Preamp Idea, Relay question

[Hellawatt]

Hi,

Im planning on either converting my Deville, or building a new head. I want something like a 2204 or Hoffman plexi hot switch. Im thinking about using a relay to make it act like a two channel amp. I want two gains and two masters so I can go from clean to dirty with a footswitch.

Ive attached a schematic of my idea. Would this work? Or would the 'disconnected' volume pot still be draining signal to ground? Would I have popping issues?

Thanks

[RicharD]

Your 2 pots are in parallel so the preceding stage will see a 500k load, not 1M.   Design for a 500k load or use 2M pots.   Pop will be determined by how drastic the change is combined with the quality of the relays.  I'd add a high value grid leak on V1-B so it has reference during that split second between switching states.

[92Volts]

Your 2 pots are in parallel so the preceding stage will see a 500k load, not 1M.   Design for a 500k load or use 2M pots.   Pop will be determined by how drastic the change is combined with the quality of the relays.  I'd add a high value grid leak on V1-B so it has reference during that split second between switching states.

A 500k load is easy enough to drive. If this circuit is otherwise designed for a single 1M pot, just double the 0.022uf cap (0.047 is close enough). A grid leak on V1-B is good advice. I'd use a very large value to avoid changing the behavior of the pots.

I've found out the hard way there is capacitance within switches and relays. If you have one volume set high, and you're connected to another that's very low, you'll hear high frequency sound bleeding from the other "channel". This is noticeable enough that production amps usually have fully separate channels, and actually mute inactive channels. If you never expect to use the 2 settings at low/max, just as a small boost for a guitar solo or something, your method is probably fine.

[Hellawatt]

If you have one volume set high, and you're connected to another that's very low, you'll hear high frequency sound bleeding from the other "channel". This is noticeable enough that production amps usually have fully separate channels, and actually mute inactive channels. If you never expect to use the 2 settings at low/max, just as a small boost for a guitar solo or something, your method is probably fine.

That might be a problem. I will definitely be driving this amp hard, with both channels at similar volume. I probably would run one master all the way up for clean and on the other channel I would have the gain more than half way up. I would be using this in a very loud practice space with a drummer and another guitarist.

If noise bleed was a problem, I guess I could put another switch in front of each volume, so that it will be completely out of the circuit when unused. But then I would need a second relay right? Is that worth it or should I just try this first and see if it works?

[Hellawatt]

Thanks for the help guys, I've attached an updated schematic for a preamp with one relay with 0.047 coupling caps and grid stopper on V1B, and another schematic with two relays and grid stopper. Do these look OK?

[PRR]

Looks OK.

> grid stopper...

Those are grid leaks. When electrons leak from the grid grid-leaks carry the drips to ground (or bias). With few exceptions you *always* need a grid-leak (sometimes a pot or other path will do).

Grid stoppers are *in series* with grid. When you try to shove 10 pounds of potatoes in a 1 pound sack, this puts some limit on the mess/mash. Or like a shower restrictor.