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3C24SE amp...

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Was checking out Valve World last night and came across this amp...

Just curious as to what you guys think of this circuit.

Thanks
 

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Guiness said:
Was checking out Valve World last night and came across this amp...

Just curious as to what you guys think of this circuit.

Thanks

Built a similar amp last year . Used 3A/108A input stage and 6AH4 driver . LCL DC filaments all round (only ac for the 6AH4) , feedback implemeted anode-to-anode between input and output valve , also the output transformer secondary was in the cathode circuit of the 3C24 . It sounded pretty good but never made it past breadboard stage , the 3C24 is an impressive looking valve running full cherry :) One point to note is that A2 amps sound very bright and can sound unusual on bass reflex speakers . If attempting a similar amp , the grid and plate dissipaters need to be used

cheers

316a
 

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Hello 316a , how do these DHT die ? or when are they dead ? filament failure or something else ? i have some HK54 and VT127a left . They're not new , they light on but i dunno about the vacuum . Should a sherry red heating of the plate clean them new ? :xeye: cheers Pierre
 
The weak points of all those valves are the seals . If the seals get too hot then air gets in and it's game over for the valve . I have over a dozen VT127A and there are a few really gassy ones so these could probably do with a good baking either in the oven or by lighting the filament . Incidentally it's a nasty one to light up and will need DC , you will need some huge chokes for the filament supply , regulators , resistors etc just drop too many volts . HK54 are a handsome valve , I've some of these but never used one , the same will apply , bring them up slowly on a variac and run the fils for a few hours . Better still run your HK54 and VT127 from an external filament supply and run on a valve tester using the filament as the cathode connection . From memory the VT127 should have gm of around 1.8mA/V at 0V / 400V . See picture for a messy breadboard using VT127 , the chokes at the back are the LCL filament supply

cheers

316a
 

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316a said:


The weak points of all those valves are the seals . If the seals get too hot then air gets in and it's game over for the valve .
for sure , they go hot hot hot , even after 5 minutes . I have done black clips from ... transistors coolers :D Do you think these are worth the effort ? Not with AC heating ? huh ? ! best regards
Pierre
 
You can try with AC heating , I could not completely remove the hum though . VT127 will be more robust than the 3C24 without dissipators . Get yourself some HR6 caps for the 3C24 which are only $5 per piece (Google it , think it was Nebraska Surplus who had some) . My father turned me some caps for the VT127 , I'm sure you could drill out an HR6 to suit otherwise ;)

cheers

316a
 
I use HR-2 heat dissipating connector on 3C24s. The HR-2 are made
for .0625" pins which is the size on the 3C24 that I have. HR-6 have a
bigger hole for 4-65A and such. Maybe some 3C24s have the bigger posts?
Anyway make sure you get the right size dissipators for your posts.

You need to run the plate red to activate the getter. This is a required
operating condition to keep the tube healthy inside. It takes about 18W
to get to the right colour on a 3C24.

The 3C24 has about 9K ohms internal resistance. There are 2 approaches
I've seen to deal with it. One is to use a 10-15K OPT and live with low
damping factor. Another is to use feedback. Some have feedback and low
DF. I personally like 316a's use of local plate feedback and cathode
feedback to lower the effective Ri.

I have not run in constant A2 (mine might be called "occasional" A2
where the first 20% of power or more is in A1) and I haven't noticed
any characteristic sound.

Cheers,

Michael
 

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I'm not certain about the the plate acting as a getter. The 3C24 is a close relative to the HK54. It uses a cylindrical tantalum plate and a vertical braced bar tantalum grid with a thoriated-tungsten filament. The design features result in no getter due to a much higher vacuum, no insulators of any kind (except the glass) and the tantalum parts don't out-gas. With thoriated-tungsten tubes, the thorium helps promote good health of the cathode and must be run at the right temperature (voltage), unless of course you stumbled onto some information I've not.

I've got several 3C24s, but I envy you having the Eimac metal base ones. Here's a pic with 6.3V on the filament and about 14 watts of dissipation.

An externally hosted image should be here but it was not working when we last tested it.


Regards, KM
 
"Tantalum was a different story. Eimac, and H&K before it, had developed
their reputations on tubes that could "take it." Tantalum was the
material that made this reputation possible. There was something we
had overlooked. Tantalum was a great getter when it was red hot; a
lousy getter when it was cold. Here was a paradox. Those customers
that "Beat" their tubes thought Eimac tubes were the greatest, but
those customers that ran their tubes conservatively, got very poor
life. In the search for long life, we finally eliminated all tantalum
in our tubes. We substituted molybdenum, coated at first with pure
zirconium powder. Later we developed a more rugged coating called
Pyrovac that was the best of both worlds. We still use Pyrovac in
our tubes."

Jack McCullough -- THe History of Eimac

http://www.rptslawrence.w0uk.net/newsletter/Aug-2002.txt

So I guess Tantalum does need to run hot to function as a getter.
But some tubes will have great vacuum and great seals and need no gettering.
Some tubes will have imperfect vacuum or years worth of one molecule
at a time seal leaks that will need some gettering. And some will need
more still...

A good compromise policy might be regular cooking periods ;-)
 
Nice link! Thanks for that. I recall reading somewhere that both Bill ad Jack worked for HK before leaving to form Eimac... and they developed the HK54 while at HK. If the tantalum components (plate and grid) don't out-gas anything then the only remaining places for contamination is the filament and possibly the tungsten support wires. Makes me want to find some of their later tubes sans tantalum that might also be viable for an audio amp.

I still like the 3C24 and am working on a design for using it in A2 mode... I was always planning on plate dissipation in the 20-watt area. Tossie's (Valve World) 3C24 amp at the start of this post does the same and seems to get good performance from it with a pretty low plate load (2.5K). Will see how this works out. Again, thanks for the link... a good read.

Regards, KM
 
Re: 3c24 work fine

aldovan said:

Other fine tube is the 15E.

15E has a similar sound to 3C24 as a driver , to date I've not gone full cherry with mine but if using this in A2 , I would lower the dissipation . I used 15E fairly recently in a mosfet hybrid . Getting the filament quiet enough is the main issue I found , then again I did use filament bias which requires a very clean supply . The 5V at 4A filament is not the easiest to light up , solid state and chip regs are out , choke input/choke smoothing is the way to go . Mounting is also an issue , I drilled some teflon sheet for the filament pins and used Farnstock clips for the anode/grid connections . Pins are non-standard , the common heat dissipators will need drilling out . This valve also goes gassy in the box , even more so than 3C24 . Out of a dozen in stock , only four would good :(

316A
 
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