WOW!
What a line of.... I bet the 4CX1600 was the best thing going too? Now
that they are TOTALLY not available anymore.
I tried to build an amp out of a pair of them... One tube shorted. Before it
did, they would not put out the power advertised on the published data
sheet.
Changed to 3CX800s and all is well 15 years later.
73
Jim W7RY
-----Original Message-----
From: Leigh Turner
Sent: Monday, October 22, 2012 7:49 PM
To: amps@contesting.com
Subject: Re: [Amps] Question re GU-74B grid current
Agreed Carl; but the labelling is unimportant. Quibbling over whether the
tube's plate dissipation is 600W or 800W is rather pointless. It's a
function of air supply CFM anyway as the cathode emission conservatively
supports an 800W rating.
All ham Tx tubes are gradually going the way of the dodo bird; some just
faster than others. Just look at the latest layoffs at CPI/Eimac's tube
division where the writing is on the wall as to what will happen next.
Prudent owners of GU74B based amps would have stockpiled a few spare tubes
when they were dirt cheap. The Svetlana St Petersburg plant have
periodically made recent vintage new tube production batches to meet OEM amp
market demand so the GU74B/4CX800A will likely be around longer than many
others. There's nothing in need of improvement with the time-proven GU74B
tube design; if one wants a stronger/higher Po mid-sized tetrode then go for
the Chinese FU728F or the Chinese 4CX1000A clone.
By way of interest, the GU74B tube in my oldest amp is now in its 10 year of
service and showing no signs of power degradation, so at that rate I suspect
my spares in the cupboard will probably see me out.
Prudence and good sense would suggest one does not use a single GU74B at
greater than 700 to 800 Watts PEP output. A pair of them will do the US
legal limit effortlessly and with great longevity.
Leigh
VK5KLT
_____
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: Tuesday, 23 October 2012 12:30 PM
To: Carl; Leigh Turner; amps@contesting.com
Subject: Re: [Amps] Question re GU-74B grid current
See correction below:
----- Original Message -----
From: "Carl" < <mailto:km1h@jeremy.mv.com> km1h@jeremy.mv.com>
To: "Leigh Turner" < <mailto:invertech@frontierisp.net.au>
invertech@frontierisp.net.au>; < <mailto:amps@contesting.com>
amps@contesting.com>
Sent: Monday, October 22, 2012 9:56 PM
Subject: Re: [Amps] Question re GU-74B grid current
Yeah sure. A 600W rated tube fraudently labled 800W to grab US dollars. At
least ACOM has NOT erased the GU74B for a 4CX800A even tho running them
just as
hard.
They will soon go the way of the dodo bird unless the Chinese copy and
maybe
improve it.
Carl
KM1H
----- Original Message -----
From: "Leigh Turner" < <mailto:invertech@frontierisp.net.au>
invertech@frontierisp.net.au>
To: < <mailto:amps@contesting.com> amps@contesting.com>
Sent: Monday, October 22, 2012 9:28 PM
Subject: Re: [Amps] Question re GU-74B grid current
Yes, the ACOM-1000 and ACOM-2000A are two good examples of what good amp
designers can do with good tubes. These GU74B tube based amplifier
products
have been immensely successful for ACOM and others.
Jim you are absolutely right to be interested in what amp builders are
actually getting and experiencing from these fine tubes, rather than rely
only on information gleaned from the translated GU74B Russian data/spec
sheets. Unlike the savvy folk, many are not aware of the full picture in
respect of how good these conservatively rated mil-spec tubes actually
are.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Carl
Sent: Tuesday, 23 October 2012 11:19 AM
To: Vic K2VCO; <mailto:amps@contesting.com> amps@contesting.com
Subject: Re: [Amps] Question re GU-74B grid current
Thats being rather polite about it (-;
Carl
KM1H
71% overall efficiency is remarkable for a class AB1 amplifier.
On 10/22/2012 12:10 PM, Val wrote:
Jim,
ACOM1000 and ACOM2000A are without doubt the most popular amplifiers
based on GU74B. Many thousands of them are working under the following
conditions at1000W output:
Plate 2350-2400V
Screen 340V
Anode current 510-580mA
Bias between -45 and -65V for 220 mA idling current (the EBS keeps the
bias between -65 and -75V for idling current of 70 mA when no RF drive)
Normally the grid current is well below 10mA where the grid protection
trips.
73, Val LZ1VB
Could somebody advise me as to the resting plate current and operating
ctrl grid bias voltage they are using on a GU-74B (plate voltage is
2500V, screen grid 300V)? Also, does the tube draw grid current? I
realize that in AB1, no grid current should be drawn, but the tube
characteristic curves predict about 15mA grid current at full output
(1000W output, 600mA anode current (per tube) and 250W resting
dissipation. I am interesting in what builders are actually getting,
and not information from data or spec sheets.
Tnx and 73,
Jim W8ZR
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