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Old 18th July 2012, 07:05 PM   #1
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Location: Pembrokeshire, UK
Default 5879 to 12AX7 conversion

Guys and gals,

I have a phono pre-amp that uses a 5879 pentode as the first stage from the magnetic cartridge. I need to squeeze a bit more gain from the circuit to compensate for the differences in the newer stereo compliant needles and the 5879 cannot be squeezed any harder. I was thinking of a two stage triode 12AX7 conversion.

Anything to watch for as I have not done this type of modification before?

5879 plate voltage is 70v, gain is about 70. I would like the gain to be about 100, but the 12AX7 would need a slightly higher plate voltage. Or could I run a 12AX7 at a low anode voltage to retain a flatter curve? I don't really want to change the pin setup on the amp chassis so I am thinking of an interconnect adaptor between the original 5879 wired socket and the 12AX7. Might even try a 7025 or an E83CC. Output is coupled to the grid of a 6SN7GT.

Any ideas as I really have to retain the 5879 chassis wiring.

Cheers,
Martin
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Old 18th July 2012, 07:33 PM   #2
DF96 is offline DF96  England
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12AX7 is not too linear at low voltages. Depending on the current circuit details, you might need to redesign the RIAA network because a 12AX7 will have a different anode impedance from a pentode. 7025 and E83CC are just different versions of 12AX7.
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Old 18th July 2012, 07:52 PM   #3
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Thanks for the advice DF96. There is no RIAA network. The original circuit dates from 1955 and the input is taken from a Seeburg Pickering cartridge.
Supply voltage is 195v, dropped via plate resistor to 70v (180K), and dropped for the screen grid at 40v (750K) decoupled to ground by a 0.15uF capacitor.
Cathode has standard bypass of 1K and 100uF capacitor running at 1v.
Input load is 220K.

The 2 triodes in the 12AX7 would have the same 220K input load direct to the grid, with a 100K plate resistor with decoupling via a small bass lift RC circuit (to compensate for the lower bass response of the newer stereo compliant needles as opposed to the non compliant 'spikes' from 1955) and the 2nd triode having a plate resistor of 140K.

Problem is I cannot change the chassis components to increase the anode voltage. A very difficult project.
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Old 18th July 2012, 08:12 PM   #4
DF96 is offline DF96  England
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You said magnetic cartridge. Is it actually for a ceramic cartridge?

Why not convert for an EF86?
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Old 31st July 2012, 09:06 AM   #5
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Location: Pembrokeshire, UK
I am not so familiar with the EF86. But a good idea. I will check the specs of an EF86.

The cartridge is a Seeburg (Pickering) Redhead magnetic type used in the V200/KD200 series of jukeboxes.

The problem is when these were designed in the early 1950's there was no such thing as stereo... The needle was just a 'spike' and this spike had enough compliance for mono records only.

The 'soloution' was to make a new design of armature with an offset needle. This gave stereo compliance but reduced output and reduced bass. I am trying to compensate for this electronically.
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