Re: Selenium rectifiers in Tek equipment


Dave Wise
 

?Coming to the party late, but I seem to recall that the instructions for one of the Se-to-Si Mod Kits stated that the purpose of the added resistance is to avoid excessive regulator input which will overheat the pass tubes. At least in some instruments, there are already resistors ahead of the reservoir caps which (a) reduce inrush and (b) furnish a handy point to estimate current.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee=ee.stanford.edu@groups.io>
Sent: Monday, February 08, 2021 11:50 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Selenium rectifiers in Tek equipment

Exactly. I never followed the lore because, as a teenager, I wasn't aware of the lore. I foolishly replaced blown seleniums with whatever rectifiers I had on hand from Poly Paks ("U test 'em and save"). Never had a problem, but perhaps it was just dumb luck. When I later worked in TV repair, I learned that I had violated proper procedure, but by that time, most gear with seleniums had either been junked or upgraded. I've only replaced a few since then, sometimes with added resistance to drop the voltage, but sometimes not. Always in my own gear, so that I alone bear the risk.

Tom

Sent from an iThing, so please forgive the typos and brevity

On Feb 8, 2021, at 23:13, "Joe Laffey" <joe@thestable.tv> wrote:

On Mon, 8 Feb 2021, Tom Lee wrote:

The short answer is resistance. It's not only a matter of forward drop at low currents (which the oft-quoted 1V number refers to; the drop at rated current can easily be an order of magnitude higher). Selenium rectifiers have considerable forward resistance (hundreds to thousands of ohms, depending on the specific device), compared to the small number of ohms typical of silicon rectifiers. So if you need to emulate selenium rectifiers with silicon ones, you have to degrade the latter by adding series resistance.

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