[Amps] Question re GU-74B grid current

Tue Oct 23 09:36:11 EDT 2012

The 1600B was a decent tube and a few manufacturers, including Ten Tec, 
jumped on it. Unfortunately there wasnt enough Russian surplus to go around 
so manufacturers and owners were left holding the bag.

I built one into a fried SB-1000 cabinet using Amp Supply RF section parts 
and a Dahl transformer.

It made a bit over 1500W right thru 10M but IMD was no better than -28 
to -30dB 3rd and was sold soon after for an Amp Supply LK-780 that was 
partial payment for some consulting work.

Carl
KM1H

--- Original Message ----- 
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
To: <amps at contesting.com>
Sent: Tuesday, October 23, 2012 4:00 AM
Subject: Re: [Amps] Question re GU-74B grid current

> 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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>
>
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