I'd like to build a simple tube-type FM broadcast tuner. So far it
looks like a 6BA7 is the best bet for oscillator/mixer but I need a
circuit to avoid the pitfalls and/or reinvention of the wheel.

Other possibilities are triode oscillator/mixers using 12AT7, and
triode/pentode using 6BL8 or 6U8, even 6X9. 6HG8, which has very high
transconductance and triode and pentode on a common cathode seems to
have possibities too.

I'd appreciate it if you could share your favorite VHF/FM converter
circuits with me.

Thanks,

John VK3XAO

VK3XAO wrote:
I'd like to build a simple tube-type FM broadcast tuner. So far it
looks like a 6BA7 is the best bet for oscillator/mixer but I need a
circuit to avoid the pitfalls and/or reinvention of the wheel.

Look in the back of the RCA Receiving Tube Handbook. There are some
nice designs in there that are pretty bulletproof.

Your problem is going to be finding good IF transformers, more than
anything else.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Jan 28, 1:43*am, (Scott Dorsey) wrote:
VK3XAO wrote:
I'd like to build a simple tube-type FM broadcast tuner. So far it
looks like a 6BA7 is the best bet for oscillator/mixer but I need a
circuit to avoid the pitfalls and/or reinvention of the wheel.

Look in the back of the RCA Receiving Tube Handbook. *There are some
nice designs in there that are pretty bulletproof.

Your problem is going to be finding good IF transformers, more than
anything else.
--scott
--
"C'est un Nagra. *C'est suisse, et tres, tres precis."

Thanks Scott,

The IF transformers was the easy bit, to be scavenged from a piece of
unused tube-type mobile radio gear that got overtaken by technology.
I've looked at the RCA Receiving Tube Handbook (RC35 in my case) but
would prefer a circuit that used more generally available (in
Australia) tubes. There *must* be some commercial FM broadcast
receivers that used a 6BA7 or a triode-pentode converter stage -
Zenith used a 12AT7 twin triode that works well.

All circuit source suggestions appreciated.

John

"VK3XAO" wrote in message
...
On Jan 28, 1:43 am, (Scott Dorsey) wrote:
VK3XAO wrote:
I'd like to build a simple tube-type FM broadcast tuner.
So far it
looks like a 6BA7 is the best bet for oscillator/mixer
but I need a
circuit to avoid the pitfalls and/or reinvention of the
wheel.

Look in the back of the RCA Receiving Tube Handbook. There
are some
nice designs in there that are pretty bulletproof.

Your problem is going to be finding good IF transformers,
more than
anything else.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Thanks Scott,

The IF transformers was the easy bit, to be scavenged from a
piece of
unused tube-type mobile radio gear that got overtaken by
technology.
I've looked at the RCA Receiving Tube Handbook (RC35 in my
case) but
would prefer a circuit that used more generally available
(in
Australia) tubes. There *must* be some commercial FM
broadcast
receivers that used a 6BA7 or a triode-pentode converter
stage -
Zenith used a 12AT7 twin triode that works well.

All circuit source suggestions appreciated.

John


There are lots of schematics of consumer radios at:
http://techpreservation.dyndns.org/schematics/

You will have to explore around if you don't have specific
model numbers.
For design data there are several good books at Pete
Milette's DIY audio site at:
http://www.pmillett.com/

The Radiotron Designer's Handbook, 4th edition, is sort
of a standard and covers the relative merits of several
mixer and converter circuits.
The 6BE6 seems to be more common in commercial
receivers than the 6BA7, both should be used with external
oscillators. There are better converters for the VHF range.
I am not sure how tubes were numbered in Oz land but,
if British types are more common you should check an
equivalent tube chart. Many US and British types were
interchangable even though they had different numbers.
Scott Dorsey's suggestion to look in the RCA tube
manuals is a good one. Many receiver circuits are nearly
generic and those in the RCA books are generally reliable,
at least as starting points and as aids in understanding how
they are constructed.
There are other editions of the RCA books on line, some
are at the Pete Milett site above.
I would also not discount the American Radio Relay
League _Radio Amateur's Hanbook_ and the various editions of
the VHF handbooks. These will have a lot of practical
information on layout and the pitfalls of working at VHF and
with high IF frequencies.


--

--
Richard Knoppow
Los Angeles
WB6KBL

Hi John:

The 6BA7 might be a poor choice. It will have the highest noise figure of
all the tubes you mentioned. I have never seen the 6BA7 used above 50 MHz,
and that was only with an rf amplifier.

On the other hand, a triode or pentode would be acceptable, though it would
hot have good weak signal performance.

The easy approach for a circuit is to look at the K4XL/ BAMA website and
steal the mixer - oscillator circuit from one of the many receiver
schematics available. There are some crystal controlled converters and you
can steal the electronics from them. Ignore the rf amplifier if you are not
going to use one. Tecraft made good converters, as did Filter King. Ameco
also made converters and National had a series to go with the NC-300.

After awhile, you can build stuff like converters without a schematic - you
just start picking up parts and start soldering. The triode has a bit lower
noise operating as a converter. Decades ago, there was a 6J6 converter
known as a ZL converter. That was simple and worked well.

You could also look around for an old tubed vhf mobile rig that would be
available for hauling away. Steal the converter out of one of those. I
have one here in Western Oregon you can haul away.

73, Colin K7FM

73, Colin K7FM