Odd problem - using a combo amp as a cab

cadence · #1 07-30-2011, 02:37 PM

Hi folks.

I have a late 70's (foam grill) Poltone Minibrute IV (15"). My bass 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.

**B-string** · #2 07-30-2011, 03:15 PM

Lets see if I understand what you are doing. When you plug a seperate amp head into the workable combo at the ext. speaker jack on the workable combo it goesn't sound good? If I understood you, then you are loading the seperate head by not only the speaker but the good combo amps output section (this could also damage the workable combos amp output). The "dead" combo amp would present (or possibly present) no load on the seperate amp head.

fdeck · #3 07-30-2011, 03:15 PM

I think the EXT SPKR jack is an output, not an input. It is probably wired to connect the external speaker in series with the internal speaker. The external head is fighting with the innards of the Polytone amp.

cadence · #4 07-30-2011, 03:51 PM

Quote:

Originally Posted by B-string

Lets see if I understand what you are doing. When you plug a seperate amp head into the workable combo at the ext. speaker jack on the workable combo it goesn't sound good? If I understood you, then you are loading the seperate head by not only the speaker but the good combo amps output section (this could also damage the workable combos amp output). The "dead" combo amp would present (or possibly present) no load on the seperate amp head.

Yup, that's the scenario. That is interesting. I took out all of the internals on the old (dead) amp so it is just the jack and the speaker. On the working 15" amp, the power is not on and it is unplugged, but the internals are still there.

fdeck - yes, it does act as an output. It is wired right to the speaker though, so I would think it could work as an input too, just as it did on the smaller dead amp. I think you are correct though...so even if the amp is powered off and unplugged it will still be an issue? If that is the case I could remove the internals and just use it as a cabinet then?

nashvillebill · #5 07-30-2011, 04:05 PM

fdeck is correct, it will not also work as an input because the amp's output circuitry is also hooked to that jack. Even turned off and unplugged, the amp section is still there. Of course you could gut it also.

Or sell it and buy a cab alone...

**B-string** · #6 07-30-2011, 04:08 PM

Even without the working combos amp plugged in it will cause problems. Yes remove the working combos "internals" and it will be fine.

cadence · #7 07-30-2011, 07:16 PM

That is very good to know - thanks.

One more question, would it be possible to wire up that jack or another to act as an input so that I can use it as a cabinet? The built in amp is not stellar but all things considered I would rather keep it in there as a backup if at all possible.

**B-string** · #8 07-30-2011, 07:24 PM

Quote:

Originally Posted by cadence

That is very good to know - thanks.

One more question, would it be possible to wire up that jack or another to act as an input so that I can use it as a cabinet? The built in amp is not stellar but all things considered I would rather keep it in there as a backup if at all possible.

Yes you can. Neutrik NMJ4HC-S 1/4" Mono Phone Jack Switched use a jack like this to interupt the power amp lead from the combos amp unit. No plug the amp connects to the combo speaker, with plug the combos amp is cut off.

cadence · #9 07-30-2011, 07:53 PM

That looks perfect. In the mean time, I've just disconnected the wires that lead to the amp itself while leaving the wires from the jack connected. Works with no problems! I think I will put in that jack though, because not having to unscrew the back to reconnect the wires and use the combo amp would be nice.

Thanks again

**B-string** · #10 07-30-2011, 07:54 PM

My pleasure