90's Amp + External Cab

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OldSoul
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90's Amp + External Cab

Post by OldSoul » Fri Apr 28, 2017 9:44 am

Hello crew! Question for all you tech-ohms-wattage heads..........

The amp in question is a Peavey Classic Chorus 212 which has no external-speaker jack. Is it possible to run wires off the internal 212 speakers to an external 212 or 112 cabinet without overloading or damaging the amp?

Classic30inCincy
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Re: 90's Amp + External Cab

Post by Classic30inCincy » Sat Apr 29, 2017 11:32 pm

I would NOT recommend trying to use external speakers

Amp is apparently set up to run on-board speakers ONLY

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tele twister
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Re: 90's Amp + External Cab

Post by tele twister » Sun Apr 30, 2017 12:50 pm

OldSoul wrote:Hello crew! Question for all you tech-ohms-wattage heads..........

The amp in question is a Peavey Classic Chorus 212 which has no external-speaker jack. Is it possible to run wires off the internal 212 speakers to an external 212 or 112 cabinet without overloading or damaging the amp?
Years ago, I used a single CC212 in a outlaw country band, and I couldn't go past 3 before the drummer, steel, and bass player started crying about my volume :?

ampegasaur
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Re: 90's Amp + External Cab

Post by ampegasaur » Sun Apr 30, 2017 2:37 pm

You can set up any amp to use an external cab. A jack has to be wired in, and a cab with the proper ohms used. What ohms are the current speakers?

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tele twister
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Re: 90's Amp + External Cab

Post by tele twister » Sun Apr 30, 2017 3:31 pm

AFAIK, the stock CC212 had two 8 ohm Scorpions, each one connected to it's own power amp (stereo amp), providing 65 watts -75 watts*, to each speaker, independently.

*depending on the year;65w-early/75w-later

(130-150 watts total, and FWIW, it is plenty loud, as is, IMHO)

OldSoul
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Re: 90's Amp + External Cab

Post by OldSoul » Sun Apr 30, 2017 7:33 pm

Hey fellas, thanks much for the feedback. The amp is pretty loud.....I just like having the option if it's possible. But I'll definitely take heed to your advice!

kukovec
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Re: 90's Amp + External Cab

Post by kukovec » Mon Jun 05, 2017 4:22 pm

OldSoul,
The thing about amps, speakers, watts, ohms, is this:

An amplifier circuit runs *most efficiently* when its own circuit impedance is exactly matched by the impedance of the load connected to it.
Notice I say "most efficiently". It's not that they won't work if there's an impedance mismatch...but there will be an efficiency loss. That loss often takes the form of HEAT...which we know is not something you want present in an electrical circuit any more than it already is. Therefore, an amplifier is designed to operate with a specific load impedance - to make it run efficiently, and not degrade circuit components by excessive heat which might lead to failure.

Impedance has two components, resistance and reactance.
Resistance is the electrical part, reactance is the capacitive or (in this case) magnetic part.
That impedance is not purely resistive is why "load boxes" and direct-to-the-board inputs don't usually sound like the amp as it would through speakers.

Anyway, speakers - and speaker cabinets - generally come in default impedance values...4, 8 or 16 ohms.
Amplifiers are designed specifically to match those commercially standardized impedances.

Most tube amps have a selector switch on the back that allows you to change the impedance of the amplifier circuit to match, as closely as possible, the impedance of the load, because it's common to use different loads at different times (such as playing at smaller venues vs larger ones).

When combining multiple components with electrical impedance, how they are wired determines how the final circuit is "seen", electrically. When connecting loads in series, the impedances just add. 4+4 = 8. 8+8 = 16.
When connecting loads in parallel, impedances add as the reciprocal (1/"x")...of the sum of the reciprocals (1/a + 1/b + 1/c + ...)...of each impedance.
4+4 = 2
8+8 = 4
16+16 = 8
16+16+16+16 = 4.
In parallel, 8+16 = not a nice number. 4+8 = not a nice number. That's why it's important for speaker ohm ratings within a cabinet to match, and for speakers cabinet ohm ratings (when used together) to match.

So, If you connect two 8 ohm speakers together in series, the total impedance is 16 ohms.
When you connect two 16 ohm cabinets together in parallel, the total impedance becomes 8 ohms.
If you connect four 16 ohm cabinets in parallel, the total impedance becomes 4 ohms.

Since your amp is a combo, by design it's supposed to be a "self contained" unit. It costs extra money to provide a jack for an external speaker...but more than the cost of that small part and wiring, it would require a more expensive *amplifier circuit* design, to allow for multiple source circuit impedance options...to match the potential load options you might connect across it. It really does come down to dollars and cents.

That said...if you properly wire new jacks, you can certainly connect additional speakers or speaker cabinets to your amp.

Tele Twister thought that the amp originally had two 8 ohm speakers, one for each amp channel. That would mean that the amp circuit (each "side" of the stereo amp) is designed for an 8 ohm load. The key is you have to account for the original speakers in the combo. Either disconnect them...or run the amp inefficiently and risk damage from excessive heat.
If for example you left the speakers connected...and plugged in a 16 ohm 4x12 cabinet on each side (two cabinets total)...each amp is going to see a load with impedance of 5.33 ohms
1/IMP total = 1/Imp a + 1/Imp b + 1/Imp c + ....
1/IMP total = 1/16 + 1/8 .... = 1/16 + 2/16 ... = 3/16.
IMP total = 16/3 ... = 5.33ohms

If instead you were to install "speaker jacks" and, when using extension speakers, *disconnect* the two combo speakers, you could run a single 8 ohm 4x12 speaker cabinet on EACH amp output (two cabinets total). Or, you could run TWO 16 ohm 4x12 speakers cabinets in parallel (which reduces to 8 ohms) on EACH amp output (four cabinets total). When not using extension cabs, you can reconnect the speaker wires to the combo speakers' terminals - just leave the jacks in place, because they will be an open circuit and won't impact impedance (so long as nothing's conencted to them).

Clear as mud?

Kuke

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