Anyone uses a switch turn remove the third gain stage?

[riscado]

Does anyone use a switch to be able to switch the third gain stage in and out of the circuit on a wreck type amp?

I'm thinking about including one, for blues gigs... any thoughts on this!

[mlp-mx6]

The Kxmet style fast/gradual switch is good for this purpose.

[riscado]

Hi mlp-mx6,

I realize that is a good option, but I actually am interested in knowing wether any one has had good results leaving the third gain stage out of the equation and switching it back in if needed.

I know it will sound nicely without the third gain stage, what I'd like to know is if the extra switch will give me oscilation problems, since the express lead wiring/layout is a little touchy.

I meant something along these lines...

[Bob-I]

There are a few things I'd change.

First off, ground the grid of the input on the unused gain stage to prevent to from runaway oscillation. Second put a coupler cap in before the output side of the switch, other wise you'll be switching 200+ volts.

Otherwise I thing it's doable.

[riscado]

isn't the grid referenced to ground alread on the gainstage?

I'll add the coupling thanks

[Trace]

Does anyone use a switch to be able to switch the third gain stage in and out of the circuit on a wreck type amp? I'm thinking about including one, for blues gigs... any thoughts on this!

If an Express is set up properly (IE: right values and voltages) a player can lower the volume control on the guitar to attain a clean tone as well as lesser amonts of overdrive and this is a very effective technique but to address your question, yes you can use a switch to bypass the third stage.

Upon reviewing your schematic there are a couple of things you may want to take into consideration before implementing the switching that is shown in your schematic. When you are bypassing the third stage you have a .022uF (coupling cap from second stage) in series with the .1uF cap going into the grid of the phase inverter, which are subtracting from one another (unless that is the desired effect).

As the Express was designed/voiced around being a three gain stage amp I would presume that the cap values you are using in this schematic were selected around the three gain stage voicing. If that is the case you could redesign the switching method so that both “modes” give you the best of both worlds with the least amount of tonal compromises.

The other consideration is you have the high voltage/B+ of V2A on a contact of the switch. One alternative would be to employ the use of a SPST mini toggle switch in place of a DPDT. You could incorporate a DPDT switch and use one side to ground the grid of V2A when the third gain stage is bypassed. You can email me if you like and I can send you a schematic with one or both options.

I hope this makes sense and if not I apologize in advance, I am recovering from a cold and I also had the first stage of a root canal today .


Happy holidays
Trace

[riscado]

trace, thank...

I sent a PM.

[Trace]

trace, thank... I sent a PM.

Both versions of the schematic have been emailed


Happy holidays
Trace

[lastwinj]

there is another way, but basically the same. you really shouldnt switch hot b+ at any time. bad design, IMO. and usually VERy noisy.

try the attached, caps values not optimized. 1st is basically how you have it, but shouldnt pop much. 2nd is the ideal way, and allows you to adjust each switched section for response, but modifying the values of the caps. remember, caps in series are basically like parallel resistors for figuring value (u can use a parallel resistor calc), so if you want .1uf, go with 2 .2uf in series, etc.

also, no worries about grounding the grid of the triode when not used. it is barely running. no runaway oscillations will occur. trust me. i never ground the grids in my high gain design 2 channel for the clean channel. nary a peep of noise from the clean side triodes.

germ

[funkmeblue]

it won't be phase coherent....

[txbluesboy]

lastwinj, In your second diagram, is the second 1uf cap replacing the .1uf cap leading into the phase inverter, or is this feeding the .1uf?

[Andyjoe]

Is there any way to put a pot for the gain and bypass it with a switch?
That way you can adjust to taist and bypass it for more gain.

Andy

[novosibir]

Is there any way to put a pot for the gain and bypass it with a switch?

You probably will run into serious RF oscillation problems, when you're messing with the Pot's wiper/tube's input grid line!

There Ken has placed a grid stopper sometimes, and sometimes not - what says, that this area already is hard on the border to RF and wouldn't accept additional cables IMO.

Larry

[lastwinj]

lastwinj, In your second diagram, is the second 1uf cap replacing the .1uf cap leading into the phase inverter, or is this feeding the .1uf?

yes it is replacing it. remember, caps in series are just like parallel resistors. 1.5k with a 10k acros it will be lower than 1/5k. a .022uf (or whatever) in series with a 1uf, with be lower than .022uf. why i chose that value. wont affect a smaller cap as much.

germ