[Amps] Question re GU-74B grid current

Tue Oct 23 09:10:47 EDT 2012

YEAH Jim, just shows you the Hammy Hambone engineering capability of a few.

It reminds me a bit of WW2 when the 450TL wouldnt make enough power as a 
radar pulse modulator. So Eimac increased the filament voltage from 7.5 to 
8.2V and slapped a 6C21 number on it. It should come as no surprise that 
service life was short resulting in a tidy profit for the various 
manufacturers.

Of course, George Badger, VP of Svetlana USA was from Eimac and it was he 
who championed that tube and the change. I have some of the original Russian 
test data for efficiency and IMD....it aint pretty!

And Leigh, dont bother repeating ad nauseum how well your Emtron works with 
the GU74B at the 400W VK power limit.

Carl
KM1H

----- Original Message ----- 
From: "Jim W7RY" <w7ry at centurytel.net>
To: "Leigh Turner" <invertech at frontierisp.net.au>; <amps at contesting.com>
Sent: Tuesday, October 23, 2012 12:39 AM
Subject: Re: [Amps] Question re GU-74B grid current

> 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 at 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 at jeremy.mv.com]
> Sent: Tuesday, 23 October 2012 12:30 PM
> To: Carl; Leigh Turner; amps at contesting.com
> Subject: Re: [Amps] Question re GU-74B grid current
>
>
>
> See correction below:
>
>
>
>
>
> ----- Original Message ----- 
>
> From: "Carl" < <mailto:km1h at jeremy.mv.com> km1h at jeremy.mv.com>
>
> To: "Leigh Turner" < <mailto:invertech at frontierisp.net.au>
> invertech at frontierisp.net.au>; < <mailto:amps at contesting.com>
> amps at 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 at frontierisp.net.au>
> invertech at frontierisp.net.au>
>> To: < <mailto:amps at contesting.com> amps at 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 at contesting.com] On Behalf Of Carl
>>> Sent: Tuesday, 23 October 2012 11:19 AM
>>> To: Vic K2VCO;  <mailto:amps at contesting.com> amps at 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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