[Amps] about the WARC bands of the Heathkit SB-220.

[Kouichi Ueno]

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 at 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