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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 [ 29 May 2016, 09:39 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Steph wrote:
Hello all,
I've scanned through this project thread and didn't see what I was searching for. I absolutely love this preamp but I'm considering a modification but before possibly reinventing something that may well have already been done I thought to ask. Has anyone built this with a third channel output? My thinking is to add another channel a sub output. Possible with a low pass adjustable filter and a line out level control.

Anyone tried this yet?

No. I tap off the preamp output.

Author:  tizman [ 19 Jul 2016, 06:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi All. I have just completed my 4S Universal, and have measured voltages at the last capacitor in the power supply and at pins 1 and 6 of three different preamp tubes. For every tube I got between 305 and 309 volts at the last capacitor. For the 12AU7 I got 78 volts at pins 1 and 6. For the 12AX7 I got 187 volts at pins 1 and 6. For the 12AT7 I got 134 volts at pins 1 and 6. Are these voltages correct? They all seem low to me. I haven't found a list of voltages for the 4S anywhere yet. It would be very useful to have a list of voltage test points so that newbies like myself can check voltages to see if they are correct. Thank you in advance for any information that you provide. I will post photos and listening impressions shortly.

Author:  Chris65 [ 19 Jul 2016, 08:46 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Those plate voltages are about right for your higher B+. The voltage at the last capacitor in the power supply will be constant, it is not determined by which signal tube you are using.
The expected plate voltages can be determined from the load line graphs shown on the first page of the thread.

Author:  tizman [ 21 Jul 2016, 15:15 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi All. This is my recently completed 4S Universal. Thanks to Matt for the great design and the kind support that he provides. I am very pleased with it. I have rolled a bunch of different tubes through it, and it sounds good with all of them. It provides the additional gain that I require with certain source/amp/speaker combinations, and sounds great doing it. It is also very quiet. I have attached a bunch of photos....


Attachment:
IMG_5247.JPG

Attachment:
IMG_5248.JPG

A powder coated Hammond enclosure


Attachment:
IMG_5249.JPG

The guts. I ended up using four 30uf film caps instead of a 33uf and a 100uf cap. and then I split the B+ with two resistors and two 5uf film caps, one per channel.


Attachment:
IMG_5251.JPG

Clarity MR .47uf capacitors, an Alps pot, Shinkoh tantalum 10K resistors, a bunch of 1% metal film resistors and one carbon comp bleeder resistor. The signal wires from inputs to tubes and from pot to outputs are all less than an inch long.

Author:  tizman [ 21 Jul 2016, 15:29 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi again. I'm thinking about adding the cathode resistor bypass capacitor on a switch. Other than increasing gain, is there any other audible effects of adding this capacitor? I seem to remember that these capacitors increase gain by reducing feedback. I don't need additional gain, but the idea of eliminating a feedback loop is intriguing. Adding it would mean adding the two capacitors, a switch, and some more wiring. I am always nervous about adding complexity and would prefer not too unless there is something to be gained from it. Any comments or observations would be much appreciated. Thanks.

Author:  tizman [ 21 Jul 2016, 15:57 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

These are a few of the tubes I've tried in it so far.

Attachment:
IMG_5259.JPG

From left to right, top row...
Matsushita 6CA4 - GE 12AU7 - Philips JAN 5814A - Conn organ RCA clear-top 12AU7
bottom row...
Unknown 12AU7 - GE 12AX7 - Electroharmonix 6072A/12AY7 - EI 12BH7AEG

So far, I like the 12AU7s best, and the clear-top is the best of the 12AU7s. The Philips Jan 5814A is my second favorite.

Author:  mwhouston [ 21 Jul 2016, 18:01 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Removing the bypass cap will add some local FB which, to me, is OK. It will reduce gain BUT it will increase output impedance. There's no free lunch.

Author:  tizman [ 21 Jul 2016, 19:42 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

What does it do for the sound? Is there any way to describe this circuit with and without the cathode bypass capacitor? I like it now, and I know adding the cap increases distortion and gain, and impacts the output impedance, but I was hoping that there is someone who has an on off switch for the cathode bypass capacitor on a 4S that has some listening impressions.

Author:  mwhouston [ 21 Jul 2016, 22:12 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Easy enough to do.

Author:  tizman [ 22 Jul 2016, 01:37 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

I suppose I could try it with jumpers. It's not easy to AB that way quickly though. At least not for me. Have you heard it both ways? What is your preference?

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