effects loop advice

[vaughandy]

I've been working on an old Ace Tone 601 that I've got up and running quite nicely. However, the one thing that I've been lamenting is a lack of an effects loop.

I really have no problem using a delay pedal at the guitar end of the amp, and the amp already has a reverb tank so I don't need to worry about inserting reverb effects later on. The real reason I want an effects loop is so I can throw my clean boost / EQ pedal directly in front of the power tubes to insure power tube distortion without pre-amp tube clipping.

I've been contemplating building a loop like the schematics out there for Marshals (i.e. Index of /new/schem/marshall/modnotes) But honestly, why do I need all this? I just want to be able to boost the signal right before the power tubes. Why can't I just break the circuit before the tubes and throw in a TRS jack? What's gonna happen?

Thanks! I don't claim to be an amp tech and am working with only a minimal knowledge of electronics theory, so maybe give me a break if this is a really dumb question.

[pdf64]

'insure power tube distortion without pre-amp tube clipping'
Any well designed amp should do that; it has to be like that, otherwise the amp won't realise it's maximum clean power output.
So if you keep your amp vol low, the boost should do what you want in front of the amp.
Got a schematic?
Pete.

[vaughandy]

Yeah, I'll enclose the schematic.

Hmm, I've heard differently. my understanding is that by boosting the signal at the front end with a clean boost pedal, you are almost certainly going to overdrive the initial preamp tubes, as they aren't designed to see such a hot signal initially. anyone want to confirm / deny this? I've always heard that this is exactly why you want to put boost pedals into the effects loop rather than the frontend of the amp.

[pdf64]

'by boosting the signal at the front end with a clean boost pedal, you are almost certainly going to overdrive the initial preamp tubes'
It all depends on how hot the pickups are, how much boost is on the pedal, and the headroom of the input stage. There's no voltages indicated on the schematic, so it's difficult to judge it's headroom.
'insure power tube distortion without pre-amp tube clipping'
OK, hit the front end hard enough and it will clip. But (unless there's blocking distortion in the preamp), the power tubes will contribute the most distortion and put their stamp on the tone, as they are last in the signal chain.
Thanks for the schematic.
The problem with a simple unbuffered 'fx insert' is that the high 'send' signal level will overdrive the fx, it's high impedance will roll off treble, and the low max output from a pedal may well be less than the max clean signal level that can the obtained.
So you would lose pre-amp headroom - the opposite of what was intended. Pete.