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Author Topic: DENTRON MLA 2500  (Read 4065 times)
KF8SJ
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« on: March 27, 2013, 06:35:14 PM »

i have one of these antiques in storage and not readily accessable.  thinking about resurrecting it.  misplaced the manual.  can anyone tell me what tube it uses and possible source or manual.  willing to pay repro costs

thanks for the help

73  jim kf8sj
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KF8SJ
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« Reply #1 on: March 27, 2013, 06:46:43 PM »

please disregard this post.  i thought this boat anchor was so old that nobody knew about it, and didn't realize dentron was still in business. guess i need to get my head out of you know where.
googled the damn thing--surprise...
when in doubt-google

73  jim kf8sj
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W8JX
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« Reply #2 on: March 27, 2013, 07:03:42 PM »

please disregard this post.  i thought this boat anchor was so old that nobody knew about it, and didn't realize dentron was still in business. guess i need to get my head out of you know where.
googled the damn thing--surprise...
when in doubt-google

73  jim kf8sj

Detron is long out of business and 2500 was a good amp but tubes are long out of production
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N7WR
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« Reply #3 on: March 27, 2013, 07:20:31 PM »

There have been a lot of hams that have converted 2500's to use available tubes since the originals are pretty much unobtainium
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W8GP
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« Reply #4 on: March 28, 2013, 07:39:47 AM »

If you decide to use this amp pay close attention to grid current, the tubes have very sensitive grids and virtually no grid protection.The tubes will last a long time if not abused.I would keep it below 800W output.
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K2DC
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« Reply #5 on: March 28, 2013, 07:56:07 AM »

8874's do show up on the market from time to time, but not very often.  If you look around there are plans available to convert the 2500 to use GI7B's that are cheap and fairly easy to find, but I think you have to add a screen supply for those.

73,

Don, K2DC

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W9GB
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Posts: 2626




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« Reply #6 on: March 28, 2013, 12:53:42 PM »

Quote
I thought this boat anchor was so old that nobody knew about it.
I didn't realize Dentron was still in business.
SHORT HISTORY of Denny Had, founder of Dentron.
http://en.wikipedia.org/wiki/Cary_Audio_Design

Dennis Had was the founder of the Dentron Radio Company, owner and designer of the Ameritron amplifiers, and the Amp Supply Company.
Dentron Radio Manufactured all sorts of Amateur Radio gear; Amplifiers, and Tuners from 1960s to the early 1980s.
Amp Supply Company and Dentron are no longer in business. Ameritron is now owned by MFJ Enterprises.
Dentron, Ameritron and Amp Supply Company were all based originally in Ohio.
Amp Supply moved to Cary, North Carolina (late 1980s) and soon morphed into the Cary Audio Company.
Amp Supply was eventually closed.

==
Eimac 8873, 8874, and 8875 ceramic tube data sheets.
http://www.ok1rr.com/tubes/eimac/8875.pdf

This FAMILY of Triode RF tubes only differ by the cooling and anode dissipation !!
8873 - Conduction cooling and rated for 200 Watts
8875 - Transverse cooler for forced-air cooling and is rated for 300 watts
8874 (3CX400) - Axial Flow forced-air cooling and is rated for 400 watts

The Dentron MLA-2500 used TWO 8875 ceramic tubes.  
Eimac stopped production of the 8873 and 8875 over 20 years ago.
The 8874 / 3CX400 ceramic triode tube is still produced by Eimac/CPI (USA) and Svetlana (Russia).

Dentron MLA-2500 HF Amplifier Repair
http://www.aa1zb.net/Radio/Dentron/Dentron.html

The easiest tube conversion is using the 8874 / 3CX400 or 3CX800 ceramic triodes.
You ONLY need to change the cooling for axial flow.
Uses the SAME socket.
The 8874 tube was used by ETO Alpha (Dick Einhorn era) for their
late 1970s to 1980s HF amplifiers (e.g. 374A, 76, 76A, 78)

B.N. "Bob" Alper, W6KT -- Communications Quarterly magazine, Summer 1996
Svetlana article for using Eimac 8874/3CX400A triode in Dentron MLA-2500.
http://www.tube-town.net/info/sed-technotes/docs/technoteNo14.html

Note: The economic differences mentioned in 1990s by W6KT - no longer exist - as surplus Russian 4CX800 are about same price as triodes now !

DK9IP Conversion of MLA-2500 - January 2005
http://www.dk9ip.de/projects/mla2500/mla2500.html

3CX400A/8874 or 3CX800A conversion instructions for the Dentron MLA-2500 by LX2KL
http://195.84.101.101/~goranl/shack/amp_mla2500/convert_3cx800a.pdf

NG9R performed this upgrade/change-out for his MLA-2500 in 2007
« Last Edit: March 28, 2013, 01:43:54 PM by W9GB » Logged
K5TEN
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« Reply #7 on: March 28, 2013, 01:46:03 PM »

Wow....looks like I need to put mine on ebay.  Has had filter cap probs in the past.
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K2DC
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« Reply #8 on: March 28, 2013, 01:54:45 PM »

My mistake - 8875's were used, and they almost never show up.  I knew Denny (casually) in the early 80's when I lived in the Cleveland area.  That was after Dentron and before Amp Supply moved to NC.  A lot of guys poo-poo Dentron, but I always thought he built good stuff for the price.  I had the single-tube version MLA-1200 for 18 years until replacement tubes got too hard to find, and I still regularly use the Super Tuner Plus that I bought used along with the amp well over 30 years ago.

73,

Don, K2DC
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KF8SJ
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Posts: 14




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« Reply #9 on: March 28, 2013, 05:07:46 PM »

thanks for input.  did not see much of this while surfing
actually bought this while in cleveland ohio area.  came without tubes-fellow said he had bought it for the tubes at a good price.  thought i would re-tube it.  tubes were expensive at that time, so put it aside, thinking i could possibly use the hardware, and home brew up some thing else.  always been partial to 4cx250 tubes,  hmmmm how about four in parallel?

live out in county now--nearest neighbor about two miles away, and they use satellite for tv.  tvi should be no problem
time to back up and think about it for a while.  hate to rush a project--only been about 14 years so far!
have saved the comments and data. 
thanks again


73  jim studer kf8sj
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W9GB
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Posts: 2626




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« Reply #10 on: March 28, 2013, 06:36:05 PM »

2008 Retrofit to an MLA2500 amp using the Eimac 3CX800 ceramic triodes
done by Lawson, W4EMF
http://w4emf.gs35b.com/mla2500-retro-3cx800-2008/index.html

===
Tony King, W4ZT came up with a schematic diagram (July 2007) for converting the Dentron MLA-2500 to use TWO Russian surplus GI-7b ceramic triodes.
http://gi7b.com/mla2500/mla2500mod.pdf

W4EMF and K4POZ provided assistance and suggestion.
http://w4emf.gs35b.com/mla2500-retro-gi7/index.html

http://gi7b.com/mla2500/index.html

Modifications, in no particular order, that are shown on the schematic above:

GI-7B tube retrofit.
Cathode choke move from the ALC board to under chassis cathode connection.
Input coupling capacitor change.
Cutoff resistor and bias change. Adjustable Bias Board and bypass caps added.
Glitch resistor change from a fuse resistor to a true glitch resistor.
New grid shunt and protection diodes. Resistor value not specified on schematic.
New plate current shunt and protection diodes. Resistor value not specified on schematic.
Metering calibration resistors and pots added.
Harbach SS-100 Step Start module added.
Antenna Relay change to 3PDT Relay to allow keying the amp only after Antenna contacts are made. The third set of contacts is put in series with the bias keying relay and prevents hot switching the amp.
Filament transformer added.
Filament choke and bypass caps added.
New and added filter capacitors, equalizing resistors and reverse protection diodes added.
Dropping resistor in series with delay tube filament to make up for the unloaded filament winding. This value is found experimentally.
Meter switch changes to properly switch the metering and calibration.
===
Based on this work, other amateurs have successfully documented this conversion.

Dentron MLA-2500 GI7B Modification
by N3OC, February 2013
http://n3oc.dyndns.org/GI7B.htm
« Last Edit: March 28, 2013, 06:44:43 PM by W9GB » Logged
N8CBX
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Posts: 159




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« Reply #11 on: March 29, 2013, 09:50:42 AM »

There's very little (if any) room inside these Dentron amps, so I think the best solution for tube selection is a single tube. Like a 3CX800/4CX800
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
AH6RR
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Posts: 803




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« Reply #12 on: March 29, 2013, 10:57:04 AM »

I had a MLA-2500 a few years ago that I converted to the GI-7BT tubes it is a easy mod. When I picked up the amp at a swap meet for $85 it had one good 8875 the other was toast I sold the remaining tube and with the funds I received from that I bought all the parts to do the change. I bought 6 GI-7BT tubes the 12V transformer and the bias zener and parts to rebuild the HV power supply along with a used 2 meter rig for what I sold the 8875 for. After the mod I was able to get 1200W out without a problem. I did look at the 8874 and 3CX800 mods but the tubes were too expensive and now with new 3CX800 tubes topping $700 each you might as well buy a working SB-220 but the GI-7BT tubes are sitll under $50 each so it is worth the time with them.
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