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Old 27th January 2005, 11:50 PM   #1
abj1 is offline abj1  Australia
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Default Type 46 as voltage amplifier

Hi All,

Question from left-field: has anyone any experience using this valve as a voltage amplifier with both grids tied together (as a combined control grid)?. I know this valve was used as a class B output biased at 0V and also has published characteristics for class A triode (with the g2 tied to the plate) but has anyone tried to tied the grids together and using a little -ve grid bias, say 1 or 2 V?

I'm thinking in terms of potential use as a high mu triode (but directly heated) as the first stage to be followed by another type 46 trioded (g2 to plate), or perhaps 71A, driving a type 50, single ended. I'm looking for more gain than a 26 can give (even actively loaded).

May have to consider two stages of type 26's (hum problems aplenty here, no doubt) or perhaps a type 47 pentode (with VR regulated screen) but the 46 is a fairly unusual valve and may just do the job. I don't mind straying from the well-beaten path (in fact, prefer to!)

Any thoughts?

Andrew.
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Old 28th January 2005, 12:15 AM   #2
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Hi Andrew,

The 46 is quite a nice little valve, quite capable of being an output stage on its own! The problem with trying to drive it with tied control grid and screen is that there is very little room for class A1 operation. Triode-strapped it makes an excellent driver, but its low m gives little gain.

Have a think about using 3A5 or 1LE3 DHTs for gain.
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Old 28th January 2005, 01:40 AM   #3
abj1 is offline abj1  Australia
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Thanks Jason,

Will give these valves serious consideration. Was really hoping to find a solution using AC filaments (for simplicity). The 3A5 does have pretty good curves to around -5V though which is right on the money

Agreed that the 46 curves don't show much daylight between 0V and 0mA but maybe there is enough at 250 to 300V to squeeze out a clean amplification of a 1V p-p input signal at a m of >30 (this is another unknown quantity!) which would give enough overall gain to clip a type 50 (with triode 46 as driver) with an input signal of 500mV - which is what I'm aiming to achieve. If the m was more like 50 I'd be very happy - provided the linearity was acceptable (also unknown territory!).

The 3A5 is relatively cheap to try but the 1LE3 is not quite so (especially given the loctal socket). What have you used to power the filaments on these?

Cheers,
Andrew.
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Old 28th January 2005, 05:01 AM   #4
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Hi Andrew,

I have never used these valves myself, having a tendancy to using things with real cathodes! Though I have noticed that the few schematics floating out there using the 3A5 tend to use DC on the filaments.

3A5 -> 46(triode) -> 50 will probably have barely enough gain, even with three stages. There are some more obscure DHTs out there which I can't remember that might be more suitable.

Take note of the low maximum grid circuit resistance for the 50.
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Old 28th January 2005, 10:45 PM   #5
abj1 is offline abj1  Australia
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Hi Jason,

The low max grid resistance on the 50 is a bit of a challenge and one of the reasons I have decided to use a power valve to drive it (trioded 46 most likely although I do have a pair of VT25's that would do the job nicely - 71A's probably will struggle to give enough clean voltage swing without active loading to satisfy the 50's). Even an RC coupled loadline at an AC impedance of 5K for the trioded 46 should give good results - and better at 10K (from the 50 grid) in parallel with an active load. Driving it with a more traditional smaller signal valves would be difficult! And of course the issue of driving a largish capacitance is another good reason to use plenty of power in the driver....

And in any event I would be likely to push the max grid leak value too - perhaps to a whopping 20K...!

Cheers,
Andrew.
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Old 29th January 2005, 12:05 AM   #6
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Why not try interstage transformer coupling - you get rid of the problem of the low max grid circuit resistance, and probably will get away with one gain stage since you can use a step up ratio provided the ra of the driver is low enough to not cause problems with miller capacitance.

The 50s are such rare things that I would be terrified to even light them up for fear of breaking the filament or something - let alone pushing max ratings!

50s, VT-25s.... Gee, you seem to have quite a collection there
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Old 29th January 2005, 06:34 AM   #7
abj1 is offline abj1  Australia
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Hi Jason,

The only real problem with interstage coupling is a simple one of not having any in my stash....

I was very lucky with the pair of 50's (and the VT25's for that matter) in the sense that I was in the right place at the right time and had enough available cash to snaffle them at a very good price. Rare beasts no doubt and when the amp is finished it will be a one operator only concern! Been slowly collecting a stash of DH valves and am about ready to put them to work - apart from a push-pull setup with 47's (which was really good but never got around to even using regulated screen supply) I really haven't had to opportunity until now to work with valves without cathodes. This is about to change.

Cheers,
Andrew.
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