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Old 17th January 2007, 02:37 AM   #1
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Default 7189/7189A Problem

Hi,
I acquired a Kenwood KW-70 for which I have not been able to find a schematic. My immediate problem is that the chassis is stamped 7189 next to the output tubes and it came with 7189A's installed in it.
The unit does work. My understanding is that the 7189A is not a direct replacement for the 7189. If that is true, what could have been damaged as a result of this situation and what should I do about it. Thanks, Victoria
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Old 17th January 2007, 09:58 AM   #2
pauldg is offline pauldg  United Kingdom
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Hi,

From what I recall the 7189a will work in almost all circuits designed for the 7189 without modifications but NOT the other way round. I think the 7189a has more than one pin for grids 1+2 and a slightly higher max voltage rating.

It has happened in the past that I have been wrong but I don't like to admit to it... Someone normally points it out!

You say the unit is working? Does it just concern you that the components aren't what is stamped on the chassis? If measurements taken are within component spec and it works properly I don't worry unless I'm specifically restoring or it has historical significance.

Paul
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Old 17th January 2007, 01:35 PM   #3
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They COULD be wired to be incompatible, but probably aren't. Amps are almost aways wired so that either 7189 or 7189A will work. Pinouts ARE different, but so are many different brands of 6BQ5s - on that one the pins are marked IC (internal connection), they never get connected in a circuit, so it really doesn't matter what is connected inside the tube.

The KW-55U schematic is online here: http://www.mcmlv.org/Archive/HiFi/KW55U.pdf

The Lafayettte LR-800 (made by Kenwood) is on my website: http://www.audiophool.cjb.net/Audio.html
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Old 17th January 2007, 04:15 PM   #4
ArtG is offline ArtG  United States
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Wow! that schematic looks like an old tube TV schematic!

Tom Bavis' post brought up an interesting point. All my Telefunken EL-84 tubes show internal shorts on my TV-7 tester, on 4 positions, as I recall. This occurs on all Telefunken EL-84s that I've ever tested, including NOS, but not on other brands, except perhaps Siemens (which appear to be the same tube). These tubes, of couse, work perfectly in my ST-35, which backs up his point that differant brands may have unused IC pins that may or may not affect use depending upon the socket wiring of a particular amplifier.

It makes me wonder how many many perfectly good tubes have ended up in the "can" because of this!
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Old 18th January 2007, 04:21 PM   #5
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Default 7189/7189A Problem

Hi Pauldg and Tom,
The unit is actually rather abused and I am just trying to refresh it to
learn about the process and then use it. What concerned me was
I installed 4 new Russian 6P14P's(not matched - no tube tester) and the power output dropped dramatically.
I can't find any typical bias pots and a google search return, from someone who had scrapped one out, said "...it used 7189A's,pentode connected, and fixed bias." The coupling caps had been replaced prior
to my acquiring it.
Have you any suggestions on what might be causing the 6P14P power to be so low and how to fix it.? I would really like to use this tube type because of it's availability.

Thanks for the schematic links!! Very helpful because the tuner sections
are close and the output section of the LR-800 is close.
Thanks again, Victoria
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Old 22nd January 2007, 01:38 AM   #6
pauldg is offline pauldg  United Kingdom
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Hello Victoria,

If the voltages in your unit are as per the LR-800 I would say you're getting a bit close to the operating limits of your new valves. (Sorry - tubes...) It may not be an issue depending on what revision they are. Output impedance could also come into it - the sheet I have for the 6p14p doesn't list this. If someone else has some more experience they might like to comment.

Either of those may or may not affect your power output problem. My recommendation would be to have a good poke around with a meter to really see what's going on.
After reading and understanding at least the thread on high voltage safety precautions of course - make absolutely sure to NOT kill yourself, that would make me feel bad. If you are at all unsure of messing with big volts don't do it - it's too damn easy to slip and not get time to say 'oops'.

Once you have a nice list of values for voltage/current etc. you can start to change them. Or not. Calculation and experimentation are what learning is all about if you ask me. If you fry some stuff that isn't you then you've learnt something... You have to pass it's limits to find it's limits.

I do apologise for the abiguity of this reply but without some readings or the unit in front of me I can't suggest much more.

Best of luck with it though.
Paul
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Old 22nd January 2007, 03:08 AM   #7
ArtG is offline ArtG  United States
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Default Re: 7189/7189A Problem

Quote:
Originally posted by Victoria
Hi Pauldg and Tom,


I installed 4 new Russian 6P14P's(not matched - no tube tester) and the power output dropped dramatically.


Are you saying that when you install the 6P14P tubes the "power" drops, but reinstalling your previous tubes returns the amp to normal operation?
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