[Amps] Question re GU-74B grid current

Tue Oct 23 04:00:26 EDT 2012

On 10/23/2012 12:39 AM, Jim W7RY wrote:
> WOW!
>
> What a line of....  I bet the 4CX1600 was the best thing going too?  Now
> that they are TOTALLY not available anymore.

 From what I've read the 4CX1600B really was an excellent tube. I think 
Carl has experience with them.  They certainly were popular, but not a 
large enough market to support them.

OTOH I have used the Chinese FU728F which is *relatively* inexpensive, 
and does well.  Still, the early FU728Fs were not the tube they are 
building now.

I'd like a pair on HF with a single one of 6-meters.
NOS OTOH do not have a good reputation and they are interchangeable with 
4CX1500Bs except for filament voltage and the socket centering pin which 
is only a couple thousandths different in diameter. OTOH if you run 'em 
for all they are worth it gets kinda noisy as one is capable of 2500 PEP 
out.

73

Roger (K8RI)

>
> 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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