Parallel/cascade switch for an 18 watter

[Silvio55]

Hello! Iīm building a 18 watt clone, the "lite" version (just the normal channel, no tremolo), this is my first building. I see this amp has the first gain stage in parallel, so I was wondering if itīs possible to put a switch for parallel/cascade modes, for more gain in cascade mode and the parallel mode for the orginal sound.
This is the Layout Iīm using:



Thanks!!

[tubeswell]

you need to start with a schematic before you com up with a layout. Here is an untested schematic for switchable parallel cascaded V1 triodes. Be keen to hear how you get on.

[Silvio55]

Thanks tubeswell! Of course I have the schematic, I just forgot to post it! here it is:


The 18 watt has the plate and cathode resitors shared in the first stage, do I have to split them to make this mod? Another thing, the parallel first stage on the 18 watt doesnīt have coupling caps between the plates, so can I run the CC after the switch so in parallel mode it wonīt be working?
What I want to achieve is the amp being exactly like the original in parallel mode.

[tubeswell]

The 18 watt has the plate and cathode resitors shared in the first stage, do I have to split them to make this mod?

If you want cascaded stages, you have to have a separate plate resistor (and coupling cap) for each stage. Ideally you also want a separate cathode resistor (/bypass cap) for each stage - because it you don't, you will likely get oscillation.

Another thing, the parallel first stage on the 18 watt doesnīt have coupling caps between the plates, so can I run the CC after the switch so in parallel mode it wonīt be working?

There are in fact two coupling caps after V1 in that 18Wer schematic, 10nF and 5nF.

In that schematic I posted, you ideally want the coupling cap before the switch to keep DC off the switch.

What I want to achieve is the amp being exactly like the original in parallel mode.

The schematic I posted won't be exactly like the 18Wer, because the parallel mode in the 18Wer uses the same plate resistor and same cathode resistor for both parallel stages, which doubles the current through the plate resistor resulting in bigger plate voltage swing. Whereas the 'parallel mode' in the schematic I posted is just two separate gain stages with the same grid signal and having the coupling caps joined at the AC end - and the only way to really get a tonal advantage in doing this, is to have separate Rk/Ck values for each triode to experiment with effects of blending different frequency roll-offs in 'parallel mode'.