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diesel_tech 6th October 2007 02:10 AM

using 24G and 3B28 in an od ball amp.......
I need some direction. How would I go about designing an amp using 24G's as the output, and 3b28's as the rectifiers? Filament currents arn't a problem, i can find a way to deal with those, this is more or less a kind of franken stein project, or just something to make people nervous when they enter my little work shop.:devilr: Any leads on where I should begin? Has anyone used a 24G? I searched the site but did not find anything.

Jeremy Korbe

billr 6th October 2007 05:06 AM

You need to try and get some curves for this, from that you can work out a suitable load, and therefore an output transformer.

Once you have the output sorted out, you will then be able to determine the input swing that you need.

The rest is a matter of a driver circuit to meet the input demand of the output valve.

sorry I can't give you more

pmillett 6th October 2007 05:41 AM

I've played some with them. To me they beg to be driven in class A2.

You can find curves here:

24G, 3C24, and HK24G are the same as HK24 with the grid lead out the side. I think Eimac 25T and 25TG are also very similar. From what I've seen the baby Eimac 15T is also not too different in characteristics.

Look for circuits that use the 811A, 812A, and similar tubes. The 24G curves actually are very similar to the 812A and 834, but the max power dissipation is a lot lower.

I would try a circuit like this one:

You might need lower B+ to stay under the plate dissipation limits.

The 24G has fairly high Rp so you really want a high Z OPT. 5k minimum, 8k or 10k would be better.


Gluca 6th October 2007 10:05 AM

I hope my memory doesnt fail, but I believe Jim Dowdy used the 3C24 to drive a GM70 (LC coupling).


nullspace 6th October 2007 04:43 PM

You beat me to it Gianluca. Jim has said that as a driver he's used the 3c24 at ~400Vpk, -5.5Vgk, and 6mA. Rp was up around 15k. I think he is using a choke of ~600H for the plate load, can't remeber if it was Lundahl or MQ.

I have about a dozen of them hanging around, waiting for me to find Eimac HR1 heatsinks or get a machine shop to make some.


Gluca 6th October 2007 09:49 PM

It took me half a year to spot EIMACs caps just hanging over 304TLs in a auction on ebay, the seller was kind enough to give me the caps only (for a bargain).

I have been using crocodille clips in the while.

Have fun.

diesel_tech 7th October 2007 05:40 PM

Thanks for all te suggestions, I'll gve something a try.


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