Valve Itch phono

[Salas]

Haven't listened to the radio on various iterations of this so it isn't inherent. It is some detail in your build. For MM you can put 100R grid stopper as a quick measure because such cartridges have higher impedance and it will not affect the noise significantly. There are ferrite beads to try as well. Better/shielded small signal's cabling practice i.e. narrower loop areas of it and tighter construction in the build itself should be thought of and revisited. A grounded metal box is good to transfer the build to, or dress the wooden box's inside with grounded MuMetal shielding, but if its a sensitive tubes issue some locking metal cap shield with compatible base is effective.

 

[stajo]

You can always test if its the wiring or the tube itself by removing the input wiring. Keep the 47k or whatever you’re loading to gnd tho

 

[chat72]

I'll try all suggestions and report back.
Thank
C.S.

 

[stajo]

I'll try all suggestions and report back.
Thank
C.S.

Black smoke?

 

[chat72]

Black smoke?

Not yet.
I put a metal sheild on the input tube, that cut RFI by a large margin. I have to turn the volume to really high in order to hear the radio.

 

[stajo]

Yea I have also used that on 12QAX7-like tubes for riaa inputs, usually helps. Good if the metal hugs the glass so it transports heat rather then prohibiting airflow. I have a box of those various caps somewhere. And a soldered wire as direct to earth as possible is good.

 

[615498]

Choke questions

Hi
I have a couple of orphan chokes and thought I could use them in an Itch. I plugged some values into PSUD and came up with a supply that 'hopefully' suits a SSHV2 regulated version of Rev1.2, to be housed in a separate box. The power transformer is a Hammond 370BX putting out about 300v unloaded on local mains voltage. The heater transformers are Hammond toroids (1182K9).

Does this look like it will work?

Thanks

Keith

 

[Salas]

Looks alright

 

[615498]

Thanks. I'll post as things evolve.

Keith

 

[merlin el mago]

@ Salas, what's the total power consumption of both channels: 14mA?

TIA
Felipe

 

[Salas]

Yes 14mA @ 300V i.e. 4.2W (when not counting heaters power), with small +/- differences depending on tubes samples bias deviations

 

[merlin el mago]

Thank you Salas.

How much % can vary?

 

[Salas]

I don't know... 10% (?). V2 & V4 sections are the main consumers. Try two different tubes there and measure voltage drop across any resistor in their way (R9 R10 R11) to derive mA when dividing Vdrop by the resistor's value. That will give you an idea of mA bias difference across your own tubes.

 

[merlin el mago]

Thanks again Salas.

Salas the Itch have hum volume related, any idea where to start to fix?

If increase volume increase the hum.

 

[Salas]

Didn't have in the past but it recently developed? If so I would start from tubes swap or screening or moving away if other gear transformer was introduced nearby and the hum went up or undo some sensitive modification

 

[merlin el mago]

Thanks for answer Salas, always was the hum. No other gear introduced nearby. I will try swapping tubes if hum not goes away I will try screening the tubes.

Salas could be the Belton micalex sockets some people said go for teflon? Or the 6SN7 to 7N7 adaptor?

 

[Salas]

If the contacts are clean inside them and they achieve good mechanical lock I would not think that the choice of bases can be a factor for background noise. Adapter maybe can expose more if its barrel is not grounded, but a swap from 7N7 back to 6SN7 direct would tell.

 

[merlin el mago]

Hum gone swapping the 7N7 tubes with adapters included

Salas worth the change for teflon sockets to improve SQ?

 

[Salas]

Don't know, haven't used such high quality sockets. Beyond the good insulation material they seem to have very nice machined copper gold plated contacts also and that alone can be a plus when combined with well cleaned and chemicals treated tube pins (Cramolin/Deoxit).

 

[merlin el mago]

Thanks again Salas.

Salas could I use this anti-RIAA to measure the Itch RIAA accuracy?

An externally hosted image should be here but it was not working when we last tested it.

Component values

Ideal Actual Alternative
R1 883k 909k 910k
R2 75k 78.7k 75k
C1 3.6n 3.3n 3.3n
C2 - 180p 270p
C3 1n 680p 1n
C4 - 270p -
R3 0 1.48k 1 - 1.5k