Hi there. I have a philips radiogram as mentioned in other posts. I now have the replacement volume control installed (An exact one for this unit!)

The problem I am having it there is a noticable Hiss in the amp. the noise goes away when I remove the Preamp tube (12AX7A). I have played with the hum stopping pot but this didn't change things. (There is no 50Hz hum at all)

What could cause the hiss? here is a link to the diagram Philips RF6/01

About the only thing you can do is try other 12AX7 tubes. Hiss in phono preamps is a fairly common thing in very small signal RIAA amp circuits. The tube itself generates hiss, Boltsman constant or some Physics related thing...

Lots of people over in this forum should be able to offer more and better advice.

Questions: What vintage is this?

Does it have paper type capacitors, which are still in it?

Is this hiss common to both channels?


Marcc

Hi Marc

The hiss is in both channels. it is present even with the volume turned down and the i/p shorted. (Not surprising since the volume is before the preamp!)

I am not sure of the vintage of it. I would say late 50's. the caps that are in it are the mustard coloured ones. I soldered a few of then to test on my meter and they test good for resistance and capacitance so they were left in place.

The noise completely stops when the preamp tube is removed. I have 4 new tubes and they all do the same thing!

Do you think I should just replace all caps and resistors even though they are within 5 - 10% of their marked values on a static test?

I don't think the Hiss would be normal as it is very noticeable when at low volume.

The tone control, maybe? If you've used the pot I sent, the tone control tap is at 900K. The diagram shows a tap at 400K, which might upset the tonal balance and emphasis the hiss. Maybe you need to turn the treble down a bit.

Hi there

I have been playing around with the amp. The things that I have done are change the anode, Cathode and grid resistors for both channels on the 12AX7 (R29,30,31,32,35,36 and 69). This has stopped the hiss.

The only problem I now have is one channel has a hum. I think this might be another resistor or a coupling cap that has had-it in that channel. The voltages on the anodes of the 12AX7 are a bit high on the channel that is humming. I have changed the tube and this had no effect.

I have also swapped the o/p tubes but this made no difference. With the 12AX7 out of cct, the amp is completely silent. the only noise you hear it with your ear right next to the speaker cone.

So any thoughts .

The POT you sent is working fine!

If the hiss is in both channels, the logical thing is to look at that which is common to both channels. first. and the pre-amp valve is.

C53 and C54 would have been most likelys for hiss, valves do not normally hiss but do have often a small amount of hum modulation from the cathode.

The circuit reads as no heaters should go to earth. The heaters are earthed via the humdinger R33. An old idea for preventing hum, check if R33 is OK

Hum in a single chanel suggests a bad earth probably on the grid circuit. of V1. If you moves wires (lead dress) that can be an issue. I have had to rewire parts of sets with shielded cable, due unwanted induction and / or capacitive coupling.

If those brown caps can be identified as paper, get rid of them. I f they are a physically small axial with "Hunts" on them get rid of them as well. You often find them cracked or leaking.

Ohm meters will only test a cap if its short, thereafter not reliable. I normally use an insulation tester set at as close to as I can get to the working voltage of the cap. Should I feel compeled to test as most papers are not worth the effort.: That's more realistic test.

One end of the cap needs to be out of the circuit to test.

Marc

Hi

I have tested all caps, resistors, subed then out and still didnt make a change.

After a while I came to the conclusion that I might have a bad batch of 12AX7s. I went for a completly differend brand and the hum stopped for the left channel. Now the only problem was the Right channel hummed so band it wasnt funny.

So a bran new valve (Same brand as origionally used) but different batch is in there and now all noise has completly stopped. There is no hum hiss, Just good music!

Just goes to show that even though you might buy a bran new valve they are not great and sometimes aren't worth the money.

Might pay to lift the valves and have alook at the bases of all of them & clean the top of the socket.

Some tubes had silver on the pins; This can form whiskers, and you can get silver tracking. Clean the base & pins of the valves as well.

If there is indication of pin contact problems, which is not uncommon for some types like the half circle ones.

Get a spike, like a scriber & close the socket jaws if they have sprung.

Marc