[Amps] VTX-X118 Tetrode mytery tube?

Mon Apr 4 12:39:01 PDT 2011

Hi Sam,

By the way, my original call was KB7FUT.  Never could come up with any good
phonetics that were appropriate for the radio :).

As far as what the 8410 can do with a little tuning, I have a good idea ;).
Last weekend I decided to pull the tube access plate and measure the
filament voltage as close to the sockets as possible (J1 and J2 on the tube
deck board).  I measured 5.6 volts after about 5 min of warm up with the
primary's on the 240 volt taps.  This also gave me about 2860v for the HV no
load and about 2550v at 1A.  So I decided to try the 220 volt taps,  this
brought the filament voltage up to 6.1 volts with the amp warmed up at idle,
well with in spec, and the HV to 3180v no load and about 2900v at about 1A.
The filter bank is 9x 470uF 400v caps for a max working voltage of 3600v.
Also with 18 amps of filament current there will be another slight voltage
drop from the lead feeding from J1 and J2 to the tube sockets, maybe another
50 to 100mV. For my situation even though my mains are right at 239 to 241
volts (no load) it appears the 220 volt taps may be closer to ideal then the
240 volt taps.

The screen voltage on the early 8410s was set to 200 volts and later they
bumped it up to 225 volts.  Mine measured 227v.  I tested the amp with the
screen voltage at 235v and 250v but no higher.  This is not a neutralized
design and you could run in to stability issues if you push the gain up!  As
it is 28w in = 1500w out with the increased HV and 250v screen.  Note, if
you do adjust the screen you MUST re-adjust the G1 and G2 bias.  This can be
done through the USB port using a serial com program (I use Tera-Term with
Win 7).  At the factory they set G1 (intermediate bias) to 60mA and G2 (full
bias) to 200mA.  I'm still playing with the ideal settings here but I have
G1 set to 80mA and G2 set to 250mA right now.

So what will it do now?  I have not tested it on all bands with the new
settings yet, but on 20m from the factory on the 240v taps and 227v on the
screen it would run 1500w out with 37w drive and 2200w with 60w drive.  The
grid was going in to saturation at 60 watts.  Now with a few hundred extra
volts on the plate, 250v on the screen and the 50mA of extra G2 bias the amp
will run 2800 CW with about 60 watts drive.  Its still starting to go in to
saturation but the grid current is still very low.  The green "min" grid LED
is set to light at 2ma.  I see it start to light at about 2300w out, I dont
remember exactly what it was at 2800w out but I remember I wasn't
concerned.  The max Ip I remeber seeing was about 1.35 to 1.4A at that power
level. Yes, there is probably more in it yet.  This was as far as I was
willing to take it but all the values where still well with in acceptable
limits.

I dont recommend anyone changing the values from the factory.  The amp runs
very well as it is, I just wanted to see what it COULD do.  Now I know and I
dont plan on operating it at those levels.  Its just nice to know.

Damon Stewart, KJ7E

On Mon, Apr 4, 2011 at 10:04 AM, Sam Carpenter
<sam at owenscommunication.com>wrote:

> It is a WAY cheaper foreign tube. I asked about getting one tubed in EIMAC.
> She said the difference was somewhere in the range of simply purchasing the
> EIMAC tubes. On that amp, the screen voltage is held way back and the way
> things are set up, I don't see how you would run into grid problems on
> either tube under 3k out. One tube 4cx1000 or 1500 could handle the rated
> capacity of the amp in full duty cycle. It could also do it with low drive.
> I think they only run the screen voltage about 250v. If you upped that a
> bit, you would have a 4k out amp. I don't know if the tank could take it
> but
> it looks like the PS would for casual use. I really like this amp.
> Everything seems to be conservatively built and I like the 4cx1000 platform
> because of the number of tubes out there. On top of that, there are two of
> them. I do worry about the proprietary items that inevitably accompany the
> newer amps. What chips will no longer be made, or whatever. Seems that
> lifecycles are shorter all the time. That is what I love about the big
> henry
> amps. Excpepting the tank inductor. Most everthing else was off the shelf.
>
> Sam N9FUT
>
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Damon Stewart
> Sent: Monday, April 04, 2011 12:09 PM
> To: amps at contesting.com
> Subject: Re: [Amps] VTX-X118 Tetrode mytery tube?
>
> On 4/4/2011 1:38 AM, KL7RA wrote:
> > Originally the new AP8410 used the Eimac 4CX1000A tubes but they made a
> > switch to the VTX-X118. Seem to be a mystery tube, can't find the spec
> sheet
> > and never heard of them.
> >
> > Anyone know anything about this tube? My guess is Alpha used the 4CX1000A
> > but got them with a higher grid rating?  It looks like this tube number
> is
> an Alpha
> > part number to me.
> >
> > 73 Rich KL7RA   Alpha amp owner
> >
> > > From Alpha:
> > The amplifier uses the Alpha Power brand VTX-X118 tubes, a ceramic,
> external-anode,
> >   indirectly heated tetrode that, unlike the excellent Eimac 4CX1000A, is
> qualified to handle
> > the grid current that might be experienced in the AP8410.
> > _______________________________________________
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> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
> For what its worth.  The most grid current I have ever seen out of my
> 8410, even when testing its limits, has been less then 1mA.   In the
> serial data string the grid current is read in tenths of a mA.  I
> usually see somewhere in the 0.2mA to 0.5mA range with CW output well
> over LL.
>
>  From many of the reports I have seen and my own testing,  the VTX-X118
> seems to be a slightly improved Chinese version of the Eimac 4cx1000a.
>
> Damon Stewart, KJ7E
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>
>

-- 
Damon Stewart, KJ7E