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

On Wed, 05 May 2004 20:21:45 -0400, Ken Scharf
wrote:

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).

Nice. Got any KT88s?
--

The BBC: licenced at public expense to spread lies.

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

On Wed, 05 May 2004 20:21:45 -0400, Ken Scharf
wrote:

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).

Nice. Got any KT88s?
--

The BBC: licenced at public expense to spread lies.

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).

Paul,

Check out the Advanced Power Technology web site. They have some ap notes on
using some more or less regular power FETs for RF. Nice thing is that the
higher voltage ones will run off straight 170 V rectified AC.

Tam
"Paul Burridge" wrote in message
...
Hi all,

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?

Thanks,

p.
--

The BBC: licenced at public expense to spread lies.

Paul Burridge wrote:
Hi all,

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?


Paul,
what is the frequency range and the amplitude ?
I assume a 50 Ohm system.


Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Paul Burridge wrote:

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

Paul,
assuming that I understand your question and assuming you want a
50ohm-50ohm HF amplifier for CW, I have seen several designs using an
IRF510 MOSFET for amplifiers up to 30 MHz.
On a QST article using 2 IRF510 in a push-pull fashion some 30W could be
achieved over the entire HF range. For a simpler design check the NB6M
Miniboots (http://www.amqrp.org/kits/miniboots/miniboot.htm).

To my understanding, switching MOSFETs can be used as HF C-class
amplifiers as long as their gate capacitance is low (few pF).

As an example, IRF530 has a way too high gate capacitance to allow
acceptable HF results. On 80m it might be good, thought.

HTH,
Paolo IK1ZYW

Paul Burridge wrote:

Hi all,

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?

Thanks,

p.

You are aware that there are power RF MOSFETs that go up to VHF,
probably higher? There's a number of manufacturers out there -- I know
Motorola makes them, probably any one that is in the cellular base
station business makes them. You can pretty much build an amp off of a
Motorola app note.

As far as using 'regular' power MOSFETs they tend to have very high gate
capacitance as a result of getting the Rds down. Since Rds isn't the
main efficiency driver in an RF amp the chips aren't well suited to RF
application.

I know the ARRL handbook has information on MOSFET amps from 'standard'
power MOSFETs, and there have been a few articles in QST and QEX.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Paul Burridge wrote in message . ..
Hi all,

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?

Thanks,

p.

Here's a link to a MOSFET transistor that will do RF.

http://www.macom.com/data/datasheet/DU1260T.pdf

Jim