Hi,

I should introduce myself. I am a Ham radio operator who likes to tinker
with audio (hollow state gain stages), and Amplitude Modulation. I have
several transmitters that run AM on 80-40 meters. Getting the audio low in
distortion, and maximized in terms of modulation percentage without making a
signal broad from over modulation is my goal. A lot of what I do has overlap
into what this group is interested in. I discovered this group quite by
accident, and I was amazed at the content, and expertise seen here. I posted
a few replies to some entries. Please point out to me when I am off base, or
just plain wrong.

I have several Central Electronics transmitters type 20A. These were
built in the 1950's, and produced Single Sideband using what is known as the
phasing method to filter out the carrier, and adjacent sideband. The CE
company pioneered SSB to the hams as ready made as well as kit transmitters.
Back then SSB was produced basically from CE equipment, or from "home
brewing". These transmitters (hams call them rigs) could also produce AM,
PM, CW and the RF output was about 15 watts from a pair of 6AG7's.

The CE 20A transmitter needed a boost in power, and I opted for
beefing up what was there by changing tube types, and power supply changes.
I needed a good linear tube with high gain that also had very low
interelectrode capacitance (particularly Cgp). I did not want to go through
the complexity to neutralize the Cgp capacity on a multiband transmitter.
Searching through my tube manuals I stumbled on the 7868 Pentode with a
Novar socket. High gain, linear, low Cgp (abt. .15 pf), dual Grid two
connections (desirable for RF). It was small, and capable of 45 watts of
audio (ab1) with 450 volts on the plate. I only had about 18 volts of peak
grid drive available, and the 7868 gain seemed a match for Ab1 operation. To
use this tube I needed to change the sockets to Octal, so being lazy I
proceeded to search for another tube that was octal. Seems like the 7868,
6GM5, and 7591 are all in the same family with similar if not identical
ratings. The 7591 was Octal. I started going through my tube crates, and I
had three, and another four in an old Fisher stereo amplifier. All tested
good. I had no idea at the time that these were fast becoming valuable to
the audiophile, and prices for them have went into the stratosphere.

I am getting long winded so I will wrap this up. Bottom line is that a
pair of paralled 7591's (Ab1) with 550v on the plates, 375 (regulated) on
the screens, and -18 on the control grid will put out about 55 watts RF
carrier, 60 watts PEP SSB, and 12 watts AM carrier (PEP is 4 X carrier at
100% modulation). The linearity is incredible. It seems that these tubes
work up to 7 Mhz, and still behave like they were in my old Fisher audio
amplifier. I haven't tried going higher in frequency yet.

I also tried some Sovtek EL-34's, but the Cgp was over 1 pf, and the
gain was lower (had to set G1 at about -25 volts), so I could not fill out
the Ab1 operating range with my RF driver. They did work on 1.8 Mhz, but I
had to remove one tube at 4 Mhz, and no way at 7 Mhz. To use tubes like that
I must neutralize the Cgp, and increase the drive to the output tubes.

I have done a lot of searching, and I have not found much about using
high gain (Gm) Hi-Fi audio tubes at RF frequencies. One exception is the
6550 where several transmitters have been built using these up to 30 Mhz
(requires a neutralizing circuit though).

Does anybody have experience with the 7591-EH? I know it is larger than
a 7591, and works fine in audio amplifiers (if it fits). I wonder if this
new Electro Harmonic type of 7591 will work at RF frequencies like the plain
old 7591 of yester year does? So far my first set of 7591's are hanging in
there. These are a very stable tube if the G1, and G2 voltages are held
constant. The DC operating point shows almost zero drift. The Plate B+ idles
at 600 volts (a little high I know), and I set the cathode current at 30 ma
per tube. A little DC computer fan keeps the air moving.

Regards,
Jim Candela
ARS WD5JKO

Welcome to the group :-)

IMO, the best way of beefing a rig's output is with an amp, not changing
the innards ;-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*

Hi,
One thing to keep in mind is that the primary markets for new manufacture
tubes are guitar amps, and a little bit of hi-fi. They are not always the
same as NOS tubes even if they share the nomenclature.

I would stick with the old tubes for best results, 807s and 1625s are always
around and cheap as chips. IIRC, they are good to 30MHz...

Anyway welcome to the group!

Jim Candela wrote:

Hi,

I should introduce myself. I am a Ham radio operator who likes to tinker
with audio (hollow state gain stages), and Amplitude Modulation. I have
several transmitters that run AM on 80-40 meters. Getting the audio low in
distortion, and maximized in terms of modulation percentage without making a
signal broad from over modulation is my goal. A lot of what I do has overlap
into what this group is interested in. I discovered this group quite by
accident, and I was amazed at the content, and expertise seen here. I posted
a few replies to some entries. Please point out to me when I am off base, or
just plain wrong.

I have several Central Electronics transmitters type 20A. These were
built in the 1950's, and produced Single Sideband using what is known as the
phasing method to filter out the carrier, and adjacent sideband. The CE
company pioneered SSB to the hams as ready made as well as kit transmitters.
Back then SSB was produced basically from CE equipment, or from "home
brewing". These transmitters (hams call them rigs) could also produce AM,
PM, CW and the RF output was about 15 watts from a pair of 6AG7's.

The CE 20A transmitter needed a boost in power, and I opted for
beefing up what was there by changing tube types, and power supply changes.
I needed a good linear tube with high gain that also had very low
interelectrode capacitance (particularly Cgp). I did not want to go through
the complexity to neutralize the Cgp capacity on a multiband transmitter.
Searching through my tube manuals I stumbled on the 7868 Pentode with a
Novar socket. High gain, linear, low Cgp (abt. .15 pf), dual Grid two
connections (desirable for RF). It was small, and capable of 45 watts of
audio (ab1) with 450 volts on the plate. I only had about 18 volts of peak
grid drive available, and the 7868 gain seemed a match for Ab1 operation. To
use this tube I needed to change the sockets to Octal, so being lazy I
proceeded to search for another tube that was octal. Seems like the 7868,
6GM5, and 7591 are all in the same family with similar if not identical
ratings. The 7591 was Octal. I started going through my tube crates, and I
had three, and another four in an old Fisher stereo amplifier. All tested
good. I had no idea at the time that these were fast becoming valuable to
the audiophile, and prices for them have went into the stratosphere.

I am getting long winded so I will wrap this up. Bottom line is that a
pair of paralled 7591's (Ab1) with 550v on the plates, 375 (regulated) on
the screens, and -18 on the control grid will put out about 55 watts RF
carrier, 60 watts PEP SSB, and 12 watts AM carrier (PEP is 4 X carrier at
100% modulation). The linearity is incredible. It seems that these tubes
work up to 7 Mhz, and still behave like they were in my old Fisher audio
amplifier. I haven't tried going higher in frequency yet.

I also tried some Sovtek EL-34's, but the Cgp was over 1 pf, and the
gain was lower (had to set G1 at about -25 volts), so I could not fill out
the Ab1 operating range with my RF driver. They did work on 1.8 Mhz, but I
had to remove one tube at 4 Mhz, and no way at 7 Mhz. To use tubes like that
I must neutralize the Cgp, and increase the drive to the output tubes.

I have done a lot of searching, and I have not found much about using
high gain (Gm) Hi-Fi audio tubes at RF frequencies. One exception is the
6550 where several transmitters have been built using these up to 30 Mhz
(requires a neutralizing circuit though).

Does anybody have experience with the 7591-EH? I know it is larger than
a 7591, and works fine in audio amplifiers (if it fits). I wonder if this
new Electro Harmonic type of 7591 will work at RF frequencies like the plain
old 7591 of yester year does? So far my first set of 7591's are hanging in
there. These are a very stable tube if the G1, and G2 voltages are held
constant. The DC operating point shows almost zero drift. The Plate B+ idles
at 600 volts (a little high I know), and I set the cathode current at 30 ma
per tube. A little DC computer fan keeps the air moving.

Regards,
Jim Candela
ARS WD5JKO

If I was going to build my own transmitters, I'd use tubes that had top caps,
such as 807, 6146, TT21, 6CD6,or 6CM5.
These suit the typical layout where there is an above chassis mounted plate
coil,
and the plate wire can be short, and the chassis is a natural screen against
parasitics.
Another good tube would be 13E1, and two could make about 300watts,
with a supply that isn't too high.

I am yet to move up the band beyond audio
to the world of hf.
I might have to move house, if I wanted a decent antenna,
which to me seems vital for ham use.

Patrick Turner.

Jim Candela wrote:
[...]

Does anybody have experience with the 7591-EH? I know it is larger than
a 7591, and works fine in audio amplifiers (if it fits). I wonder if this
new Electro Harmonic type of 7591 will work at RF frequencies like the plain
old 7591 of yester year does? So far my first set of 7591's are hanging in
there. These are a very stable tube if the G1, and G2 voltages are held
constant. The DC operating point shows almost zero drift. The Plate B+ idles
at 600 volts (a little high I know), and I set the cathode current at 30 ma
per tube. A little DC computer fan keeps the air moving.

Hi Jim,

I've looked around to see if there are spec sheets on the new EH 7591's
and have come up empty-handed. Maybe Ned Carlson (Triode Electronics)
will step in if he's around and find you some specs (especially
capacitance). I wouldn't be surprised, though, if the newer tubes are
not as suitable for RF as the vintage ones. For example, I *do* know
that the newer JJ/Tesla and similar 12AT7's do not work very well in RF
circuits, being optimized strictly for audio.

You mentioned the 6550, I think that would be a good choice as a high-mu
RF pentode. You probably know about the 6146, which is not as high mu as
7591 but still quite a bit higher than 6L6, 807 etc. Another one is the
6159, which is a 26 volt version of the 6146. This one is quite readily
available at low cost, presumably because of the oddball filament voltage.

Good luck!
Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Jim Candela wrote:
Hi,

[....]

Does anybody have experience with the 7591-EH? I know it is larger than
a 7591, and works fine in audio amplifiers (if it fits). I wonder if this
new Electro Harmonic type of 7591 will work at RF frequencies like the plain
old 7591 of yester year does? So far my first set of 7591's are hanging in
there. These are a very stable tube if the G1, and G2 voltages are held
constant. The DC operating point shows almost zero drift. The Plate B+ idles
at 600 volts (a little high I know), and I set the cathode current at 30 ma
per tube. A little DC computer fan keeps the air moving.

PS - also consider 832A's or 829B's, with both sections connected in
parallel.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Hi Jim,

Thanks for the interesting post. I can't really vouch for 7591's
because I don't have a lot of experience with that tube. I would be
interested in your results with EH tubes at RF.

Many audio tubes do make good transmitting tubes, especially the GE
6550A; two of my upcoming projects include an AM/CW transmitter based
on the Heathkit DX-35 but with GE 6550A in the final and an all-tube
FRS base station with 6550 ouput. 6L6's and 5881's were also popular
in homebrew single-tube transmitter projects. Also, many of the well
known audio triodes have pretty low interelectrode capacitance and are
good performers at RF. I've used an RCA 2A3 at 80 meters with pretty
good results.

Regards,

C.W.

Casino wrote:
[...] an all-tube
FRS base station with 6550 ouput. [...]

FRS? As in "Family Radio Service", 450 mHz? I think you'll have a heck
of a time trying to get 6550's to work in that frequency range.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Behold, Fred Nachbaur scribbled:

Casino wrote:
[...] an all-tube
FRS base station with 6550 ouput. [...]

FRS? As in "Family Radio Service", 450 mHz? I think you'll have a heck
of a time trying to get 6550's to work in that frequency range.

Cheers,
Fred

Sounds like 6J6 drivers and 4(CX)-xxxx power to me 8-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*

Fred Nachbaur wrote:

Casino wrote:
[...] an all-tube
FRS base station with 6550 ouput. [...]

FRS? As in "Family Radio Service", 450 mHz? I think you'll have a heck
of a time trying to get 6550's to work in that frequency range.

Why? 6550s work nicely at DC, so should 450 milliherz be a problem? ;-)

Best regards,

Mikkel C. Simonsen

Mikkel C. Simonsen wrote:
Fred Nachbaur wrote:

Casino wrote:

[...] an all-tube
FRS base station with 6550 ouput. [...]

FRS? As in "Family Radio Service", 450 mHz? I think you'll have a heck
of a time trying to get 6550's to work in that frequency range.


Why? 6550s work nicely at DC, so should 450 milliherz be a problem? ;-)

Best regards,

Mikkel C. Simonsen

haha, very funny. :-

Old habits die hard. I meant MHz, of course.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Yes, that's right. FRS. That's 462 MHz and/or 467 MHz with either a
GE 6550A or Tung-Sol 6550. It will work, but maybe not efficiently.
I've had no problems generating microwaves at 2.2 GHz with 2N3904
transistors (rated to 300 MHz) and a single 9 volt battery, and 6550's
at 460 MHz shouldn't be a problem at 5 or 10 watts either.


FRS? As in "Family Radio Service", 450 mHz? I think you'll have a heck
of a time trying to get 6550's to work in that frequency range.

Behold, Fred Nachbaur scribbled:

OK, well let us know how you make out. It'll be interesting to see what
kind of power you get from these.

Cheers,
Fred

Or he pulls a Tim W. and makes little ingots ;-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*

Behold, Patrick Turner flashed in code from a keyed 4-1000A filament:



Jim Candela wrote:

Group;

Hey thanks a lot for all the comments. I have a lot to chew on.

Regards,
Jim

Watch out when you chew up KT88 from russia, they cause indigestion
unless you wash them down with vodka.

Patrick Turner.

Da!

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
----------------------------------------
| GeeK Zone * scripts * articles * forum |
| http://geek.scorpiorising.ca |
----------------------------------------

"Gregg" wrote in message
. ..
Behold, Patrick Turner flashed in code from a keyed 4-1000A filament:


Watch out when you chew up KT88 from russia, they cause indigestion
unless you wash them down with vodka.

Patrick Turner.

Da!

--

Ne! Bolje su kad se zaliju sa sljivovicom!
)

Gregg
*It's probably useful, even if it can't be SPICE'd*
----------------------------------------
| GeeK Zone * scripts * articles * forum |
| http://geek.scorpiorising.ca |
----------------------------------------

--
Choky
Prodanovic Aleksandar
YU