Dentron hf amplifier Valve board upgrade, why?

[jazzy]

Hi all again
As you probably know by now i am rebuilding an old dentron gla 1000b hf amplifier,this uses four 6lq6 sweep valves .dentron had sent out new boards as a upgrade to people who had the first gla 1000 amplifier,this board was the same as the newer 1000b and had pins 2,3 and 6 connected at the valve base, the old board had 1,2 connected together and then to ground.
What does the achieved as now pin 3 (k)is connected up to pin 2,6 grid?
Thank you
Garry

[Herald1360]

No idea why they've done it, though if the original had grid and screen(1&2) connected to ground it suggests it was being used as a sort of grounded grid triode amplifier, a common enough linear RF amp mode.

Connecting the grid to the cathode suggests that the screen is now being used as the grid, still in a grounded grid triode configuration.

It all rather looks like the valve is being used in ways not thought of when it was designed as a line output bottle! Hopefully the Dentron developers got it right......

[jazzy]

New dentron gla 1000b pin out upgraded board

old dentron gla 1000 pin out
Hi Here are some pictures
Thanks garry

[glowinganode]

Sounds like a poor man's zero bias triode.
Rob.

[Herald1360]

No further "why" but an illustrative "how" below.

Both circuits are variations on zero bias grounded grid triode configuration even if in the "new" version the physical grid is now tied to the cathode.

[jazzy]

Hello
Thank you both for getting back to me,What in your mine is the best to use? and will they work and act in the same way?
Thank you Garry

[glowinganode]

My only thoughts are;
1. To reduce the effects of C k-g1
2. To eliminate the chance of k-g1 flashover
3. To eliminate excessive g1 dissipation.
It might be a combination of these or none at all.
G8HQP Dave is probably the best person to answer this.
Rob.

[swordholder]

Where does the grid bias come from.
In the first diagram the cathode can be made positive and give grid bias, but in the second the grid and cathode are connected.

Mike

[Herald1360]

A look at Jazzy's other thread on the Dentron explains everything except "why"- there's a circuit there too.
https://www.vintage-radio.net/forum/...ad.php?t=54206
Bias is provided by sitting the cathode up on a RF bypassed zener diode. Presumably the level chosen (around 33volts) works with the current arrangement where the screen is being used as the grid. So it's not zero bias after all, just grounded "grid".

[glowinganode]

Originally Posted by Herald1360

So it's not zero bias after all, just grounded "grid".

You're absolutely correct Chris, it isn't strictly zero bias. The bias required (and associated power dissipated by the zener) is a lot less though than if the screen grid were at a couple of hundred volts.
I seem to recall reading that the screen grid gives more linear control of anode current.
Looking at a circuit that uses 4 x 6HF5 sweep tubes which have a similar anode dissipation, with 800V ht the standing current is 120mA.
So with 1200V ht, 80 - 90 mA seems about right.
Rob.