line.. doesn't it still pass through the DC bias point? It doesn't look like you drew it that way in your attached image..

Also, since I think I'm going to go cathode biased, I can actually have a 250Kohm grid leak resistor following the driver stage rather than just 100K. So that would make the AC load line rotate less.
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Old 3rd February 2015, 05:30 PM   #9
thehoj is offline thehoj  Canada
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Oh.. Or were you saying that would have been a better bias point to shoot for..?
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Old 3rd February 2015, 06:39 PM   #10
Ketje is offline Ketje  Belgium
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The crossing of the blue and white line is the bias point (draw a white circle there).
And yes, with a grid leak of 250k the white line gets closer to the blue one.But an excursion of 120V gets you near Vg=0 ,best to shift the bias somewhat to the right,say Vg=-5V.
Mona
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