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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

Showing 1 response by 4krowme

4krowme's avatar
4krowme
542 posts
 
It is a coincidence that at the moment I am choosing a plate voltage for tube phono preamp. In my case 12AT7 tubes are being used and the max plate voltage shows 300 VDC, but typical running for this tube is at 250 VDC. 
 In your case, if the schematic is showing 270 VDC and you are measuring much lower, it seems possible that something is wrong and should be sought first. What VAC reading do you get at the input of the circuit? It shouldn't be much different than at the output. 
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