Golborne Vintage Radio

(22-09-2021, 06:53 PM)AdrianPH Wrote: [ -> ]The Rod valves I ordered from Ebay have arrived today, I also have 4 extra pieces of glass included 3 x 6C7b Triodes and 1 diode 6D6A, these both being 6.3 volt heaters.  It will be some time before I get chance to delve into any of these bits of glass.

Adrian

Those are sub-miniature or pencil tubes and simply miniature versions of conventional valves. Rod pentodes are called that not because  of the envelope, but because instead of spiral wound grids all three grids are actually two sets of pairs of rods or thin rod-like plates. They are not really pentodes as they work on electrostatic focusing of two electron beams (though at least one model has one beam). So rod pentodes need a very narrow electron beam which means a thin direct filament. They can't use an indirectly heated cathode.
One half:
[attachment=20981]

Top view
[attachment=20982]

Neither is to scale

(22-09-2021, 03:48 PM)Kalee20 Wrote: [ -> ]

(22-09-2021, 02:18 PM)Mike Watterson Wrote: [ -> ]But the issue is magnified by the rod pentodes because when Ia= max, the Ig2 is nearly zero, uA anyway, but at Ia nearly zero the Ig2 is maybe a 1mA. Thus ONLY a series resistor to 80V HT varies Vg2 too much.

That's interesting, Mike.

I've certainly found that Ig2 is low with rod pentodes. What I don't have data for, is how constant, between samples, is Ig2 as a fraction of Ia.

Ia being nearly zero brings Ig2 up to a maximum surprises me - again, for a given valve, I've found the ratio to be constant. The only circumstances where Ia falls and Ig2 rises, is if anode voltage drops to a low level (below the pentode 'knee') - or if the suppressor voltage is made very negative so that electrons are repelled back to g2 and can't make it to the anode.

And low anode voltage can mean low current.
[attachment=20980]
See how there is a mirror effect?

Ah. Yes, Mike - though this is indeed at low anode voltages, which I alluded to:

(22-09-2021, 03:48 PM)Kalee20 Wrote: [ -> ]The only circumstances where Ia falls and Ig2 rises, is if anode voltage drops to a low level (below the pentode 'knee')...

The cathode current is virtually constant as anode voltage is reduced, it's just that electrons don't make it to the anode and they are caught by the screen. Whereas with higher anode voltages, once in the vicinity of the screen, the electrons, rather than being caught by the (rather tenuous and shielded) screen, go on to the anode.

If you reduce anode current by making g1 more negative, while keeping Va above the 'knee' voltage, what happens to Ig2, does it stay the same fraction of Ia then?

I don't know. I do know that on some sets where I replaced the DF96 by 1j24p that it was only stable with an added resistor from g2 to f- (or f+) to set the voltage at about 40V. It was motor-boating.

I did mean to try the 1j24p at 12 to 18V (2x PP3 to exhaustion) with a centre tap transformer on g2 and anode for RF, IF and also AF preamp to see did it improve performance. But I never actually even thought it through fully, maybe a silly idea. Certainly they do work tying g2 to anode and pretending it's a triode.

It's a unique type of valve that is pentode like. The susceptibility to magnetic fields is significant and the versions with the g1 plates connected separately are strange, where a differential voltage in EITHER direction reduces the output.

(23-09-2021, 10:22 PM)Mike Watterson Wrote: [ -> ]... and the versions with the g1 plates connected separately are strange, where a differential voltage in EITHER direction reduces the output.

Yes I can visualise that - applying a differential voltage will steer the electron beam to left or right. And if the anode plate is straight ahead, then either way, the anode current will reduce as electrons go off-course (again, I'll bet that screen current increases).

Slightly off topic but was mentioned within the thread. I am picking up Radio Caroline tonight, reasonable with some fading!

Adrian

Hi.
I've modified the original again and now down to two 1ZH24B valves. The first one is an RF amplifier with positive feedback derived from the G2 fed back to the ferrite rod with 8 turns of litz wire around the ferrite rod slightly off the centre of the rod. Tuning remains the same as previously. The anode of the RF amp is loaded with 2x 2mh chokes in series, they are modern ones which have a high capacitance, using two improves the low end of the band. The HT end of the chokes are decoupled by a 100pf cap and the HT is dropped by a 15k resistor. We need to have a lower Anode volts to use the G2 for feedback. The feedback winding is directly connected to the G2, the earthy end goes to a 5k pot the other end goes to HT via a 1k resistor the junction is decoupled by a 1nf cap.
The detector is as the original with a small addition of an added 1m resistor from filament plus to the G1, this removes distortion caused by non linearity in the detector. The AF amp is as previously without the output valve. I'll redraw the circuit soon.
On strong stations with minimal reaction it's a bit too loud on headphones, audio quality is clean with little or no distortion, weak stations come up well advancing the reaction with Radio Caroline coming in well at 20.10 this evening with the radio sitting on the arm of my chair in the living room.
HT is 54v

Hi.
I've done a video with the modification down to two valves driving a speaker.
It easily fills a room with sufficient volume.
https://youtu.be/48iGsVd5yUs

Hi.
Here is the diagram. I discovered the resistor from the G1 on the AF amp to deck was O/C so there is no need to bias the G1. a 1m to deck works perfectly

I watched the video - impressive!

But what are the valves? Video title says 2 x 1zh24b as does the 8- October post; video commentary says the output is a '29b as does the diagram.

(I can't see the '24b driving a loudspeaker without being over-run).

One Qu: running the output valve without bias, and setting anode current by Vg2 gets you where you want to be. But does this allow sufficient headroom for the AG swing on g1 without running into grid current? Could you cope with stronger signals with a bit of -ve bias, and higher Vg2?