12AX7 instead of 5879?

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12AX7 instead of 5879?

Postby sitedrifter on Sat Jul 16, 2016 12:58 am

I have an amp (vintage) that the preamp tube is supposed to be a 5879 (per the label inside the amp) but checking it, someone put in a 12AX7 preamp.
I cannot find a schematic for this particular amp so I do not know if rewiring was done. My question, would a 12AX7 replace a 5879 without electrical issue or was there some rework of the wiring done?
I would love to put in the 5879 preamp tube but I will not try it without knowing what could happen if reworking of the wiring was need to accommodate a 12AX7.


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Re: 12AX7 instead of 5879?

Postby OldSchoolDave on Sat Jul 16, 2016 1:22 am

sitedrifter wrote:My question, would a 12AX7 replace a 5879 without electrical issue or was there some rework of the wiring done?
I would love to put in the 5879 preamp tube but I will not try it without knowing what could happen if reworking of the wiring was need to accommodate a 12AX7.


They are not equivalents, so I would assume some rewiring had been done to swap out a 12AX7. It's a little late in the day for me to chase down the differences. Here are the data sheets:

http://www.r-type.org/pdfs/5879.pdf

http://www.r-type.org/pdfs/12ax7.pdf

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Re: 12AX7 instead of 5879?

Postby sitedrifter on Sat Jul 16, 2016 11:16 am

Thanks Dave!!!


Looking at the socket connections the are very different.

5879 12AX7
1 Grid 1 Plate 2
2 Not Used Grid 2
3 Cathode Cathode 2
4 Heater Heater
5 Heater Heater
6 Not Used Plate 1
7 Grid 2 Grid 1
8 Plate Cathode 1
9 Grid 3 Heater Center Tap

Is it a matter of changing the wiring on the socket and making sure voltages and resistance are correct such as: (things I think need to change for the 5879 to work)

5879 12AX7
1 <-- 7
2 disconnect wire
6 disconnect wire
7 <-- 2
8 <-- 6
9 ?
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Re: 12AX7 instead of 5879?

Postby OldSchoolDave on Sat Jul 16, 2016 12:18 pm

sitedrifter wrote:Is it a matter of changing the wiring on the socket and making sure voltages and resistance are correct such as: (things I think need to change for the 5879 to work)


Hopefully, the socket rewire was the only change. Trace down the other ends of all connections, though, and look for other clues (like newer solder joints).

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Re: 12AX7 instead of 5879?

Postby sitedrifter on Sat Jul 16, 2016 12:35 pm

There are new resistors, caps and lots of new solder.
I will just leave it for now. I am not experienced enough to revert it.
Thanks for your help!

Mark
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Re: 12AX7 instead of 5879?

Postby mozz on Sat Jul 16, 2016 6:23 pm

5879 is a pentode. What is the amp brand and model number? Someone here may have the schematic.
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Re: 12AX7 instead of 5879?

Postby sitedrifter on Sat Jul 16, 2016 11:58 pm

mozz wrote:5879 is a pentode. What is the amp brand and model number? Someone here may have the schematic.




1954 Magnatone Varsity 108

Would be great if someone could fig up a schematic as the only ones I found are from later 50s that used 12AX7 preamp tubes.


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Re: 12AX7 instead of 5879?

Postby mozz on Tue Jul 19, 2016 8:35 pm

Gibson GA40 used a 5879 so that would be a good starting point.
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Re: 12AX7 instead of 5879?

Postby Papa Dog on Mon Jul 25, 2016 7:12 pm

the 5879, as mentioned, is a pentode
12ax7 is a dual triode.
someone, it seems, made a major conversion on that amp.
gur shots would help to sort it out
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Re: 12AX7 instead of 5879?

Postby sitedrifter on Sat Aug 06, 2016 6:48 pm

duplicate post. sorry
Last edited by sitedrifter on Sat Aug 06, 2016 6:58 pm, edited 1 time in total.
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Re: 12AX7 instead of 5879?

Postby sitedrifter on Sat Aug 06, 2016 6:51 pm

Here are some pics

Image

Image
Image
Image
Image
Image

Below is the 12ax7 socket
Image
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