Re: [Amps] Screen supply for 4 paralleled 4-125A's?

[R.Measures <r@somis.org>]

On May 1, 2004, at 7:24 PM, charlesmorris@direcway.com wrote:

> > ð  More power would be produced if the 4-125As were run in Class
> > AB1 linear mode.
>
> true, but then what happens to the linearity?

**  It degrades slightly, but it is still better than the IMD level of 
most transceivers.

> I can always experiment with grid bias settings, anyhow.

**  In AB1, one can not adjust the grid bias to the point where grid 
current flows -- i. e., the grid must not be allowed to go positive 
with respect to the cathode during max signal conditions.
> >   If I were attempting such a project, I would use c. 500V on the
> > screens.
>
> Isn't the max screen rating 400 volts?

**  My Eimac spec sheet says 600V max for Class AB1.  In AB1 one needs 
to use high screen potential in order to utilize the full emission 
ability of the cathode since the grid potential never goes positive.

> My HV supply will be around 3 kv.
> Any good place to find the curves online for a 4-125A?

**  I have a paper copy of curves for E-screen = 350v.  With 0V peak  
on the grid, the anode draws c. 0.11a-peak, so for an AB1 4-125A with 
350V on the screen, the max average anode-I would be c. 38mA - which is 
hardly good news.  Since I-anode is a function of E-screen ^1.5. with 
600V on the screen, the peak I-anode would be (600/350)^1.5 x 0.11a = 
225mA-peak which is still only about 75mA average..  Thus, my guess of 
using 500v on the screen was a bad one.  Even 600V on the screen would 
allow using only c. 1/3 of the max rated emission potential of 225mA 
average.
-  A work-around would be to use 4-250As in AB1 with 800V on the 
screens.  Another way to go would be to use the 4-125As in g-g.
> ...
> >   I would avoid tuning the
> > amplifier with a dead carrier because of limited anode dissipation.
>
> Well, I wasn't planning to push the ratings anyway. If not using my 
> HW-101, I will be playing with an ARC-5 transmitter with a homebrew 
> screen modulator, which will only produce about 15 watts carrier. I 
> figure at rated 500 watts dissipation (125 per tube), at 33% 
> efficiency worst case would give me 250 watts carrier in Class A, 
> which should be plenty (about +14 db)...
>
> And with no signal, or pure carrier, a class A amp will still 
> dissipate the same 500 watts - so no heat-related tuneup problems 
> unless I want to go above 250 watts out, right?

**  The characteristic curves suggest otherwise -- at least for AB1.
> I decided to use 125's because they were readily available and cheap 
> (in fact the sockets can cost more than the tubes!) but I don't intend 
> to be playing games with Eimac's ratings.
**  pushing ratings is like unto pushing greenbacks into the crapper.

cheerz, Charles.
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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