Amp tone help

[gordon]

I found my amp at a garage sale and it seems to lacking bass. When I turn up the treble nob, it seemingly cuts bass frequencies. Anyone know whats up?

[Jazz P Bass]

Can you supply the make & model of the amp?
Lack of bass response can be attributed a number of things.
A small speaker, an intentional design (ie: low powered amp) or maybe a problem with the amp.

[gordon]

It is a Randall RT-30 (supposed to be a knock off of an ac30). I had some work done on it to fix a problem when it kept blowing fuses and when I got it back I seemed to lack bass frequencies and accentuate high end. I had a buddy tell me that this could be a circuit that was put on backwards? I really don't have much knowledge of circuitry or anything but maybe you have an idea.

[olddawg]

It is a Randall RT-30 (supposed to be a knock off of an ac30). I had some work done on it to fix a problem when it kept blowing fuses and when I got it back I seemed to lack bass frequencies and accentuate high end. I had a buddy tell me that this could be a circuit that was put on backwards? I really don't have much knowledge of circuitry or anything but maybe you have an idea.

If you just had it repaired and you have a problem with lack of bass response, maybe you should go back and ask the tech someting like,"Why does my amp lack bass response after your repair?" It could be a lot of things. Commonly, aging filter capacitors in the power supply can cause this, but it could be a lot of things including what you are driving it with.

[gordon]

Yeah the problem is I'm back home from college so I need to find someone new. Typically though, turning up the treble nob shouldn't cut other frequencies, correct? When I turn up the treble, the bass gets lost and the volume drops.

[Enzo]

It has two speakers? What do you want to bet they are wired out of phase?

[gordon]

I know that the wiring was changed when I took it in. Would that explain the odd functionality of the tone knobs?

[Enzo]

Sure, why not?

it only takes a couple seconds to check.

[jmaf]

As Enzo said, losing bass when you turn it up is classic phase inversion. But without hearing how exactly that bass is attenuated it's hard to tell. Checking phase is a matter of visual inspection, 2 minutes...

Some loose ideas:
Lower notes demand more current from the amp, the filter caps may not be holding up - common issue in older amps.
Other capacitors to look for problems are the tone stack caps, check for DC voltage on the tone stack pot terminals, there should be none.
Reversed wiring on the bass pot is also common.

[gordon]

I believe it is wired in series. A quarter inch is plugged into the "Int" speaker slot. From the quarter inch there are 2 wires, one black and one white. The white one is connected to the positive on Speaker 1 and the black is connected to the negative on speaker 1. Speaker 1's negative is connected to Speaker 2's positive and Speaker 1's positive is connected to Speaker 1's negative. Is the battery test the best way to check if they are in phase?

[gordon]

One speaker goes out, one goes in. That would be a phase issue correct?

[g1]

You are correct, the speakers are out of phase. The way you have described is parallel wiring but the phase is wrong. Speaker 1 pos. should go to spkr.2 pos. and the 2 negs. should connect to eachother.

[gordon]

Alright, thanks a lot!

[gordon]

I fixed the wiring and it sounds amazing now! But here's the interesting development, there is a rather strong (electrical?) smell that comes from the back of the amp above the tubes. I have 2 8 ohm speakers and I have the load selector set 4 ohms. I'm assuming this is probably a bad thing and not the smell of a working amp? Once I fix one problem on this amp another one arises...

[jmaf]

There are many possibilities for the smell, not all are entirely bad.

If the power tubes are brand new, the paint on the tube smells for a while during the first uses - no worries there. Sovtek tubes for example are coming with a big red silk screened logo nowadays, the paint toasts and smells for a while.

If the BIAS is wrong, you'll smell burning plastic from the bases of the power tubes working overheated - this is not good.

If the sockets are loose and a tube is arcing it'll burn the plastic socket and eventually cause a more serious problem. But you'd hear pops and crackle. Does smell bad too.

If the amp was stored in a garage or something, there could be a dead bug(literally, an insect) in the amp.

Open it up and run it with the chassis open side up, have a look, the source of the smell might be obvious. If it's coming from either one of the transformers, that's the worst news....I hope not.