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12ax7 phono pre-amp tubes - plate voltage


Hello,

I have a H-K TA5000x that I’ve been slowing restoring. The only schematic I have found is in two parts, Part 1 has the right side cut off and Part 2 has the left side cutoff, which results in the phono preamp being shown on Part 1 and the power supply on Part 2. Unfortunately, while very similar, Parts 1 and 2 are not same version of the schematic. The specific problem I have is the phono preamp plate voltage feed from the power supply (Part 2) is listed at 190vdc, which I have measured to be 188vdc. This same supply is listed on at the phono preamp in Part 1 as 270vdc, which is then reduce to 170vdc and 120vdc before going to the plates. I have measured the plate voltages to be about 120vdc and 78vdc; this seems very low as these tubes are rated up to 300vdc plate voltage. It plays okay but I think it could be better.  I definitely have much lower volume from phono then the radio or Aux and this receiver seems to have a little bit of low volume in general for the watt rating. I’m currently running reissue russian TungSol 12AX7/ECC803S gold pins and I have some used mismatched vintage tubes that work but strength is unknown; there are differences but in general new and old tubes work the same. Should I adjust the power supply voltage to the plates by changing resistor values and if I do what tube performance parameter should I being trying to achieve?

https://www.radiomuseum.org/r/harman_ta_5000xta5000.html

Thanks for reading and any input is welcome.
Bill
wgh64
imhififan's avatar
imhififan
2,203 posts
 
https://www.thegearpage.net/board/index.php?threads/question-about-preamp-plate-voltages.1770030/

Please take a look at the harman kardon A-500 schematic, which is quite similar to TA-5000x minus tuner section, its only supply 170V to the phono stage!

Harman-kardon-A-500-OM - vintage hifi


wgh64 OP
28 posts
 
imhifhifan - thanks.  That was helpful.  I will look elsewhere for RIAA-NARTB switching issue fix.  This thing's a head scratcher.  I feel like I've checked everything.   I guess the next step is to follow the signal and see where is drops out unless somebody has any other ideas.  
wgh64 OP
28 posts
 
So I was troubleshooting Channel B cutting out when switched to RIAA (works fine in NARTB).  I found the plate voltage difference on the schematic while measuring the voltages and checking resistors on the entire phono pre-amp circuit.  I didn't find any problems.  I powered it back up and the problem has resolved itself.  
imhififan's avatar
imhififan
2,203 posts
 
So I was troubleshooting Channel B cutting out when switched to RIAA (works fine in NARTB)... I powered it back up and the problem has resolved itself.
Could be simply dirty switch contacts, btw, a receiver that old it is better to refresh all solder points to eliminate cold/dry solders joints.

Perhaps you already knew, the phono section of TA-5000x high input is for 200mV ceramic or crystal cartridges, and low input is for 4mV MM/MI or high output MC cartridges.

If that is mine, I’d modify the high input for 4mV cartridges, and add a pair of SUT to low input for low output MC cartridges.

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