Help with simple amp question, please?

[markeebee]

OK, I know this isn't a stompbox, but I was hoping somebody here could help me anyway.

I'm building an amp for a friend.  He wants a real low volume valve amp for home use, but he also wants it to be loud enough to use on small gigs.  Probably wants it to make breakfast as well.

So I thought I'd go with this simple 3W ECL86 design for low volume use, and then bypass the pentode in favour of a TDA something something something chip amp when he wants it louder:



The bits in red are where I was thinking of putting some kind of ganged switch TO ONLY EVER BE OPERATED WITH THE POWER OFF.  I'm having a bit of a brain blindspot - can anybody think of a better way, because there surely must be one?

Thanks!

[jacobyjd]

It's always a little hairy to add conditional switches to something you're building for someone else--it makes you just like the technician who installs the 'Do not press this button!' button.

What about using an extra pole or two to switch in a dummy load? you wouldn't have to qualify when the chip/tube switch should be used

[markeebee]

Yeah, good point Josh.  I was thinking of putting an L-pad on the output anyway, so that it could go REALLY quiet, so maybe a could rig up a rotary switch that always connects the L-pad as a dummy load before the pre-amp output is transferred over.

Also, I was thinking that the switch knob should have a long paddle and be situated such that you have to withdraw the mains connector in order to operate the switch, thusly:

[jacobyjd]

Yeah, good point Josh.  I was thinking of putting an L-pad on the output anyway, so that it could go REALLY quiet, so maybe a could rig up a rotary switch that always connects the L-pad as a dummy load before the pre-amp output is transferred over.

Also, I was thinking that the switch knob should have a long paddle and be situated such that you have to withdraw the mains connector in order to operate the switch, thusly:

Not a very good friend, huh?

lol...having to pull the mains is a good idea--it would just make me angry though. Take away my ability to A/B something without unplugging it, and it starts.

Here's the progression:

"Ah! A switch! How convenient! Like 2 amps in one!

...

What!?

MOTHER@(#*&^! 

WHERE'S MY DREMEL!?"

...which would defeat the purpose.


If you add the L-pad, would the chipamp be necessary? That sounds like a solution right there

[markeebee]

Oh wait......listen....

*PPPPSSSSSHHHHHHTTTTTTTTT!!!*

That's the sound of a whole other can of worms being opened.

Even though the output of the valve section is only 3W, it's still surprisingly loud.  The L-pad is to reduce the output to "the baby's asleep" levels.  The idea of the chip amp is to give, I dunno, 30W or something so that he can play small gigs and compete with a drummer.  I'm guessing that after the first few minutes of enthusiastic A/B-ing, the switch won't be used that much.

The can of worms - I'm not sure how much "louder" a 30W chip amp would be compared to the ECL86.  I guess it would be kind of interesting to find out, but I reckon I'm probably going to put myself through a bunch of shag and hassle for no real return.  And that, children, is how we do science.

Thanks again, J.

[jacobyjd]

Ah, I was thinking that the tube amp portion would be for gig use, and the chip would be for the...ahem...bedroom.

That makes more sense now. I'd say go for it. Even if you can't do the mains-unplug thing, you could just use a switch with a cover, so when you A/B, it would be like launching a rocket!

[bancika]

I would do something different - leave the power amp in the circuit at all times, it contributes to the overall sound of the amp so it's cool to have it. If you want to use another power amp just add a line out jack, put a dummy load resistor on the output jack (you need a switching jack to do that). That way, if you want your pentode to drive the speaker just plug it in speaker out. If you want to re-amp the signal, plug in the line out and leave speaker jack unplugged.
Search for firezog circuit on ax84.com, it's just two resistors and a pot...

Cheers,
Bane

[markeebee]

Thanks Bane.  That would be a cool solution, except that this is a combo amp and I want to use the same speaker for either amp.  If I use the line out, it means that the guy will have to plug the input of the chipamp to the line out, then unplug the speaker from the valve output and replug into the chipamp output.

Actually, that's not difficult, is it? 

[jacobyjd]

huh...there you go! If you can't switch, patch.

[markeebee]

Hey Josh - I'm going to see the Great Lake Swimmers tonight.  Poetic love rock?