Help to get rid of hum please.

[catalin gramada]

Is the cathode bypass cap large enough, say 100µ+?

There is no cathode Bypass cap on any stage. The circuit sound very nice without it except the noise. I tried also with its tube noval equivalent without this noise issue. (meant 6n2p) . I don't know how Russians used this tube, it is supposed to be an 6sl7 equivalent, which it is...

[catalin gramada]

Question: may be the split plate resistor 220k + 220k responsible please ?

[Mike K]

Question: may be the split plate resistor 220k + 220k responsible please ?

I'd remove that - I don't see what that is for TBH.

I've built amps without a bypassed first stage. They didn't have that sort of hum but a very, very small amount.

[catalin gramada]

This is the second triode 220k plate with 2.2k unbypassed cathode. Same thing...

[Helmholtz]

Question: may be the split plate resistor 220k + 220k responsible please ?

While the split load doesn't make much sense to me, I don't see how that could cause hum.

I think the tube has a cathode to heater insulation problem.
You could try pure DC heating.

[Mike K]

You could try pure DC heating.

He said he did and it makes a high pitch tone.

I honestly have never heard of anything like this. I'm not really understanding if he resolved it with a different tube though i.e. the 6SL7

[Helmholtz]

He said he did and it makes a high pitch tone.

Yes, but was the DC really clean?
Maybe try a 6V battery.

[catalin gramada]

....and Voila ! same tube, same plate recordings without hum noise.



The heater was elevated at 50V ...negative this time from bias supply, the modulation is gone. !
Thanks for support.

[catalin gramada]

Yes, but was the DC really clean?
Maybe try a 6V battery.

No Helm it was not quite clean with 1000u had at hand. The high pitch refers to 100 cycle hum I may suppose. But seems negative voltage elevation solved the problem. Just thinking now to optimize the voltage it needs to reference, 50V from my bias supply seems pretty high...

[catalin gramada]

OK. I put a large potentiometer over the bias supply to not load the source and get a surprise. Not sure if a coincidence or not ( you tube guys may get a conclusion) but the modulation stops suddenly at -2.5 V elevation around meant almost exactly the bias voltage over cathode resistor.
This makes me happy, I have a whole box of russian "6sl7" tubes for my projects we know to use.

[Helmholtz]

Unusual effect.
Typically both negative and positive heater bias works.
Seems like the cathode coating of these tubes acts like a semiconductor diode which needs to be reverse biased to prevent leakage.

[catalin gramada]

I got a 6.3V tranny and 15000uF cap and want to try to see if DC supply do a difference. As have not experience using dc on heaters supply couple of questions occurs as follow, please:
1.using a separate winding only for this stage the reference to ground should be on AC or DC side please ?
2.the reference must be compulsory tied to the ground or may be elevated like in AC heaters supply ?
3.what is minimum capacitance to use to get DC ripple free for a 300mA filament please ?
Thanks

[Helmholtz]

http://valvewizard.co.uk/heater.html

[catalin gramada]

http://valvewizard.co.uk/heater.html

All clear, except the fact I don't understand if it still be necessary to elevate the reference at least -2.5V negative in case of my leaky tubes , even using a DC supply please ? Thanks.

[catalin gramada]

There are any problems using tube shields with octal tubes please ? I found shielding the input preamp tube cure the noise to absolutely none. But did not found any device to use it. I.m talking about 1W tubes. Any concern about please ? Thanks.