Roller inductor or band switch

[WB2WIK]

A big negative for the 3k ultra and the 8k ultra is the fact that the l1 and l2 roller inductors were made by Henry Radio. So if they end up having a serious failure then your in trouble. Unless you want to spend big bucks to have new ones custom made.

Its also a common for the roller wheel on the l2 to have problems.

Im getting a little off topic. This has been a great topic with a lot of very informative replys

Thanks everyone

The roller in my 3K Premier (vintage 1986, I had the first one ever made) was also fabricated by Henry.  However, they had them in stock.  They still might, I don't know.  The amp shop is still there, even though they're not cranking out tube amps anymore: They do still service them.

[KB1SEL]

They dont have any more roller inductor parts unfortunately.

[WN7T]

If you are building your own amplifier from scratch, I recommend the use of a HEAVY DUTY roller inductor as the main tank inductance for a pi-network output. I have had one in use for 30 years in several amplifier generations. There is almost no maintenance associated with the use of a well designed roller inductor, they are easy to rebuild/re-lube when necessary, and give you the ability to tune your amplifier very efficiently without the worries of "did I find the right point to put the band tap on the coil?/was that popping noise my band switch arcing over again because I miss-tuned the amp?".

My favorite roller inductors are the big edge wound Johnson (Multitronics) 222 series. They are easy to recognize on eBay and appear on a regular basis there for auction. If you plan on running 160 meters, make sure you find a 19 uH version.  Newly manufactured versions from Multitronics Corp. cost a bloody fortune, so stick to eBay or hamfests to find one.

I strongly recommend you consider the use of a heavy gage 2" or 3" diameter 3 or 4 turn copper tubing pre-coil in front of the roller inductor if you wish to load efficiently on the upper bands (10 meters).

Also, make sure to use a tuned pi input network with a Q of around 3 for each band of operation. You could indeed use a small roller inductor as part of the input network. By creating a faux tube impedance (using a fixed "non-inductive" resistor in place of the amplifier tube) with value based on the impedance charts for your particular tube at a given plate voltage, and using an antenna analyzer set to 50 ohms on the amp output (with no plate supply voltage turned on!!!), you can easily map the tuning coordinates for the roller inductor, plate tuning and load capacitors. Things get a bit more complicated if you decide to implement a pi-L tank circuit, but the roller inductance range is smaller.

I have a few pictures up on QRZ of my roller inductor RF deck implementation, if you care to look.

73
Paul WN7T

[K8AXW]

Paul:  Checked out your work.  Very nice!!  It's always a pleasure to look over such beautiful construction.   

Al - K8AXW

[KB1SEL]

Nice amp paul you do really clean work. Over the years I have seen both band switch and roller inductor failures. Most band switch failures were do to parasitic oscillation and roller inductor failures were do to abuse and lack of maintenance.

It sure is a lot easier to build a multi band amp with a roller.

I dont think I have ever seen a failure with the  johnson 222 series roller inductor. But this roller inductor is different then the others . The contacts ride on the outside of the ribbon . It doesn't use a conventional roller.

I think a roller inductor is more appropriate for use in an amplifier which will seldom see band changes.

[WN7T]

Thanks for the kind words, guys.

So if you really want frequency agility, go solid state.
The cost per watt has been steadily dropping and the number of high power devices increasing.

I love tube amps and their care and feeding. However, the supply of inexpensive Soviet era tubes and components isn't going to last forever and eBay is the great equalizer as far as pricing is concerned. When VOA shutdown most xmitter sites, there was a glut of high power components on the market. Now those same Sangamo G3 any G4 mica caps that went for $20 to $30 in auction are now commanding over $200 on eBay. Same for roller inductors. When is the last time you went to hamfest/flee market and found that pristine $20 vacuum variable cap?

Components to support high power solid state amps OTOH, seem to be more abundant these days. Couple that with light weight switching power supplies with clean output and "new" high efficiency modes of operation (i.e. class E) and you have a winning combo. 

Some of the new high power RF transistors are mind blowing with their specs and low cost. And we as hams, have not even begun to tap into the coming supply of military-grade diamond film high power semiconductors.

It's fun to see computer engineering savvy hams meld their expertise with micro controllers with high power solid state amps. I think we are going to see some beautiful sophisticated homebrew solid state amp designs in the years to come.

There's my 3 cents worth!

73
Paul WN7T