Bass Practice Amp Question

[Paleo Pete]

Howdy guys, I have a question or two about a small bass amp a friend dropped off for me to look at. The amp is fixed, or 99% fixed anyway, it's a Dean Markley K-20BX solid state practice amp for bass, 26 watts according to their specs, one of the transistors (Q5) had cold solder joints (apparently from the factory, they looked like they had never been re-soldered), two legs were loose and moving around, re-soldered them and it works fine. I've only played it at low volume with a guitar, can't crank it around here, but it sounds good and seems to work fine, I played it for 2 hours a few days ago and left it turned on another 3 or 4 hours so it should hold up pretty well.

However...

Two things bug me, both probably in the "it works don't fix it" category, so I'd like some opinions. Schematic Here

The volume pot (VR 1; B50K) was replaced before I got it, with a 50k Audio pot, the schematic (and circuit board) are marked 50k Linear. Should this be replaced with a proper pot, or will the Audio pot work fine? I think replace it, to get it exactly right but not sure.

The two 5 watt .33 ohm resistors (R26, R27) both test around .5 ohm or a bit over, when isolated from the circuit, will those be OK or should they be replaced? I'm thinking it works, don't fix it on this one...that may not be far enough out of tolerance to cause trouble. And it works and sounds good, no distortion at all, tone controls work, it has to be plenty loud, I can only turn it to about 3 and I'm afraid the neighbor will hear it (this place has really thin walls, I can hear dogs a block away bark anytime day or night, I have to play really quiet.) You can hear my Fender Champ next door if I crank it.

Otherwise, everything looks good, I found the main problem while trying to check the transistor, and realized at the same time I had forgotten the chopstick test, which should have been the first thing I did...

And while I'm at it, thanks for all the help on my Ibanez SD 9 thread, if you haven't checked, I found out it was the AC adapter all along, not the pedal, and since I use that same one all the time and it runs all the other pedals just fine through a power splitter I never suspected it. I'm now using the other (backup) wall wart and just got another that works for a backup, the one causing the problem won't be used at all any more.

[Alex R]

If the pot gives good control over the volume I'd leave it. A linear pot will in most if not all situations bring up the volume sooner on the dial, that's all.

Those emitter resistors are in fact probably 0.33 - unless you have a really specialised meter most values under 1 ohm are a bit approximate, this being such a low value that the resistances of the solder, wire, probes etc start to matter.

I'd leave both and go catch a crawfish!

[Paleo Pete]

Hehe, you saw that huh?

Thanks, that's pretty much what I thought about the pot, figured my cheapo meter might not be all that close, but didn't take into consideration the low resistance and the materials possibly affecting it too. My train of thought was if it works, don't fix it. I haven't been able to crank it and see what the volume pot does, but I'm not expecting it to be much of a problem. Have to wait and see I guess.

Thanks.

[Alex R]

Oh it'll be the same cranked, the only difference is on the way up, the linear volume comes up faster.

[Enzo]

All the way up and all the way down will be the same. The middle setting will be different. If you can get useful volume settings with the existing control, what else do you need?

0.33 ohm resistors are just 1/3 of an ohm. Your meter is measuring that plus the resistance of your probe wires, PLUS the resistance of the probe tip contact to the resistor . I expect such readings top be a little high. When you are reading 100k ohms or something, your probes adding a tenth of an ohm will not be noticed. And the tip of your probe adds anothe tenth of an ophm or so trying to make contact on the resistor wire. Again at 100k or 10k or whatever, it doesn't matter. But when you are measuring only three tenths of an ohm to start with, that same couple of tenths added makes the reading look way off.

Those wire wound resistors will for the most part weither be burnt open or they are OK and close enough. In other words, just having continuity is close enough.

[Paleo Pete]

the linear volume comes up faster

That's what I thought, but wasn't positive it wouldn't cause trouble.

Enzo: I understood about the probe tip and so forth affecting readings at really low resistance from the earlier post, but thanks for a more in depth response and I like your comparison between very low and very high resistance. I just didn't take that into consideration when I questioned the readings originally. I did figure on my cheap meter to not be extremely accurate at that low a reading, but didn't take into account the differences the probes and wires would make. The meter probably is about the same, at higher levels it might be close enough to be negligible but at very low readings it's probably not that precise at all.

I always wondered if the probes and wires would make a difference at tiny readings though. And your skin too, if you're holding a component with your fingers would they affect it? I suspect so at fractions of an ohm.