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-   -   "The Fisher" tube amp 7189 output tubes (http://www.diyaudio.com/forums/tubes-valves/40078-fisher-tube-amp-7189-output-tubes.html)

laflamme 27th August 2004 09:36 PM

"The Fisher" tube amp 7189 output tubes
 
I have an old tube amp that my father gave to me. It is called "The Fisher" and if memory serves me correct it has 20 or so watts per channel. The output tubes are 7189 and the preamp tubes are 12ax7s. The amp is pretty old (probably from the late 60's) and I am considering restoring it. I would like to re-tube and re-cap the amp and have a few questions.

First, I have looked around and see that there are alot of variations of the 12ax7 which ones would be compatible with the ones in my amp (the markings on the amp show 12ax7 only)? Second, it seems real hard to locate the 7189 output tubes, however I have seen that they cross reference to the el84 or the 6qb5. The el84s are easy to find, can I use those instead?

Lastly, the capacitors on the amp are in this big metal can and it seems that the can contains 4 capacitors. Are these easy to find, or am I going to have to design a mini ckt board to house these caps?

I appreciate any help you can give me on this.

Thanks,
Peter

SY 27th August 2004 11:41 PM

Check the pinouts in the tube manual to see if that has to be modified. And you may have to change the bias a bit. But otherwise, the EL84 will work fine. If you're REAL lucky, you might stumble into some 6CZ5s or 6973s, which are even better but pretty much impossible to source anymore.

fdegrove 27th August 2004 11:54 PM

Hi,

Quote:

First, I have looked around and see that there are alot of variations of the 12ax7 which ones would be compatible with the ones in my amp (the markings on the amp show 12ax7 only)?
Any 12AX7 and its equivalents would work fine: 12AX7 is pretty old, nowadays you'll easily find USSR equivalents, Ei made in Yugoslavia and some Chinese 12AX7As.
You may want to try the close equivalent of the 12AX7, the 5751 which often gives excellent results as a replacement.

Quote:

The el84s are easy to find, can I use those instead?
Most of the time these will work as a drop in replacement but the interanl connections are laid out differently meaning that the manufacturer way have used solder lugs on the noval socket for hooking up a component or two.
Check this out and correct if needed.
From the NOS EL84 equivalents that are still available the Siemens 7320/E84L is my favourite choice.

DATASHEET 7189 (A)

DATASHEET 6BQ5

BTW, that a fine little amp you have there, the diagram can be found on the web.

Cheers,;)

SHiFTY 28th August 2004 12:49 AM

Don't buy new tubes unless you really need to. The output tubes may need replacing if they are worn or really mismatched, but don't do it unless you need to, whereas the preamp tubes will be fine unless they are noisy. BTW 12AX7s last for ever in a conservative circuit, and any 12AX7 will work fine in there.

The old tubes are the best, and if the amp is in reasonable condition it is likely the tubes are just fine. I would suggest just cleaning them with some paper towels and rubbing alcohol, don't clean the writing on them as it comes off very easily.

However, what you should definitely do is replace all coupling capacitors (they will be around 0.1uF 600V) and any resistors that have gone out of spec (eg 100K has become 120K) and any small value electrolytic caps (eg 100uF 50V)

The metal can caps might be ok if they haven't leaked. I use several old ones that work just fine.

Good luck! And post a pic!

fdegrove 28th August 2004 01:40 AM

FISHERMAN'S FRIEND...
 
Hi,

If it's the Fisher X-100 you have:

X-100

Cheers,;)


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