Classic 30 Extension Cabinet not loud as combo

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Jbowen16
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Classic 30 Extension Cabinet not loud as combo

Post by Jbowen16 » Sat Nov 27, 2021 8:55 am

Hi,

I have a 90s model Classic 30. Sounds great with JJ tubes and 50 watt C Rex speaker. I just bought a Marshal extension cabinet with one 16 ohm, 80 watt cellestion 70/80 speaker. Unfortunately, when I plug it into the extension speaker Jack of the classic 30, the volume of this extension cabinet seems to only be ~a third the loudness of the combo. You almost can’t hear it. What’s wrong?

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Enzo
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Re: Classic 30 Extension Cabinet not loud as combo

Post by Enzo » Tue Nov 30, 2021 12:01 pm

Aybe nothing, compare the efficiency ratings of the two. ALso possible is that the extension cab is wired "backwards", ie positive and negative are reversed on the speaker. Try reversing those for a test.

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65 SG
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Re: Classic 30 Extension Cabinet not loud as combo

Post by 65 SG » Sat Mar 19, 2022 9:32 am

Old post, I know but according to the schematic the C30 has a TRS ("stereo") jack for the extension speaker and depending on what cable is used and how the cable is connected, you might have the 16 ohm cab connected to the 8 ohm output transformer tap, and/or you could be shorting the 8 ohm tap to ground. Depending on the jack condition, this short could be high enough resistance to allow the external speaker to work, weakly. It goes without saying, this is bad for the OT and tubes.

Schematic says:

"Shield" of the TRS jack is connected to ground

"Tip" is connected to the internal speaker and switched to the 16 ohm tap (disconnects the internal speaker from the 16 ohm tap when cab is plugged in).

"Ring" is connected to the 8 ohm tap - NOTE a standard TS cable Shield will short to Ring when plugged into a TRS jack

Bottom line - you MUST use a TRS cable for the extension cab, and wire it like this:

Shield to cab (-)

Ring to cab (+), Tip not connected, for an 8 ohm cab where the cab runs alone (internal speaker is defeated)

Ring and Tip connected together to cab (+) for a 16 ohm cab where the cab and internal speaker BOTH run together

There is no way to run an external 16 ohms cab only (no internal speaker) on the 16 ohm transformer tap without modding the amp's jack wiring
'03 SG Special, '08 LP Special DC, '60s Dano/Silvertone, Epi Moderne, Epi EB-0, '60s Telestar semi-hollow,
Classic 20 w/6V6 mod, much modded VK112, Windsor Studio combo, Transtube Envoy II, '61 Reverberocket

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Enzo
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Re: Classic 30 Extension Cabinet not loud as combo

Post by Enzo » Sat Mar 19, 2022 12:24 pm

I fear you have misread the schematic. Compare the extension speaker jack with the footswitch jack. The footswitch jack IS a TRS, not the tip and ring contacts are in different positions. Now look at EXT SPKR jack. The two contacts are in the same position. This is a "two-tip" jack, not a TRS. Plugging into EXT shorts the two tip contacts together, thus shunting the inner speaker to the 8 ohm tap, and also disconnecting it from the 16 ohm tap. I find this clever.

PV loved two tip jacks and used them in speakers a lot, just using the double tip blades as more contact surface for the higher current speaker needs.

A TRS cable wwill work, but has nothing to do with the ring contact. And I do not recall seeing TRS speaker cords, only signal cords in TRS. But that is only my experience.

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