Re: 4 Grids

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike" <induction-at-comcast-dot-net> 

Hi,
      When I saw the initial post about the 304TL I went to our tube supply 
area here at the lab to see if I had one, as I thought I did.
It turned out to be a 304TH, which I think is generally the same tube but 
with a different gain. This one I had pulled out of a working low power 
induction heater some time back. What you could do is series 2 sets of the 
filaments, add power resistors to balance out the current draw across both 
tubes you have in series and run with the lower emission. You will loose 
some power output but that may be better than nothing.
What I do not understand is why are you using such a hard to replace tube 
when you can get 4-400's, and triode connect them or 3-500 z triodes which 
are very available, even 4-1000A's are out there plenty and just triode 
connect them.
833A tubes are still around, we have lots of them in stock at work, even 
have a bunch here and they are still in production. That is a 10 volt tube 
at I think 10 amps, ~450 watt dissipation. If you plan on high current 
pulses, the 4-400PR (pulse rated) tetrodes are great, I have a pair of 
those, 5 volt filament. They fit the same base as the 4-125, the 4-250, 4-400A
For those larger commercial tubes ( where I work we also used to rebuild 
tubes) they are often done in filament sections and the sections are in the 
end parallel connected to the filament terminal posts. It is easy to see if 
you've lost sections of filament yet the pieces have fallen where they do 
not short the tube elements out. Like the water cooled 6696A or in the air 
cooled version the 6697A, an air radiator is simply soldered to the anode. 
These are used up to 80 kW, have 12 sections of filaments and a typical 
filament current of ~205 amps. So, if your amp probe shows a reduction in 
current of 17 amps, you lost one section and so forth. In tech support, 
when the complaint is the tube is lit but the output has dropped off, we 
look for that. Also, the filament transformer voltage to the tube will 
start to soar as the load backs off and that is another sign.
Sockets are so available for the 3-500z, the 4-400 family, though all of 
them demand forced air cooling even with no plate power. Have you 
considered these?
Regards,
               Mike

----- Original Message ----- From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, October 03, 2004 7:46 PM
Subject: Re: 4 Grids


>Original poster: "Steve Ward" <steve.ward-at-gmail-dot-com>
>I have run across many partially dead 304TLs.  I did in fact run 1/2
>of the tube in a VTTC circuit.  I think the most i got was about a 12"
>spark and the plate was very cherry red.   I didnt care about the tube
>so i didnt mind how hot the plate was getting.
>
>Anyway, im not sure if you can actually run just 3 of the filaments,
>unless somehow the 4th was shorted out?  From what i remember, they
>wired the filaments in strings of 2, and there is no center tap
>between each single filament, but there is between all 4.  So you
>could either series all 4 filaments at 10V, or you could parallel 2
>strings of 2 at 5V.  Anyway, it may be worth your time to just play
>around with half the tube, its probably not good for anything else.
>
>Steve
>
>
>On Sun, 03 Oct 2004 14:46:43 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:
> > Original poster: "Rich" <rdjmgmt-at-socket-dot-net>
> >
> > I was bringing a 304tl up to operating voltage on the filaments.  Well I
> > guess they call the 304 for a reason, 3 for triode and 4 for the 4
> > filaments. One tube has one bad filament. I have the tube set up on a
> > series circuit because my main filament transformer is 10v. I found out
> > quickly I can't run it that way. One filament will pull to much
> > voltage.  Before I junk the tube, I could wind another 5v trans, has any
> > one ever run one with just three of the filaments.
> >
> > Rich
> >
> >
>