Speakers and amps and ohms and magic

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clubhouse
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Speakers and amps and ohms and magic

Post by clubhouse »

Last night I swapped out a G12M (16 ohm) driver in my Marshall 1974x combo for an Anni G12H (8 ohm) driver. I set the multi-tap ohm selector for the OT to 8 ohms to match the speaker. Pretty straight forward and all went well.

Playing through the amp I've noticed a substantial reduction in volume. The amp otherwise sounds great and I much prefer the G12H to the G12M (a little too lower-mids congested). Just the volume drop concerns me. It used to rattle my room at about 7 but now it's quite comfortable being 1m in front with it on 10 and no feedback. Am I about to do damage to something or do I need to adjust anything else? What's going on?

I quite like the volume drop in terms of 'bedroom' practise volume.

I'll await advise before flipping it back on again.

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Re: Speakers and amps and ohms and magic

Post by robthemac »

clubhouse wrote: Sun Feb 19, 2023 1:15 pm Last night I swapped out a G12M (16 ohm) driver in my Marshall 1974x combo for an Anni G12H (8 ohm) driver. I set the multi-tap ohm selector for the OT to 8 ohms to match the speaker. Pretty straight forward and all went well.

Playing through the amp I've noticed a substantial reduction in volume. The amp otherwise sounds great and I much prefer the G12H to the G12M (a little too lower-mids congested). Just the volume drop concerns me. It used to rattle my room at about 7 but now it's quite comfortable being 1m in front with it on 10 and no feedback. Am I about to do damage to something or do I need to adjust anything else? What's going on?

I quite like the volume drop in terms of 'bedroom' practise volume.

I'll await advise before flipping it back on again.
That sounds very off. Have you checked the efficiencies of the speakers? Should be available online. From memory they're both middle of the road.
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Re: Speakers and amps and ohms and magic

Post by sizzlingbadger »

G12M 98db G12H 100db. so if anything it should be louder. Could just be the OP transformer doesn't match so well at 8ohms. What's it like on the 16ohm tap ?
Last edited by sizzlingbadger on Sun Feb 19, 2023 2:37 pm, edited 1 time in total.
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Re: Speakers and amps and ohms and magic

Post by RectifiedAmps »

Maybe the new speaker is just a bit ‘tight’ and needs to be broken in? You could measure it with an ohmmeter just to be sure it’s actually 8ohms as well but an impedance mismatch wouldn’t change output volume much anyways.

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Re: Speakers and amps and ohms and magic

Post by clubhouse »

sizzlingbadger wrote: Sun Feb 19, 2023 2:34 pm G12M 98db G12H 100db. so if anything it should be louder. Could just be the OP transformer doesn't match so well at 8ohms. What's it like on the 16ohm tap ?
Is it ok to run the amp on 16 ohm tap into an 8 ohm speaker? I thought that was verboten and would cause OT damage.

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Re: Speakers and amps and ohms and magic

Post by clubhouse »

RectifiedAmps wrote: Sun Feb 19, 2023 2:35 pm Maybe the new speaker is just a bit ‘tight’ and needs to be broken in? You could measure it with an ohmmeter just to be sure it’s actually 8ohms as well but an impedance mismatch wouldn’t change output volume much anyways.
The G12H was out of a 5E3 clone (8 ohm OT) and has been well played in. I'll measure it to confirm ohms :thumbup:

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Re: Speakers and amps and ohms and magic

Post by clubhouse »

Ok...speaker measures about 9.6 ohms on m'old analogue mm. Close enough for jazz?

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Re: Speakers and amps and ohms and magic

Post by sizzlingbadger »

the DC resistance of the 8ohm speaker should be appx 6.7 ohms at 20C
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Re: Speakers and amps and ohms and magic

Post by clubhouse »

Diagnosis; imminent speaker failure?

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Re: Speakers and amps and ohms and magic

Post by sizzlingbadger »

Could be just your meter isn't very accurate measuring low resistance. The 16ohm should measure appx 13ohms DC resistance.

It usually more common for a speaker to burn out a coil and read open circuit, or fuse a coil and read lower DC resistance. Your reading is a little odd in that respect.
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Re: Speakers and amps and ohms and magic

Post by clubhouse »

sizzlingbadger wrote: Sun Feb 19, 2023 3:19 pm Could be just your meter isn't very accurate measuring low resistance.
Yeah...it's about 40 year old Hioki and has seen quite a few battles. I just use it to test general continuity these days and don't really measure anything of consequence.

Wanna recommend a DMM that'll do service testing electronics as well as automotive (my main use)?...was looking at Flukes some time ago but got distracted.

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Re: Speakers and amps and ohms and magic

Post by sizzlingbadger »

Fluke 117 if you have deep pockets ;-)

I have a 17B+ it's good, but the response is a little slow, fine for what I do though.
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Re: Speakers and amps and ohms and magic

Post by Bg »

No-one ever got fired for buying Fluke. or IBM.
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Re: Speakers and amps and ohms and magic

Post by clubhouse »

I measured the G12M using my amm (for what it's worth) and that tests about 12.5ish. The G12M is rated at 15 ohms (supposed to have special sauce tuning to suit the 1974x...probably just the magnet sticker was changed so it looks more authentic to the OG). The scale graduations are pretty compressed so it's my eye dividing up the space between the grads. Tested the G12H and the needle initially measures about 7 before stabilising just over 9.5.

Question still lingers though...is it safe to run the amp at 16 ohms into a speaker load of 8 ohms?

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Re: Speakers and amps and ohms and magic

Post by clubhouse »

Bg wrote: Sun Feb 19, 2023 3:54 pm No-one ever got fired for buying Fluke. or IBM.
Jeez...I've often though of firing myself...especially after chopping off a digit :crazy:

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