Triode Connected?

[Aub]

Hi all,

When we talk about triode connection of a pentode output stage, does it mean that the screen grid is directly connected to the anode, or can it mean something else?

Thanks

Aub

[GrimJosef]

There may be a low-value resistor between g2 and a, but that's basically what it means. The Williamson amp http://www.r-type.org/articles/art-107b.htm and the Leak TL/12 Point One http://www.44bx.com/leak/Leak/tl12_cct.gif are classic examples.

On the rare occasion that a pentode has g3 brought out separately questions can arise about what to do with that.

Cheers,

GJ

[ms660]

Screen grid strapped to anode is triode connected, sometimes a small amount of resistance between the screen grid and anode is ok.

EDIT: post crossed.

Lawrence.

[G6Tanuki]

I tend to think of it more in the context of strapping control- and screen-grids together in high-power valves; the attached description is fron the early-60s RSGB "Radio Communication Handbook" and refers to audio amplification, but I've seen the conttrol-and-screen-grids-strapped approach also be used in 'cathode driven' [a.k.a. grounded-grid] RF linear amplifiers using TV line-output valves [or 'Sweep Tubes' as they're always called in the US]

[G6Tanuki]

Here's the 'triode connected' [all grids connected together] grounded-grid linear amp from the 1982 ARRL handbook; using a pair of 6KD6 it's described as a "Quarter Kilowatt" amp.

I guess sweep-tubes were cheaper than the likes of a pair of 811-A triodes. Come to think of it, they still are!

[Aub]

Ok thanks all. That's what I thought. Saves on components I suppose, not having to use decoupling caps on the screen grids, or resistors, if there aren't any small value ones.

Cheers

Aub

[ms660]

For the same Va and Vg1 the mutual conductance will be slightly higher, gm(triode) = gm(pentode)*Ik/Ia.

Lawrence.

[GrimJosef]

I don't think component count was the main driver.

Triodes have lower inherent distortion than tetrodes or pentodes, but they're capable of delivering less output power. So in amps trying to achieve low distortion it was worth buying a large (=expensive) valve and running it as a triode. If what mattered was power, though, then you'd be better off running the biggest valve you could afford as a pentode.

The discovery of ultralinear operation, with a tapped output transformer primary, allowed valves to run with more power than triodes but lower distortion than pentodes, as in the Dynakit Stereo-70 http://www.thehistoryofrecording.com...nakit_ST70.pdf or the Mullard 5-20 http://www.r-type.org/articles/art-003d.htm.

Cheers,

GJ

[Radio Wrangler]

The Dynakit ST70 was a David Hafler design, I believe.

There was a need to keep the open loop distortion of amplifier stages fairly low because the build-up of stray phase shifts and the desire for flat gain across the audible spectrum put a seriously intractable limit on the amount of feedback which could be employed.

Cathode-driven push-pull grounded-grid with a step-down interstage transformer would have been an interesting route to low distortion, but a second transformer within the loop would have raised issues.

If I ever had to have designed a good amplifier but with fashion-dictated lack of overall loop, it's the path I'd have taken.vIt would have been interesting to compare power triodes with beam tetrodes or pentodes with all grids grounded. It might shoot a few holy cows.

David

[joebog1]

With valves that have grid 3 available at the base, such as the 6CA7/EL34, most people strap it to cathode, but a much more elegant design is to tie it to the negative bias voltage. By that I mean if we have a source of negative grid bias voltage available say 70 or 80 volts, that we tap of with either a resistive divider, or a pot to get our 40 or 50 volts to bias the grid 1, we tie grid 3 to the raw minus 70 or 80 volts through a relativly low value resistor. By doint that the output valve can not be driven into hard saturation that melts the anodes as it holds the valve at maximum output and cant go "on " any harder. If grid 1 bias is lost for some reason, like a burnt grid bias resistor or pot, it holds the valve at maximum but no higher.

Yorkville guitar amps are configured this way and they are esentially "blowup proof ".


Joe

[G6Tanuki]

For me the big thing I dislike about ultra linear operation is that it means that the HT for the anodes is limited to the maximum allowed for the screens. Which is generally quite a bit below the maximum permitted anode voltage.

I was always brought up to believe efficiency and linearity were enhanced by running with the anode voltage nice and high, ultra linear operation goes against the grain for me...

[joebog1]

6CA7 maximum voltage on the plates is 750 volts.
Screens are 225 or perhaps 250 volts.
NOTHING to do with grid 3!! also known as the supressor grid as I know YOU know well.
6CA7 is a horrible valve anyway in my opinion. Works superbly at 350 or 400 volts HT for an audio amp.
NOT much good for nought else except maybe a modulator.

It fitted Mullards lineup for valves, and it won by being "mo-bedda " for audio. Just check Marshall guitar amps??!!.
NOT the nicest amp around, apart from brute forse . ( no spello )
NOT very reliable, not very "sweeet sounding ".
Yep!! I will get kicked and beaten again, BUT its true.

Anyway, origional question is "what to do with grid 3 ", and why is triode strapped pentodes/tetrodes called "triode strapped ".

Joe

[Robert Gribnau]

The 6CA7 has 800 V as maximum anode voltage and 425 V as maximum screengrid voltage. These numbers are 800 V and 500 V for the classic EL34 (many or even most 6CA7's are not true pentodes but beam tetrodes).

The original question was not about grid 3 but about grid 2.

[barrymagrec]

In the case of the Williamson amplifier, which normally used KT66 Beam Tetrodes (Pentodes) triode strapped (with a 100 ohm resistor) an alternative version was published using PX4 triodes - the circuit was identical.

[GrimJosef]

Originally Posted by barrymagrec

... an alternative version was published using PX4 triodes ...

PX25s I think https://en.wikipedia.org/wiki/Willia...ypes_and_tests but your point about triodes still stands, of course.

Cheers,

GJ

[Radio Wrangler]

Originally Posted by Robert Gribnau

The 6CA7 has 800 V as maximum anode voltage and 425 V as maximum screengrid voltage. These numbers are 800 V and 500 V for the classic EL34 (many or even most 6CA7's are not true pentodes but beam tetrodes).

The original question was not about grid 3 but about grid 2.

I'd be cautious, I think those voltage ratings are for normal pentode mode operation and the screen voltage rating is likely intended for power amplifier service where the anode voltage will dip significantly below the screen voltage, screen current will go up and screen dissipation is likely the physical cause of the rating.

In triode-strapped service, the screen can't be above the anode voltage, so it never appears more attractive than the anode. It will just collect a proportion of electron flow based on the fraction of area subtended. So I think the ratings would be somewhat different if the valve was to be re-assessed for triode strapped service.

Some testing with metered g2 and g1 currents would be needed , and also some high voltage breakdown tests would be needed to find out for sure.

David

[barrymagrec]

Originally Posted by GrimJosef

PX25s I think https://en.wikipedia.org/wiki/Willia...ypes_and_tests but your point about triodes still stands, of course.

Cheers,

GJ

PX4s in the GEC "Art and Science in Sound Reproduction" 1953.

My uncle built one which much later I converted to KT66s after the mains transformer burnt out and the free replacement only had 6.3 volt heater windings.

[GrimJosef]

Originally Posted by barrymagrec

... PX4s in the GEC "Art and Science in Sound Reproduction" 1953 ...

I guess that would be the PX4 circuit on p26 https://dalmura.com.au/static/Art%20...ook%201953.pdf ?

The small-signal stages conform exactly to the Williamson topology, I think, but the output stage differs. Instead of connecting the cathodes/filaments together and including adjusters for the valves' total and individual quiescent currents, the GEC circuit separates the filaments and simply takes each one, via the CT of its supply, to ground via its own capacitor-bypassed resistor. Current-matching will depend entirely on how well the valves are matched. The HT voltage is a great deal lower too, of course.

Cheers,

GJ

[Robert Gribnau]

Originally Posted by Radio Wrangler

I'd be cautious, I think those voltage ratings are for normal pentode mode operation and the screen voltage rating is likely intended for power amplifier service where the anode voltage will dip significantly below the screen voltage, screen current will go up and screen dissipation is likely the physical cause of the rating.

I agree.

The attached Mullard datasheet for the EL34 gives data for UL operation (taps at 20% of primary turns) with Vb being 450 V. At no signal the voltage drop over the cathode resistors is 32 V. At full power this voltage rises to 41,5 V. The dc-resistance of the primary will cause some additional voltage drop. I think Vg2(max) in UL operation is atleast 400 V.

There are relatively few valve types of which the datasheets state Vg2(max) for UL operation. But when they do, part of them show a somewhat higher Vg2(max) for UL operation than for pentode operation. Three examples:

6L6GC: Va(max) = 500 V ; Vg2(max) pentode mode = 450 V ; Vg2(max) UL mode = 500 V
7027A: Va(max) = 600 V ; Vg2(max) pentode mode = 500 V ; Va(max) = Vg2(max) UL mode = about 550 V (being 600 V minus the voltage drops in the cathode resistor and the primary)
7189A: Va(max) = 440 V ; Vg2(max) pentode mode = 400 V ; Vg2(max) UL mode = 415 V

The last page of the attached Tung-Sol datasheet for the 5881 has an interesting diagram for UL operation in that it shows some values of Vg2 at different values of Va.

[G6Tanuki]

I've pondered in the past whether you could use a higher anode-voltage in an ultra-linear amp by wiring something [Zener diodes? voltage-regulator valves?] in the connection between the primary-taps and the screen-grids, so essentially subtracting a fixed voltage from what gets applied to the screens?