suppressor grid

[Detroit47]

  Why is it that some sweep tubes have the suppressor grid tied to the cathode internally?   This precludes the use of a grounded grid situation. I was never could understand why they did this. Is it to keep the path short to ground for the suppressor?  Or did want to make sure the tubes where ran grid driven? It is really apparent in the old octal base stuff. But the Compactron stuff comes both ways.
 
73 N8QPC

[K1JJ]

I think you pretty well have it figured out.

My first guess would be the suppressor internally goes to the cathode for a short, stable path. The second complementary reason is it was designed for a specific service in TVs so might as well configure it ready-to-go as a grid driven, suppressor grounded, sweep tube.  They probably had huge custom orders tailored to the TV manufacturers.

This is akin to some big power grid tubes (ext anode) that are designed to be mounted with the grids bolted directly to the chassis. No socket needed.  They were meant to be GG config and that's it.

Suppressor modulation of any kind was out of the mainstream, so they weren't losing much business by doing so I suppose..

[Tom WA3KLR]

My $0.02 worth -

Most tubes were originally designed for TV and entertainment radio service.  As Tom said the sweep tubes were intended for the TV horizontal sweep circuit.

Some of the “pentode” tubes drawn as such are beam power tubes.  The beam-forming plates are drawn as a suppressor grid even though they are not truly a suppressor grid.  Perhaps this is all just a matter of semantics, as they can be called beam-power tetrodes or beam-power pentodes, same thing, and I think everyone understands this.  The requirement for the suppressor grid and beam-forming plates is that they need to be at the most negative potential part of the tube’s circuit.

One popular sweep tube in linears was the 6KD6.   This is usually drawn as a supressor-grid coming out separate to pins.  It is a beam-power tube though and this element is really the beam-forming plates.  The 6146 is in the same boat, with the beam-forming plates, drawn as a suppressor grid and tied to the cathode internally.  At the power-level of a 6146, it is usually grid-driven.  But off the top of my head I don’t see why it would necessarily preclude this type of tube’s use in a grounded-grid configuration.  Perhaps someone else can chime in on this detail.

One nice thing about the suppressor coming out separate is that in a cathode bias configuration, with the suppressor grid tied to ground, it gets the additional negative bias in relation to the cathode.  This configuration was done in some low-level tubes also.  But I’ve never read that this was necessary or provided an additional benefit.  But again off the top of my head, as long as the suppressor or beam-forming plates are at the cathode potential, this is o.k. as far as I know.

[K1JJ]

My $0.02 worth -

Some of the “pentode” tubes drawn as such are beam power tubes.  The beam-forming plates are drawn as a suppressor grid even though they are not truly a suppressor grid.  Perhaps this is all just a matter of semantics, as they can be called beam-power tetrodes or beam-power pentodes, same thing, and I think everyone understands this.  The requirement for the suppressor grid and beam-forming plates is that they need to be at the most negative potential part of the tube’s circuit.

Good points, Tom.

Maybe if tube technology stayed with us they would have come up with different symbols to indicate a simple suppressor vs: beam forming plates.

The 813 is in a similar boat, with the G3 beam forming plates disconnected for universal use.

http://www.nj7p.org/Tube4.php?tube=813

T

[k4kyv]

Some 1625's have the beam forming plates exiting the glass envelopes with a separate wire lead, but tied to the cathode at the socket.  A leenyar described in the late 50's handbooks uses modified tubes in which a small hole is drilled in the bakelite base, and the lead is separated from the cathode pin and brought out via one of the other unused pins.

A commercial version of that same amplifier was produced, using 837 pentodes, which do have a separate lead for the suppressor.

Most audio beam power tetrodes like the 6v6 and 6L6, as well as power pentodes like the 42, 6F6, 6K6, etc. also have the suppressor/beam-forming plates internally connected to the cathode.

One buzzardly BC rcvr audio power pentode does have a separate pin for the suppressor: the type 59. I use one of those in my HF-300 rig as a pentode xtal oscillator, switchable to grounded grid vfo buffer.

An rf shielded, 6.3v version of the 59 is the 802.  The 802 is to the 59 as the 807 is to the 6L6.

The 837 is a heavy-duty version of the 802, with a 12v filament.

[KM1H]

P&H used 4 modified 1625's or 837's, buyers choice. The 837 is a more rugged tube but who knows what the IMD was of those early linears.

Pentodes such as the 803, 814, 813 make excellent GG amps. The 803 can handle a lot more than the specs show, its bigger than an 813 yet has lower ratings. One can easily run a KW input and laugh at 3500V on SSB. I dont know how high it can be pushed on AM as I havent hi-potted any.

[WQ9E]

Some 1625's have the beam forming plates exiting the glass envelopes with a separate wire lead, but tied to the cathode at the socket.  A leenyar described in the late 50's handbooks uses modified tubes in which a small hole is drilled in the bakelite base, and the lead is separated from the cathode pin and brought out via one of the other unused pins.

A commercial version of that same amplifier was produced, using 837 pentodes, which do have a separate lead for the suppressor.

I picked up one of these amps (P&H LA-400C made in Lafayette IN) as part of a 2 for 1 deal (the other amp was a CE-600L) at a little hamfest last year.  The amp came with two spares sets of tubes NIB, one set of 1625 tubes labeled by P&H as modified for the amp and another set of 837 tubes.  In the pile of awaiting restoration I have a similar homebrew amp (built from QST plans I believe) using the tuning circuit from a command set using modified 1625 tubes.  

These are good little amps for low power homebrew SSB rigs and provide a tad over 10 db gain with a rated 400 watts input.  The latest versions were rated for 800 watts and you better have a good supply of modified 1625 tubes to try that!

[Opcom]

Because the supressor is there to prevent secondary emission from the screen, its natural potential is cathode level or near DC GND or in some cases slightly negative. Perhaps it was simply a matter of convenience to attach internally to the cathode.

[KE6DF]

Because the supressor is there to prevent secondary emission from the screen, its natural potential is cathode level or near DC GND or in some cases slightly negative. Perhaps it was simply a matter of convenience to attach internally to the cathode.

That seems to be true for a lot of tubes, but the 803 is an exception. The suppressor runs with up to +500v with that tube. It was designed for suppressor modulation and used that way by the Navy during and right before WWII.

The 828 usually runs with +50 - 100 volts on the suppressor as well.