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Re: [Amps] GI-7B

To: "MIKE DURKIN" <patriot121@msn.com>, "amps fourm" <amps@contesting.com>
Subject: Re: [Amps] GI-7B
From: "Jeff Blaine AC0C" <keepwalking188@yahoo.com>
Date: Sun, 6 Dec 2009 13:18:27 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Guys,

My amp is just one data point - maybe other guys have data on amp efficiency 
mapped against plate voltage.  So don't make your final decision considering 
my comment - I could be wrong.  Lot of other much more smart guys on the 
board than I am.  I can say, without reservation, that the tube gets much 
hotter once the B+ gets above 2650V.  It's actually (efficiency considered) 
happiest at about 2100V - but the trouble is the grid current is pretty high 
(this is relative - because it's nearly impossible to map the grid 
dissipation limit over to grid current on a meaningful basis) so I run the 
amp at a higher voltage, and have went overboard on the anode cooling - so I 
may melt the anode eventually, but the grids will not be damaged.  Well, 
that's my hope anyway.

One thing I can state as a fact is, with respect to the Russian tubes, is 
that while there are a lot of guys using the various Russian tubes, the 
number of documented articles that show a characterization of the tube/amp 
over various performance parameters is almost zero.  It seems that most guys 
get the amp working, write up a little bit on what they did - maybe include 
the operating point info - but beyond that, there is not much detail.  So 
(for example), a guy has a 3KV plate voltage on his amp, there is not any 
explanation as to WHY the B+ is at 3KV or WHY that is actually a good value.

I can understand it - characterization (if it can be done by the builder) 
takes a lot of time, is not nice to the tubes, and it seems most guys may 
not care about it so why do it.  For me, I ran over the ranges because I 
wanted an amp that would run 1KW RTTY service without having to worry about 
melting the tubes - and to keep the entire thing inside the original SB200 
enclosure.  If you have more space, or bigger tubes, or a lighter duty cycle 
mode, then these things are not as big of an issue.  The end result is 
great - and the tubes are wonderful (especially compared to the hopeless 
572B eggshells) - but the costs of the conversion - well, I could have 
picked up a decent used Alpha for what I spent on this mod...

For the SB220, I have often thought that a very neat conversion would be to 
put a GS35B in there to replace the 2x3500Z.  Mount the transformer 
externally.  Pull the fan and replace with a rear mounted blower.  Remove 
the dividing wall between the RF cage and the backwall where the fan inlet 
is.  Rework the aluminum to support the cooling channels and sockets. 
Voila, a heavy duty 1.5KW amp (well, the SB220 PS is not very good for a lot 
of reasons) but it's interesting.  I suppose there are easier ways to go...

Good luck!

73/jeff/ac0c


--------------------------------------------------
From: "MIKE DURKIN" <patriot121@msn.com>
Sent: Sunday, December 06, 2009 9:02 AM
To: "amps fourm" <amps@contesting.com>
Subject: Re: [Amps] GI-7B

>
>
>
>>
>> > -----Original Message-----
>> > From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>> > On Behalf Of Ron Youvan
>> > Sent: Saturday, December 05, 2009 8:58 PM
>> > To: amps
>> > Subject: Re: [Amps] GI-7B
>> >
>> > > The tube will run at that voltage (it's pulse rated at something like
>> > 7KV or
>> > > whatever so the problem of internal arc should not be the factor), 
>> > > but
>> > in my
>> > > evaluation of a pair running from 1800v - 3100v, the "sweet spot" of
>> > > efficiency is around 2500-2650.  I'm not sure why, but voltages much
>> > above
>> > > this the tube efficiency seems to drop.  And in my application (RTTY
>> > > mostly), efficiency is the name of the game because you only have 
>> > > 700w
>> > of
>> > > total Pd.
>> > /*snip*/
>> > >> GI-7B    just a question about what the true limit of plate voltage
>> > is??
>> >
>> >    Increasing the plate Voltage allows a tube/s to produce more RF
>> > power, (once you reach the
>> > current limit of the tube) within limits.
>> >    My experience with high power transmitting tubes (IOT-D2100) is; the
>> > tube runs cooler and lasts
>> > longer at the lowest supply Voltage that the tube can make the required
>> > power.
>> > Higher Voltage at a lower current means the tube is operating at a
>> > higher average resistance,
>> > resulting in a greater IR drop across the tube.  Dropping the power
>> > dissipated in the tube
>> > increasing the overall efficiency.
>> > --
>> >     Ron  KA4INM - The only real problem is death is so final.
>>
>>
>> I don't think that is so.
>> True that a tube can make more power with higher voltage because it has 
>> more
>> room for plate voltage swing before cutoff.
>>
>> But how cool a tube runs has nothing to do with IR drop in the tube.  How
>> cool it runs depends upon how much power is dissipated in the plate. That 
>> is
>> determined by the power input verses efficiency.
>> Some tubes may be more efficient at certain plate voltages but that is 
>> not
>> normally at lower voltages.
>>
>
> Ok, the reason this came to me as a question is that i was thinking of 
> replacing my 3-500Z's (one is arcing over - and had destroyed bias 
> circuitry - which im almost done repairing) with a pair or 3 GI-7B's 
> because the filament -with out the dropping resistor installed is what the 
> Gi-7b calls for and im fairly sure that the bias voltage range will be 
> suitable - But the plate voltage i have is in the range of 3600V - 3800V 
> (not measured but im running a transformer meant for 50Hz@200V on 
> 60Hz@240V - so i have higher plate voltage)
>
> Or sell the one GI-7b i have and get a pair of 3-500ZG's from RF parts 
> ..... its some what a cost issue....
>
> Mike
> KC7NOA
>
>
>
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