Hi Steve,
You're right, tubes ain't cheap anymore but few manufacturers respect the
HAMS right to know and investigate.
There's an amplifier manufacturer out there that DOESN'T provide full
schematics for their amplifier even after you've purchased one. What do they
want to hide? Probably hate for you to surmise it's more advertising than
substance.
I know of one manufacturer who DOES provide for filament voltage measurement
(and FULL schematics).
HENRY provides pin jacks on the back for you to measure the filament voltage
idling and under load.
My amplifier is the HENRY 3K Classic X MKII and it has these pin jacks on
the back of the RF deck.
I even made a fun page for it at
http://radiodan.com/Henry/misc/Radiodans_3K.htm
On my old HENRY 4K Console I put a 25 ohm 50 watt reostat in the primary of
the filament transformer and adjusted the filament voltage to suit me. My
method is to lower the filament voltage slowly to just below where the power
starts dropping off and then back up a few tenths of a volt. You will find
as the tube(s) ages you may need more filament voltage to maintain the power
out. New good tubes will operate at full power below their rated filament
voltage.
Another bonus of this is that you get a "step start" effect whereby the
onrush current is absorbed to a large degree by this reostat.
Now the bad news:
For well designed console amplifiers: No bad news!
For your typical "let's jam everything in real tight and light" desktop
amplifiers which use a single transformer for both high voltage and
filament: Sorry, you can't do it because of the huge voltage drop and
inability of the reostat to handle all the power being drawn through it.
Even HENRYs desktop HF amp, while well designed and spacious in the RF deck,
also uses an all in one type transformer.
Simply put, if you want your amp and power supply on the desktop in one
small light package, something has got to go.
There are no shortcuts or good ways to get around solid amplifier design.
p.s. I think we all wish we had AC coming in like yours!
73, Dan Magro
Radiodan W7RF, Portable Clinic
High quality Henry Radio RF Power Amplifiers for LESS!
New, used, unusual, classic radios, accys & RF POWER!
http://www.radiodan.com RFpower@radiodan.com
A trip to our web site is worth the click!
-----Original Message-----
From: Steve Jackson <Steve_Jackson@BayNetworks.COM>
To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
Date: Sunday, September 06, 1998 1:49 PM
Subject: [AMPS] filament voltage questions
>
>Scenario:
>
>I have a Fluke 77 DVM (not RMS-reading, AFAIK). I show 243.5 VAC on my
>dedicated 20A, 240V line in the shack, amp unplugged, no other load on this
>branch. Run is about 45 feet of 10/3 copper Romex to the breaker panel,
>adjacent to the meter.
>
>I have 400 amp service at my home, with no other home sharing the service
>transformer. I split the feed to two separate 200A panels, and run about
>1/2 the house off each one. The local power substation is 1/2 mile away.
>Pretty stiff supply, doesn't budge much.
>
>After I send this message, I am going downstairs to measure the filament
>voltage on my L7 amp. I haven't done this before; I probably will
>tack-solder leads onto the fil pins so I can meter the voltage from outside
>the amp, with the amp sitting level, and covers on. Safety first, you
know.
>
>(Why don't amplifiers have filament volt-and/or-ammeters built in, or at
>least test points for this? Tubes ain't cheap anymore.)
>
>I intend to look at the voltage on the tube side of the choke, to account
>for any I2R loss in that component.
>
>So, there's the situation. Here are the questions:
>
>1. I assume the Fluke measures P-P voltage, and is this an issue? I can't
>think of a reason why it would be, but... perhaps the 3-500Z 5-volt
>filament spec is in RMS volts?
>
>2. Suppose that the filament voltage is a bit high. What are some good
>ways to cut it down? I was thinking a small resistor in series with the
>primary, as the L7 has a separate fils transformer, and the primary wires
>are easy to get to.
>
>3. What if it's too low? ;-)
>
>TIA
>
>Steve KZ1X/4
>
>
>--
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>
>
--
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