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Subject: [AMPS] 4-1000 IDLING CURRENT
From: (km1h @
Date: Mon, 30 Mar 1998 13:23:43 EST
On Mon, 30 Mar 1998 09:31:10 -0500 (David E Sinclair)
>Jon- The 400V plate supply voltage drop doesn't seem extreme.
>Just remember that the 800+ ohms you calculate *is* effectively
>there somewhere. What Bob is seeing is probably related to the
>voltage regulation of the transformer. In addition to any copper 
>losses (winding resistances), there are additional losses that 
>affect regulation. The leakage reactance of the transformer, a
>magnetic parameter, has a big effect on what the overall voltage
>regulation turns out to be. Transformers with lower leakage 
>reactance have better voltage regulation, if I am remembering 
>right. I believe this parameter is usually more important to reg-
>ulation than winding resistance.

I have used the ESR parameter to cover both the DC and magnetic losses
David....perhaps an over simplification.

An item that seems to bear regular repeating here is the actual
transformer design and its intended application. With hams often looking
at price first many of the 30 year old bargains may not be the correct
A transformer designed for a FW CT circuit or even a FW bridge design
with tube rectifiers will often perform horribly in a SS capacitor only
filter enviroment. 

I learned this back in the 60's and 70's when I was actively building one
and two tube 4-1000A amps for sale. The transition from WW2 or commercial
surplus and 872 rectifiers to SS and big filter caps was a painful one at
first!  I have experienced over 1000V drop on otherwise similarly  KVA
rated xfmrs in those amps. 

OTOH I have a xfmr and SS FW bridge that are part of a water cooled
triode RF heater. It is rated at  12.4 KVA and with the original 4MF
filter cap it would drop from 7800 to 6500VDC under load. With a 50MF
150+ pound oil filled cap it drops to only 7300V under a higher
load...due to the increased HV.   All I need now is the right tube for
linear service!

73  Carl  KM1H

>Two examples: my 3kv supply for my 813's idles at 3200-3300V
>and drops to around 2500V at 500mA. Terrible regulation, but it
>works fine, and has for years. Yes, I have more than enough
>filter capacitance. I also have a rebuilt Heath Warrior amp with a
>pair of GG 813's in it. That supply is so stiff it drops from 2600V
>down only to 2500V at 500mA. It's all in the transformer! (No, I'm
>not using the old Heath plate transformer, it is a replacement.)
>73, David K3KY
> ======================
> David Sinclair         K3KY
> e-mail:
> ======================
>On Sun, 29 Mar 98 23:12:59 -0600 Jon Ogden <> writes:
>>>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 
>>>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.  
>>>the B- line for stray resistance; none.  Have tried it with the 
>>both in
>>>and out; no difference.  Power supply is capable of more than enough 
>>>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 
>>80 mA, so it sounds like you have it correct.
>>Your plate current of 450 mA sounds low.  According to Eimac again, 
>>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 
>>As for power out, from my understanding, you are getting about the 
>>output.  The 4-1000 needs a lot of HV for it to really get good gain. 
>>have had people recommend 4500 V or more. 
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 You are only running at 
>>volts under load.  Somewhere you are dropping 400 Volts from the 4600 
>>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 
>>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 
>>supply is dropping by 400 volts, that seems a little much, but maybe 
>>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 
>>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 
>>class C.  It really determines what drive level will cause the tube 
>>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 
>>plate RFC for resonances that can steal power.
>>There is a guy on the sig, Jim Thomson, who has helped me out a lot.  
>>has some 4-1000 amps.  His address is:  If 
>>his call, but he can help possibly as well.
>>I am still learning mine.  When I get it figgerd o
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>ut, I'll let you 
>>73 es gud luck,
>>Jon Ogden
>>"A life lived in fear is a life half lived."
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