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Old 22nd February 2011, 03:03 AM   #1
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Post Senior Project Tube Preamp

Hello everyone! I'm new here, but I thought this may be a good place to ask for help regarding my Senior Design project. I'm looking to build a low-voltage (48vDC) line-level tube preamp. This project originally started as a full class AB amp, but has been dialed back big time...
I've attached my schematic design, and I'm looking for any general input at all. I'd like to use the tubes listed, and to keep the overall design as-is (barring any major issues I may have overlooked). Any and all help is greatly appreciated!

Regards,

John
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Old 22nd February 2011, 03:25 AM   #2
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John,

A 48 V. B + rail is WAY too low for the types you've selected. Look at the data sheets, please.

If you really want to work with a 48 V. B+ rail, types intended for use with batteries will serve you better. The 1U4 and 1S5 are sharp cutoff pentodes, which will work well at the low B+ voltage.
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Old 22nd February 2011, 03:35 AM   #3
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I was afraid of that, yeah. I'll look into redoing the PS, but I may have some 1U4s kickin' around (not sure about the 1S5s - I'll have to check). Also, I've added a 22nf coupling cap between V1 and V2, but I'd also considered moving the tonestack between the two stages and taking the output directly from the cathode follower. Thoughts?
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Old 22nd February 2011, 03:42 AM   #4
SY is offline SY  United States
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Don't mean to be the grumpy old engineering manager, but... have you formally set down the design goals? Step One in any engineering project. It's hard to see what's on and off target when the target is unclear.
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Old 22nd February 2011, 03:49 AM   #5
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Sparing you the detailed proposal (which, in truth, outlined the project as it was intended - not what it is now), the design goals are really as simple as "low voltage line-level preamp", I'd like it to distort to some degree, but I'm more or less designing it to my advisor's specs...not mine :\ unfortunately the project has lost much in the way of cohesion, and is now just something I need to finish before early May. The design is still intended somewhat for studio guitar use...I'm sorry I can't be much more helpful. At this point I just need something that works reasonably well, sounds reasonably good, and isn't overly complicated.
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Old 22nd February 2011, 04:01 AM   #6
SY is offline SY  United States
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You're fortunate that you're not my student.

Gain? Use (MI, PA, hifi...)? Sources driving it? Load it needs to drive? Bandwidth? Distortion? Power consumption or other constraints?
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Old 22nd February 2011, 05:41 AM   #7
Knarf is offline Knarf  Denmark
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Drop the standby switch I'd say. Running tube cathodes for any significant length of time without anode current flowing is likely to damage the cathodes due to a process called cathode poisoning.

Switch everything on, or everything off (inrush current protection and/or slow charge not included. Those are OK, of course).
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Old 22nd February 2011, 09:34 AM   #8
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The first 5751 triode is DC coupled to the anode of the 2nd 12AU7 triode. You 'might' want to use a coupling cap there.

Quote:
Also, I've added a 22nf coupling cap between V1 and V2, but I'd also considered moving the tonestack between the two stages and taking the output directly from the cathode follower.
22nf sounds a bit small for a decent LF response, but it might be sufficient for guitar.

Moving the tone controls to between the 12AU7 and the 5751 seems like a good idea to me; I'm not really sure if you'll still have line level output in the current design.
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Old 22nd February 2011, 12:20 PM   #9
DF96 is offline DF96  England
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Don't forget to mention the forum in your list of thanks on the Acknowledgements page of your write-up.
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Old 22nd February 2011, 02:55 PM   #10
ilimzn is offline ilimzn  Croatia
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How do you expect to get the required current to drive the heaters through a 10k + 10k divider off the 48V line? This suggests that basic things such as ohms law are an unknown to you.
The tubes yous elected are not very good at 48V... with careful design you might get away with something like 6DJ8s. Anther possibility would be certain TV tubes with higher voltage heaters, which you could series-connect and work off the 48V power supply. Even so, it would be MUCH easyer to simply add a dedicated transformer for the heaters...
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