[Top] [All Lists]


To: <>
Subject: [AMPS] 4-1000 IDLING CURRENT
From: (Jon Ogden)
Date: Sun, 29 Mar 98 23:12:59 -0600
>My Home brew 4-1000 amplifier works great except for on thing.  The power out
>seems low.  I believe it may have something to do with the low idling 
>Plate voltage is 4,600 VDC and idling current is about 80 ma.  The max plate
>current I can load it to is 450 - 500 ma.  At 4,200 VDC under load and 450 ma
>plate current, gets 1,450 watts out with about 130 watts drive.  Have checked
>the B- line for stray resistance; none.  Have tried it with the zener both in
>and out; no difference.  Power supply is capable of more than enough current
>and I have 200 watts available.  I have 3 tubes: all 3 operate the same in my
>amplifier, full output in another amp.
>Any suggestions!
>Thanks, Bob

Well, Bob, I have a 4-1000 amp that I've been struggling with too!

Here's what I can figure out:

I am assuming you are running grounded-grid.

Idling plate current at 4 KV according to Eimac's specs should be about 
80 mA, so it sounds like you have it correct.

Your plate current of 450 mA sounds low.  According to Eimac again, the 
single tone plate current with 100 W in should be 680 mA.  I've seen 
lower than expected plate current in my amp as well.  Not sure about it.

As for power out, from my understanding, you are getting about the right 
output.  The 4-1000 needs a lot of HV for it to really get good gain.  I 
have had people recommend 4500 V or more.  You are only running at 4200 
volts under load.  Somewhere you are dropping 400 Volts from the 4600 V 
idling to the 4200 V drive. 

This bothers me.  I assume that under load, you are still seeing 4600 
Volts at the supply and 4200 volts at the plate.  With 450 mA of current, 
that would me that there is a resistance of (4600-4200)/.45 = 400/.45 = 
888.88 Ohms somewhere in your system.  This wouldn't be right.  If the HV 
supply is dropping by 400 volts, that seems a little much, but maybe not.

Your idling current really doesn't have anything to do with PA output 
under drive anyhow.  Really all it sets is basically the bias point of 
the cathode (someone correct me if I am mistaken).  The more idling 
current, the close to class A you are, the less, the closer to class B or 
class C.  It really determines what drive level will cause the tube state 
to go from cutoff to active mode.  It has no bearing on ouput power.

Also, what band are you working with this on?  You may wish to check your 
plate RFC for resonances that can steal power.

There is a guy on the sig, Jim Thomson, who has helped me out a lot.  He 
has some 4-1000 amps.  His address is:  If forget 
his call, but he can help possibly as well.

I am still learning mine.  When I get it figgerd out, I'll let you know!  

73 es gud luck,


Jon Ogden

"A life lived in fear is a life half lived."

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>