RadioBanter - Amplifier with a pair of 6LQ6's

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Amplifier with a pair of 6LQ6's

I recently got a good deal (five bucks) on a "ten meter" amp with two 6LQ6
tubes (no, I don't intend to go below 28.3 mHz with it). It seems to work,
more or less, but I was wondering something.

At about 28.5 mHz, it puts out 43 watts with 4 watts driving it on AM (using
a CB converted to 10 meters). Is this normal, or are the tubes getting
soft? I understand a pair of 6LQ6 tubes are rated for a lot higher output,
but I don't know how much I can feed into it without overdriving it. If I
feed in 7 watts AM or 25 watts SSB with an HTX-10, will I earn the splatter
award of the year?

Thanks.

Brian

Yep, they are weak. The worst part is the price they want for replacements!
Rob
"Brian" wrote in message
...
I recently got a good deal (five bucks) on a "ten meter" amp with two 6LQ6
tubes (no, I don't intend to go below 28.3 mHz with it). It seems to
work,
more or less, but I was wondering something.

At about 28.5 mHz, it puts out 43 watts with 4 watts driving it on AM
(using
a CB converted to 10 meters). Is this normal, or are the tubes getting
soft? I understand a pair of 6LQ6 tubes are rated for a lot higher
output,
but I don't know how much I can feed into it without overdriving it. If I
feed in 7 watts AM or 25 watts SSB with an HTX-10, will I earn the
splatter
award of the year?

Thanks.

Brian

Brian:

How much output you get out of the amp does depend on the condition of the
tubes, but you didn't say whether the tubes were grid driven or cathode
driven (grounded grid).

If they are run grounded grid, 4 watts in to get 43 watts out is very
reasonable for a pair of healthy tubes. Depending on plate voltage and the
bias setup, you could very well drive it to higher output. But keep in mind
that in AM operation the peak output can be up to 4 times the unmodulated
carrier power, so with 43 watts of carrier output you can be running over
170 watts peak output at 100 percent modulation.

You may be able to drive it with 25 watts of SSB and get close to 250 watts
out. Be sure to get some stations to tune around your signal and let you
know if they hear anything bad.

73,

Roger K6XQ

"Brian" wrote in message ...
I recently got a good deal (five bucks) on a "ten meter" amp with two 6LQ6
tubes (no, I don't intend to go below 28.3 mHz with it). It seems to work,
more or less, but I was wondering something.

At about 28.5 mHz, it puts out 43 watts with 4 watts driving it on AM (using
a CB converted to 10 meters). Is this normal, or are the tubes getting
soft? I understand a pair of 6LQ6 tubes are rated for a lot higher output,
but I don't know how much I can feed into it without overdriving it. If I
feed in 7 watts AM or 25 watts SSB with an HTX-10, will I earn the splatter
award of the year?

Thanks.

Brian

Well, using it as an AM linear isn't too good in the first place, as
the overall efficiency is less than 30%.

AS in the other posts, check plate input power (plate voltage x plate
current). I'm sure it is running grounded cathode, and you should be
at about 200 mils plate current or better if the tubes are good.
Plate voltage should be anywhere from 600 to 800 volts.

Use the CW mode and tickle it with a watt or two of drive and figure
out the power gain (Power out divided by power in). Advance the drive
until the amp saturates i.e. no further output for increasing input.
If the plate current is less than 200 mils then you've got flat tubes
and they'll set you back almost a C note for a pair. That's probably
why you got the amp cheap.

Also beware that these amplifiers weren't built to exacting
specifications for linearity, or spurious products. I'd always use a
LP filter on the output, and an antenna tuner to provide another tuned
circuit to clean things up a bit.

What you might do, is find a 'higher' voltage 6LQ6, one with a
different first digit (filament voltage). These quite often go very
cheap. Then provide a filament transformer to light them. For
example, 25LQ6 ---- 25.2vac transformer. If it is a mobile (12vDC
input) amp, use a pair of 12LQ6's and tie the filaments in parallel.

If you can't find tubes, I'd gut things out and put in a pair of
6146's. They're readily availabe, rugged, and can quite often be had
for $5-10 a pair. You'll have to change the tube sockets of course to
octal, but the operating voltages and the Pi network should be close
to the same. And you won't detract from the value of the amp --
you'll gain.

Gary Hildebrand WA7KKP

I'd gut things out and put in a pair of 6146's.
6146's can run grounded grid ?

"Gary hildebrand" wrote in message
om...
"Brian" wrote in message
...
I recently got a good deal (five bucks) on a "ten meter" amp with two
6LQ6
tubes (no, I don't intend to go below 28.3 mHz with it). It seems to
work,
more or less, but I was wondering something.

At about 28.5 mHz, it puts out 43 watts with 4 watts driving it on AM
(using
a CB converted to 10 meters). Is this normal, or are the tubes getting
soft? I understand a pair of 6LQ6 tubes are rated for a lot higher
output,
but I don't know how much I can feed into it without overdriving it. If
I
feed in 7 watts AM or 25 watts SSB with an HTX-10, will I earn the
splatter
award of the year?

Thanks.

Brian

Well, using it as an AM linear isn't too good in the first place, as
the overall efficiency is less than 30%.

AS in the other posts, check plate input power (plate voltage x plate
current). I'm sure it is running grounded cathode, and you should be
at about 200 mils plate current or better if the tubes are good.
Plate voltage should be anywhere from 600 to 800 volts.

Use the CW mode and tickle it with a watt or two of drive and figure
out the power gain (Power out divided by power in). Advance the drive
until the amp saturates i.e. no further output for increasing input.
If the plate current is less than 200 mils then you've got flat tubes
and they'll set you back almost a C note for a pair. That's probably
why you got the amp cheap.

Also beware that these amplifiers weren't built to exacting
specifications for linearity, or spurious products. I'd always use a
LP filter on the output, and an antenna tuner to provide another tuned
circuit to clean things up a bit.

What you might do, is find a 'higher' voltage 6LQ6, one with a
different first digit (filament voltage). These quite often go very
cheap. Then provide a filament transformer to light them. For
example, 25LQ6 ---- 25.2vac transformer. If it is a mobile (12vDC
input) amp, use a pair of 12LQ6's and tie the filaments in parallel.

If you can't find tubes, I'd gut things out and put in a pair of
6146's. They're readily availabe, rugged, and can quite often be had
for $5-10 a pair. You'll have to change the tube sockets of course to
octal, but the operating voltages and the Pi network should be close
to the same. And you won't detract from the value of the amp --
you'll gain.

Gary Hildebrand WA7KKP

In article , Brian wrote:
I recently got a good deal (five bucks) on a "ten meter" amp with two 6LQ6
tubes (no, I don't intend to go below 28.3 mHz with it). It seems to work,
more or less, but I was wondering something.

At about 28.5 mHz, it puts out 43 watts with 4 watts driving it on AM (using
a CB converted to 10 meters). Is this normal, or are the tubes getting
soft? I understand a pair of 6LQ6 tubes are rated for a lot higher output,
but I don't know how much I can feed into it without overdriving it. If I
feed in 7 watts AM or 25 watts SSB with an HTX-10, will I earn the splatter
award of the year?

What B+ is is running?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Roger Leone" wrote in message
...
Brian:

How much output you get out of the amp does depend on the condition of the
tubes, but you didn't say whether the tubes were grid driven or cathode
driven (grounded grid).

If they are run grounded grid, 4 watts in to get 43 watts out is very
reasonable for a pair of healthy tubes. Depending on plate voltage and
the
bias setup, you could very well drive it to higher output. But keep in
mind
that in AM operation the peak output can be up to 4 times the unmodulated
carrier power, so with 43 watts of carrier output you can be running over
170 watts peak output at 100 percent modulation.

You may be able to drive it with 25 watts of SSB and get close to 250
watts
out. Be sure to get some stations to tune around your signal and let you
know if they hear anything bad.

73,

Roger K6XQ

"Roger Leone" wrote in message
...
Brian:

If they are run grounded grid, 4 watts in to get 43 watts out is very
reasonable for a pair of healthy tubes. Depending on plate voltage and
the
bias setup, you could very well drive it to higher output. But keep in
mind
that in AM operation the peak output can be up to 4 times the unmodulated
carrier power, so with 43 watts of carrier output you can be running over
170 watts peak output at 100 percent modulation.

You may be able to drive it with 25 watts of SSB and get close to 250
watts
out. Be sure to get some stations to tune around your signal and let you
know if they hear anything bad.

73,

Roger K6XQ

That's assuming this amplifier is capable of remaining linear all
the way to 250 watts output... I'd tend to doubt it.

Pete