First 2-channel amp build

[John]

So, built a prototype harp amp for a fellow using 2 6L6's for output. He likes the way both the harp and acoustic sound through it, so I'm planning on doing 2 channels, both basically the same circuit but with the ind. t/s and volume controls. My question is, when playing through it with both harp and guitar simultaneously, there's no reason the audio signals will cancel each other out is there? I'm having them converge just before the PI of course.


My next question is, he won't overload the power tubes by doing so?


Thanks in advance!

[pompeiisneaks]

There would be two things to consider with your suggestion.  1. you'd just need some mixing resistors between the two channels merging at the PI, usually 220k on either side as they hit the PI.  2. Phase Cancellation should only happen if there are a mismatched number of gain stages from one channel to another.  I.e. if one goes through two triode stages and the other 3 AND they're the same signal.  I.e. jumpered with the same guitar input.  Otherwise, if they're two distinct tones, they shouldn't cancel out at all, as they're entirely different. 

Without an actual schematic, it's hard to say 100% but since you're saying both channels would be 100% identical, then you'd be fine, as they'd be in sync by the time they got to the PI.

~Phil

[pompeiisneaks]

Oh and for the second question, no it shouldn't overpower the power tubes in any specific way more than any other two channel amp design has.  The output of the preamp tubes, unless already overdriven, is designed by past amp designers to be at the right level for the PI.  Now if you decided to add 5 more gain stages, that may be a problem you'd have to look at.  Since you're likely reusing existing channel design, it should be just fine.

~Phil

[sluckey]

My question is, when playing through it with both harp and guitar simultaneously, there's no reason the audio signals will cancel each other out is there?

The two signals are totally unrelated to each other. There will be NO phase cancellation. The only time phase cancellation becomes an issue is when you send the ***same signal*** through two preamps and combine them later. That's not the case here.

My next question is, he won't overload the power tubes by doing so?

Not likely, unless he just gets crazy. You could easily 'overload' the power tubes with just the guitar or just the harp.

[PRR]

> with both harp and guitar simultaneously, there's no reason the audio signals will cancel

That's why there are NO recordings with both harp and guitar.

Oh, wait, there are! Beyond Bob Dylan, Gibson made a single instrument<g?>

The "issue" is prolly that when the guitar hits a big whang, slams the 6L6es, the weaker harp signal will be overwhelmed and lost. (Or vice-versa, depending on the harp.) If amps are not clipping, mixing is fine. If amps are clipping, two signals will mess with each other. The trend has been against having multiple inputs, each player has his own amp. But the Beach Boys got famous with a monster 4/8 input single amp (hardly clipped); maybe you can too.

[jjasilli]

Yes.  Note that the '60s Fender 2 channel amps were primarily for a perfomer to play an instrument & sing through one amp.

[John]

Thank you all. I didn't see why there'd be a huge problem, but so much easier to ask now.


Sluckey, thanks for the explanation about "why not". I had read something on here a while back about the cancellation, but couldn't remember what it was.


PRR, thanks for the chuckle! Chuck is a pretty good musician with a soft touch on his acoustic, and runs his harp through an old green bullet. I just "ass"umed he's a rack guy, but didn't think to ask. He might only be playing one at a time anyway.


Jjasilli, I had looked at a couple Fender schems, but I thought "meh, I'll ask". 


Pompeii, thanks for the mixing resistor comment. I'll be sure to look at those schems again and plagiarize.