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4S Tube Preamp with 12AU7 Cathode-Follower Output Stage
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Author:  mwhouston [ 28 Oct 2018, 16:29 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

One other time I found hiss was when the filament voltage was too low. I'll assume that has been checked.

Author:  tizman [ 28 Oct 2018, 18:42 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Thanks Matt. i'll try moving the output capacitors to other spaces in the chassis using shielded hookup wire to see if I can make some room next to the tubes. That way I can increase the distances between components around the tube sockets, and space things more in general. I'll post with the results.

Author:  tizman [ 29 Oct 2018, 18:10 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Attachment:
IMG_8906.jpg


mwhouston: The heater voltage is 6.6 Volts. I will add some resistance to each leg to get it down to 6.3 Volts and let you know what happens. This may be a contributing factor to the hiss. Thanks!

Matt: I rearranged the circuit and moved the output capacitors away from the area above the tube sockets. They are now located next the the power supply capacitors. This gave me more room around the tube sockets, and I was able to arrange the wiring so that the wires and component spacing are now closer to what is normal for my builds. It's still on the tight side, but there are gaps between all components and wiring with nothing touching. It's hard to tell from the photo, but things are spaced from top to bottom as well. I tested the preamp in my system, and found the hiss noticeably reduced, but still objectionable, with exaggerated sibilants. Also, the hiss increases as volume is increased and then goes down at full volume. The reduction in hiss caused by the modification tells me that the close proximity of wiring and components was, and still is, one of the causes of the hiss as you suggested in your last post. Do you feel that given the required tight layout, it would be prudent to add a grid resistor to the 12AU7 cathode follower tube in order to help get rid of oscillation? If so, what value would be advisable for that resistor? I have attached a photo of the modified interior for your consideration.

Author:  tizman [ 29 Oct 2018, 18:11 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Attachment:
IMG_8906.jpg

Author:  Gab [ 06 Nov 2018, 08:40 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Suncalc wrote:
Gio, for your preamp. I have a design I’ve drawn up for a 4S preamp with a nice 12AU7 cathode follower output. The overall output impedance is Ro=1.43kΩ, Co=0.65pf. This should be low enough to drive just about anything. I have included a formula for choosing the final coupling capacitor based on desired low end rolloff. It is a function of the input impedance of the equipment to which it’s hooked up. You should be able to choose a value that suits your needs. I know that you said “low gain” but the cathode follower stage is such that even a 12AX7 at full output cannot overdrive the buffer. As such, the preamp is totally free of blocking regardless of the preamp tube used in the gain stage. I also moved the volume control to the gain stage input to simplify the design. I don’t know whether you need a control in this unit, so whether you use it is up to you. You’ll note that I have the B+ listed at 300v. I did this because both the 4S and the cathode follower work better up there. But in reality, you could go anywhere from 250v to about 330v on the power supply without any issues.

Take a look at the attached schematic and let me know what you think.
Attachment:
4S-Buffer Preamp.jpg

Talk to you soon.
Matt


Hi everyone. I have a few questions that stops me from finally starting this build. First, the power supply voltage. Is there a simple way to bring the original 4S PSU B+ from 250V to 300V ? I already have all the parts to build it to the specs of the original 4S. Are the advantages of getting an additional 50V worth it ?

The next one is a simple one : is the cap between the gain stage and the buffer a 0.1uf or 0.1pf ? Is the voltage rating the same as the original 4S ?

Author:  mwhouston [ 06 Nov 2018, 16:39 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Firstly I'm running a new 6SN7-SE preamp on 215V where 250V is recommended. It produces nearly the same bias voltage and current drawn. I wouldn't stress 250V is stacks. Don't try and increase it there is no advantage.

Also interstate caps are 0.1uf for a number of different tube amps I've never seen 0.1pf.

Author:  Gab [ 06 Nov 2018, 18:47 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

mwhouston wrote:
Firstly I'm running a new 6SN7-SE preamp on 215V where 250V is recommended. It produces nearly the same bias voltage and current drawn. I wouldn't stress 250V is stacks. Don't try and increase it there is no advantage.

Also interstate caps are 0.1uf for a number of different tube amps I've never seen 0.1pf.


That kind of answer both questions at the same time ! Thanks !

This also means that I will be building this preamp soon. I'll take some pictures along the way !

Author:  mwhouston [ 06 Nov 2018, 20:55 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Always interested to see others vision in real life. I always have an image in my mind how something will look once finished. 9/10 times it comes out exactly as I saw it. How it sounds can vary. Thank the Lord it is usually good with only a few duds along a very long road.

Author:  tizman [ 06 Nov 2018, 23:10 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

I added grid resistors to both tubes, right at the grid pins. 1K at the first tube and 470 Ohms at the cathode follower. This reduced the hiss to an acceptable level. This is something to consider if you have a tight build as I did. It was unusable without these resistors in my case.

Author:  Gab [ 07 Nov 2018, 11:19 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

I am starting to wonder if I should buy a bigger enclosure. I might also build one myself or use one of the steel hammond enclosures I have here. Anyway I need to order the parts for buffer... and I really like stepped attenuators wich are getting more affordable these days...

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