Dual PI's utilizing one PT?

[Decko]

Hi Folks,

I built two express preamps and included the PI on each. I connected them both to the same power section. I don't run them at the same time but manually toggle between the two using a switch. When I do this I get a nice pop sound.

Is it possible to toggle between two PI's that share the same OT using a relay without getting noise/pop?

Cheers!
Decko

[Leo_Gnardo]

I'm thinkin' you have a DC level building up on the switch contact from the unused amp section. By using a resistor you can equalize the charge on that contact so there's little or no racket when you throw the switch. Sure would be nice to have a schemo of your switching circuit then we can show you where to put that resistor. Its value will probably be a couple megohms.

[Stevem]

May be install a duel gang 1 to 2 meg log taper pot, this would let you blend the two or mix them.

[Decko]

I'm thinkin' you have a DC level building up on the switch contact from the unused amp section. By using a resistor you can equalize the charge on that contact so there's little or no racket when you throw the switch. Sure would be nice to have a schemo of your switching circuit then we can show you where to put that resistor. Its value will probably be a couple megohms.

Hi Leo, please see schematic. Keep in mind that both pre-amp sections have post phase master volumes which are not shown. I am using a double pole double throw switch.

Please note that I am experimenting with different topologies.

May be install a duel gang 1 to 2 meg log taper pot, this would let you blend the two or mix them.

Hi Steve, that would be the next step. Great idea!

Thanks for the feedback!

Regards,
Decko

[gui_tarzan]

The circuits look identical, why would you switch between them into the same power amp?

[Decko]

The circuits look identical, why would you switch between them into the same power amp?

This is pure experimentation so I am not sure if this is going to work.

(Keep in mind that I have PPIMV's and pre-master volumes on both channels.)

What I am planning to do is:

Set Express preamp 1 to high gain.

Set Express preamp 2 to lower gain for more clean.

The thought is channel switch between the two. I want to play with dynamics on the two PI's.

Decko

[Leo_Gnardo]

I'm thinkin' you have a DC level building up on the switch contact from the unused amp section. By using a resistor you can equalize the charge on that contact so there's little or no racket when you throw the switch. Sure would be nice to have a schemo of your switching circuit then we can show you where to put that resistor. Its value will probably be a couple megohms.

Hi Leo, please see schematic. Keep in mind that both pre-amp sections have post phase master volumes which are not shown. I am using a double pole double throw switch.

Please note that I am experimenting with different topologies.

Yikes, as you throw the switch, for an instant your output tube grids are attached to - nothing. No wonder there's a noise. Quick 'n' dirty solution, solder a resistor across each of the connections the switch makes & breaks. What value, I'd start with 220K and work up from there. That way your output tube grids will always have some connection to bias voltage - even if not ideal - as you throw the switch. Tack on a couple R's and see if that doesn't calm it down, should take 3 minutes.

[Decko]

I'm thinkin' you have a DC level building up on the switch contact from the unused amp section. By using a resistor you can equalize the charge on that contact so there's little or no racket when you throw the switch. Sure would be nice to have a schemo of your switching circuit then we can show you where to put that resistor. Its value will probably be a couple megohms.

Hi Leo, please see schematic. Keep in mind that both pre-amp sections have post phase master volumes which are not shown. I am using a double pole double throw switch.

Please note that I am experimenting with different topologies.

Yikes, as you throw the switch, for an instant your output tube grids are attached to - nothing. No wonder there's a noise. Quick 'n' dirty solution, solder a resistor across each of the connections the switch makes & breaks. What value, I'd start with 220K and work up from there. That way your output tube grids will always have some connection to bias voltage - even if not ideal - as you throw the switch. Tack on a couple R's and see if that doesn't calm it down, should take 3 minutes.

Thanks for your response Leo. I will try this.
Is there a more elegant design to this endeavor or is this completely crazy?

Decko

[Stevem]

The blend pot thing I posted about will not work if one setting of the preamp will have a unequal amount of gain stages as then you will have two out of phase signals you are trying to blend!

[Smokebreak]

Is there a more elegant design to this endeavor or is this completely crazy?

Decko

You could have 2 switchable volumes, linked to trigger 2 PPIMVs.

[Garthhog]

Move the grid stoppers and grid bias supply to the power tube side of the switch (then you don't need a 2nd set). Also, tie 220k resistors to the 0.022uF caps of the PIs to ensure they still "see" ground when not is use (shouldn't pop this way). I think you may need to ties them to the Bias supply. You may need to re-evaluate the bias resistor values as the extra 220k resistors to ground makes a voltage divider and your fixed bias power amp would need an adjustment. Alternatively you could go with a cathode bias power amp.

This was my 1st idea and there may be issues with implementation that I didn't consider... (Translation: I might be full of BS and need someone to perform a rectal-cranial extraction).

[Decko]

Move the grid stoppers and grid bias supply to the power tube side of the switch (then you don't need a 2nd set). Also, tie 220k resistors to the 0.022uF caps of the PIs to ensure they still "see" ground when not is use (shouldn't pop this way). I think you may need to ties them to the Bias supply. You may need to re-evaluate the bias resistor values as the extra 220k resistors to ground makes a voltage divider and your fixed bias power amp would need an adjustment. Alternatively you could go with a cathode bias power amp.

This was my 1st idea and there may be issues with implementation that I didn't consider... (Translation: I might be full of BS and need someone to perform a rectal-cranial extraction).

Hi Garthog, I'll digest your post as well as what Leo, Smokebreak and Steve has posted. First I will draw it out then experiment on the breadboard. I look forward to the implemtation!

The image is my Naked Amp Breadboard. You will notice both channels which are laid out back to back for ease of wiring. Not the final version but for testing circuits. Underneath the setup resides the Power tubes and transformers. You will also see the switch that I installed to toggle PI's.

Decko