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Re: [Amps] Supply question

To: 'AMPS' <>
Subject: Re: [Amps] Supply question
From: "Dr. David Kirkby" <>
Date: Wed, 20 Jul 2011 08:11:27 +0100
List-post: <">>
On 07/20/11 01:47 AM, Michael J Talkington wrote:
> Hello,
> I am building a single band hf amp. I am using two 3-500Z tubes. The specs 
> for the tubes say
> 4000 Volts dc max. I keep hearing people volting these tubes in standby above 
> this voltage
> and when they transmit the voltage sags to 4000 vdc or a little below. Is 
> this bad for the tube?

That is quite acceptable. Tubes don't mind that. Strictly you are going above 
the voltage, but I don't think it will harm any tubes. I've seen that written 
some data sheets, but I would certainly not worry about it, even if it does not 
say so.

However, the max voltage is not always the same as the "typical operation" 
voltage. In some tubes it is, but in others the data sheet will show typical 
operation at a lower voltage. I would in that case pick a voltage that shows 
"typical operation" rather than absolute max.

> If I figure correctly 2800 volts ac full wave bridge and after filter caps I 
> should get  4032 volts dc
> 2800 X 1.44 am I correct?

No. You are not. If you have a 2800 VAC transformer, off-load the transformer 
will put out more than 2800 VAC. Exactly how much depends on the transformer. 
Very big transformers drop less than 1 % on load. Very small transformers drop 
more than 10%. How much yours drops is hard to say, but I'd guess a few percent.

I will be bombarding you with questions cause I want to do this right.
> Thanks Mike N8FWD

I normallly reckon on the on-load DC voltage being about 1.2 x that of the AC 
voltage, so for 4000 V, I'd be wanting a transformer of 4000/1.2=3333 VAC.

More accurate calculations are possible, taking into account the effect of 
capacitance, voltage drops on the diodes and sag of the transformer.

If you intend getting a transformer wound, I would ask them to suggest the 
voltage, saying you will  be using a full-wave bridge, capacitance and need 4 
on load. They will be in the best position to know.

Note there is no "free lunch" here. Although you get more DC output voltage 
the bridge than the AC voltage, of the transformer, the AC current is higher 
than the DC current. So the transformer needs to having a current rating of 
about 1.25 x that of the DC current. (The slightly higher factor is due to the 
fact the load on the transformer is not purely resistive).

There is a program called "psudesigner"

which will help with these, but it relies on you knowing the characteristics of 
the transformer which is a big unknown. That's why I would suggest telling the 
supplier what DC voltage and current you want, and letting them decide on the 
actual voltages and currents of the transformer. Don't specify them yourself.

If you are looking to find a transformer used, I'd be looking for one around 
3333 V mark assuming typical operation of the tube is 4 kV. If typical 
for the tube is only 3 kV DC. than aim for about 2500 VAC.

Just my 2c.

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