Heater elevation behaviour, and 6V6 plexi noise woes

[liddokun]

Hello all,

Currently working through a 6v6 Plexi build that I finished recently.  I am working off the schematic posted by Sluckey, though I am using a slightly larger chassis so opted to make my own layout. 

The amp fires up and sounds lovely, voltages are roughly similar to the ones posted in the Sluckey build, though a bit higher because of the HT coming in from the old tube PA transformer I used.

There is hum in the preamp section.  I've pretty much narrowed it down to the V2a.  It sounds like noise is being picked up via the mix resistors/bright cap.  I have it wired up similar to the sluckey and hoffman layout, with a terminal attached to the back of a pot, the 2 resistors+cap going to the terminal, then a shielded wire from that terminal to the grid of V2a.  It's not really noticeable until the MV is half way up, touching either of the mix resistors or bright cap significantly makes it worse (pretty suce my body is an antenna).  Any suggestions on how to quell this? 

Secondly, a question about DC elevated filaments.  I'm using an artificial center tap via 2 120R pots, connected to about 56vdc from a voltage divider.  Without anything connected to the voltage divider, I can measure 56vdc fine, but as soon as I hook up the artificial center tap, that measurement drops to about 100mv.  I have no idea what causes this behaviour, hoping someone can shed some light on this for me.

[liddokun]

Here are some photos of the guts, for reference.  Don't mind the inputs at V1, I was testing out grid blocker resistor values.

[Latole]

I would shorten these wires at the two resistor at V1.
Long wire may act like antenna to noise

[JB]

I would shorten these wires at the two resistor at V1.
Long wire may act like antenna to noise

Here are some photos of the guts, for reference.  Don't mind the inputs at V1, I was testing out grid blocker resistor values.

[liddokun]

I would shorten these wires at the two resistor at V1.
Long wire may act like antenna to noise

As JB has pointed out, I do intend to shorten this when I finalize my grid blocker resistor value. Though, even as is, it is not contributing to my noise. 

I've all but isolated it to V2a; if I pull V2 the amp is completely silent.  The mix resistors+parallel cap seem to be acting as an antenna and then going into the grid. I'm going to try grounding the input to the grid of V2a and see if V2 is inducing noise anywhere else.  To be clear, the noise I'm concerned about is hum.
I don't know if moving the mix resistors+cap location in the layout will make a difference.  I know many people have built both the hoffman layout and sluckey layout and it lays these parts out similarly.  Putting the chassis on top of a sheet of aluminum does not change the issue. 

There's also hiss but this is only with both volumes fully open, which I presume is normal. 

[sluckey]

There is hum in the preamp section.

Does the hum disappear when the normal and/or bright volume controls are set to zero?

Secondly, a question about DC elevated filaments.  I'm using an artificial center tap via 2 120R pots, connected to about 56vdc from a voltage divider.  Without anything connected to the voltage divider, I can measure 56vdc fine, but as soon as I hook up the artificial center tap, that measurement drops to about 100mv.  I have no idea what causes this behaviour, hoping someone can shed some light on this for me.

This sounds like your old PT already has a heater center tap that is internally connected to the HT center tap (red/yel) wire. That was quite common back in the old days. To verify if this is the case simply measure resistance from a 6V6 pin 2 and/or 7 to chassis. If you measure a low resistance then you already have a hard wired center tap and will not be able to use your elevated circuit.

[liddokun]

Does the hum disappear when the normal and/or bright volume controls are set to zero?

The hum does not disappear when either/both of those volumes are set to zero. 

This sounds like your old PT already has a heater center tap that is internally connected to the HT center tap (red/yel) wire. That was quite common back in the old days. To verify if this is the case simply measure resistance from a 6V6 pin 2 and/or 7 to chassis. If you measure a low resistance then you already have a hard wired center tap and will not be able to use your elevated circuit.

This is really helpful to know.  I will take a measurement tonight to confirm this. 

[liddokun]

Updates:

Pins 2 and 7 of the 6v6 measure 1.5 ohm to chassis, indeed there is a built in CT for filaments. 


I relocated the mix resistors and cap closer to the tube socket. It seemed to help a little. But the noise still remains;  Again, pulling V2 completely eliminates the noise. I've run the circuit off DC heaters with no change, so I've ruled out filament induced noise. 

I recorded some clips of the noise in case it's helpful to pinpoint it better.  Noise with master volume at full, and both channel volumes at zero, then noise with all volumes full.

[sluckey]

I'm going to try grounding the input to the grid of V2a

What were the results?

[tubeswell]

And you swapped V2?

[liddokun]

I'm going to try grounding the input to the grid of V2a

What were the results?

The noise gets exponentially louder when I ground V2a grid.

And you swapped V2?

Yes, I've tried several different known quiet tubes in V2. 

[liddokun]

In my new layout I'm ignoring one of the 20uf caps in my multicap can and relocated that cap on the board instead. 

I swapped out the preamp filter cap and the filter cap directly filtering the B+ going to V2 with new ones.  No dice. 

I did ground the V2a grid again, I think earlier I wasn't making great contact with the alligator clip.  Properly grounding the V2a grid cuts a significant amount of hum, mostly mid and high frequences, though a hum remains.  I recorded more sounds to illustrate.