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4S Universal Tube Preamp Project (12A*7)
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Author:  jayveesea [ 12 Aug 2018, 18:12 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Could this "4s Universal Tube Preamp" work as a phono preamp paired with a gainclone? Thanks, -jvc

Author:  Suncalc [ 12 Aug 2018, 21:49 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jayveesea wrote:
Could this "4s Universal Tube Preamp" work as a phono preamp paired with a gainclone?
Nope. A phone preamp needs a compensation network. The 4S doesn't have that.

Author:  mwhouston [ 12 Aug 2018, 22:11 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I have built three Radi0kit tube phono preamps and they are excellent. Go for the one with the better parts. Cheap and complete. You will need a really quiet PS but they are cheap and easy to build.

Author:  jayveesea [ 13 Aug 2018, 06:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

thanks for clarifying, and the suggestion!

Author:  Woodo [ 19 Aug 2018, 21:49 ]
Post subject:  Re: Load Lines - 4S Universal 12A_7 Tube Preamp

Suncalc wrote:
As promised, here are the load line designs for the 5 different tubes in order of increasing gain.
Attachment:
12AU7 4SU.jpg

Attachment:
12AV7 4SU.jpg

Attachment:
12AY7 4SU.jpg

Attachment:
12AT7 4SU.jpg

Attachment:
12AX7 4SU.jpg

As always, question and/or comments are more than welcome.

Hi Matt. Is there an available reference you recommend for an amateur to learn how to plot these load lines and understand what they mean? I would like if possible to understand more about the operating points used in the pcb based amp I'm assembling.
Regards

Author:  Renaissance Man [ 10 Sep 2018, 08:48 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I have a 12 AU7 4S Preamp hooked up to a Pass F5. In my reading I some people have said that there is a possibility that during power up the preamp might have a voltage spike on the output that will damage the input transistors on the F5. Suggested solutions have been a zener voltage clamp on the preamp output or a time delay relay on the preamp output (I already have a time delay relay on the F5 output protecting the speakers during power up).

My questions are how much of a problem is this surge? And if it is a problem what the best way to deal with it?

Author:  Suncalc [ 10 Sep 2018, 10:12 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Renaissance Man wrote:
In my reading I some people have said that there is a possibility that during power up the preamp might have a voltage spike on the output that will damage the input transistors on the F5.
From where exactly would this voltage surge be coming? What's the PS look like on your 4S?

Author:  laurie54 [ 10 Sep 2018, 21:15 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

A quick glance would suggest 250vdc applied to the 100k B+ point would allow a spike to get threw the .47uF series cap (until it charges) and damage the SS it is attached to.

BUT, the power supply is not at 250vdc it is at 0vdc. The time it takes for the power supply to come up to 250vdc is much slower than it takes the 0.47uF to be charged to the nominal 125vdc. Therefore, there is no spike at the output to get passed on to the SS's input.

Author:  Renaissance Man [ 11 Sep 2018, 00:11 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I have a CRCRC filtered SS power supply with a NTC resistor on the B+ transformer primary so it takes about 20 seconds for the B+ to reach full voltage. During this time I measure about 5 Volts DC with a multimeter on the preamp output which drives the F5 to a DC offset of 25 volts at the output. After the B+ stabilizes there is no measurable DC on the preamp out and less than 1 mV offset on the F5 output.

Since I have a 30 second delay on the speaker protection relays this is all no problem. My only concern is that there might be a brief voltage spike when the B+ is first applied to the tubes before the output capacitor is charged that might damage the input transistors on the F5. I don't have an oscilloscope and I don't have enough knowledge to know if this would happen or be a problem.

Author:  ILoveHiFi [ 11 Sep 2018, 01:49 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Do you have a pot or any resistor fitted to ground on the output of 4s pre amp.
If not then the 5v your seeing should be due to the output end of capacitor floating.

There should be zero reason on why you see 5v at pre amp output, the only reason is because you have not fitted any resistor to ground at output of pre amp.

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