Sorry forgot to copy to the tentec community
----- Forwarded Message -----
From: Eduardo Araujo <er_araujo@yahoo.com>
To: A R <raf_3@msn.com>
Sent: Sunday, October 9, 2016 1:32 AM
Subject: Re: [TenTec] Titan 425 plate current meter needed - Floyd & Allen
Hi Allen, About meter cost, yes, it should be similar, 28 +
shipping. When done I will confirm you the exact amount they charged me.
I don´t know the centurion circuit but on the 425 Q1 is also the bias
transistor, having a 4.7 ohms in its emitter which is use to monitor Ig and
also as a fuse when things go wrong, also to light the excessive Ig current led
(at 120mA), and is also part of the Ig foldback protection when it reaches
160mA.In Q1 collector is the 8.2 zener for polarization and 1.5 ohms where the
Plate current is measured.In the 425 I just bought, there was obviously a huge
current peak across grid and plate blowing many things and also the plate
meter, probably caused by a tube flash over. The 1.5 ohms 5w resistor which is
used to monitor the plate current opened so, if the meter didn't burn with a
many amperes (volts) spike when the resistor opened, I think it could have
ended with +B across its terminals.!!!, so, no chance to survive, the circuit
was not designed to prevent this.
Similar of what was suggested by your friend which is in my opinion a good
advice, I plan to install 3A diodes in parallel with the instrument, because I
think they will resist better a flashover impact than a zener. Also I will
install at least a 100 ohms small resistor in series with the meter to act as a
fuse in case diodes has to clamp and the plate meter adjust pre-set is at its
extreme .
As I should have about 2.6Vdc full scale, I will place about 4 diodes in one
direction (4x0.7) 2.8vdc to clamp at that level and one in reverse direction.
The good thing with the 425 is that they do an indirect Ig measurement with an
LM358 in between, so it is less risky to loose Ig meter in the event of a flash
over as typically happens in amplifiers like SB220 and others.
'73.... Eddie.- LU2DKT
From: A R <raf_3@msn.com>
To: "er_araujo@yahoo.com" <er_araujo@yahoo.com>; "tentec@contesting.com"
<tentec@contesting.com>
Sent: Sunday, October 9, 2016 12:31 AM
Subject: Re: [TenTec] Titan 425 plate current meter needed - Floyd & Allen
Eddie,Glad you're able to get the 425 healed. It's my favorite TT amp. Was
the meter cost from Jewell-Modutec comparable with the cost from Galco.com
($34)? I'll certainly be interested in how you do. BTW, my Centurion with the
blown meters (really low and inconsistent readings) also had suffered a failed
Q1 bias transistor on the 81537 FILAMENT-AC Board. The ham that I got the
meters from told me that the original meters in his Centurion failed as a
consequence of (or, at least coincidentally with) the failure of the Q1 bias
transistor in his amp (must have created one helluva spike). He is a very
accomplished, seasoned EE, and suggested installing "snubbing" Zeners across
the meter leads. Per his suggestion, I have installed 1N748A 3.9v zeners across
the meter leads (cathode on + lead) in both my Centurions and Titans, with no
adverse effects on normal meter function. Whether or not limiting the "spike"
current thru the meters to ~160% of rated full-scale current will prevent
damaging the Modutec meters is yet to be confirmed. Fortunately, I haven't lost
another Q1 bias transistor. But, the zeners are cheap, easy to install, don't
affect normal meter function...so may be worthwhile protection for the Modutec
meters. Best 73,Allen--W7GIF
----- Original Message ----- From: Eduardo Araujo via TenTec To:
tentec@contesting.com Cc: Eduardo Araujo Sent: Saturday, October 08, 2016 6:48
PMSubject: Re: [TenTec] Titan 425 plate current meter needed - Floyd & Allen
Hello Floyd, Allen and rest of the group,
First of all many thanks for the valuable information.Let me add my two cents
commenting on what I have found and done so far.
". I suspect the Titan meters are the same or from the same source. Google
"Modutec Jewell 2MAS" and takea lookat those meters. "
Titan 425 definetely use Modutec 2MAS-DMA-001 with a custom scale. The factory
does not have original scale but they could make a customized one for 170 US
above meter cost.
I though about two options, one is described by Allen and it is to replace the
new electrical/mechanical elements into the old body, that would allow me to
use the original scale. As I already did an autopsy of the dead meter, Allen
comments confirmed me that the exchange could be done relative easily, Many
thanks Allen too.
The second alternative was to print a meter scale with a SW same as you suggest
Floyd but the factory print the scale directly on the meter plastic body
surface, so I would have to remove the printing before use a printed paper
scale on it, otherwise and as the light is from behind, both scales would be
superimposed.
As I was not able to get from the factory a catalog with scales they have
readily available at no extra cost in order to find which one could fit more or
less similar to the original, and also to avoid both scale superimposed, I
requested the factory to do not print the scale on the meter, so they will
(hopefully) provide me with a Modutec 2MAS-DMA-001 with blank scale (at no
extra cost) which would be useful for both alternatives.
"On my Centurion, I found that the plate current meter full scale current
should have been 1 ma. but my meter (and those of some other people) actually
measured 1.33 ma for full scale."
I don't know the original because it was already dead when I recently both the
amplifier but I have another 425 of same generation and the meter measures
1.022 mA FS with 2.60vdc across its terminals which means an internal
resistance of 2540 ohms.
I swear I saw 2MAS-DMA-001 spec as having 2300 ohms which would need 2.3vdc to
FS
Considering what you and Allen wrote, that makes me wonder because on the 425
they measure the voltage drop across a 1.5 ohms resistor and the meter scale is
1.5A FS, so, 1.5 x 1.5, would provide 2.25 vdc. That would be 86.5% FS of my
other meter or aprox 1.3A so if did the math right, something doesn't match....
I will check that
"A few weeks back I emailed Tentec with a question about the Centurion and it
took just over two weeks to get a response. They told me that all the amplifier
stuff was still in a warehouse somewhere and they were not in a position to
supply parts or information at that time"
Many thanks for the great update, at least you got feedback, They didn´t answer
me (yet).
So as a resume, a Modutec 2MAS-DMA-001 is being produced for me with blank
scale and would be ready for first or second week of November....
When I get the meter and check it electrically I could update here if you guys
think it could be useful.Eddie, LU2DKT
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|