开云体育

I measured my line voltage here in central NC and it’s usually 123VAC or thereabouts. I decided that, rather than going through a dozen or so rigs, it was and is much easier to buy a 20A Variac and connect all of the non-amp equipment to it.?

I never have more than 2-3 sets in at any given time and so this works for me. It keeps everything more original and cuts down on stressed parts.?

The Variac I bought was one of those red Chinese ones on Amazon or eBay. Well under $100. And yes, I scraped some paint off some grounding points first. I’ve had no trouble in the several months that it has been installed.?

73,

Steve Wedge, W1ES/4

Time flies like an arrow. Fruit flies like a banana.

Sent from Proton Mail for iOS

On Thu, Mar 23, 2023 at 07:17, David <david.w5xu@...> wrote:

Some of the earliest discussions were by the US Navy that was studying why tubes did not last long. Filament voltage was generally the culprit and gave rise to the +_ 10 % voltage range. The major culprit is that the older radios were designed for lower input voltages ranging from 110 volts to 115 volts and then to today's standard of 120. So a power transformer in an older set was optimally sized for the input range of that day and use in todays world leads to all voltages in these radios being higher than intended. For the filament consideration according to the navy study there then was a reduction in useful lifetime. Tube manufacturers used this fact in designing filaments more tolerant of various voltages ( the 10%) so the result is they last longer but are still challenged in older sets in use today.

David Assaf III
W5XU VP8RXU

On Thu, Mar 23, 2023, 3:38 AM Richard Knoppow <1oldlens1@...> wrote:

The extension of lamp and tube life applies to pure tungsten filaments. These were common in large transmitting tubes. For receiving tubes the general reccomendation is to keep filament voltage to within about +/- 5%. This applies to tjoriated tungsten also but voyage can be lowered somewhat if emission is monitored. Eimac has authoritative information in their tube application notes.

A place where lowered voltage is often found is the filament of the 6H6 and 6AL5 twin diodes when used as detectors an noise clippers. These tubes have a tendency to produce hum which is reduced by lowering filament voltage by about a volt. However in general its not a recommended practice.

during WW2 RCA published charts showing the effect of filament voltageon tube life for pure tungsten filaments. I don't off hand remember the numbers but a relatively small reduction increases life substantially with relatively small reduction in emission.

-------- Original message --------

From: Doug Crompton WA3DSP <wa3dsp@...>

Date: 3/23/23 12:04 AM (GMT-08:00)

To: [email protected]

Subject: Re: [DRAKE-RADIO] R4C power supply question

I am picky about things, but the twins are not going to be rigs I will use a great deal and probably seldom. No doubt the Sherwood updates are top-notch but it just wouldn't be worth it to spend the time and money the way I would operate the receiver. I will leave that to the next guy. I did update the power supply and audio amp which are probably the first things someone with an R4C should tackle.

As for the filaments, I did add a self wound resistor to reduce the voltage by about .5 volts. It is now sitting at about 6.5 @ 122 line volts. From what I have read, you don't want to run the filament voltage too low, but too high by a volt is not a good idea either. This greatly depends on your line voltage. Typical today is 120-122, but in some areas it can vary greatly. I remember in the old incandescent light bulb days seeing a chart that showed a bulb would last 10 times longer with 10% less rated voltage and 1/10 the time with 10% more. I don't know if that is true, and tube filaments would probably not directly correlate to the old bulbs.

There is a lot of misinformation on the Internet so it is hard to get accurate information since tubes other than in hobby stuff are rarely used anymore. I saw many posts that warned of permanent loss of emission after running low filament voltage, and others saying that tube gain actually improved at lower filament voltage. Of course, we are talking about greatly lower. Keeping filaments between 6.0 and 6.5 volts is a good range to shoot for. It is also a good thing that Drake did not blast the hell out of plate voltage limits like Heath and other manufacturers. There are a couple of miniature tubes in the KWM2 that would give you a burn as bad as touching a soldering iron just by briefly touching. Heath often ran tubes at or higher than maximum ratings. There is no reason to run a receiving tube at 250-300 volts.

Many tubes are a limited resource so we need to take care of them!

Doug, WA3DSP