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Re: Grid bias 3-500Z

 

John and Alek, my friend just spoke to Tibi at OM Power the other day. ?He called to order some FU728 tubes. ?Tibi said to him that his supplier of tubes in China is making a design change inside the tubes ( something to do with the grid structure) and the design change is now being implemented on the production line. He added that his next shipment will have the upgrades. ?We have found that at least 90% of the amps that come to our shops for repair are tube related. ?Just about all oh the OM4000’s present with at least one bad tube. ?Some long term owners of these amps have reported going through several tube changes already. ?Of course I don’t know how they use their amps and how many hours they put on them in a year but non the less it’s definitely been an issue. ?Hopefully if this is the case that there has been an engineering change, maybe this will be the fix. ?Remains to be seen. ?On another note, I have yet to use a Chinese GU74b so I can’t comment on them yet. ?Maybe some has?




On Thursday, February 27, 2025, 4:46 AM, John Sparkes via groups.io <vk6jx1@...> wrote:

Hi Francois.

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In my experience, the 7.5V Zener provides a little bit of bias to limit the quiescent current of a pair of 3-500Zs running at around 3kV to 150mA. This is the requirement for Class AB2 linear operation.

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You could run them with Zero Bias (as was Eimac’s recommendation) to reduce circuit complexity, but the quiescent current would be much higher – for no improvement in linearity – hence the Zener diode.

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Most of us shy away from Zener diodes these days as a few decent size silicon diodes (1N5408 or 6A10) can be placed in series and provide the same bias voltage as the expensive Zener.

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Good luck with it. I do love all of my amps – 2 x 3-500Z on 6m, FU728F tetrode in Emtron DX-2SP for HF, GS35B triode in homebrew 2m amp for EME and YC156 triode for 160m/80m. All the triodes have silicon diodes providing the quiescent current bias voltage.

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Best 73,

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John VK6JX

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From: [email protected] <[email protected]> On Behalf Of Fran?ois via groups.io
Sent: Thursday, 27 February 2025 4:32 PM
To: [email protected]
Subject: Re: [ham-amplifiers] Grid bias 3-500Z

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Hello

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Now that both of my 3-500Zs have been de-gassed, I have a new question.

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  • In the L7 (Drake) the bias is at 0 volts
  • In the Kenwood TL-522 it is -7.5 V as shown in the following diagram

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We read a bit of everything

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The objective is to put the L-7 into operation in SSB at medium power.

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F1AMM

Fran?ois
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