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4S Universal Tube Preamp Project (12A*7)
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Author:  JerryB [ 13 Aug 2016, 11:28 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Thanks for the reply Mark. I found other discussions on this topic elsewhere (including posts by you). I am totally out of depth here, but it sounds that a tube buffer or cathode follower will not add/benefit much.
Such a pity - I was really looking for a good reason to build this 4S!

Author:  Reaper996 [ 13 Aug 2016, 15:39 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Here are a couple of pictures of the pre-amp completed and hooked up to my Park Audio SE Chubby Clementine running through some Mourdant-Short Pageant speakers.

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Author:  mwhouston [ 13 Aug 2016, 19:09 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

JerryB, Suncalc designed a two stage tube preamp which has a buffered output. This would work with low impedance SS amps. Build that instead.

Author:  tizman [ 15 Aug 2016, 03:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi. I have been using my completed 4S for a few weeks now, and I am really enjoying it. It plays well with all my tube amps, and sounds great with my Pass Amp Camp Amp as well. The Amp Camp Amp is completely silent on its own, and the 4s adds almost no additional noise. It's great being able to tube roll so many different tubes through it. An excellent project and highly recommended. Thanks for the schematic and build guide!

Author:  harrydutt [ 18 Aug 2016, 07:08 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Chris65 wrote:
Just a bit of info for those who run tube rectifiers: I discovered another rectifier tube that would be suitable for the 4S - the 6BW4.
It is fairly close to the EZ80/6V4 in specifications, the only difference being the cathode comes off pin 9 instead of pin 3.

It appears that if you jumper pins 3 & 9, then you should be able to run the 6CA4/EZ81, 6V4/EZ80 or the 6BW4. Comments on the web suggest this is possible, but also say that some datasheets have pin 9 as an internal connection on the 6CA4/EZ81 & 6V4/EZ80.
Inspecting my stash of these tubes, it appears there is no connection. Some have the wire from pin 9 terminating just inside the glass envelope, others have the wire going up to the bottom mica but no further.

I've ordered some to try, as they can be found cheap, just a few dollars for NOS.


Did you get the 6BW4 rectifier to try ? Are they working well in the 4S preamp ?

I have to order some tubes soon so I'll get a couple of these if they work well in the preamp.

Author:  Chris65 [ 18 Aug 2016, 08:04 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Hi, yes I have the 6BW4 now, but not had a chance to modify the 4S to try them. Hopefully in the next day or two.

Author:  harrydutt [ 18 Aug 2016, 08:35 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Chris65 wrote:
Hi, yes I have the 6BW4 now, but not had a chance to modify the 4S to try them. Hopefully in the next day or two.


Okay, Thanks. Let's know how it turns out.

Author:  Chris65 [ 24 Aug 2016, 20:23 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Found some time to do the quick mod. The 6BW4 works well as expected. A little more voltage drop, similar to the 6V4/EZ80.
My 4S runs about 270Vdc B+ with a 6CA4/EZ81 and about 260Vdc with the 6V4 & 6BW4. So, a good alternative that can be bought cheap. No audible difference that I could detect.
Quite different structure to the other tubes as well, with the 2 separate plates.

For interest's sake, I took some voltage readings on the unused pins before & after the modification. With one Miniwatt Holland EZ81 there were a few volts on the unused pins before the mod, after the mod the unused pins were at B+ on two & half B+ on another. So this particular tube seemed to have some internal connections after all, whether this is detrimental long term I don't know.
Other tubes, including a different Miniwatt EZ81 & some EZ80/6V4, all measured zero on the unused pins before & after. Odd...

Author:  harrydutt [ 25 Aug 2016, 08:43 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Chris65 wrote:
Found some time to do the quick mod. The 6BW4 works well as expected. A little more voltage drop, similar to the 6V4/EZ80.
My 4S runs about 270Vdc B+ with a 6CA4/EZ81 and about 260Vdc with the 6V4 & 6BW4. So, a good alternative that can be bought cheap. No audible difference that I could detect.
Quite different structure to the other tubes as well, with the 2 separate plates.


Great ! Thanks for the info. So the only mod you had to do was to jumper pins 3 and 9 ?
I'll be getting a couple of these to keep as alternative spares.

Author:  Chris65 [ 25 Aug 2016, 11:26 ]
Post subject:  Re: 4S Universal Tube Preamp Project (12A*7)

Yes, jumper pin 3 to pin 9. In my 4S there wasn't quite enough room around the socket, so I just ran a wire from pin 9 to the first cap.

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