From the N4UQ e-mail archives

Alpha 76, Alpha 78, Alpha 374-A

• This is the most asked question:  "Why don't you simply use the cheaper 4CX800 Russian tube in your conversion instead of the Eimac 3CX800?"
  A pair of 3CX800 tubes is a natural replacement for the 8874 trio.  The characteristics of the tubes are very similar.  These triode tubes are found mostly in grounded grid amplifiers.  They use the same inexpensive socket.  Only the filament voltage needs to be changed.  This is accomplished with a small transformer mounted inside the tube deck.
  
  The 4CX800 uses a different socket and therefore requires a different mounting scheme requiring serious metal fabrication.  The 4CX800 is a 'tetrode' tube.  It has an additional grid called the screen.  This is why the tube has so much gain.  The screen grid is designed to have a high potential applied, on the order of several hundred volts positive.  Special attention must be given to the screen power supply.  The control grid must also have a separate power supply that produces a negative potential of perhaps 60 volts.  The first obstacle to overcome is finding a place to put all this power supply hardware in the already crowded amplifier.
  
  By creating a 'grid driven' amplifier instead of the former 'cathode driven' amplifier we now have a volatile circuit on the verge of oscillating.  A circuit with so much gain is apt to 'take off' on its own at any time, since the input is in phase with the output.  Some form of negative feedback is usually seen in well designed amplifiers to reduce distortion, lower the gain and stabilize the system.  The amount of negative feedback tends to vary over a wide frequency range and is not easily implemented in an HF amplifier.  On the other hand the grounded grid, or cathode driven circuit is inherently stable.  It's gain is much lower.  Even a poorly designed grounded grid amp will likely be well-behaved.  Most tubes designed for this service will even work well with zero bias.
  
  The cathode driven circuit requires considerably more drive than its grid driven counterpart.  But who among us does not have at least 100 watts available?
  
  Now, there is the possibility the 4CX800 can be operated as if it were a triode.  Many years ago I built an amplifier using a pair of 4-400 tubes.  All grids were grounded.  The tubes were old broadcast 'pulls' and didn't perform very well.  It took lots of drive and would only run about 800 watts.  I later installed a pair of 3-500s and all was well.
  My Dad also built a 4-400 rig and ran it for years.  My point is, you can ground all the grids of a tetrode and treat it like a triode.  I don't know what the parameters are.  And I don't know if 100 watts drive would get you any where near full power. 
  
  It is also important to note the Russian 4CX800 is larger than the 3CX800.  There is barely room for a pair of 3CX800s in the 76 and even less space in the 78 and 374A.
  
  
• When I tune my Alpha 76 to 1500 watts output, the PA current meter is off the scale.  What's up with that?
  The fix for that is a simple meter doubler circuit.   It's a resistive divider that doubles the reading on your PA current meter.   Full power will then be about .6 amps on the meter.

• I'd like to upgrade my Alpha 76 to run a pair of 3CX800's.
  What are the advantages to making the switch?
  If your present trio of 8874 tubes is healthy, there is very little reason to upgrade.  Only a bit more power is available due to voltage limitations in the PA power supply.  My upgrade does nothing to change the power supply.  Only the filament voltage is changed.  If your 76 has only two 8874 tubes, there
  is a real improvement in performance by changing to the 3CX800 pair.
  
  
• I read some information on your Web site about converting the Alpha 76 to run a pair of 3CX800 tubes instead of the three 8874's.  Are the 8874's still being manufactured? 
  Yes, they are available both new and used.  They are used in higher powered communications gear.
  
  
• Why did you finally offer the 3CX800 upgrade on the Alpha 78?
  The 78 is similar to the 76 but the RF deck components are packaged differently to allow for the pre-set bands.  Conversion of the 78 requires relocation of at least one of the tube sockets.  The project costs more than the 76 but a few customers have been willing to pay the premium.  
  
  
• Is 12 meters the only WARC band that doesn't work on the 76PA?
  That's correct.  Actually it will work there.  It may cause the last few turns on the PA choke to fuse together from heat.  That fixes the resonance problem but causes problems on 160 meters.  The 'shortened' choke can cause the RF bypass capacitors to overheat and change value.
   
• Does the Alpha 76 have tuned input?  What is a "Low Drive Mod" ?
  The 76 has no switched input tuning.  It does have a toroid on the input.
  The 'proper' configuration for resistive loading is five, 470 ohm, 2 Watt resistors in parallel placed across the tube's inputs.  This is the infamous 'low drive kit'.  less than 100 watts will produce at least 1500 watts out.
  Many rigs were shipped with a resistive network which requires 200 Watts to get it to full output!  15,  220 ohm resistors.
  (The low drive mod is not for 2-tube rigs.)
   
• What is a good source for 3CX800 and 8874 tubes?  How much do they cost?
  Here are a couple of guys to check with:

  Allen Bond, WB4GNT  "MAX-GAIN SYSTEMS"  http://www.mgs4u.com
  email Allen at mgs@mgs4u.net  or call 770 973 6251 before 9pm Eastern.

  Alan Emerald, K6GA     acemerald@earthlink.net
  8956 Swallow Av
  Fountain Valley, CA
  Alan has a personal ad in the back of QST each month.
  Of course there are many other sources and prices will vary greatly.

• What about the 3CPX800?  Is this tube compatible with the 3CX800?
  What is the difference?
  The tube is totally interchangeable with the 3CX800.  It is a more robust
  model of the same tube.  Detailed information is available at the Eimac
  Web site: http://www.cpii.com/eimac/index.html
  I have seen very good performance from the 3CPX800A7.  It sells for around
  $445 at RF Parts.  http://www.rfparts.com/
   

Alpha 77

• During your rebuild pursuits, have you ever come across a supplier for the brushed aluminum knob inserts as used on the 70, 77, 77Dx series amps?
  Kurz-Kasch was the O.E.M. but the product line was sold to Cosmo Industries in the mid-'80s, who now only makes the knobs and inlays on demand.
  K3ICH now makes those.  Charlie Talbott, 13192 Pinnacle Lane, Leesburg VA  20176-6146.
  Home phone is:  540-822-5643.    E-Mail: k3ich@arrl.net
  
  
• Can you give me more information on the Alpha 77 2-tube kit?
  The kit consists of everything you need to install the second tube in a standard Alpha 77 except the tube.  Some rigs need more preparation than others depending on the vintage.  A qualification sheet is available on the Web site.
   http://www.n4uq.com/qualified.htm

  Here's the bill of materials for that kit:

  2  25-amp, 250v ceramic fuses
  1  Plate suppressor / anode connector
  1  Silicone Rubber exhaust stack
  2  Cathode ballast resistors, 25 Ohm, 50 W
  4  fiber washers
  2  2 Ohm, 2 per cent resistors (meter shunt)
  1  Custom input toroid
  1  Silver Mica capacitor, .01 Mfd, 500 volts
  1  4.5" x .25" stainless steel bolt and hardware
  1  15 microhenry choke
  1  Eimac SK2210 with mounting clips
  1  Eimac SK2216 chimney
  1  10-kOhm, .25 watt resistor
  1 .02 mFd bypass capacitor (filament)
  3  1N5404 diode
  1  3.75" stainless steel hose clamp
  1  2.5" stainless steel hose clamp
  1  Heat exhaust plenum (trade-in required)
  4  .5" 6 x 32 stainless steel screws and hardware

  The socket is shipped prewired.  All that remains is to bolt it in and make a few solder connections.  The two 50W resistors are prewired with the three-diode stack.  All
  hardware is included.  The instructions are very thorough.
  To qualify, the installer should feel comfortable working on electronic equipment.  A voltmeter is the most exotic equipment needed.  An external wattmeter is highly recommended for tuning.  A Bird model 43 with a 5 kW slug is a nice addition to the shack.

• What is the difference in a real two hole 77 and one converted in the field?
  It was never legal for the factory to sell a 2-holer in the USA! Therefore the only ones they made went overseas or to the Non-amateur service. Most field modifications look as good, if not better, than the factory. Also most operators who claim their 77 was a factory 2-holer are simply misinformed. I've seen at least 40 and only ONE was a factory 2-holer.

• I have been reading the archives from the amp reflector and getting somewhat confused as to what can be done to improve 77DX....I've been advised to replace the plate choke with a B & W 802 but I can't find one.
  The original choke in the 77 is a B & W 800, roughly 88 mH.  It's only rated around 700 mA.  To make matters worse, the manufacturer removed 12 turns from the cold end of the choke to move it's series resonance frequency (to 11 meters perhaps).  The B & W 802 would seem a better choice but unfortunately 15-meter performance is degraded with this choke in place.  Some models
  won't work at all on 15 meters with the 802 in place.  "RF Parts" sells the RFC-3 but it is resonant on 30 meters.  While it is sold as a high powered choke, the wire is awfully small.
   
• My front panel has sticky paint which is dirty and easily scratched and marked.  Can I replace the front panel?
  Yes, You can.  RF Concepts LLC has some panels so you might check with them.  By the way, the sticky paint went away at some point in production.  Later model 77, 76, 78 and 374A rigs have a totally different hard surface which is much more durable and looks great.  Most hams will have no problem doing the front panel change.
   
• I just purchased a 77.  A friend told me of a heftier transformer which mounts outside the amplifier.  How can I find out more about that?
  Your friend was probably making reference to a 2-Amp continuous duty high voltage transformer manufactured by Peter Dahl.  The idea is to leave the existing transformer in place for filament and low voltage cicuits.  The new unit mates with the HV connector and is placed nearby.
  Peter Dahl also manufactures a direct replacement transformer.  It will fit but it's quite snug.  Even though the rating on the Dahl unit is the same as the original, I consistently see around 200 to 300 watts more output with the Dahl transformer.
   
• How can I tell if my single tube Alpha 77 is qualified for the upgrade kit for the second tube?
  Simply print the qualification sheet and follow the instructions.
   
• More questions to follow.