Joerg wrote:
For rock bottom bill of materials budget it has always paid off to
check what TV manufacturers use in the horizontal deflection finals.
That's as low cost as it gets. But I guess with the advent of big
plasma screens that may fizzle over the next 10 years.

Horizontal deflection transistors generally have an FT way too low
to be usefull much higher in frequency than the 160 or 80 meter ham
bands. Sweep tubes are now rare and expensive bottles costing more
than 'common' 6146 types. Surplus 1625 tubes used to be only a few
bucks each, can be found for about $5-10 each at hamfests and fleabay
(Still cheaper than 6146's though). 807's cost a little more.
The best bargin in price vs power might be the 811A, or the 813.

BTW I have a bunch of 813's I'd be willing to sell. Someone make me
a good offer on a lot of 5 of them. (Used, but don't look too bad).

Hi Ken,

It depends on the TV but all this has been 20+ years ago. In those days I
found a few transistors that made great shortwave amps. I believe their
part numbers pretty much all started with BU. When one type fell out of
favor with the TV manufacturers they showed up on markets by the carton.
Usually at dump shops or discounters.

The other types that worked even better were the video transistors that
drove the CRT. You had to secure a cheap source, no scavenging out of TVs
because each set only had three. But they were low power so that required
a soldering marathon and it only made sense when you could buy them a dime
a dozen. We used to do some crazy things such as running them oil cooled
because these weren't easy to mount on a heat sink.

Except for the CRT drivers the data sheets mostly didn't spec FT, just
switching times. But that used to be the same with small signal types such
as the BSS123 that I have used heavily in front ends.

Regards, Joerg
http://www.analogconsultants.com

Hi Ken,

It depends on the TV but all this has been 20+ years ago. In those days I
found a few transistors that made great shortwave amps. I believe their
part numbers pretty much all started with BU. When one type fell out of
favor with the TV manufacturers they showed up on markets by the carton.
Usually at dump shops or discounters.

The other types that worked even better were the video transistors that
drove the CRT. You had to secure a cheap source, no scavenging out of TVs
because each set only had three. But they were low power so that required
a soldering marathon and it only made sense when you could buy them a dime
a dozen. We used to do some crazy things such as running them oil cooled
because these weren't easy to mount on a heat sink.

Except for the CRT drivers the data sheets mostly didn't spec FT, just
switching times. But that used to be the same with small signal types such
as the BSS123 that I have used heavily in front ends.

Regards, Joerg
http://www.analogconsultants.com

Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Steve VE3SMA

Paul:

There is an interesting two part article in QST May & June 1997
describing 300 W and 500 W output Class E (more efficient, non-linear)
amplifiers using a single IRFP440 (300 W) or IRFP450(500 W) at 7 MHz.
Since you are in the UK, there is a brief mention in Radcom (Technical
Topics) August 1997.

Ordinary garden-variety power FETs can also be used for HF linear
amplification using appropriate bias settings - though exactly how
linear they can be in practice I don't know.

Steve VE3SMA

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?
=================================
MOSFETS can be used for RF applications provided the input and output
capacitance is sufficiently low.
I recently bought a power module from a surplus store with 48 pcs HUF76143P3
Hi-speed MOSFET in a TO220 package ,all for Euro 4.00 (just under US$5.-)
The relevant spec sheet says Input cap.= 3900pf , Output cap.=1600pF
,useful up to 1 MHz.

Most MOSFET studded RF power amps circuits I have come across are equipped
with IRFxxx range of devices.

Frank GM0CSZ / KN6WH

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at
least - ideally suited to the task. I'm just a bit concerned about
whether even the fastest ones would be fast enough, even given
adequate gate drive. I'd be surprised if they weren't good for at
least a few Mhz., but am quite frankly clueless as to MUF. Anyone
know?
=================================
MOSFETS can be used for RF applications provided the input and output
capacitance is sufficiently low.
I recently bought a power module from a surplus store with 48 pcs HUF76143P3
Hi-speed MOSFET in a TO220 package ,all for Euro 4.00 (just under US$5.-)
The relevant spec sheet says Input cap.= 3900pf , Output cap.=1600pF
,useful up to 1 MHz.

Most MOSFET studded RF power amps circuits I have come across are equipped
with IRFxxx range of devices.

Frank GM0CSZ / KN6WH

"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

"John Larkin" a écrit dans
le message news: ...
On 5 May 2004 16:41:11 -0700, (Tom Bruhns) wrote:


Back when hexfets first came out (1981 or so), I was having trouble
with them self-destructing. Back then, at least, if you read far
enough in the fine print, you'd find a maximum drain dv/dt rating.

And the substrate diode made a nice step-recovery diode, making it
possible to generate lethal dv/dt's in totally non-obvious ways!


If the drain load is resonated and you want some control over the resonant
frequency (i.e. you have external to the mosfet tuning Cs) I don't see much
discontinuities in inductors current. Of course when all the stuff is well
"wired" and that's an entirely different matter, isn't it Paul ? ;-)

John, can you suggest some refs that nicely snap ?

Thanks,
Fred.

Paul Burridge wrote in message . ..

Okay, thanks all and keep it comin'. I had no idea there were power
FETs capable of UHF. I'll follow any links given and investigate.

Check out www.polyfet.com; SR706: 300 watts at 225MHz. SR705: 200
watts at 400MHz. LX401: 60W at 1000MHz. Also some very terse "ap
notes" and some somewhat less terse but very to the point technical
papers.