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

[David G4FTC]

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 at wine.ocn.ne.jp>
Sent: 09 October 2024 14:48
To: David G4FTC <g4ftc at hotmail.com>
Cc: Amps reflector <amps at 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 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