Phono Stage tube PCF802 from VLAD ~ any info?

[damiena]

Been away a lot and only just got round to this.

I put two 5K6 resistors in parallel - so 2.8K ohms.

Orig voltage 333v, dropped voltage 322v

That's only an 11v drop. So to drop 30v a 10k resistor sounds about right.  Interesting.

So voltage drop / R = 11v/2.8k = 3.93 mA

In which case, to drop 30v I’d need a 7.6k ohm resistor.

[damiena]

Then spotted I had some 15k resistors. So I put two 15k resistors in parallel and got 7.45k ohms.

I’ve put a pair of those on each of the 300+ inputs. Its dropped the voltage to 296v (from 333v).

Was still getting a lot of noise on the right channel. So resoldered the RCA socket that looked dodgy, but that didnt help.

So poked everything with a chopstick, everything had no effect until I pressed the left output capacitor which fixed the noise. That didn't make any sense so I tapped the valves again, this time the right valve was microphonic (which it wasn't before), so I tapped and pushed that with the stick a little bit more til the microphony was gone, and the sound was still good.

So not sure what that was all about. Its fine now though so will close the box up and look into it deeper if the noise or microphony comes back.

Been away a lot and only just got round to this.

I put two 5K6 resistors in parallel - so 2.8K ohms.

Orig voltage 333v, dropped voltage 322v

That's only an 11v drop. So to drop 30v a 10k resistor sounds about right.  Interesting.

So voltage drop / R = 11v/2.8k = 3.93 mA

In which case, to drop 30v I’d need a 7.6k ohm resistor.

[niclaspa]

So poked everything with a chopstick, everything had no effect until I pressed the left output capacitor which fixed the noise. That didn't make any sense so I tapped the valves again, this time the right valve was microphonic (which it wasn't before), so I tapped and pushed that with the stick a little bit more til the microphony was gone, and the sound was still good.

Maybe the solder joint on the left capacitor was bad?  I think you should reheat it and add a little new solder.

For the microphonic valve, could it be dodgy contact between the valve pins and the socket?  Maybe you should scrape the valve pins to remove any oxide?

[damiena]

Maybe the solder joint on the left capacitor was bad?  I think you should reheat it and add a little new solder.

That was my first thought, bit I tapped it again and it didn't go bad so decided to hold off from resoldering for now

For the microphonic valve, could it be dodgy contact between the valve pins and the socket?  Maybe you should scrape the valve pins to remove any oxide?

So, oddly, when I put the case back on, the left valve became the microphonic one, and the right one wasn't. However the right one was much lower in volume and lacked bass. So I swapped it for a spare valve, and the volume and bass was back with no microphinics if tapped. So I decided to swap the left one for the other spare. Now both sound amazing - the tube rush sound on both is now the lowest its been, and both are good volume and lovely full bass. However they are both now microphonic, but only if tapped.

The pins did look a little oxidized. I shall clean them next.