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Protoamp project.

Posted 29th January 2012 at 08:52 PM by DigitalJunkie

So,I've been pondering this for a while,and I'm going to finally build it.

Basic idea- Fairly simple Push-Pull amplifier,that will deliver ~50Wpc,Using common/available tubes.
I decided to go with the 'usual' octals,since they are all a similar basing/pinout(7ac?).
This leaves us open to use 6L6,EL34,6550,KTxx,etc. for output tubes.
We'll have to have a -Vg1 supply with a wide voltage range to cover the needs of all those tubes,shouldn't be a problem.

The power transformer is a big 1kva/500W beast,used in the Tektronix 545A. (P/N 120-120)
Pete Millett was kind enough to post some data on this transformer,and a few others here:
Tek power transformers

The output transformers are rated ~100W,they came from a pair of old RCA MI-12188A amps.
They are ~3.5-4.0K (p-p) primary impedance.
There is some information on these amplifiers/transformers here:
The Shrine of the Ancient and Most Excellent RCA MI-12188A Power Amplifier

I'm thinking I might want to incorporate George's 'Powerdrive' setup in this amp.
I have two main thoughts behind this.
First, it gives our input tube(s) a nice high impedance to drive - we won't have to worry as much about whether it has enough drive for the chosen output tubes.
Second,It will be able to drive whatever output tubes we can fit in the output 'holes',without breaking a sweat.
All we need to do is make sure our driver can swing enough voltage.

To fit the kind of modular/prototype theme,I'm going to use a ST-70 board as a driver. This way we can use the original Dynaco circuit/board,or one of the many other "upgrade" boards available. We can even spin our own custom boards/circuits to experiment with.

Here's the power supply schematic,as viewed from my mind's eye. Some values and things are just kind of ballpark/placeholders for now.

And here's the basic amp schematic,again some values and things are just kind of ballpark for now.

A couple of questions,

I need to find some MOSFET's to use for the source followers,any thoughts on FET's that are currently available?

Also,I'm wondering about the +V supply for the source followers. Since I'm not planning on driving the grids very far positive (though,it could happen) would it be okay to lower the +V for the source followers,to help lower dissipation? Right now I'm thinking of using something like a +/-150V supply. Could I maybe swap that for,perhaps, +12V and -150V supplies?

I'm sure I'll have more to ramble about later..but for now,any thoughts?
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  1. Old Comment
    wintermute's Avatar
    Hi DigitalJunkie I hope you start a thread as well! perhaps a summary and a link to your blog post? People seem timid when it comes to leaving comments on the blogs. I'm not sure why! I'm not a tube guy, but it certainly sounds like you have a plan!

    My limited knowledge of tube amps tells me that at 50W / channel this will be somewhat of a beast!

    Posted 2nd February 2012 at 10:06 AM by wintermute wintermute is offline
  2. Old Comment
    Hey Tony,

    I'm thinking I'll make a thread about it sometime soon and see what kind of input everyone has. Right now the blog post is kind of for my own note-keeping/sanity,but comments are always welcome!
    Posted 2nd February 2012 at 05:19 PM by DigitalJunkie DigitalJunkie is offline
  3. Old Comment
    Slowly making progress on the top plate of the chassis. It's 16-gauge steel,and I still need to trim one end off. The plate is (going to be) 16.5 x 12inches. There will be another plate for the bottom cover of the chassis,and I'm going to build a wood (pine,probably) frame for everything,and stain it a nice color. (probably something light)

    Kind of a tight fit for everything,but I think I can make it work.

    Here's a couple quick snaps,while I was test-fitting the transformers.
    Posted 6th February 2012 at 09:54 PM by DigitalJunkie DigitalJunkie is offline
  4. Old Comment
    Okay,I've got the top plate of the chassis painted,and have started some assembly.

    I have yet to order Mosfets for the source followers. I'm thinking something in a TO220 "fullpak" (insulated case) would be ideal,as far as mounting them goes,no worries about insulators,etc.

    This is where I'm currently at a loss.
    Before I delve into the Digikey/Mouser catalogs,does anyone have any suggestions for the source follower/powerdrive mosfets?

    I'm planning on ~+/-150Vdc for the Source Follower supplies. So anything 500V and up would allow plenty of overhead.(maybe even a little overkill.)

    Here's a couple pics of some of the progress.
    Posted 21st March 2012 at 03:11 AM by DigitalJunkie DigitalJunkie is offline
  5. Old Comment
    Well,after a short search,I've come up with FDPF5N50NZ


    Low Ciss :~400pf
    Low Crss:~4pf (-lowest I've come across yet.)
    And around $1 each.

    I'm thinking these will be the Fet's I go with,at least initially.

    Posted 21st March 2012 at 03:26 AM by DigitalJunkie DigitalJunkie is offline

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