[AMPS] Problem with SB-220 Bias Diodes

[measures]

>
>I need some advice and insight from the amplifier gurus in this group.
>
>I have a Heathkit SB-220 which I purchased two years ago.  I upgraded the
>linear with Harbach's mods (Sorry Rich) as follows:
>
>1. Replaced the capacitors with new capacitors and bleeder resistors.
>2. Replaced the existing rectifier board with a new board.  This board
>includes the replacement of the zener with 8 - 1N4005's.
>3. Installed the Parasitic Suppressor Kit.
>4. Replaced the meters.
>5. Replaced the Fan.
>6. Installed the inrush current limiter upgrade.
>7. Installed the Soft key switching module.
>8. Installed a 'home-made' H.V. fuse using #30 wire .. 1/2" long.
>
>You must be thinking ... why on earth did he do all this stuff?

Because you are not aware of the situation.  

>Well I like the SB-220 and it was less expensive to buy the amplifier and
>make the mods instead of buying a new amp.
>Bottom line is ... economics?
>
€  Harbach does not use enough L-supp in his 3-500Z suppressor.  This 
increases VHF gain.  The popped bias diodes may have caused by 
intermittent parasitics.  The parasitics may have been caused by not 
enough L-supp.  

>Here is my problem. About a few months ago I was operating normally and the
>H.V. power supply failed.
>There was no "flash" or "Bang" ... just quit.  I have had the "flash" and
>"bang" when I first fired up the amp when I bought it and operated it
>without any of the mods being installed.  That's one on the main reasons why
>I initially installed the Parasitic suppressor kit.
>
>I opened up the amp and found that one of the 8-1N4005 bias diodes had
>failed.  It was quite obvious ... charred and black circuit board at the
>diode location.  

3A diodes are way more rugged .  

>The H.V. fuse was still intact.  

What is the voltage rating of the fuse?

> I called Harbach and he
>couldn't think of anything obvious that would make the diode fail and he
>concluded that possibly the diode may have been faulty.
>
>I replaced the diodes and operated normally for a few days and the same
>thing happened.  A bias diode had failed again.
>
>It should be noted that I always operate the amp in CW/Tune (1kW) position

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