Golborne Vintage Radio

Hi.
The total current is 8ma between RF and AF.
I've tried different biasing arrangements and this one gives the best overall performance with minimal distortion.
The addition of the 1ZH29B was a last minute change to the audio stage.
Grid current doesn't effect these valves at all. If you look at the Rod Pentode Theramin diagram posted elsewhere you'll see that at least one is running quite high grid currents with no detrimental effects. I often feel we get overly concerned about grid current and there were some valves designed to operate that way. I believe this is less of an issue with rod construction vs very fine spiral grid in conventional valves. The 1ZH29B ran yesterday for some hours. Remained cool and no decline in performance until the LT battery dropped to 1v.
I actually listened to Radio Caroline for well over an hour and a half last night using the speaker.
Next is to makes sides and top and a small speaker cabinet.

Re over running the 1ZH29B if you remember I really pushed the 1J24P's very hard indeed with the rods glowing and cathode current over the max spec. Neither of the two valves I used failed and were then tried as a line timebase output with over 500v pulse on the anodes, they again survived and when I did some comparisons re an unused one were in fact giving a higher slope, these are tough little valves, I think the ratings are very conservative. I've at least 60 1ZH29B valves here and wouldn't mind trying out how much they can take. The 1J42A pantry tx ran virtually consistently for a year with no issues and again it was run at the maximum ratings.

Typo above.
1J24P should read 1P24B

Hi.
A quote from Radicalvalves.

All these valves can run with G1 (control or signal grid) up to 0.5V more positive than the positive end of the cathode/filament with under 1uA grid current. Even more at low HT.

Yes, I ran some tests to see where grid current starts. It looks like about 1.6V positive of (negative) filament terminal, with 1.2V on the filament. My leaky-grid AVC detector, for instance, is biased at +1.4V.

It doesn't hurt the valve, but I was just wondering in your circuit that running into grid current could cause AF distortion at the grid. Obviously, it is a non-issue!

(10-10-2021, 11:42 AM)Murphyv310 Wrote: [ -> ]Hi.
A quote from Radicalvalves.

That's my site. Anyone want it?

How much? Swap for a 1ZH26A

You have my address & email already?

(10-10-2021, 08:51 AM)Kalee20 Wrote: [ -> ]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?

At low levels grid current seems to me to only really be an issue with distortion or less predictability in results. Don't forget that transmitting valves are often intended to be run into grid current, the level being metered to check (Ok they are designed for it and usually power valves and RF). With a rod electrode, as previously comments, there is perhaps less likelihood of problems. Maybe the limitation is really just overall cathode current?

I have an article on one particular Russian radio (R-326) which used a 1j29b input to 1j24b then 1j37b mixer was with signal level. The suggestion is that a large antenna results in blocking. Perhaps not particularly a problem with the sparsly populated bands these days. I have never seen any details with these valves though. Interestingly, when I look at the Russian manual I have for it, valve voltages are listed. They are pretty much as may be expected but maybe I'll look a little deeper.

Tracy

The 1j29b would have a bigger dynamic range for RF input than the otherwise brilliant 1j24b.
The 1j24b can do maybe 250mW audio and a pair in push pull with and ordinary miniature 110-0, 110-0 dual i/p mains transformer works fine as the bias currents cancel, roughly like 1/2 a DL96 (i.e. economy mode). That will give more audio than one 1j29b.

I think the main limitation on Rod Pentodes and possibly all battery / direct filament valves is the cathode current. Parallel operation allows a little more power than series and in any case a "centre tap" (dual filament) on a direct type needs a decoupling capacitor when in series mode.

Series battery sets also have ballast resistors due to the varying cathode currents and a simulator for B7G series needs suitable resistors from g2 & anode to f- and f+

I have a box of Russian, USA and British subminiature valves, if anyone is in need. I think the russian ones use rods?

Stephen Walters
G7VFY