ECC88 mic pre - thump problem

[ghiatorino]

Hi guys,
i've built a tube mic pre with an EF86 and a cascode ECC88 output stage.
I added a control over the 2 cathodes (EF86 and lower ECC88 cascode triode) to change simultaneously the bias point of the tubes, to get different sounds. A rotary switch selects between 2 Led bias and Res+Cap.
This works as i am expecting, except from a big thump in audio while turning the switch.
part of the schematic in attachment.
I tried to leave a big cap always connected from cathode to ground with no result.
Can you suggest a way to eliminate this?
Cheers,
T

 

[abbey road d enfer]

Hi guys,
i've built a tube mic pre with an EF86 and a cascode ECC88 output stage.
I added a control over the 2 cathodes (EF86 and lower ECC88 cascode triode) to change simultaneously the bias point of the tubes, to get different sounds. A rotary switch selects between 2 Led bias and Res+Cap.
This works as i am expecting, except from a big thump in audio while turning the switch.
part of the schematic in attachment.
I tried to leave a big cap always connected from cathode to ground with no result.
Can you suggest a way to eliminate this?
Cheers,
T

I would think it's impossible, since you're brutally changing the idle current. A MBB switch may attenuate the problem, but the only solution would be to find a way to transition smoothly from one cathode load to another; this may be done with a potentiometer, but I don't know of 3-way potentiometers. I you were dealing with higher impedances, you may use photoresistors, but here it wouldn't work.

 

[outoftune]

you might have luck rigging up some sort of timing based mute / relay with a 555 chip, i know some tube units do this to delay the audio output on power up while the tubes warm up

 

[Audio1Man]

The simple way is to add a "MUTE SWITCH".  Mute, Change mode , Un mute.  The MBB switch may still have some type of pop.

 

[walter]

I'm not sure if  putting the switch at the ground side would change anything, something to try. How about a  high value resistor from cathode to ground, or 3 resistors across each switch contact. Maybe put a 1M pot across and dial it in.

 

[ruffrecords]

I'm not sure if  putting the switch at the ground side would change anything, something to try. How about a  high value resistor from cathode to ground, or 3 resistors across each switch contact. Maybe put a 1M pot across and dial it in.

The are two effects here that can cause a pop. One is capacitors no charged up when the switch is operated. The 'standard' way to fix this is to use high value bleed resistors across adjacent contacts - large enough no to upset the required operation but small enough to trick charge the caps top the same value.

If all you were doing was changing caps then this would work. But in this case the dc operating conditions are also being changed and the bleed resistors will not help with this.

Cheers

Ian

 

[ghiatorino]

Thanks for helping,

A MBB switch may attenuate the problem

this is the first thing i changed (from BBM) and this helped a little bit indeed

you might have luck rigging up some sort of timing based mute / relay with a 555 chip, i know some tube units do this to delay the audio output on power up while the tubes warm up

This is one solution i was thinking too, saw recently in a Pendulum OCL psu stage for soft start

If all you were doing was changing caps then this would work. But in this case the dc operating conditions are also being changed and the bleed resistors will not help with this.

i am changing between 3 bias positions: Red Led, Germanium diode and a resistor, while a 220uF cap is always connected from cathode to ground.
Another thing i'm the process to try is to add a 100K resistor fixed, from cathode to gnd. Will post results, thanks again everybody