Triode vs tetrode, Class C plate modulated

[PA4WM]

A recent question in this board about switching a plate modulated amplifier (K1JJ cathode bias switching) and the question what to do with the screen supply?... made me wondering....

Modulating the screen is another issue; Through a dropping resistor from the modulated B+, or from the unmodulated B+, or through a voltage divider from both, or from a sepperate screen supply with a chocke or....

So I was wondering, with all the "hasle" to get the screen of a tetrode modulated in the correct manner, without destroing a "perfect" AM signal... What would be the disadvantage to use a triode.

My goal is to build a 100 to 200 watt carrier output transmitter (legal limits here in Holland), and the idea was to use an 813.
But an 810, 100th or maybe even an 811/572b and something simmilar would give me the same amount of power, without a screen having to worry about....
OK, they need a bit more driving power, say 15 instead of 5 watt... but make life easier on the modulating side.

Or am I mistaken??

Martin

[W2PFY]

At the power level that your talking about with a triode I feel that it's not a big deal. Oh the other hand, if it were a KW running a 450TH , You may need in excess of fifty watts to drive it when considering circuit losses. However with a transceiver to drive a larger tube and since most transceivers will make 100 watts of RF, it's a non issue IMHO. I think it would be pushing it a bit when using an 811A tube for 200 watts output. The other tubes would be fine that you have suggested.

[K1JJ]

Martin,

It's a little like Ford vs: Chevy...  Porsche vs: Mercedes...


Many prefer tetrodes for finals and triodes for modulators.  I do in most cases. The tetrode is easier to drive and appears to modulate more fully in plate modulation service without adding in some grid modulation to obtain 100% modulation, like some triode schemes require. With tetrodes, the added screen modulation is probably the equivalent of the triode grid modulation added.  

But, certainly, eliminating the screen supply is an advantage of the triode.

The tetrode is inherently more stable (due to the screen grid shield between the grid and plate) and may not need neutralization - a triode always needs it unless heavily swamped and laid out well. I always neutralize my tetrodes anyway.


When axed, WA1HLR, one of the AM gurus, has usually recommended using a tetrode final for plate modulation.

That said, triodes make excellent finals, as shown by the many 833A broadcash rigs out there.


I once used a pair of 24G's modulated by a pair. (triodes) These are old buzzard 35 watt triodes. They put out more power than I would expect when the triode grids were driven heavily. What a great rig.  Other than that, I run a 4-1000A tetrode in the final, mostly cuz that's what I had available.

Really, you can't go wrong choosing either one.

T

[k4kyv]

A recent question in this board about switching a plate modulated amplifier (K1JJ cathode bias switching) and the question what to do with the screen supply?... made me wondering....

You pretty well covered the reasons why I have always used triodes for anything running more than 50 to 100 watts.

An additional concern with tetrodes and pentodes is protecting the screen in case you lose plate voltage. Running full screen voltage without the presence of the higher plate voltage to pull some of the electrons away from the screen grid may cause the screen to become damaged due to overheating. In addition, one must pay attention to screen current while tuning and loading high power tetrodes and pentodes. Some tuning and loading conditions can result in excessive screen current, risking damage to the tube.

Tetrodes and pentodes are more power sensitive and require less driving power, but that power sensitivity also makes them more prone to self-oscillation, requiring more attention to shielding and isolation between input and output circuitry. Neutralisation is less an issue with s.g. tubes, but usually some form of neutralisation is still required with grid-driven amplifiers.

OTOH, the more extensive neutralisation circuitry makes band-switching more difficult with a grid driven triode amplifier; triodes are more conducive to plug-in coils.

From my experience, vhf/uhf parasitics are just as much a problem with triodes as they are with s.g. tubes.

The main trade-off with triodes versus s.g. tubes is the neutralisation circuitry and extra grid drive versus the hassle with the screen grid and associated circuitry. As Tom said, it boils down to personal preference.

[Opcom]

You can set up your relays so that the plate voltage is applied first, then the screen voltage, and when un-keying, the screen voltage is removed first, then the plate voltage. It may not be necessary to go that far. In my TX, the voltages are applied and removed simultaneously and there has not been an issue with a 4-1000. The TX also has G2 overload circuits appropriate to a tetrode so don't forget those.

[flintstone mop]

You can set up your relays so that the plate voltage is applied first, then the screen voltage, and when un-keying, the screen voltage is removed first, then the plate voltage. It may not be necessary to go that far. In my TX, the voltages are applied and removed simultaneously and there has not been an issue with a 4-1000. The TX also has G2 overload circuits appropriate to a tetrode so don't forget those.

That would be the turn-off for me to consider a tetrode. Besides a screen supply you need to watch and protect the screen element in the tube and devise protection circuits. 833's modulated by 833's for visual pleasure of bright glowing tubes. 
And a transceiver would not like to be running full tilt at 100 watts in an 'ole buzzard' transmission. Even 25 watts driving another TX is going to heat it up that the cooling fans will activate.
Copy a broadcast transmitter RF driver and use a VFO.
Fred

[PA4WM]

Well... that answered my question quite a bit...

When wondering about the advantage vs disadvantage when using a triode as modulated amplifier, I overlooked a few things...
Thanks for putting my mind in the right direction.
I wasn't aware that the grid of a triode gets some modulation help as well...  Neutralizing is another thing.
So yes,... I was a bit to quick starting this topic

I will stick to my original plan, and use the 813 instead of a triode.
I just finished the modulator, using 2 x 811a, running at zero bias (1250V HV)

Regards, Martin PA4WM

[w1vtp]

An intriguing possibility is the Russian GU-48 which is very similar to the 833.  A pair can be had for had for slightly above $250.  A quick check on ePay and an 833A can be had for $109.  If one is going for a triode class "C" plate modulated deck.  An 833A makes a lot of sense to me.

I'm still sticking with the venerable 813 mostly because I have a baker's dozen of 'em in my parts room and a a couple (at least) of fil xfmrs that will light two 813s.  Just power both the RF deck and the modulating stage with the same PS and going from RX to TX should be less complicated.  Hammond makes a real nice 20 HY choke for the screen connection and a self-modulating screen setup

Al

[KM1H]

It takes an awful lot of mistakes and poor construction for an 813 to act up. Its one of the best tubes ever made. Just be sure to use one with a graphite anode, the sheet metal ones dont like being driven into serious color.

Carl

[W2VW]

It takes an awful lot of mistakes and poor construction for an 813 to act up. Its one of the best tubes ever made. Just be sure to use one with a graphite anode, the sheet metal ones dont like being driven into serious color.

Carl

Audiophools buy those for large bux.

[k4kyv]

So the audiophools prefer the graphite anodes or the sheet metal ones?

[W2VW]

So the audiophools prefer the graphite anodes or the sheet metal ones?

The rectangular shaped thin ones.