Putting a 4-250 or 4-400 into a BC-610

[K1YTG]

I purchased a BC-610 that turns out to have a final tube that is bad.  But I have a lot of 4-250's and 4-400's .  I have heard of using these in the final, but I don't know if the results were good.  Or how many changes are needed.  Would the filament transformer take the higher current?  Does the 4-250 match the modulation transformer?
Will adding a screen resistor off the HV work?
Will making the change degrade the BC-610?  Would it be worth it to put out $100 to $200 for a 250th?
It looks like the supply of the 4-400a is going to remain good while the 250th is going to be harder and harder to acquire.
This is my first experience with a BC-610.  So I want to take advantage of all the experience out there and see what others have learned.
Thanks, Norm

[KA5N]

Why would you ever consider modifying this antique by putting a tetrode in place of a triode?

Restored the BC-610 is a handsome reminder of bygone days, poorly and inadvisedly modified
it becomes a real "Boat Anchor."

Just my opinion

Allen

[K1YTG]

I was assuming that any changes could be removed and it could be put back as original if it were to go into a museum. 
I was hoping it could use a different tube without major changes.  That way putting it back to original would not be difficult.
But that might not work.
Perhaps there are 250th tubes available.  In that case I would run it just as designed.
If it is practical to do so.
I am looking for more information on the subject to help point me in the right direction.
So I do thank you for your response.
Norm

[KA5N]

The problem with tetrodes is that they require a screen supply with excellent voltage regulation not just a resistor to the high voltage.  If the screen supply isn't properly done the tube will be ruined.  You could more easily use a 3-500Z if there is room for it and of course
would have to get a socket for it.  Also the filament voltage has to be correct  etc. etc.
The price for a 250TH would probably be a lot less than the mods you would need and a heck of a lot easier. 
Good Luck whatever you decide
Allen

[K1YTG]

I have placed a bid on a 250th tube on ebay.  Perhaps I can get one for under $100.

Norm

[K4MFN]

Norm

I think I may have a 250TH in my "stuff". If you do not get the one @ auction, let me know if interested. I think I recall testing it for emission about 7 yrs ago on my "big bottle tester".

Harold
k4mfn

[K1ZJH]

pair forsale over on the 'Zed too.

http://forums.qrz.com/showthread.php?342311-250th-TUBES

[G3RZP]

You would have to change the socket, and you would have way too much drive for either a 4-250 or a 4-400 as a tetrode. You  might get away with strapping grid and screen grid and run it as a triode, although it would have a higher mu than a 250TH.

Whether the filament transformer would stand it is another matter. It's 40% more current, so that's a pretty hefty overload.

I'd just buy some 250TH tubes, and regularly swap them around.

[K1ZJH]

Triode connected 4-400 tubes might work in GG amplifiers, but for the BC-610
I'd wonder how you would get them into Class C (assuming you are going to
use high level plate moduation...) and in grounded cathode I'd also wonder if
you'd be able to neutralize them?

Just musing.

Pete

[G3RZP]

I haven't looked in detail at the  data sheets. Let's assume that you could strap screen and grid and run grounded cathode. You'd probably want zero or very little bias to put it into Class C, and hopefully the present neutralising would have enough range to be OK. Even assuming the filament transformer can stand a 40% overload.

I'd still bite the bullet and buy 250 TH tubes.

[WA5VGO]

3-500Z would be a much better choice.

Darrell

[K1ZJH]

A 4-400 is a drop in replacement for a 3-500Z in grounded grid linear.

Pete

[G3RZP]

We aren't talking grounded grid or linear. It's grounded cathode, grid driven, neutralised, Class C and the 250TH has a mu of about 35.

[K1ZJH]

The tube doesn't know if it is grid or cathode driven

I suspect with the grids tied together it is running closer to AB even with no cathode bias.
I wonder if the tube will be fully in the Class C region unless some screen voltage is used.
I could be wrong, but   I recall the 3-500Z was advertised as being a zero
bias tube.

Pete

[G3RZP]

The difference is in the drive requirements. Although the 250TH requires fair amount of drive, it's nothing like the requirements to drive a 4-250 or 4-400 in GG. So GG is out - besides which, there's no filament choke. Then add the fact that the impedances are totally different, as are the capacities, so it may not neutralise in grounded cathode. Even a 3-500Z would need bias to make it Class C, and the tetrode will need some - but not as much as at present. All in all, I stick to the position that biting the bullet and buying several good 250THs (call it insurance for the future) is the way to go.....

Run them up the same way as advised for the 100TH in another thread here.

If I REALLY had to change tubes, I'd look at a 833A. Overkill, yes, but slightly less like rocking horse manure...