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4S Universal Tube Preamp Project (12A*7)
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628
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Author:  mwhouston [ 11 Feb 2018, 17:26 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I have half a dozen 6N3Ps. I have one in a chip amp running on 90V.

Author:  oloppolo [ 12 Feb 2018, 08:15 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi,

hi, I have the pcb attached. it is noisy at 200 volts dc and silent at 100 volts dc.

Author:  Geek [ 12 Feb 2018, 15:00 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Noisy at 200, but quiet at 100?

I'd say something is wrong and the regulator can't do its job. Is it designed to go to 200V?

Author:  oloppolo [ 13 Feb 2018, 13:41 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I'm studying the valves and I try not to
make mistakes.
Thanks.

Author:  jocelys1 [ 18 Feb 2018, 12:03 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi,

I'm building the 4S to front-end a Brook12A (6J5@Anode-Choke loaded | 2A3 PP) which, also is under build. I can't figure if I must use the design as-is or tone down the gain by putting the pot at the input, next to the grid.

Can I please get some help to understand how to go about it ?

P.S: The brook does not have a pot.

Author:  Suncalc [ 18 Feb 2018, 16:25 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jocelys1 wrote:
I can't figure if I must use the design as-is or tone down the gain by putting the pot at the input, next to the grid.
As with most general questions, it depends.

The main gain driver of the 12A is a 6SN7 (µ≈20). The following concertina adds no gain and the drivers look to be tuned to drive the 2A3s. It also has a bucket load of feedback. As such, it's probably not very sensitive. Probably takes at least a few volts to drive it to full power; maybe more. The design looks like it was intended to be used with a preamplifier of some type.

So the real question is, what's your audio source? If it's a high output driver, then move the 4S volume control to the front end to help keep from overdriving the 4S. If it's just a line level (or adjustable) input device, then build the 4S as is.

Remember that a volume control at 50% setting is ≈-20dBv and the 4S with a 12AU7 is ≈+20dBv of gain. This means that with the 12AU7, the preamp actually attenuates the input below the half volume setting and amplifies it above the half volume setting. The other triodes will obviously, provide more gain.

Author:  jocelys1 [ 19 Feb 2018, 03:16 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Thanks Matt. The source is a Pass B1, which also is where the sources are connected to.

Remember that a volume control at 50% setting is ≈-20dBv and the 4S with a 12AU7 is ≈+20dBv of gain. This means that with the 12AU7, the preamp actually attenuates the input below the half volume setting and amplifies it above the half volume setting.

This makes a lot of sense about the gain bit. Do I have to consider the output impedance to the Brook12A ?

Author:  Suncalc [ 19 Feb 2018, 19:59 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

jocelys1 wrote:
Do I have to consider the output impedance to the Brook12A ?
The 12A has an input impedance of 470kΩ. This is high enough that you shouldn't need to worry about it.

Author:  jocelys1 [ 19 Feb 2018, 23:45 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Awesome. Thanks Matt.

I will complete the standard 4S design.

Author:  oloppolo [ 24 Feb 2018, 15:12 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi, I built 2 4S for 12at7 and 6cg7, based on your schematic.
I use 50K pot. I am waiting for 250K pot.
My impressions with smartphone Nokia and Spotify Premium.
12at7 "Sine China selected"- the sound is beautiful and fun.
6cg7 "RCA clear top NOS" - the timbre is perfect and low distortion, but the low frequencies are attenuated. It is perhaps the small output capacitor. Can i try with 1uF?

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