loud pop with boost switch

[dscottguitars]

Hi,  I have a new preamp I'm working on for a bassman clone power amp.  Everything works great except for the boost, it pops really loudly on and off.  It's on the footswitch with a relay.  I am trying to add another tube stage for the boost, an idea a friend of mine had.  Is this something I can do w/o the pop noise?

Thanks,

Daniel

[sluckey]

I can't tell what you're trying to switch. Does V2B cathode really connect directly to ground?

[dscottguitars]

No... sorry that's a mistake in the drawing.  The cathode goes through the resistor to ground.  The boost switch: from the master to the relay, closed- it goes directly to the PI (phase inverter); or open- it goes to V2B, back to the relay to the PI.

[dscottguitars]

here's a correct drawing and showing the gain/lead boost a bit clearer - I hope...

[PRR]

TMI, also NEI. Can't even find "P1".

Needs bleed resistors.

[sluckey]

Can't even find "P1".

P1 is really PI.

[dscottguitars]

it's not P1, it's PI as in phase inverter, and it's not included.  As I stated it's a bassman clone with this preamp.  What are the 1meg resistors for?  The pot should be sufficient for the link to the tube and then straight to the phase inverter.

[dscottguitars]

Well, I can't figure out why this wouldn't work, so I'm going to move the boost circuit and put it between the input and channel circuits.  Any thoughts about where the pop comes from?

[PRR]

> What are the 1meg resistors for?

Go to any DIY Pedals forum, search "popping", lots of answers.

> The pot should be sufficient

When in "Lead". What's the situation when not in Lead?

Capacitor at B5 leaks. It may only leak like 1000Meg, but in not-Lead you have infinite Megs holding it to zero volts. Point B5 will, eventually, charge-up to 170V or more. Now when you switch, that 170V POPS, then eventually (though more quickly) drains-off through whatever resistace (IF any) is after "PI".

Tube V2-B grid leaks. Again like 100Meg. In stages like this, when NOT in Lead and NO bleed resistor, typically the grid leakage drifts the grid negative until it cuts-off the tube. Plate rises from 170V normal to 300V. When you switch in a grid resistance (or pot), the plate drops rapidly from 300V to 170V, POP!

Spend the 24 cents, try the resistors.

Moving things around is not the sure path. You may accidentally incorporate bleeders, but you won't understand the real difference.

[dscottguitars]

Thanks, that makes sense about where the popping comes from.  I'll try the resistors first, changing the path won't be easy...

[sluckey]

Take a look at how Doug switches a gain stage in this 18 watt Stout. Notice that the grid of the gain stage is grounded by the relay when the gain stage is switched off. I would still use the resistors that PRR suggested.

[dscottguitars]

I like the stout switch configuration.  I'm going to try it that way.  Here is the drawing...

[PRR]

> grid of the gain stage is grounded by the relay when the gain stage is switched off.

That works too. Ways to skin cats.

Note that he also has my 1Meg, except he used 470K. No real difference.

Walking in starlight I estimated the leakage resistance of a capacitor. Good caps leak less than 1uA per uFd. So 1uFd at 100V is like >100meg. You have 0.05uFd, so 20X higher, 2,000Meg. You probably have more like 200V, so 2X higher, 4,000Meg.

With 200V through a 4,000Meg leak into a 1Meg bleeder you have 0.05V. Most places in a tube amp, that's a small pop. Caps are better than they say, the leak voltage is smaller, usually no problem.

Your first plan showed "infinite" bleeder when not-Gained. 4,000Meg/infinity is the full 200V. It takes about 3 minutes to get most of the way there. Say your solo comes 2 minutes into the song, there's over 100V DC waiting at B5; compared to ~~1V signal. POP.

[dscottguitars]

Thanks guys, it works great now.  I decided to change the position of the gain tube anyway, much better distortion response at lower volumes this way.  I have a little bit of gain issues-too much yet even with the lead boost off and I'll be adding some resistors to the gain pots to equalize that.  Or I may change V1 to a 12 AU7 and V3 to a 12AV7 first to see if that works.  

Here is the full schematic...