About 10 years ago I used this tetrode driver which I loved. The 24a. It has hungry filaments, albeit a lovely sound. I probably lost a few readers by now, but don’t care. The 24a into the 45 Single ended, was a great choice, no wonder why Torsten praised this valve.
I settled with the following driver which worked well for me. The 24a is carefully operated at 8mA and 80V on the screen to minimise distortion. This way you can get about 35.5dB (x60) gain stage with about 160Vpp output at 0.5% THD. Yes, more H3 component than a triode, but sounds really nice:
Simple and effective gain stage for a 2-stage amplifier. The output valve could be anything of your choice: 2A3, 45 or even 300B.
The hybrid mu-follower use R1 to set the gain of the tetrode stage (I covered this before many times) and the stable mu-output also feed the screen regulator (Reg 1). I made also a PCB for this regulator, so the driver footprint is very small.
Hi Ale,
Nice to see this post. I built a 24A/45 Loftin-White according to Thorsten’s London Live DIY Circle Newsletter 42 article of 2002-01-20. It is playing in my systeme as I write this. I had a failure of one of the National Union 24As just a few days ago…some kind of short that made the valve look like a “glow tube”. Dug into my stash with rememberance of where the next NOS pair of 24As were and found them… Westinghouse ones. Replaced a Bourns 1K pot in parallel with the left channel R1 and it was good to go, so I’m listening, mostly to Biber as I row each day. And looking to get my TD-121 back and running to get my LPs back.
I have upgraded my 45 to an EML 45 Mesh.
I’m not sure where you are going with this intro to the 24A, but I will certainly follow it, looking for improvements.
Thanks and Very Best,
Robert
Hi Robert,
Decade ago I breadboarded a variation of Thorsten’s amp and really like it. Experimentation took me in another direction but at that point I bought many 24-As. I need to look at my stash but remember having several Radiotron 24As, 324As, UY224As and even some NOS Arcturus blue globe ones. I loved their sound so I kept them.
No specific project in mind but likely to put them back into service at some point.
Cheers
Ale
Hi Ale,
Thanks for posting the schematics.
Could you clarify what the Reg1 is, the screen regulator? Is it a voltage divider followed by a buffer?
Thanks,
Mike
Hi Mike,
A bit more elaborated with a voltage reference created by a CCS, but a voltage divider with the buffer works perfectly as well.
Cheers, Ale
ALe, I’m looking at the 36 tetrode as a driver for an EL34 amp build. Have you ever looked at the 36? Also, What PCB did you use for the screen supply and do you have any to sell? I might like to include that in my design.
Hi Jay,
I’ve not experimented with the 36 so can’t help there am afraid.
I used a PCB I designed for myself with a CCS-based reference and a follower. Never offered for sale as didn’t have the time to pull together documentation etc. Ping me if you’re interested
cheers
ale
I did more comparing of the 36 to the 24a and I am now convinced that the 36 is a 6.3v 24a with a smidge lower plate resistance. I would be intrested in that regulator. I did see that PMillett has one on ebay, but I don’t know about his circuit. I did use his bias regulator and it works great. I don’t know what your work load is. If you have time and the inclination, that would be great. I got myself a pretty good stash of 24a and 36 tubes. Thanks, your website is a gold mine btw.