Ken Scharf wrote in message ...
N2EY wrote:
Ken Scharf wrote in message ...

I have several ARC-5 rx three gang variable
caps, these have a real nice vernier drive on them. Just attach a
larger dial, or a drive pulley for a slide rule dial and you have
something as nice as the Eddystone.


Yep! Note, however, that except for the 6-9.1 MHz version they have
plate shapes that won't yield a linear dial on the ham bands. The
6-9.1 MHz version is almost pure straight line capacitance, and is
only 62 pf per section or so.

I would think you want a 'stright line frequency' where the capacitance
changes as the square of the rotation. IE: at half open position the
capacitance is down to 1/4 of the value available at full mesh.

That only works with a tuning range of about 2:1. The type of ham band
rx being described has a much more limited tuning range, and needs an
almost-linear capacitance curve to get a linear dial.

For the ultimate, though, use the capacitor from an ARC-5 tx, BC-221
or LM freq meter. Nice gear drives and an even bigger dial than the rx
versions. Only one section though, but good for "unit oscillator"
construction of the HFO. (Only needs to cover the range 5.2-5.7 MHz)

I have a few of those ARC-5 tx caps in my junk box, and at least one
with the dial drive. It binds just a bit though as if the drive shaft
is slightly bent. Just enough to be noticed while turning it with a
good sized knob attached.

Somebody hammered on the shaft to get the knob off, or it was dropped.
Ruined unless you machine a new shaft. *sigh*


With such a design, it would be a good idea to use a Pullen mixer and
no RF stage in the 80/40 bandimage section (at least). There's also
the problem of secondary images - you need a lot of selectivity in the
front end and 1.7 MHz IF section to avoid signals 170 kHz from the
desired one seeping into the second IF. The 2B used a 455 kHz
first-fixed-IF for this reason. IM performance is compromised by the
fact that the selectivity is so far from the antenna.

With a good 'roofing' filter ahead of the final mixer you should be able
to knock down the images from the 1.7mhz if before reaching the 85khz
one.

Exactly!

I have three 1.7mhz double tuned cans available, with all three in
series top coupled with gimick caps I should be able to achieve enough
selectivity to avoid secondary image problems. The use of a 455khz IF
is also a good idea, and I have a few Collins mech. filters that could
be used there as well (a 2.0khz bw out of an R390, a 2.7khz that looks
like an S line filter, and a large unit of 1.8khz bw that came out of an
if adaptor for an HRO-50 or 60).

NICE!

With proper layout and shielding I don't see a problem using the 6AR11
in the two stage IF. Hell, they were designed for use at 47mhz in a
dual stage TV if where cross coupling would be even more of a problem!

Not really. The TV applications were broadband and low gain compared
to what you're trying to do at HF. And the manufacturers could do a
whole bunch of not-obvious tricks and PC board prototypes to get what
they wanted.

The 6AR11 is an excellent semi-remote cutoff amplifier with good
overload and cross mod specs equal to the pentodes used in the HBR.

I agree 100% - it's just that cascading them at 85 kHz may prove
troublesome.

OTOH, in a 455 kHz design where the selectivity comes from the xtal
filters, you may be OK.

Some alternatives to consider:

1) Get some xtals in the 1700 kHz range and build a filter or filters
so that the 85 kHz IF is not needed. Perhaps a variable-bandwidth
filter using a multigang variable capacitor could made, using 4
crystals and a three-gang capacitor. This approach solves the
secondary image problem, too.

2) Have a single tuning range of 3.5 - 4.1 MHz and the fixed IF at 455
kHz or thereabouts. Would require dual conversion on 40 but would also
allow use of standard 455 kHz IF filters. Or make your own from
FT-241A crystals (which is what I did way back when).

3) Use the filters and heterodyne xtals from a junked transceiver as
the basis of a homebrew rx. Hangar-queen/basket case HW-100s, -101s,
and SB-line units show up on ePay and at 'fests for quite low prices -
far below what the filters and xtals would cost separately. Other
types of transceiver can also be good parts sources (Tempo One comes
to mind - nice VFO mechanism in them, and the IF is 9 MHz IIRC). KJ4KV
turned an early-version FT-101 into a pretty interesting receiver this
way.

As for a new design with todays parts, well I have several ideas here
begging to be tried. I have quite a few 'Samples' from Analog Devices
including many DDS chips. The 400mhz DDS parts would make a great HFO
for a single conversion receiver with an IF at 9mhz (again more junkbox
filters, including about 1/2 dozen 9mhz 3.2khz 8 pole units out of
Gonset Sidewinder rigs purchased at Dayton years ago).

The big question with DDS is the spectral purity of the output. Even
weak artifacts can cause all kinds of birdies and other troubles in
today's RF environment. This is one reason so many folks find the old
designs so appealing - they are "clean" except for the obvious things
like images.

Thought I'd use
three IF stages with a filter between EACH stage and a final one before
the detector.

Very good idea. The best filter goes first, then "cleanup" filters.

Probably use MC1350's or ancient CA3028's as the IF amps.
The front end would use a quad DFET (Siliconix) switching mixer that
was in the handbook for several years driven at twice the required HFO
frequency (to get push-pull drive using a D Flipflop). Bowing to the
junkbox, the front end would be a double or triple tuned filter using
toriods bandswitched using a standard coil tuner chassis as the switch.
(The toriods fit nicely in the tuner strips). An 8051 series micro
drives the DDS, frequency display on 7 segment LEDs (I have enough of
these to choke an alligator) and a rotary encoder drives the micro to
select frequency.

And the LEDs glow!

Maybe I'm crazy, but I still wonder about puting in a second conversion
down to 85khz to use those ARC5 IF cans! Anything wrong with a hybrid
radio using the latest IC's and microprocessors along with 60's
compactrons! (Just what kind of bandwidth will a properly aligned 85khz
if strip using arc5 cans give?)

The original design gave a decent SSB passband if the rods were pulled
up. But the shape factor isn't the best and the selectivity winds up
so far from the antenna....

The above radio would probably end up being a transceiver (cause the
extra circuity isn't much) but the finals would end up being 1 or 2 1625
bottles 'cause I have at least a dozen of 'em in the junk box.

Great bottles but the sockets are a pain. Unless you hack up an ARC-5
tx.

The biggest headache I've encountered in transceiver design is finding
a heterodyne combination that works in both directions and uses
available components. All of the classic ones are compromises in one
way or another, either on rx or tx.

73 de Jim, N2EY

The big question with DDS is the spectral purity of the output. Even
weak artifacts can cause all kinds of birdies and other troubles in
today's RF environment. This is one reason so many folks find the old
designs so appealing - they are "clean" except for the obvious things
like images.

The big problem with DDS is getting a good anti-aliasing filter on the
output to satisfy Mr. Nyquist. Trying to build a good 'brick wall' low
pass filter is insane. Running the DDS with a clock many times greater
than the highest output frequency helps, with an injection frequency of
39mhz on 10 meters with a 9mhz if a 400mhz DDS clock is 5x the Nyquist
requirement. Rather than using a low pass filter, I'd use switchable
bandpass filters, much more rejection of unwanted spurs and a cleaner
output. Not as simple to build, you have to switch filters to change
bands, but hey, I can always GANG several tv tuner chassis for my
band switch! (I knew those old TV sets I caniblized would come in handy!)


Thought I'd use
three IF stages with a filter between EACH stage and a final one before
the detector.


Very good idea. The best filter goes first, then "cleanup" filters.

Actually those filters are identical. But another idea is a double
conversion with both 9mhz and 455khz if's. Then by tuning the second
conversion oscillator you can 'overlap' the bandpass of both sets of
filters to use only a portion of them, and if you also move the bfo
frequency you can move the position of the signal within the resulting
bandpass.


The above radio would probably end up being a transceiver (cause the
extra circuity isn't much) but the finals would end up being 1 or 2 1625
bottles 'cause I have at least a dozen of 'em in the junk box.


Great bottles but the sockets are a pain. Unless you hack up an ARC-5
tx.
My junk box has a few 1625 size sockets, two ceramic 'plate' types and a
large saddle mount. Actually these sockets are easy to find and not too
expensive. Antique Electronic Supply has them, and they show up on Ebay
often. No need to use ones out of ARC5 xmtrs.

I also have a good number of 6AG7 tubes, they would make a good driver
stage for the 1625s. (BTW 6AG7 is sorta an octal version of the 6CL6).
If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).
Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2 voltages).


The biggest headache I've encountered in transceiver design is finding
a heterodyne combination that works in both directions and uses
available components. All of the classic ones are compromises in one
way or another, either on rx or tx.
If you have enough filters you can put separate filters in the tx and rx
chains, and even use double conversion on the rx but single conversion
on tx. Then you can use a separate tunable bfo for the receiver. The
bandwidth and center frequency of the tx filter can be selected for good
audio, while the rx filter(s) for selectivity. Tx bandwidth is a
function of audio channel filtering, alc, and NOT overdriving anything.

Ken Scharf wrote in message ...
The big question with DDS is the spectral purity of the output. Even
weak artifacts can cause all kinds of birdies and other troubles in
today's RF environment. This is one reason so many folks find the old
designs so appealing - they are "clean" except for the obvious things
like images.

The big problem with DDS is getting a good anti-aliasing filter on the
output to satisfy Mr. Nyquist. Trying to build a good 'brick wall' low
pass filter is insane. Running the DDS with a clock many times greater
than the highest output frequency helps, with an injection frequency of
39mhz on 10 meters with a 9mhz if a 400mhz DDS clock is 5x the Nyquist
requirement.

I agree 100%.

Rather than using a low pass filter, I'd use switchable
bandpass filters, much more rejection of unwanted spurs and a cleaner
output. Not as simple to build, you have to switch filters to change
bands, but hey, I can always GANG several tv tuner chassis for my
band switch! (I knew those old TV sets I caniblized would come in handy!)

A good scrounger approach if you can find tuners with good contacts!

Here's another approach, shamelessly copied from Elecraft: Bandswitch
the low-level circuits with small latching relays and some logic
circuitry (a PIC or TTL can easily do the job). These relays are
available from Digi-Key - Omron is one mfr. - and are not too
expensive in quantity.


Thought I'd use
three IF stages with a filter between EACH stage and a final one before
the detector.


Very good idea. The best filter goes first, then "cleanup" filters.

Actually those filters are identical. But another idea is a double
conversion with both 9mhz and 455khz if's. Then by tuning the second
conversion oscillator you can 'overlap' the bandpass of both sets of
filters to use only a portion of them, and if you also move the bfo
frequency you can move the position of the signal within the resulting
bandpass.

Good ideas but getting less and less simple.

The above radio would probably end up being a transceiver (cause the
extra circuity isn't much) but the finals would end up being 1 or 2 1625
bottles 'cause I have at least a dozen of 'em in the junk box.

Great bottles but the sockets are a pain. Unless you hack up an ARC-5
tx.
My junk box has a few 1625 size sockets, two ceramic 'plate' types and a
large saddle mount. Actually these sockets are easy to find and not too
expensive. Antique Electronic Supply has them, and they show up on Ebay
often. No need to use ones out of ARC5 xmtrs.

Agreed. The ARC-5 ones are good for submounting, though.

I also have a good number of 6AG7 tubes, they would make a good driver
stage for the 1625s. (BTW 6AG7 is sorta an octal version of the 6CL6).

Oh yes, used them in several rigs.

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).

But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2 voltages).

I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.

The biggest headache I've encountered in transceiver design is finding
a heterodyne combination that works in both directions and uses
available components. All of the classic ones are compromises in one
way or another, either on rx or tx.

If you have enough filters you can put separate filters in the tx and rx
chains, and even use double conversion on the rx but single conversion
on tx.

Sure, but by that time you're actually building a separate transmitter
and receiver that share a VFO and not much else.

Then you can use a separate tunable bfo for the receiver. The
bandwidth and center frequency of the tx filter can be selected for good
audio, while the rx filter(s) for selectivity. Tx bandwidth is a
function of audio channel filtering, alc, and NOT overdriving anything.

Agreed!

But since I'm primarily a CW op, the problem is greatly simplified in
some ways. My current homebrew transceiver uses a different het scheme
for tx and rx, sharing a common VFO. Both paths are single conversion.
Requires that the het xtals be dead-on freq (handled by trimmers
across each xtal).

73 es GL de Jim, N2EY

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).


But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00. Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long). I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).



Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2 voltages).


I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.


The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG. If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.

In article , Ken Scharf
writes:

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).

But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

Agreed 100%. Note, however, that the driving power figures in the books are
usually theoretical, derived from tube characteristics. Usually, there's no
provision made for losses in matching circuits, etc.

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

Again, note that those output figures don't include circuit losses.

But in general I think you'd be OK with a single 1625 driving up to a pair if
811s if the matching circuits are lowloss.

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00.

I've got a bunch of used and a bunch of new. Also a big pile of 807s, which
have been my homebrew final of choice for quite some time now.

Here's a hint: Look at the 6BG6-G sweep tube. They are still very common (the
GA's are liked by some audiophiles but the Gs aren't) and their ratings are
very similar to the 807/1625. Looking at their construction, one wonders if the
6BG6-G isn't just an 807 with an octal base.

Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long).

Yup.

What about the Russian 811s?

I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).

Same here. The lowest I ever saw them was 19 cents in quantity (I think the
quantity was 100). And of course there was a pair of them in every ARC-5 tx,
which could be had for a few dollars from Fair Radio.

Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2
voltages).

I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.

The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG.

The B&W L-1000 amp used a pair of 813s in GG, as I recall. How did they get 'em
to work?

If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.

Makes sense, but the other problems (socket, filament voltage, high C, need for
higher plate volts to get full power) make it a second choice compared to, say,
a pair of 811As. (A pair of 811As needs only 1500 V to do 450 W input, while a
single 813 wants more like 2500 V)

73 de Jim, N2EY

Ken Scharf wrote:

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd
build a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP
output).



But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00. Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long). I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).



Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and
they have such HIGH output capacitance that making a good tank
circuit in a single ended amp is rather a pain. Still a grounded
grid tetrode circuit looks interresting! (cathode driven with normal
g1 and g2 voltages).



I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.


The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG. If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.

Antique Electronics Supply, http://www.tubesandmore.com, has 1625s for
$5.00 a pop.

--

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

In article , Tim Wescott
writes:

Antique Electronics Supply, http://www.tubesandmore.com, has 1625s for
$5.00 a pop.


I can beat that, both NOS-in-the-box and used-tested-good.

73 de Jim, N2EY

In article , Tim Wescott
writes:

Antique Electronics Supply, http://www.tubesandmore.com, has 1625s for
$5.00 a pop.


I can beat that, both NOS-in-the-box and used-tested-good.

73 de Jim, N2EY

In article , Ken Scharf
writes:

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).

But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

Agreed 100%. Note, however, that the driving power figures in the books are
usually theoretical, derived from tube characteristics. Usually, there's no
provision made for losses in matching circuits, etc.

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

Again, note that those output figures don't include circuit losses.

But in general I think you'd be OK with a single 1625 driving up to a pair if
811s if the matching circuits are lowloss.

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00.

I've got a bunch of used and a bunch of new. Also a big pile of 807s, which
have been my homebrew final of choice for quite some time now.

Here's a hint: Look at the 6BG6-G sweep tube. They are still very common (the
GA's are liked by some audiophiles but the Gs aren't) and their ratings are
very similar to the 807/1625. Looking at their construction, one wonders if the
6BG6-G isn't just an 807 with an octal base.

Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long).

Yup.

What about the Russian 811s?

I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).

Same here. The lowest I ever saw them was 19 cents in quantity (I think the
quantity was 100). And of course there was a pair of them in every ARC-5 tx,
which could be had for a few dollars from Fair Radio.

Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2
voltages).

I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.

The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG.

The B&W L-1000 amp used a pair of 813s in GG, as I recall. How did they get 'em
to work?

If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.

Makes sense, but the other problems (socket, filament voltage, high C, need for
higher plate volts to get full power) make it a second choice compared to, say,
a pair of 811As. (A pair of 811As needs only 1500 V to do 450 W input, while a
single 813 wants more like 2500 V)

73 de Jim, N2EY

Ken Scharf wrote:

If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd
build a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP
output).



But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00. Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long). I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).



Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and
they have such HIGH output capacitance that making a good tank
circuit in a single ended amp is rather a pain. Still a grounded
grid tetrode circuit looks interresting! (cathode driven with normal
g1 and g2 voltages).



I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.


The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG. If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.

Antique Electronics Supply, http://www.tubesandmore.com, has 1625s for
$5.00 a pop.

--

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