[Heath] Your typical idling plae current on your Heath SB 220 amp

[Kenneth G. Gordon]

On 23 Mar 2017 at 18:16, John King via Heath wrote:
 
> I have a number of kw rated amps, one of which is a  Heath SB 221, and when keyed up, with Zero driv, has 
> an idling plate current of 140 ma.. I have looked up the specs for the Eimac 3-500Z but my tubes are 
> Chinese 3-500 ZG tubes. I think 140 ma is about right for two tubes???
> 
> I am asking about the plate current  meter readings of YOUR SB SB221/220 amplifier keyed  up with NO 
> RF DRIVE. My 140 ma plate current keyed up with Zero drive  sounds about correct but I would like what 
> you are experiencing with your amp keyed up with no drive . Thanks and 73, John, K5PGW

Hello, John:

I own an SB-220 in which I have a pair of Amperex 3-500Z tubes. 

Your idle, zero-drive, plate current depends on your plate voltage. The SB-220 has two plate 
voltages available, one is labled CW/TUNE and the other is SSB. 

In the CW/TUNE position, the plate voltage is about 2500 VDC, while in the SSB position, 
plate voltags is around 3000 VDC.

According to Eimac, for a grounded-grid operation of a pair of 3-500Zs, the "idle current" 
should be 130 mA at 2500 VDC, and 160 mA at 3000 VDC.

My SB-220 reads about that current.

Your 140 mA is midway between those two values and in my opinion is about perfect.

Remember that this idle current is not exact. A few mA above or below the specified current 
will not effect linearity. WIDE excursions from that will be a bad idea. If idle current is 
substantially low, linearity will suffer. If it is too high, tube life may be shortened as the tubes 
will be continuously dissipating a lot of power in heat.

I think your setting of "about" 140 mA is perfect, and I wouldn't worry about it at this point.

In plain language, if it works, don't fix it.

vy 73,

Ken W7EKB




---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus