572B. Anyone tried it?

[rabski]

Seeing as I'm Christmas shopping, working and putting up decorations, my mind started wandering to looking at valve spec sheets. As sometimes happens. Anyway, one of the first New Year projects is destined to be another SET and the plan was/is 845 again, as I like them and I've got most of the bits.

But... I've stumbled on the 572B and it's tweaked my interest. It's another primarily RF amplifier valve, but the specs look interesting. 250mA plate current is tidy, though it does need a few volts to get there (max anode is quoted at 2500). Not out of the question though. The main thing that's peaked my interest is that they're surprisingly low cost.

One or two US and Far East audio amps seem to use them, but nothing mainstream.

Anyone tried it? Unlikely, but you never know.

 

[pmcuk]

Off my radar, I'm afraid. I don't build with higher voltages. 2a3 is about my maximum, so sorry no help here. I seem to recall from looking at 811 literature that it's not so easy to implement. There are a few other larger DHTs out there but they may not be any better than your existing 845. How about shifting your curiosity to the driver tubes? Just an idea to spark some research. With the 845 a lot of the sound is going to depend on what you drive it with. Plenty of nice tubes you could look at there.

 

[dave]

With the 845 a lot of the sound is going to depend on what you drive it with.

For that 572B, a nice big cathode follower or mosfet by the looks of it.

 

[rabski]

For that 572B, a nice big cathode follower or mosfet by the looks of it.

That's why some existing applications help, of course. The data suggests your suggestion is right, but much beyond that, and getting it into the sweet spot, there's a fair bit of 'try it and find out'. I'm discouraged by the lack of commercial uses and whilst getting near 2kV is doable, it's a lot of work in terms of suitable smoothing.

I suspect this will go in the 'could have been interesting but too mad' pile. It's potentially an awful lot of faff to end up with something that could sound far from good.

A diversion really. I have no idea why I'm bothering to look elsewhere. I've got all the iron for an 845, and all the valves and other bits.

 

[toprepairman]

Phew, would have hated seeing you get zapped by 2.5kV and get your head 'blown clean right off'.

 

[rabski]

Phew, would have hated seeing you get zapped by 2.5kV and get your head 'blown clean right off'.

Hahaha. To be fair Henry, anything above 80 volts is 'enough' if it's got any current behind it and if you're careless. My learning days in DIY stuff were courtesy of some of the amateur radio crowd, where a few kV was par for the course, as were mercury rectifiers and high-current RF. I gained a very healthy respect for anything that can cause a spark and those lessons have stayed with me.

I can be a bit cavalier with drills and soldering irons, but I take extreme care with anything that has volts in it. Even testing 'ordinary' circuits, it's one hand only, latex gloves and never working tired.

We laugh and joke here, but I never, ever take risks with electricity or electrical safety.

 

[steve 57]

Have you tried your vt75 's as output valves, I once used mine and was surprised how good they where, close to px4.
Transmitter vales are great if your speakers are lacking efficiency, but they are not as resolving as some of the proper audio valves in my experence

 

[bencat]

Okay this just me talking and most of you on here know I am not in any class to be able to judge on this but I would like to comment . @rabski I would love you to just go out there and give this a try . Just because it has not really been done and it is going to be hard does not mean it should not be attempted.

There are lots of examples with 845 ,300B , and others and while it is nice to refine them and get what for you is a really sweet sounding result but doing this with a strange and mostly unused valve would be such a better achievement in my view . It would also offer a real challenge and not rambling down things you have already done a number of times before .

If I go to buy a bottle of wine and see a grape type I have never seen before I almost always have to buy a bottle and at least try it and see . I know full well that after years of tasting tempranillo is my favourite and I know what i will get from any given bottle without too much surprise and I will enjoy it . But when you come across something not common and it tastes great there is a much deeper warmth about the find .

 

[rabski]

Not quite like a quick bottle of plonk though, unfortunately. We'll see.

I've nothing against 'try it and see' builds, but 2kV power supplies and suitable output transformers don't grow on trees, and there's potentially a lot of cost involved to end up with something that sounds like a pile of sawdust.

 

[dave]

Not quite like a quick bottle of plonk though, unfortunately. We'll see.

I've nothing against 'try it and see' builds, but 2kV power supplies and suitable output transformers don't grow on trees, and there's potentially a lot of cost involved to end up with something that sounds like a pile of sawdust.

The bit I am not very good at would be first to try and select a semi sensible operating point and see what sort of primary kohms of transformer it wants, after that it's all parts bin down hill, with the hope the output TX can handle the volts without flashing over. (back to front baseball cap emoji)

 

[rabski]

The bit I am not very good at would be first to try and select a semi sensible operating point and see what sort of primary kohms of transformer it wants, after that it's all parts bin down hill, with the hope the output TX can handle the volts without flashing over. (back to front baseball cap emoji)

Me too. The pencil and paper bit is time consuming and messy. I'm fine with small signal stuff, but much slower when it comes to anything glued to a transformer.