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

[R.Measures]

Charles -- It would be better to turn on the attribution-level marks in 
your e-mailer application so readers can keep track of who said what, 
when.

On May 2, 2004, at 9:34 AM, charlesmorris at direcway.com wrote:

>> 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
>
> yes, that is in fact part of the definition of AB1 (180<x<360 
> conduction angle but no grid current).

***  Agreed.
>
> **  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
>
> ok, about what grid bias would be required at 600v screen, 3kv anode?

***  It's difficult to tell since no curves are given for 600v on the 
screen, but by E ^1,5 interpolation of the E-screen = 350v curves, my 
guess is about -90v.
>
> **  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.
>
**  note -- there is an error in the above calculation.  The peak 
anode-I should be 246mA and the average anode-I about 82mA.

> however, with 3kv anode, the average per-tube anode current has to be 
> kept below 41 ma to avoid exceeding the 125 watt plate dissipation 
> rating...
>
***  Agreed

> .Even 600V on the screen would
> allow using only c. 1/3 of the max rated emission potential of 225mA
>
> agreed, but this amp appears to be dissipation limited, not emission 
> limited, since I will be using it in AM (100% duty cycle).

>> Another way to go would be to use the 4-125As in g-g.
>
> probably requires too much driving power (I'm assuming I only have 10w 
> carrier to drive with).

***  So, use a modern 100w transceiver with AM mode built-in to drive 
4, 4-125s in g-g.  The AN-612 low level modulator IC used in my TS-440 
produces delightsome-sounding AM with typically 10-times less 
distortion than older modulation methods.
>
>> 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.
>
> please explain further?

***  Typical efficiency in AM linear service is c. 33%.  In order to 
get 250w of AM-linear  carrier output and 1000w pep under 100% 
modulation, (4) 4-125As would seem to lack enough emission for AB1.   
(4) 4-250As will go in the same sockets and they definitely have the 
needed emissive ability since each 4-250 will produce 1200mA of peak 
emission instead of the 4-125's 246mA-peak.  My guess is that the 
4-250A was designed for more versatility than the 4-125A.

later, Charles.

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