2 inputs - cascade/single - possible?

[WRC34]

I know the Marshall JCM800 does something like this - single channel amp w/2 inputs where the "Hi" input cascades the two halves of the first 12AX7 and the "Lo" input is just a single half/triode - but I'm reading that it can only be done with the plastic Cliff jacks.

Is there a way to wire an amp like this using Switchcraft type 3 conductor jacks?

[martin manning]

https://tubeamparchive.com/viewtopic.ph ... 004#358004

...but nobody uses the low input.

[WRC34]

Thanks Martin!

Reading the thread helped me understand a bit, when you mentioned the "Hi" jack un-grounding the second triode...but I'm still a bit confused.

I built an amp recently with one dual triode feeding another as long tailed pair phase inverter. Originally the first tube (dual triode) was wired parallel but there wasn't enough gain to get any useful overdrive sounds from the amp, so I opted to cascade the two halves and ran into an issue that some lovely guys here helped me solve (Firestorm, JazzGuitarGimp and Littlewyan https://tubeamparchive.com/viewtopic.ph ... c&start=15)

The fix was to add a 1M resistor between the grid of the second triode to ground. Is the un-grounding you mention the equivalent of breaking this connection, and if so, how does the amp stay stable?

I know no one uses the Lo input on a JCM800, but I'd like to have a cleaner input option for some flexibility (amp wont be just for hard rock, and may even be used for acoustic instruments like mandolin, etc)

[xtian]

I do SO use the Low input. 😡

[JazzGuitarGimp]

I do SO use the Low input. 😡

Pay no attention, folks - he's got Jumbo-tron pickups!

[martin manning]

In the original schematic (using Cliff jacks) both the input and the output of the unused first stage are grounded when the low input is in use, where in my diagram only the input is grounded. Both stages always have a ground reference for their grids, so no problem there. I don't know but the first stage's output coupling cap might make a pop when it is reconnected. If it does, you could put a large resistor to ground, 4M7 say, across the low jack's switch and sleeve lugs. I'm interested to hear how it works if you try it.

[WRC34]

I will post updates as the build progresses!

[WRC34]

I finished the build! The friend I made it for wanted maximum clean headroom so I opted for parallel/single triode inputs as opposed to the cascade/single arrangement I started the thread about. Separate cathode bias resistors & bypass caps on each triode of the first 6SL7 (1.5K/1.6uf & 820R/25uf) The parallel input breaks up a bit when cranked and the single input sounds clean all the way. 6SL7 LTPI into two fixed bias 7581A Tung Sol Russian made re-issues. US made GE 5AR4 rectifier.

I still would like to implement the cascade/single set up, maybe on a prototype amp I build for myself where I won't be as concerned with 'making a mess'

Photos of the finished amp