Two Stroke Boost Switch Question

[Tremolux55]

Just finished building Dave Hunter's Two Stroke Circuit - basically a Princeton 5F2 circuit with some mods. Great little amp or not so little in my case!

Anyway my question is that when I engage the boost switch which brings a capacitor into the bias circuit of the 12AX7 I get a loud pop. Any ideas how I can reduce this or is it just a fact of life with this style of mod.

All halp and thoughts appreciated

[Enzo]

You mean a cathode bypass cap? Post a schematic or a link so we can see the circuit.

In most cases, the cathode bypass cap is switched into parallel with the cathode resistor. If this is what you have, the cap need s charging path to keep it charged while switched out. A 1 meg resistor should do. ANy resistor large enough that when placed in series with the cap, the cap essentially disappears. SO with the cap and resistor in series place them across the cathode resistor. Now to switch the cap in, have the switch short across the 1M resistor. In fact right now you could most likely clip a 1M rsistor across the switch right where it sits to see if it helps.

A lack of a charging path is most often the reason things pop when channel switching, feature switching, etc.

[Tremolux55]

Hi Enzo,

Thanks for the repsonse.

Yes that's correct it is a switching arrangement whereby there is either no capacitor, 0.047uf or 25uf in parallel with the 1.5K resistor. The two capacitors are hooked up to an on/off/on swtich so that one on switches in one cap, the off has neither cap in the circuit and the 2nd on has the other cap.

Just wanted to be clear on the 1M resistor and where it should be placed. Should it be placed between the cap and ground with the switch at that point, ie it goes to ground via the resistor until it is switched "on" and then it grounds out through the switch circuit being the path of least rersisitance.

I've tried to upload a circuit layout for reference with no luck. I'll try again later.

Much appreciated

Mat

[Tremolux55]

Hi Enzo,

Thanks for the suggestion. Got to try it on the weekend, worked well took the pop right away.

Cheers,

Mat

[6stringer]

I am unclear as to where exactly the 1M resistor goes in the switching circuit. I patched in a resistor to ground in several places with no resolution to the "pop". Can you
please instruct me as to the correct placement of the resistor?
Thanks for your help.

[Tremolux55]

Hi 6Stringer,

The resistor(s) go on the switch between the connection point where the wire from the capacitor goes and the connection point which then goes to earth. This is done for both capacitors. My understanding is that it allows a minute draw of current through the capacitor in question even if the switch is closed thus keeping the capacitor charged at all times. The pop you hear without this resistor is the capacitor charging up as the switch is opened.

Hope that helps.

[6stringer]

Thanks...so, you put a resistor on each side of the switch?
I noticed that when once I've charged the circuit by switching to one side or the other, the pop goes away on both sides. Is that how it works for you?
Thanks again for your reply