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Author Topic: Linear Amp "Technical" Questions - I  (Read 3649 times)

Posts: 73

« on: December 30, 2009, 07:50:10 AM »

Hi All:

I am going to build a linear amp with a 1kW input using 4 811A's.  In thinking the project through, and considering some adaptations, I have some questions.  I have listed some below, but I am sure others will pop up along the way.  Here is what I have so far:

1.  The rectifier circuit is older style using vacuum tubes, which I have (866A substitutes).  The filter capacitors are 4x 100MFD in series, so a total 25MFD filtering capacitance.  Is there any advantage to increasing the overall filtering capacitance?  I could put 4 or 5 470MFD's in series instead, which I have on hand.  Are there advantages/disadvantages to having an overall filter capacitance of say 100MFD?

2.  Should I include an in-rush protection circuit?  This would be easy to do, although not called for in the original schematic.  Are they used only with solid state rectifier circuits, or should they also be used with vacuum tube rectifier circuits?

3.  Any harm in designing a PC board to mount the filter capacitors and bleeder resistors?  Can I assume that a PC board is OK as long as the tracks are as wide as possible and spaced out appropriately, to handle the voltage/current, or should I use solder lug terminals and do direct wiring?

4.  The circuit merely states for the load capacitor that it be a "broadcast replacement style" air variable.  I bought one of these but it looks.....well....wimpy.  It's smaller in physical size than the one in my Swan 1200X.  Is the size/spacing critical for the load as opposed to the plate capacitor?  The circuit specified the handling voltage for the plate capacitor, but not for the load capacitor.

5.  With a true 1kW input/say maybe 800W PEP output on SSB if I'm lucky, what do you think about my using schedule 40 PVC pipe to wind the tank coils on as the coil form?  My Swan amp has a ceramic form in it, but white PVC is readily available, and cheap.  I've seen homebrew antenna tuners using PVC but would welcome comments/thoughts. It would be easier to use a form than to make air wound coils.

That's about it for starters.  I am trying to be thorough as I go through this project, so any and all comments are welcomed.



Posts: 1209


« Reply #1 on: December 30, 2009, 10:24:53 AM »

Although I have worked on many amplifiers and transceivers, I can't provide direct answers to your questions about materials and such. I would recommend that unless you just have to 'roll your own', from what you have described, you're going to end up with something in the class of a Ameritron AL-811H.

From my own experiences with design, building and fabrication, unless you have a full set of tools to do quality metal work for the chassis, you'll end up with a 'Frankenamplifier' unless you outsource the metal work for added costs. Also unless you can spec out all the required components from one or two competent sources, piecemeal shipping of parts is going to push your project budget up and over what can be had ready built.

Just some things to consider in managing a project. Years ago when there were radio parts 'palaces' everywhere, you may have been able to get parts cheaply but today it isn't uncommon to pay more for shipping than the cost of the part(s) when buying as onesey-twosey method.

Good Luck on the project. If you do keep with it, I'd suggest photos and videos along the way to provide insight for others and maybe you'll consider an article for some mag (QST, QEX, CQ, etc.)


Gene W5DQ
Ridgecrest, CA - DM15dp

Posts: 73

« Reply #2 on: December 30, 2009, 11:00:49 AM »

Thanks for the insights.  I understand the comments re ugly amplifier.  Fortunately, a friend gave me some old IT equipment from where he works, that contains some nice sub-boxes that I can likely use to make a fairly good looking sub-chassis.  I have a metal bending brake and a metal cutting bandsaw, and a drill press.  I also have a screen printing press and other printing equipment.  I doubt my finished cabinet will look as good as factory-built, but I suspect it won't look bad.

I've been lucky so far in collecting parts.  Some of the used parts have come inexpensively from the local ham community.  I have shopped around for other parts.  The amp is likely to have cost in the region of $400 when finished, but (a) that money is being spent over time, as I can afford it, and not all up-front, and (b) this has already been a tremendous learning project.  I'm really doing it for the experience and learning.  I also want (for the heck of it) to build something that is totally vacuum tube - I just love older technology.

Best regards


Posts: 7718

« Reply #3 on: December 30, 2009, 01:56:21 PM »

The 866 in a full wave center tap capacitor input filter is rated for 0.25 amps average. Increasing the filter capacitance will reduce the AC ripple while increasing the stress on the 866 mercury vapor rectifier tubes. If the AC ripple is low enough with 25 uF I would use that.

I would not bother with inrush current limiting.

A PCB will be fine. Keep the stress at or below 5 volts per mil (0.001") and it will adhere to the UL60950 safety standard (and others). I used to use 10V/mil in HV circuits with no problems.

Posts: 2483

« Reply #4 on: February 02, 2010, 07:52:05 AM »

Using the tube type HV rectifiers will work, but for around $1.00 worth of 1N4007 silicon diodes you can do the same thing without having to use tubes, a filament transformer, ultimate tube failure, and the associate heat generated by the tubes.

Dick  AD4U
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