HF amp using 1625 tubes

[KC5HMC]

It even mentions that you have to mod the tubes as mentioned by K4DPK and N2EY.It says you first remove the base by applying heat from a large torch, seperate the cathode and beam-plate leads, and reinstall the base using Tube-base cement. What can I use as a sub for the cement?

[VE3LYX]

I just bought 10 of them NOS. I needed one for my ARC5 T18 . Thought also about building a quad four.Grounded grid is no good here. The homebrew Am stuff here wouldnt have the suds to drive one. it will have to be a grid driven amp. Have built a few , even a parallel 6l6 amp that is sitting be side me right now. Other then top plate lead shouldnt be much difference if any circuit wise.
Half the gird bias for two so 1/4 for four I guess. Seperate parasistic suppressors for all with a common tie point. Make a Rf plate choke to handle all 4 and a gimmick cap per tube for neutralization issues.  They cant be all bad. The Arc 5 has only two tubes ahead of them . A vfo and a buffer I guess of sorts. There is a parallel pair of 807 in the ARRL book. Specs are not a whole lot different then a plain jane 6146 except I dont think they like higher freqs like the 6146. A few years late but mirror cement for replacing automotive mirrors that are glued to the windshield work pefectly and has no resisdue like some of the other popular fixes do. I use it for all my loose tubes.
Don VE3LYX

[G3RZP]

Parallel tubes need the same grid bias. But if the tubes are in Class C, then the grid leak needs to be one quarter of that which you would use for one tube. The ARC5 T18 used a 1626 triode oscillator to drive the 1625s - the other tube was a magic eye for frequency checking.

For a linear amplifier, I would go for grounded cathode swamped grid as WB6BYU suggests, although I would go for a separate bias pot for each tube, as they don't necessarily match.

[N2EY]

Thought also about building a quad four.

Think carefully about that. Getting 4 1625s to be stable in grounded-cathode is not going to be easy. It can be done but you have to be very careful with layout, bypassing, shielding, and neutralizing. If all you want is 160, 75 and maybe 40, you may be OK. Note that the pi-net output values will include lots of C and not much L.

Half the gird bias for two so 1/4 for four I guess.

No! Same bias voltage, four times the current.

Seperate parasistic suppressors for all with a common tie point. Make a Rf plate choke to handle all 4 and a gimmick cap per tube for neutralization issues. 

No, one big neutralizer for all 4. Not a gimmick, either.

They cant be all bad. The Arc 5 has only two tubes ahead of them . A vfo and a buffer I guess of sorts.

The ARC-5 uses a 1626 power oscillator to drive a pair of 1625s. No buffer, though many hams modified them to include a buffer because they can chirp and FM pretty bad if you don't.

There is a parallel pair of 807 in the ARRL book. Specs are not a whole lot different then a plain jane 6146 except I dont think they like higher freqs like the 6146.

Max power input of a 6146B is 120 watts. Max power input of a 6146 is 90 watts. Max power input of an 807 or 1625 is 75 watts. (Class C maximum ICAS ratings).

Hams used to beat the tar out of 1625s because they could be had surplus in almost unlimited quantities for 19 cents each. That's not the case any more, and those old bottles should be treated with more respect.

btw, in AM linear a quad of 1625s is only good for about 50 watts carrier output.

73 de Jim, N2EY

[G3RZP]

1956 ARRL handbook has a design (p.303 et seq)  from QST for April 1954. 2 paralleled 1625s in push pull with 2 more. On page 306, it has the 4 off 1625 parallelled grounded grid amp from QST June 1955.

[N2EY]

1956 ARRL handbook has a design (p.303 et seq)  from QST for April 1954. 2 paralleled 1625s in push pull with 2 more.

A decent design. Lots of parts for the power level - and not a pi-net.

On page 306, it has the 4 off 1625 parallelled grounded grid amp from QST June 1955.

Which became the P&H L-400.

Note that the design uses modified 1625s. Only some brands can be modified to separate the beam-forming plates from the cathode.

Note too that the design exceeds tube ratings quite a bit....

73 de Jim, N2EY

[G3RZP]

>Note too that the design exceeds tube ratings quite a bit....<

But, Jim, 1625s and 807s are tough babies, so on SSB, they'll take it. Just don't tune up with single tone for too long. Or run SSTV, RTTY or AM.

I don't think the audiophools have discovered them yet....

[VE3LYX]

Might be difficult. Well I hope so! Otherwise it would hardly be fun.
Mod the tubes? That is only for grounded grid. Grounded grid requires a pretty stout exciter to drive it. I dont have that kind of power in my exciter.
Don VE3LYX

[WB3COB]

   1625 amplifier, 150 watts QST April 1961 "Surplus tubes + an Old TV Set = 150- Watt Amplifier by McCoy. 
  The1625 is a good tube, but needs a base shield.  Old soup can works well with vent holes cut in the bottom for air circulation and spade bolts to mount the can.  Millen used to make the base shields.  Salvaged two from an old low power AM brodcast band Gates transmitter.  I have also seen them on the old Millen exciters and larger Gates AM broadcast transmitters from the
early 1960's.  BC-1T is an example.  Also the ARC-5 transmitters.
Trust me, use a base shield.  It will save you a world of problems.
Also, use a parasitic supperssor.   2 Watt 47 Ohm carbon resistor with 6 to 8 turns of # 20 wire wrapped on resistor body and even spaced. 
   I have used 1625 up to 18 MHz.,  with no problems.  But using the above listed precautions. 
    QST November 1972 shows a nice 14 / 21 MHz transmitter
using 1625 as well.

73's,

WB3COB
 

[KH2G]

If you can find the schematic or a live ARC-5 Command transmitter, they used 2 in them and the sockets were ideal as they have the shielding built in -hi
73
Dick KH2G