Valve Itch phono

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#2,761
Salas said:
<snip>
That's about 100kHz pure Miller, no input wiring extra from RCA input to grid. Kevin, is there a chance is less at 20kHz? Any rework suggestions for losing some parasitic from seeing his base wiring layout photos?
Click to expand...

I wouldn't expect there to be any significant differences, at higher frequencies the inductance of the wiring may start to cause some issues, 100kHz should be low enough for this effect to be small.

Seems like the choice of tube socket might be surprisingly critical, stray capacitance on the grid is not good news, but strays between the A and G pins like on a 12AX7/6N2P is critical and is actually going to increase the resulting miller capacitance substantially. This might actually argue for the use of ceramic sockets which I detest. Teflon might be an alternative.

Higher accuracy of measurement is possible with better test equipment but factoring in scope accuracy, resistor tolerance, and human factors I felt that 10% accuracy was a reasonable swag. I think the additional effect of the 600 ohm source impedance can be ignored in most cases unless really accurate equipment is available.
 
#2,762
In terms of building a new one I would recommend the following approaches that have worked well for me.


  • Enclosed metal chassis. (Inexpensive Chinese made aluminum boxes work great.)

  • Phono stage and power supply in separate chassis. No possibility of electrostatic or magnetic coupling to supply components means less need for shielding and shielded wire.

  • Input tube closest to input jack, keep the run short, and do not use shielded wire inside the box. Counter intuitively do not dress input wiring against chassis, in free air away from everything else reduces stray capacitance.

  • Use phono jacks with teflon or polypropylene dielectric, measure the capacitance and select ones with lowest capacitance.

  • Use tubes with anode pin not adjacent to grid in first stage in your design.

  • Consider the use of ceramic sockets or teflon to reduce dielectric constant and hence capacitance.
More flippantly select an HOMC cartridge which likely performs better and is not too sensitive to input capacitance. (within reason)


I sometimes do these things or some combination of these things, but honestly until Merlin mentioned the problems he was having did not consider them too seriously. My Muscovite was built to have relatively low input capacitance, the later minis actually have higher input capacitance due to strays, tube choice and build technique..
 
#2,763
Hi,

kevinkr said:
In terms of building a new one I would recommend the following approaches that have worked well for me.


  • Enclosed metal chassis. (Inexpensive Chinese made aluminum boxes work great.)

  • Phono stage and power supply in separate chassis. No possibility of electrostatic or magnetic coupling to supply components means less need for shielding and shielded wire.

  • Input tube closest to input jack, keep the run short, and do not use shielded wire inside the box. Counter intuitively do not dress input wiring against chassis, in free air away from everything else reduces stray capacitance.

  • Use phono jacks with teflon or polypropylene dielectric, measure the capacitance and select ones with lowest capacitance.

  • Use tubes with anode pin not adjacent to grid in first stage in your design.

  • Consider the use of ceramic sockets or teflon to reduce dielectric constant and hence capacitance.
More flippantly select an HOMC cartridge which likely performs better and is not too sensitive to input capacitance. (within reason)


I sometimes do these things or some combination of these things, but honestly until Merlin mentioned the problems he was having did not consider them too seriously. My Muscovite was built to have relatively low input capacitance, the later minis actually have higher input capacitance due to strays, tube choice and build technique..
Click to expand...

+1 On all of the above. Good thinking and building techniques, Kevin.

Personally I do like ceramic sockets but definitely NOT the Chines junk.
I too prefer PTFE sockets but there too the quality can vary. Used to use mil spec British made ones with silvered contacts but those are long gone now.

For wiring I try to use PTFE insulated pure silver solid core wire, as thin as possible, say no more than 1 mm in diameter for signal wiring.

Cheers, 😉
 
#2,764
kevinkr said:
Normally shielded wire is a good idea, at the input of a phono stage any appreciable length can contribute additional capacitance. I suspect the wire you are using might be contributing some capacitance - would be good to measure it. You can get close with your meter with nothing powered. Anything over 40 - 50pF would I think be justification for trying something else.
Click to expand...

The cable measures 47pF.
 
#2,765
Answer in red

kevinkr said:
In terms of building a new one I would recommend the following approaches that have worked well for me.


  • Enclosed metal chassis. (Inexpensive Chinese made aluminum boxes work great.)

Used enclosed aluminium chassis

  • Phono stage and power supply in separate chassis. No possibility of electrostatic or magnetic coupling to supply components means less need for shielding and shielded wire.

PSU and phono stage are in separate chassis

  • Input tube closest to input jack, keep the run short, and do not use shielded wire inside the box. Counter intuitively do not dress input wiring against chassis, in free air away from everything else reduces stray capacitance.

Used Cardas 2x24awg shielded as input cable between the RCA socket and the phono stage

  • Use phono jacks with teflon or polypropylene dielectric, measure the capacitance and select ones with lowest capacitance.

WBT & Neutrik I guess teflon dielectric

  • Use tubes with anode pin not adjacent to grid in first stage in your design.

  • Consider the use of ceramic sockets or teflon to reduce dielectric constant and hence capacitance.

I use Belton with Micalex material

More flippantly select an HOMC cartridge which likely performs better and is not too sensitive to input capacitance. (within reason)


I sometimes do these things or some combination of these things, but honestly until Merlin mentioned the problems he was having did not consider them too seriously. My Muscovite was built to have relatively low input capacitance, the later minis actually have higher input capacitance due to strays, tube choice and build technique..
Click to expand...
 
#2,766
merlin el mago said:
The cable measures 47pF.
Click to expand...

That's 267pF + 47pF = 314pF based on your LCR indications for build's pins parasitic and internal wire length capacitance, also on Vgk Cga from your tube's datasheet

You also have indicated 300pF before with Kevin's table gen & scope method when including the RCA to grid wire run.

Those figures look agreeable and very well possibly valid to me especially when for 10% error allowance
 
#2,768
Measured again the internal wire directly to LCR without LCR wires measures 27pF, attached pics LCR without wire + RCA connected & with wire & RCA connected.

Worth to change the wire to reduce capacitance using not shielded wire and not twisted signal & ground?
 

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#2,769
267+27=294pF i.e. even nearer to the 300pF you found with oscilloscope and table

Better measure the TT wiring also from cartridge attachments (cartridge not connected) to male RCAs.

Add that to the 300pF and see if its right range for your for your Shure cart. If its way off then you decide what cables to trim or change.
 
#2,776
Hi Merlin,
Pull the 24 ga cardas litz out of the jacket and use nude from grid to RCA jack, that should reduce the capacitance by possibly as much as 30pF with a bit of care. (No sound quality hit)

The belton micalex sockets are quite nice, and I use them in some applications as well, but so far not in a phono stage. Not practical to mess with them I'd guess.
 
#2,777
kevinkr said:
Hi Merlin,
Pull the 24 ga cardas litz out of the jacket and use nude from grid to RCA jack, that should reduce the capacitance by possibly as much as 30pF with a bit of care. (No sound quality hit)

The belton micalex sockets are quite nice, and I use them in some applications as well, but so far not in a phono stage. Not practical to mess with them I'd guess.
Click to expand...

I have to keep the shield also?

I will look for teflon, do you know a not expensive seller with good quality sockets?
 
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