1П24Б 1p24b

[Murphyv310]

Will do.
Might not get time today though.

[Murphyv310]

As promised.

[Kalee20]

Ah. But that's got a floating battery, independent of all other DH valves.

If you allow that, you can use the traditional way, whether filament is tapped or not - you just go to one side of the battery (which side it is, is immaterial in principle because you can alter bias to suit).

[Murphyv310]

That's what I said earlier in the thread that it would need a floating supply.
You can use a very small mains TX with separate windings and individual rectification and smoothing. Not ideal but another way to get this to work. To be fair a PP audio amp is messy with filament valves IMHO, no matter what way you achieve it. The power valves are really just a bit of fun. Where the rod pentodes score for me is how good an oscillator they make, good for radio projects, pre stages, pantry transmitters and the likes, for power a "normal" valve is by far the best way.

[Kalee20]

Apologies - I missed the words 'and isolated' in your earlier post. For me, isolated filament supplies are definitely non-preferred.

I'm very much aiming to have just ONE filament supply. Eventual aim is to have a switchmode converter supplying HT, LT, and GB. And while adding extra outputs is simple in principle, filtering thereof is very much not - and I'm looking at this AF amplifier being part of a rod-pentode radio.

To be fair a PP audio amp is messy with filament valves IMHO, no matter what  way you achieve it. The power valves are really just a bit of fun.

They certainly are a challenge at times, but then so are indirectly-heated valves where significant h-k voltage exists. And as for filament valves - I have a pair of 2A3 triodes which are DH, some day I'll make an amplifier with (and these WILL have separate filament supplies, but this will be mains operated with AC on the filaments and the 'cathode' connection taken to a CT on the filament winding).

Somehow, the symmetry is more appealing than the essential asymmetry of a DC filament.

[Murphyv310]

Hi Peter.
No need to apologise.
I've been repairing amps with the 300B triodes.
They feed AC to the filaments with separate supplies for each valve, four on a stereo push pull amp. Each have hum bucking pot, they are a nightmare to set up and really can only be done on a scope. Unfortunately they drift which is no good for the customer.
Another had DC feeds to the filaments but its a horrible design. This of course is to satisfy the audio fools.
I've made up a reasonable amp for a battery radio with the 1j29b, it's around 200mw and with an efficient speaker and baffle it's plenty loud enough.
Good luck with your PP amp, I'm genuinely interested in the end results.

[ppppenguin]

Directly heated valves are a real problem if you want the cathode to sit at other than ground potential. When you look at the high power cathode followers in the Ally Pally "Black Book" you can see the trouble they had to go to in making the filaments sit not just at high DC potential but also with many volts of wideband video.

[BusyBee]

Apologies - I missed the words 'and isolated' in your earlier post. For me, isolated filament supplies are definitely non-preferred.

I'm very much aiming to have just ONE filament supply. Eventual aim is to have a switchmode converter supplying HT, LT, and GB. And while adding extra outputs is simple in principle, filtering thereof is very much not - and I'm looking at this AF amplifier being part of a rod-pentode radio.

My apologies if I have missed something earlier but, although it was with an RF circuit rather than audio, in the past I did experiment with a RF differential preamplifier using 1j29b's with cathode bias and heaters supplied via switch mode isolation. I did find the noise level limited small signal performance even with fairly good power supply filtering. Obviously this would be less of a problem with lower frequencies and a power amp but yet another nagging issue to be addressed.

I do presume that directly heated valves would be less resistant to heater borne noise than indirectly heated with their greater thermal inertia. Possibly another good reason not to float the heaters, but to ensure a low impedance signal path to ground?

Tracy

[Kalee20]

Yes, DH valves are particularly susceptible to noise on the LT line. Not because of temperature fluctuations (although on 50Hz that could be an issue), but because any noise on the LT line is effectively in series with the grid input.

Hence, my wish to have all filaments tied to 0V. That way, I have only ONE line to keep 'clean.'

I do have experience of making switchmode supplies for HT, LT, and GB, and noise avoidance and filtering is definitely the hardest nut to crack...

[Kalee20]

Here is the final circuit of my 1p24b push-pull, ultra-linear amplifier. A second output transformer has been wound, incorporating the new feedback windings, and it works well (albeit that the C cores are held in place with a strong elastic band for now). It's generously-sized, weighing in at about 720g, and secondaries are sectioned, either side of primary, for reduction in leakage inductance and better HF response. After the first transformer, I took the opportunity to change the UL tapping points from 20% to 22%. The grid drive requirements are correspondingly increased, but there is still a couple of volts safety margin away from the critical positive-grid region at full output, which would seriously upset the driver stage.
   
The amplifier is built over two pieces of plain matrix board, the first-stage and phase-inverter being on one board, and the output stage on the other. Sharp-eyed readers may note a heatsink with a TO220 device bolted on, with a trimmer beside it - that's a TL783 regulator to regulate HT at 120V. With fixed battery bias (the array of lithium coin cells), the circuit needs a similarly stable HT voltage. Eventually, it will be battery-operated from a stabilised switchmode converter.
   
When commissioning, although the amplifier was stable, there was some (not unexpected) ringing with square-waves. So, I added an RC roll-off (47pF and 27kΩ, optimised empirically) from anode of the voltage amplifier pentode to 0V. The phase inverter, of course, just follows what it gets fed with.

To give the same 6mA quiescent current, one of the 1p24b's needs 19.5V bias, the other 20.4V, so there is a bit of variation between specimens. As there's no self-adjusting action whatsoever, unlike cathode bias, the balance trimmer is needed for best balance. At full output, current rises from 2 x 6mA to 2 x 16mA. Reducing the no-signal current further starts to show evidence of cross-over distortion, and although connecting feedback compensates for this, I was happy with the no-signal current drain.

Distortion is unknown, although the output just below onset of clipping looks pretty good (and it sounds good, too).
   
Frequency response, with 8Ω resistive load, is within 0.5db from 28Hz to 20kHz. The 3db point is 60kHz, so it's quite capable of annoying the odd bat! The low-frequency response isn't really useable down to 28Hz because I designed the transformer for 50Hz minimum at full output, so saturation-induced distortion starts to set in below 40Hz unless output is reduced, even though there is sufficient inductance to maintain small-signal response.
   
A close look at the driver board will show there's an unaccounted-for 1j24b, that's part of the project's next stage, which will be an infinite-impedance detector for an as-yet unstarted superhet front-end!