X-202 phase inverter question

Fuzzbuster

Addicted Member
Hi everybody. New to the Fisher forum...friends call me Fuzz. I recently picked up a Fisher X-202...not a "B" model. It has 13AX7s in the phase inverter positions. What, if any, ill effect can that have on performance? Thanks in advance.
 
I am not aware of any 13AX7 tubes. If they are indeed rated at 13 volts for their filaments, they may have a controlled warm-up characteristic for the filament with the intention of being used in a radio or television which has a filament string connected in series to the 120VAC power input. The filament of a 12AX7 is rated for .15A when connected across the full filament or .3A if its filament pins are connected for 6.3VAC filament operation. A 13 volt filament could have a higher or lower current rating. Without detailed information as to its filament ratings I would not be inclined to leave them in this amplifier.

Joe
 
I am not aware of any 13AX7 tubes. If they are indeed rated at 13 volts for their filaments, they may have a controlled warm-up characteristic for the filament with the intention of being used in a radio or television which has a filament string connected in series to the 120VAC power input. The filament of a 12AX7 is rated for .15A when connected across the full filament or .3A if its filament pins are connected for 6.3VAC filament operation. A 13 volt filament could have a higher or lower current rating. Without detailed information as to its filament ratings I would not be inclined to leave them in this amplifier.

Joe

Sorry. Obviously did not proofread before I posted...12AX7 I meant.
 
All versions of the X-202 use a 7247 in that position. Fisher went through a number of changes in the phase inverter section, which really amounts to why there were the various versions made. All of the versions however count on the low impedance of the 7247's inverter section to deliver enough low impedance drive to the output stage to develop full power output. Using a 12AX7 will certainly "work" in the inverter position, but will not allow the amplifier to reach its rated performance capability. Definitely change to a 7247/12DW7 when you can.

Dave
 
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All versions of the X-202 use a 7247 in that position. Fisher went through a number of changes in the phase inverter section, which really amounts to why there were the various versions made. All of the versions however count on the low impedance of the 7247's inverter section to deliver enough low impedance drive to the output stage to develop full power output. Using a 12AX7 will certainly "work", but in the inverter position, but will not allow the amplifier to reach its rated performance capability. Definitely change to a 7247/12DW7 when you can.

Dave

Thank you. The tech who went through it said no problem, it might just be louder. I will take your suggestion based on your reputation on this site. Do you think the JJ or EH are ok? I see AES has Phillips for $35 and GE for $39. What is your recommendation. Thank you.
 
I don't have any experience with the JJ or EH tubes to give any guidance. I think as far as small signal tubes go, the JJ and EH are good tubes. I do know however that in certainly applications, (receiver phase inverters for example), the JJ tubes don't like the stock design and perform poorly -- poorly as in clipping at slightly above normal volume levels. In the receivers however, Fisher switched to a 12AX7 that is operated nearly into the cut-off region. The JJ tubes apparently don't do well as that region is approached in equipment designed for the original vintage product. In the X-202 however, the design is much more traditional, so that any tube attempting to emulate the original 7247 specification should do fine.

Using a 12AX7 in place of the specified 7247 won't change the relative volume level of the unit's operation at all, as the section of the tube that operates as the inverter section inherently operates at a unity gain, regardless of what tube type is installed.

Dave
 
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I don't have any experience with the JJ or EH tubes to give any guidance. I think as far as small signal tubes go, the JJ and EH they are good tubes. I do know however that in certainly applications, (receiver phase inverters for example), the JJ tubes don't like the stock design and perform poorly -- poorly as in clipping at slightly above normal volume levels. In the receivers however, Fisher switched to a 12AX7 that is operated nearly into the cut-off region. The JJ tubes apparently don't do well as that region is approached in equipment designed for the original vintage product. In the X-202 however, the design is much more traditional, so that any tube attempting to emulate the original 7247 specification should do fine.

Using a 12AX7 in place of the specified 7247 won't change the relative volume level of the unit's operation at all, as the section of the tube that operates as the inverter section inherently operates at a unity gain, regardless of what tube type is installed.

Dave
Thank you again..
 
I ran an EH 7247 in my TA-600 for a while, it seemed to do OK but honestly I never really measured it. Audibly it sounded fine, and still sounds fine with the Mullard tubes it now lives with. I'd offer to swap them back and measure it, but frankly its cold out in the shop and the original filter cans have finally given out so its currently out of service until I replace them.
 
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