[Amps] MLA-2500 retrofit.

[Joe Subich, W4TV]

> Probably the GI7B although I don't think it runs as much power 
> as the the 8875s did and I am not familiar with the IM products 
> from that tube.  The plate dissipation is 350 watts or 700 watts 
> for a pair which with good efficiency would be enough for the 
> legal limit.

The rated plate dissipation for the 8875 (transverse cooled) was 
300 Watts - the cooler is not as efficient as the 8874.  From 
the Eimac Data Sheet: 

   The three types differ only in the method of cooling and 
   anode dissipation: the anode of the 8873 is designed for 
   conduction cooling and is rated for 200 watts; the 8875 has 
   a transverse cooler for forced air cooling and is rated for 
   300 watts; and the 8874 requires axial flow forced air cooling 
   and is rated for 400 watts. 

Plate voltage and current are similar for both the 8875 and 
the GI7B with the latter capable of slightly higher voltage. 
The GI7B also has a 7 W grid vs. the 5 W grid in the 8875. 

> I don't recall seeing conversions running over about 1 KW 
> which is still a healthy signal for a pair of tubes that 
> cost about $35 each.  You need some one more familiar with 
> the tube than I to come up with a real figure on what to 
> expect for output.

Although the MLA-2500 could do 1500 W output, that is really 
pushing a pair of 8875 tubes.  1500 W output at 60% efficiency 
(doing well for the MLA-2500 given its design compromises) 
represents 500W dissipation per tube and had to be really 
toasting the grids ... probably why the amplifiers are now 
dead <G>.  While the cathode may be capable of those levels, 
pushing the anode and grid that hard is not wise.  Even 
with Eimac's own typical operating values for AB2 below 
30 MHz a pair of 8874 tubes is at maximum plate dissipation 
at 1200 W peak output.  

Other than the differences in filament voltage and bias, 
the GI7B would seem to be a fairly good candidate to 
replace the 8875.  Others have used the GS36B/4CX400A 
but given the filament and screen issues along with the 
significantly higher cost, the GI7B is an appealing 
alternative. 

73, 

   ... Joe, W4TV 
  

 





> -----Original Message-----
> From: amps-bounces at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Roger
> Sent: Wednesday, March 03, 2010 1:05 AM
> To: Jim Thomson
> Cc: amps at contesting.com
> Subject: Re: [Amps] MLA-2500 retrofit.
> 
> 
> 
> 
> Jim Thomson wrote:
> > I have a buddy with TWO   MLA-2500's..... and both now with 
> flat/bad tubes. 
> >
> > I believe one of them has a tuned input, and the 2nd one does not.
> >
> > Is the tube really unavailable ?
> If you are asking if they are still in production, they are 
> not.  They 
> are occasionally available on e-bay where you pays your money 
> and takes 
> your chances.  There are a pair of supposedly unused ones 
> with less than 
> a day to go on there right now for $599 which for a pair, if 
> good, is a 
> reasonable price.
> >     If so, what are his options ?
> Modify?
> >    What other tube[s] can be retrofitted ?
> >
> >   
> The 8873 (conduction cooled), 8874 (axial cooled and 400 watts 
> dissipation) , and 8875 (transverse cooled stock tubes)  are idential 
> electrically and pin wise.  The 8873 and 8875 are no longer in 
> production. the 8874 is, but there are two problems.  They 
> are expensive 
> and would require a different cooling arrangement with 
> considerably more 
> air than the MLA stock configuration can provide.
> > What is the easiest retrofit available ?
> Probably the GI7B although I don't think it runs as much power as the 
> the 8875s did and I am not familiar with the IM products from that 
> tube.  The plate dissipation is 350 watts or 700 watts for a 
> pair which 
> with good efficiency would be enough for the legal limit. I 
> don't recall 
> seeing conversions running over about 1 KW which is still a healthy 
> signal for a pair of tubes that cost about $35 each.
> You need some one more familiar with the tube than I to come 
> up with a 
> real figure on what to expect for output.
> 
> The conversion is pretty much straight forward, the amps AND 
> tubes are 
> cheap, so to me it's an attractive conversions, but whether 
> there is any 
> one who will do it for him at what he considers a reasonable price?  
> OTOH it's likely the GI7Bs will not remain so cheap and readily 
> available forever.
> 
> I'm sure, sooner or later some one will come up with 
> specifics for the 
> conversion such as power out and the IM products (quality of the 
> resulting signal)
> 
> 73
> 
> Roger (K8RI)
> 
> >     Does anybody do these retrofits as a sideline ??
> >
> > later... Jim   VE7RF
> >
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