Re: [Amps] Want to build first amp

[R.Measures <r@somis.org>]

On Jan 19, 2005, at 9:14 PM, Steven J Fraasch wrote:

> Tomm:
>
> If it was your first, given your gain requirement (1500w out for 50w in
> or 14.8 dB !), I would recommend an 8877 in grounded grid.  That tube
> should provide 13 - 15 dB of gain depending on tube and bias.
>
> There is an excellent 8877 design by W8ZR in the 21st ed of the "Radio
> Handbook" by Bill Orr, W6SAI )late 70's edition).
>
> I modified a Henry 3K over to an 8877 and was very pleased with the
> performance, especially the gain.  One disadvantage of the 8877 is the
> relatively long warm-up, about 3 minutes.  It has a reputation for a
> fragile grid, but is much more indestructible than is given credit (ask
> Rich, AG6K).

Hello, Steve --
- note - Gold evaporates at 2966ºC at sea level, and at a somewhat 
lesser temperature in a vacuum.   Since an 8877's grid weighs c. 50g, a 
great deal of DC P is required to evaporate its gold-plating.
  The problem of gold evaporation from  the 8877's gold-plated grid is 
not due to exceeding its 25w max grid-dissipation limit in normal 
operation.  Gold evaporation is brought about by bursts of UHF grid 
current during intermittent parasitic oscillations in a HF amplifier 
where the Pi-network tank acts as a low-pass filter, thereby preventing 
the burst of UHF energy from being dissipated in the load.  An unloaded 
condition causes high grid-current.  Since UHF has a pronounced 
skin-effect, the grid current is concentrated in the atoms at the 
surface of the plating - so less energy is required to cause 
evaporation.   When I heard this explanation for the first time from 
Eimac's Willis B. Foote, I was somewhat skeptical.  However, when I 
sawed a kaput 8877 apart and, aided by a 50x microscope, observed a sea 
of gold meltballs clinging to the cathode, gold blisters on the grid 
with bare patches of molybdneum base-metal showing through on the grid, 
I realized that Eimac's 8877 development team had indeed hit on the 
correct explanation.

> I limited the current on mine with a trip circuit to less
> than 125 ma

125mA represents only about 40% of allowable grid dissipation.

> with a fairly slow time constant, on the order of 200 ms.  I
> never lost a tube to grid failure,

  When an 8877 fails from excessive grid-current, the grid is still 
functional.  The symptoms of gold migration are:  decreased emission 
caused by gold contamination/poisoning of the cathode's emissive 
coating, and internal flashovers caused by the presence of gold on the 
anode insulator.  The higher the peak anode-v, the greater the chance 
of a flashover.  For example, if an amplifier doesn't short its anode 
supply on standby, but all hell breaks loose under max signal 
conditions, gold contamination is a good bet.  This condition is easily 
found with a high-potential tester:   If the anode-grid leakage-I is 
higher with positive on the anode than with negative, loose gold is 
likely the reason.  If the leakage-I is the same with pos. or neg., 
it's due to gas.

> and used the 8877 for 16 years.  I
> popped the grid current relay a few times due to mis-tuning, but never
> lost the tube.  The Henry 4K, Alpha 77D and SX (pair), and Ameritron
> AL-1500 use the 8877.
>
> The other tube to consider is a 3cx1200, which is very rugged, has 
> quick
> warm-up, but a little less gain, 11 - 13 dB.  But, it is excellent as a
> grounded-grid choice.  The Ameritron AL-1200 and Henry 3K Ultra use 
> this
> tube.
>
> A pair of 4cx800s is worth considering, but the power supply will be 
> more
> complicated adding a screen supply and commensurate protection.  The 
> QRO
> HF2500DX, Alphas 91 and 99, and ACOMs use this tube.
>
> Good luck with the project,
>
> 73, Steve K0SF
>
>
> On Thu, 20 Jan 2005 03:58:09 +0000 Tomm Aldridge <KD7QAE@ARRL.NET>
> writes:
> > Learned listmates,
> >
> > I want to embark on building my first HF tube amplifier.  I would
> > like
> > to know what to start with, GG triode or Grid Driven Tetrode.  I am
> >
> > looking for legal limit plus and will provide 230V for operation.
> > GS35b
> > tubes have caught my eye as have GU84b units but I am open to
> > suggestions.  Oh, and this will be a deskside unit with a 19" rack
> > as
> > its basis and it will be expected to provide service from 160m to
> > 6m.
> > Lots of great material out there on both flavors.  I am looking for
> >
> > reliable operation, ruggedness (tolerance of my errors), 50 to 100W
> >
> > drive from my Kenwood to full output, simplicity and cost
> > effectiveness.
> >
> > I am an experienced EE with PSU and systems experience but have not
> > ever
> > tackled a tube amp below 10GHz (grin).
> >
> > Waiting for all teh great guidance.
> >
> > KD7QAE
> >
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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