813's as Modulators
 

My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.



I bought a little solid state audio pre-amp and driver that puts out
about 4W at 8 ohms. This is probably not enough to drive the 813's so I
need more poop and some ideas on how to drive the modulators with the 8
ohm amp.

There are some in progress pics of this project at
http://schmidling.com/radio.htm


Jack K9ACT


--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

813's as Modulators
 

Jack Schmidling wrote:

My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.



I bought a little solid state audio pre-amp and driver that puts out
about 4W at 8 ohms. This is probably not enough to drive the 813's so I
need more poop and some ideas on how to drive the modulators with the 8
ohm amp.

There are some in progress pics of this project at
http://schmidling.com/radio.htm


Jack K9ACT


If I have my numbers right, a peak envelope power of 1500 watts equates
to a carrier power of 375 watts, which requires an audio amplifier to
put out 200 watts (to round up a bit).

If the 813's were wired as pentodes they could deliver 600 watts in AB-2
with less than a watt on the grids. I'm not at all sure of what you'd
lose using them as triodes, but I'll bet they'd deliver 200W in AB-1
with no grid power consumed at all.

Antique Radio Supply (www.tubesandmore.com) has a reprint of an RCA
transmitting tube handbook that has extensive data on the 813 -- it
should have enough data in there to figure out the design stuff, and
directions in the front on how to use the data.

--

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

813's as Modulators
 

On Feb 21, 7:27 am, Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.

My sole experience is using them not in triode mode, but in pentode
mode. The RCA specs tell about using a pair in AB1 as a modulator.
Specs he

http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf

In triode mode (plate connected to screen) you'd be limited to 750
volts on the plate if you follow the spec sheet religiously, which
will really hurt you in terms of AF power ability. I'm guessing that a
pair of 813's used this way would not put out more than 120W of audio
power. Many beam power tubes will take more volts on the screen than
the spec sheet will say without arcing over but I cannot tell you
about this mode on an 813.

In pentode mode you have to do a fairly stiff screen supply (which you
might have elsewhere in your line-up). Don't forget the -80V or so
grid bias too.

The "classic" way of doing all this is with a pair or maybe two pairs
of 811A's, which are zero-bias as a class B modulator. You can easily
get 250W of audio out of a pair or 500W out of two pairs. Driving
power isn't zero but it's very moderate (less than 10 watts).

The tubes for all this is easy - but you've got a modulation
transformer lined up already? Choosing impedances to match that
transformer is probably more relevant than any dickering about tubes.

Tim.

813's as Modulators
 

Tim Shoppa wrote:
On Feb 21, 7:27 am, Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.

My sole experience is using them not in triode mode, but in pentode
mode. The RCA specs tell about using a pair in AB1 as a modulator.
Specs he

http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf

In triode mode (plate connected to screen) you'd be limited to 750
volts on the plate if you follow the spec sheet religiously, which
will really hurt you in terms of AF power ability. I'm guessing that a
pair of 813's used this way would not put out more than 120W of audio
power. Many beam power tubes will take more volts on the screen than
the spec sheet will say without arcing over but I cannot tell you
about this mode on an 813.

In pentode mode you have to do a fairly stiff screen supply (which you
might have elsewhere in your line-up). Don't forget the -80V or so
grid bias too.

The "classic" way of doing all this is with a pair or maybe two pairs
of 811A's, which are zero-bias as a class B modulator. You can easily
get 250W of audio out of a pair or 500W out of two pairs. Driving
power isn't zero but it's very moderate (less than 10 watts).

The tubes for all this is easy - but you've got a modulation
transformer lined up already? Choosing impedances to match that
transformer is probably more relevant than any dickering about tubes.

Tim.

A better way to use the 813 in triode mode is to connect the two grids
together. This creates a HI-MU zero bias triode. There is some data
for this in old handbooks for grounded grid operation. However running
them in grid driven mode as zero bias triodes would be identical, except
for lower driving power.

813's as Modulators
 

ken scharf wrote:

A better way to use the 813 in triode mode is to connect the two grids
together. This creates a HI-MU zero bias triode.

That is the way I am using them. Got it all together last night and had
a short QSO with a good audio report. It takes very little Ranger
audio to get 100% mod.

Unfortunately, the plate tuning cap started arcing so I had to shut it
down and have a look today.

js

--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com

813's as Modulators
 

On Feb 22, 8:35 pm, ken scharf wrote:
TimShoppawrote:
On Feb 21, 7:27 am, Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.

My sole experience is using them not in triode mode, but in pentode
mode. The RCA specs tell about using a pair in AB1 as a modulator.
Specs he

http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf

In triode mode (plate connected to screen) you'd be limited to 750
volts on the plate if you follow the spec sheet religiously, which
will really hurt you in terms of AF power ability. I'm guessing that a
pair of 813's used this way would not put out more than 120W of audio
power. Many beam power tubes will take more volts on the screen than
the spec sheet will say without arcing over but I cannot tell you
about this mode on an 813.

In pentode mode you have to do a fairly stiff screen supply (which you
might have elsewhere in your line-up). Don't forget the -80V or so
grid bias too.

The "classic" way of doing all this is with a pair or maybe two pairs
of 811A's, which are zero-bias as a class B modulator. You can easily
get 250W of audio out of a pair or 500W out of two pairs. Driving
power isn't zero but it's very moderate (less than 10 watts).

The tubes for all this is easy - but you've got a modulation
transformer lined up already? Choosing impedances to match that
transformer is probably more relevant than any dickering about tubes.

Tim.

A better way to use the 813 in triode mode is to connect the two grids
together. This creates a HI-MU zero bias triode.

My experiments at audio have shown that this mode causes more non-
linearity than simple pentode mode. But... maybe not anymore than
you'd get from a similar-sized high-mu zero bias triode, as those are
fairly nonlinear too!

My experiments were with much smaller tubes (807's and 6L6's and the
like) than the 813's we're talking about here. Most of the extant
SPICE models do pretty bad when you tie the screen and control grid
together (not surprising!)

Tim.

813's as Modulators
 

Jack Schmidling wrote:
ken scharf wrote:

A better way to use the 813 in triode mode is to connect the two grids
together. This creates a HI-MU zero bias triode.


That is the way I am using them. Got it all together last night and had
a short QSO with a good audio report. It takes very little Ranger
audio to get 100% mod.

Unfortunately, the plate tuning cap started arcing so I had to shut it
down and have a look today.

js

Please post a schematic on your web site!

--

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

813's as Modulators
 

On Feb 23, 10:52 am, Tim Wescott wrote:
Please post a schematic on your web site!

Tim -
You might be surprised how few parts there are in a classical zero-
bias class-B modulator.

There's a schematic in the 50's/60's ARRL handbooks, and the ENTIRE
circuit is two 811A's and an output transformer. That's it!

This simplicity is the cunning advantage and popularity of zero-bias
triodes (or zero-bias-triode operation of the other tubes). Once you
get to beam tubes/pentodes with all their bias supplies and screen
supplies and power-supply-sequencing etc. you start getting into
having to draw schematics.

Later editions added a pi filter on the output to reduce "splatter"
and a control relay to remove plate voltage on receive.

Tim.

813's as Modulators
 

Tim Shoppa wrote:

On Feb 23, 10:52 am, Tim Wescott wrote:

Please post a schematic on your web site!


Tim -
You might be surprised how few parts there are in a classical zero-
bias class-B modulator.

There's a schematic in the 50's/60's ARRL handbooks, and the ENTIRE
circuit is two 811A's and an output transformer. That's it!

This simplicity is the cunning advantage and popularity of zero-bias
triodes (or zero-bias-triode operation of the other tubes). Once you
get to beam tubes/pentodes with all their bias supplies and screen
supplies and power-supply-sequencing etc. you start getting into
having to draw schematics.

Later editions added a pi filter on the output to reduce "splatter"
and a control relay to remove plate voltage on receive.

Tim.

I guess I'm more interested in knowing what the supply and grid voltages
are, transformer ratios, a bill of materials, that sort of thing.

Yes, you are correct that the schematic of a one cubic foot, 100 pound
monster amplifier could be drawn on a business card, but still...

--

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

813's as Modulators
 

On Feb 23, 12:30 pm, Tim Wescott wrote:
I guess I'm more interested in knowing what the supply and grid voltages
are, transformer ratios, a bill of materials, that sort of thing.

I too am interested in the transformers that he's using. Tubes are
easy to come by in comparison to appropriate modulation transformers.

And it can't be the super-simplest implementation because I see some
pictures with little power transformers with diodes and electrolytics
forming some kind of bias supply.

Then you have to work hard to not burn up your favorite modulation and
plate transformers :-). Plate chokes, I can rewind them when they
catch on fire now!

Tim.