Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jul Wed 15, 2020 12:43 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 7 posts ] 
Author Message
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Wed 23, 2005 12:15 am 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20
Location: Rehoboth,Ma USA
Hello Group,<P>I was testing some tubes for my 390A and noticed my 3444 does not register any Gm. Upon further inspection I noticed my plate voltage is not correct. I.E. In measuring a 6BA6 the correct plate voltage should be 100Vdc, I get about 12Vdc. I try another tube a 6C4 (I believe) and again the plate should have been 250Vdc but I measured approx. 100Vdc. A little better but not correct. I looking at X2 the bridge tubular rectifier (Part #2250-39)as the problem. Does anyone have any comments. PS. I have not (Yet) recapped the instrument. Thanks in advance for any help -Jon<P>------------------<BR>


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Wed 23, 2005 7:39 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 25381
Location: Pocasset, Cape Cod, MA
The rectifiers seem like a good place to start. Somewhere, I've seen a reference to replacing them but I can't put my finger on it. I've never worked on a 3444 myself.<P>------------------<BR>


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Thu 24, 2005 5:56 pm 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20
Location: Rehoboth,Ma USA
Just an update to the group on my quest to fix the 3444. I called Triplett and they sent me a test circuit on how to test the bridge rect in question. So that's just what I did and guess what. It's gone. So I'm going to take a trip and pick up a bridge rect at my local radio shack and go from there. I'll let you know!!<P>------------------<BR>


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Fri 25, 2005 4:08 pm 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20
Location: Rehoboth,Ma USA
Last night I replaced the bridge rect for the plate voltage. I had good and bad results. With tubes that need a plate voltage of 100volts or less the instrument seems to work. Where before I would not get a Gm reading now I do and it seems reasonable. However with tubes that require 250Vdc, the over-load lamp goes on. Clearly I'm drawing to much current off the power transformer. Looking at the plate voltages at the various settings, they are running a bit high. I.E. 100Volts dc is approx 115volts. I'm suspecting that the 250 is more like 260volts causing to much current draw. (R is staying the same then I increases) I'm thinking of rescaling the resistor on the transformer (R27) to limit the current. Any comments? (Please) - Jon <P>------------------<BR>


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Fri 25, 2005 11:20 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 10748
Location: Mpls, Minnesota
Did you install a 400 ohm resistor in series with the bias diode? Did you calibrate the tester?<P>Dave<P>------------------<BR>Intelligence is the ability to use your knowledge


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Fri 25, 2005 11:36 pm 
New Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20
Location: Rehoboth,Ma USA
I have not installed any additional resistors. I believe you are referring to X3 next to R30. Not quite sure why you would do that. Not obvious to me yet! As far as calibrating the tester that is something I will try tonight when I get home. I really only took voltage readings last nite and did not sit down with the schematic. After reviewing the schematic, I failed to tune R31, the line voltage pot. This should reduce the voltage on the taps (on the power transformer) going to the rectifier and therefore reducing the plate voltage to the correct levels. What do you think? Thanks Jon <P>------------------<BR>


Top
 Profile  
 
 Post subject: Triplett 3444 Restoration
PostPosted: Mar Sat 26, 2005 7:36 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 25381
Location: Pocasset, Cape Cod, MA
I found the relevant emails from Chris Haedt, courtesy of Copernic Desktop Search. I've extracted some comments and combined them into one post:<P>12/11/04<BR>I often find bad selenium rectifiers in tube testers, and have yet to see a Triplett 3444 where they were still good. The design used three--line, bias, and B+.<P>7/31/03<BR>I have continued to unravel the mysteries of this tester, and have three in the shop that I am tweaking. I have solved many of the problems with the tester that I first found--like how to protect the meter, and how to eliminate oscillation of the meter when testing 5U4's. Only odd tube to test is the 6DJ8--but this throws most testers.<P>I did notice something irregular--high current tubes seems to test higher than the chart values--but not so for lower current tubes. In examining what might cause this, I realized that anything limiting the B+ would cause high current tubes to test lower, since plate current will decrease. Triplett provided BOTH the reject point, and the expected value for an average tube on their charts.<P>The full wave selenium rectifier used to supply the B+ seemed tiny--with round plates about 3/8" in diameter. Had about 10 plates per section. No part number or anything like that to go by. This thing looked more like an instrument rectifier, rather than something used in a power supply. Clearly this thing would have substantial voltage drop at full load which I measured (with my diode bridge in place testing a 6L6) at over 50 MA. The old selenium unit was dead and I could not use it to see what the voltage drop was.<P>What I did do was to insert various resistors in series with my 1N4007 bridge and measure the voltage drop across the resistor. With a 220 ohm resistor in place I was getting a 10 volt drop across the resistor with about a 50 MA load. This seems like what would be wanted. My reasoning:<P>(a) the original rectifier rated at 50 MA (a guess)<BR>(b) 1 volt per plate drop X 10 plates=10 volts<P>Now when I tested a 6L6 that previously tested at 5600, the value dropped to 4900 with the 220 ohm series resistor in place. The expected value on the chart is 4700 and this is a strong tube. So--this seems to correct the reading. When I test a 12AU7, the reading is just about the same with or without the series resistor in the B+ line--about 2000--just like the chart says.<P>My conclusion from all of this is that Triplett used a somewhat undersized selenium rectifier in the B+ supply on purpose as a current limiting/protective device. The result was that high plate current tubes got lower than expected readings, as reflected in the chart. After all--4700 is a pretty low reading for a 6L6. Another thing about the tester that suggested that the selenium was meant to limit the B+ was that the tester measures plate current--and the high scale only goes to 50 MA.<P><BR>I did the mod on another 3444 and it worked fine also. I would never have suspected that they would use the rectifier as a current limiter--but this was clearly the case--either by design or accident. I do not think you could have used a silicon diode this way--would have burned up. With the current limiting resistor the Gm matches the chart, the plate current stays within the 50 MA range, and the VR test works as it should. This was certainly not an obvious thing to do--as a 220 ohm resistor is high to place in series with a diode when replacing selenium.<P>No model on the selenium that I can see--but with plates 3/8" in diameter can't conduct much current.<P><BR>I had saved and printed information about calibrating the 3444 tester from a long time ago. This was a Test Equipment post on the Forum. Lots of BAD information on calibrating it. OH well.<P>------------------<BR>


Top
 Profile  
 
Post New Topic Post Reply  [ 7 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 8 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB