using a polytone combo as a speaker cab, problem

rio · #1 07-30-2011, 03:40 PM

Hi folks.

I have a late 70's (foam grill) Poltone Minibrute IV (15"). My bass guitar runs fine through it in normal usage as a combo amp. On the back of this amp there is an "EXT SPKR" jack. On a smaller (12") non-working polytone, I took everything out of the amp except for the speaker and this jack is wired to the terminals of the speaker. I now use that Polytone as a speaker cab (for guitar).

Back to the problem. When I plug my BG into the Polytone (the 15") it is fine. However, when I plug an amp head (Henriksen Jazzamp Convertible) into the Polytone's EXT SPKR jack, it sounds fine at low practice volumes, but a mid-high range distortion appears at any real playable volume. The tone is great with the head/cab combo though, much better than just using the Polytone as a combo amp. I would hae thought it would have been fine, being that the combination worked with the smaller dead polytone amp and the same head.

Why is this happening? I am about to take apart the amp to try to figure it out but I would be curious if anyone has any ideas.

wizard3739 · #2 07-30-2011, 04:12 PM

That may be an output jack intended to go to an external speaker, NOT an input jack to the internal speaker. My old 15" minibute IV was built that way.

wiz

mattymel · #3 07-30-2011, 05:38 PM

yeah...Ext. Speaker outs are (at least on every amp i have ever owned) meant to be used for running another cab off of that amp. not the other way around. im surprised it works at all. even if it did, i wouldnt recommend it.

Dad3353 · #4 07-30-2011, 08:26 PM

Quote:

Originally Posted by rio

...On a smaller (12") non-working polytone, I took everything out of the amp except for the speaker and this jack is wired to the terminals of the speaker...

Good evening, rio...
The answer is in the question. Do the same to the 15" as you did to the 12" and you'll be fine (but you won't have a 15" combo any more...)
Hope this helps.

rio · #5 07-30-2011, 10:26 PM

Thanks everyone for the answers!

Dad3353 - I was totally about to do that. However, after opening the amp up, I realized that I could simply remove the wires coming from the internal amp while leaving the wires from the jack. Now it works perfectly as an input! And if/when I need the internal amp, I just open up the back and reconnect those 2 wires.

You are all correct that it is meant as an external speaker jack. The external speaker would be run in series, so the jack is wired directly to the speaker itself and nothing else, which is why I mistook it for an input.

In case anyone ends up in this thread in the future after a Google search or something looking for a similar reason, you can also install one of these - Neutrik NMJ4HC-S 1/4" Mono Phone Jack Switched (thanks "B-string" from the talkbass forums) - that way you can have the internal amp still wired to the speaker as well as have it useable as an extension cabinet.