Hello Jens,
OK on your comment. Here is my explanation.
Recommended voltages are :
Cathode almost on the ground - recommended up to 10 Volts - maximum 100 Volts -
at QRO - 120 Volts.
Screen grid maximum 350 V - recommended 300 - 350 V at QRO 550 V !!!!!!!!!!!!
Grid - maximum 150, recommended 35 -60V at QRO 150V.
No fast protective circuit in screen grid - only relay.
Lousy stabilization of screen voltage.
As I have mentioned I have had 6 and 7 failures on the amps. Up to that I have
lost 2 tubes out of 4. Which in the practical life means that at least one of
the AMPs died in every contest me or OK1RF took part in, and OK1RF told me that
best would be to throw that junk away.
Since the major described reconstruction we have had no failure and the IMD is
better. Also you can not overdrive the screen since there is transistor
over-current protection in the stabilized supply. Also I do not see ANY reason
why there should be over 100 Volts on the tiny cathode of the indirect heated
oxide cathode ?!?!!? It is only and only poor design. Also it is not possible
to tune up with the parts in the PI tank in the AMP properly on 160 + 40m (I
have not solved this problem since I do not use those AMPs on those bands
anyway). If you look at the construction of the others having similar design
with SVETLANA tetrodes:
Alpha power 91BETA, EMTRON DX2, UK AMPs, one company in Ukraine -- I can not
remember the name ( they use GU74b - which is practically the same as 4CX800A).
They ALL have up to 15 Volts on the cathode and recommended voltages on both
grids and so far what I have heard the SVETLANA lamps tend to last very well
?!?!
Not like my case before the reconstruction. I (+OK1RF) do use the amps only in
the contests, we together take part in maybe 10 contests per year and we have
those AMPs on two different bands. If you loose 2 tubes in less then 8 months
of such an operation something is wrong but not with the operators but with the
design. I do believe that the lamps could withstand those rough conditions for
them since they are really heavy-duty design. According to my experience the
EIMAC tubes would not have survived those conditions at all.
I have started this discussion on the reflector well over a year ago and Rich -
AG6K told me that while rebuilding the AMP I would learn something. OK it is of
course true and fortunately I am an engineer which graduated in HF technology
(as a matter of fact running now a computer company) many years ago so I could
do this and as a matter of fact I have had fun doing it. But still I was angry
because if you spend US $3000 (more with the transportation) on a device you
want to obtain working equipment - not a box you have to rebuild plus some
parts. If I would wanted to build a home made AMP I would have bought just some
parts - some of them digged out from my cellar and started from the scratch and
would have spent considerably less.
73 !
Jiri - OK1RI
P.S. I do not believe that I do have some extra bad luck, since I have bought
three pieces at once - two for myself and one for OK1AWZ. He have had also
about the same amount of failures he have lost both tubes in the AMP. I have
fixed some of the failures since I have imported the AMP for him and I have
felt some - I do not know why - responsibility, made some minor changes. OK1AWZ
used to work in TESLA company building big radio transmitters in Vietnam,
Lybia, Sudan and other countries and he have made major rebuild himself and he
was also angry what he have bought.
----------
From: JER Jens Röhme
Sent: 17. února 1999 8:51
To: 'Jiri Sanda'
Subject: RE: Regarding your QRO 2500dx
Hi Again,
First of all, Im not very strong in the technical matters regarding amps
(and that is not an understatement!!) so of curiosity - what have you
acheived redesigning the amp - seen from a users standpoint?
Please see this as a positive request - I'll mabye learn something here hi
hi....
73's.....Jens / OZ1EEZ
> -----Original Message-----
> From: Jiri Sanda [SMTP:jirka@jimaz.cz]
> Sent: Tuesday, January 05, 1999 9:28 AM
> To: JER Jens Röhme
> Cc: 'towertalk'
> Subject: RE: Regarding your QRO 2500dx
>
> Hi Jens,
>
> I will never sort the problems with Ray. I have sorted all the problems by
> myself after 6 and or 7 failures of the device (I have two pieces). Just a
> small remark. Take universal multimeter and measure the DC voltages (both
> idling and operating) on cathode, and both grids. Than go to the
> www.svetlana.com and download the data sheet for the 4CX800 and than make
> your own decision. What I have done is - I have redesigned the whole
> control board and switching and cathode circuit. So what was left is the
> box, PI tank and transformer, everything else is replaced. There are for
> instance zener diodes in parallel two GRAETZ rectifiers (one for + and
> other for -) in parallel and other similar examples of smart and knowing
> engineering.
>
> Have fun
>
> Jiri
> OK1RI
>
>
> ----------
> From: JER Jens Röhme
> Sent: 17. února 1999 8:12
> To: 'jirka@jimaz.cz'
> Subject: Regarding your QRO 2500dx
>
> Hi Jirka,
> Just read your article regarding your experience with QRO. Im also an
> owner
> of a 2500dx and bought i last year used through the internet. When I tried
> it out, I also had problems with the swr between the station and the amp -
> even with antenna tuner, the swr on 10, 17 and 40 was unsatisfactory. I
> called Ray at QRO and told him about the problem - and we agreed on that I
> should send the amp to him for an upgrade. I received the amp 2 weeks
> after
> it was sent (he paid the shipping from the US) and it was like having a
> totally new amplifier! My swr problem was gone and the tuning on 160 was
> suddenly a breeze (was quite hysterical before).
>
> So bottom line I have no complaints whatsoever about the service and the
> amp
> itself - the problems you are mentioning are unknown to me. I discussed it
> with a swedish ham also an owner of a 2500dx - nil problems there. I guess
> you ran into som serious bad luck there, and from what iv'e heard about
> QRO
> it does not at all match your experience.
>
> Well Jirka, hopefully you and Ray are going to sort out the problems.
>
> Regards and 73's......Jens / OZ1EEZ
>
>
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|