Hi Kouichi
The SB220 was normally available as a kit for home construction and the
assembly manual and schematic are available here
http://www.radiomanual.info/schemi/ACC_PA/Heathkit_SB-220_serv_user_other_1969.pdf
and
http://www.wedophones.com/Manuals/Scanners/Heathkit%20Schematic-SB-220_sch.pdf
These documents should answer your questions. As it was designed for home
construction, it probably one of the easiest amplifiers to work-on.
Please ignore my correspondence with Victor - I got side-tracked - I used the
gas discharge tubes in another homebrew amplifier and they are not applicable
to the SB220.
As you are new to vacuum tube linear amps - just be aware that the voltages in
the SB220 are lethal - contact with an energised circuit is likely to result in
instant death.
Sorry to be pessimistic but forewarned is forearmed.
David G4FTC
________________________________
From: Kouichi Ueno <kueno@wine.ocn.ne.jp>
Sent: 09 October 2024 14:48
To: David G4FTC <g4ftc@hotmail.com>
Cc: Amps reflector <amps@contesting.com>
Subject: Re: [Amps] about the WARC bands of the Heathkit SB-220.
Hi David,
Good evening.
I'm new to vacuum tube linear amplifiers, so is the SB220 difficult to
use?
I can replace the capacitors as maintenance. However, I don't think I
have the technical skills to handle the various measures to prevent
flash-overs or the installation of gas discharge tubes.
Kouichi Ueno JI1FGX/DU9
On Wed, 9 Oct 2024 13:58:34 +0000
David G4FTC <g4ftc@hotmail.com> wrote:
> I would be cautious about using zeners - from my experience their failure
> mode is to go short circuit which necessitates taking the amp apart to
> replace them.
>
> Sometime ago I wanted to ensure that a tetrode amp I was building was
> protected from Anode-Screen flashovers which could damage the screen
> decoupling capacitor built into the tube base.
>
> I used a combination of Varistors in parallel with a Gas tubes (Spark Gaps).
> The logic being that the Varistor was fast but of limited dissipation and
> provided the initial clamping action whereas the slower spark gap which could
> sustain much higher current then provided the longer term clamping until the
> HT crowbar operated and the reservoir capacitors discharged. The Varistors
> were of a slightly higher voltage than the Gas Tubes. The crowbar circuit
> also tripped the screen supply.
>
> I have experienced flash-overs but so far never lost a tube or a tube base.
> All I have to do after a trip is to press a reset button and the HT and
> screen supplies are restored and the amp is operational.
>
> David G4FTC
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