THD attenuators and Vintage Cabs

[neikeel]

According to the very basic manual with the THD (more a sheet than a manual) -

Q. “Can I still use the Hot Plate if it has a different impedance than my amp?

A. ONLY if the impedance of the Hot Plate is EQUAL TO OR GREATER THAN that of the amp. However, this will lessen the effectiveness of the Hot Plate’s tone controls.

Q. “If I am using just one speaker cabinet, can I still use the Hot Plate if it has a different impedance than my speaker cabinet?”

A. ONLY if the impedance of the speakers is equal to or greater than the amplifier, AND the impedance of the Hot Plate is equal to or greater than the amp. Again, this will lessen the effectiveness of the Hot Plate’s tone controls.

I’ll keep searching but my main lack of understanding is the can setup. Again I have two 16 Ohm speakers (as per photo) and I do not know whether these are in series or in parallel (sorry, never rebuilt a cab before).

My current understand on impedance is that 2 16 Ohm in parallel is 16/2 = 8 ohm. If that’s correct then I get it…..so I either buy a 4x12, 8 ohm attenuator or change the speakers to be in series?

No
You are fine. Your amp matches your attenuator. Period.
You can use your cab (it will be 8ohms - pair of 16s in parallel. No one has 32 ohm guitar cabs!) out of your current Hotplate without issue.

 

[Douglas Cram]

All....

Hoping this won't open up too much of a can of worms!!!

I own a 1969 100W JMP that is literally a 'real blast' to play but my hearing, household and neighbourhood won't survive an ongoing onslaught.

It was suggested a attenuator was a good option and many suggested the 16Ohm THD Hotplate. I did, what I thought was enough due diligence, but now have some minor 'headaches' (nothing to do with the JMP!!!) about HOW this should be setup. Lots on confusing and misleading information on the web.....

Setup - Marshal 100W JMP and Marshal 2x12 with two 16Ohm Celestions. Also of note I am NOT an electrician but happy re-wiring stuff!!

The Confusion:
Point 1: General consensus (based on possible dubious web comments) and as per the manufacturer suggests the 16Ohm switch on back of JMP needs to match th 16Ohm THD and the 16Ohm cab. There is, however, evidence that the THD may put more of a load on the Head which could result in blown tubes (over a period). There is a suggestion that the Head's Ohm switch should be reduced to 1/2 the impedance of the THD and the rest of the chain (i.e. Head should be 8Ohm and the rest left at 16Ohm).

Does anyone have any concrete evidence of issues with their heads using a THD as per the manufacturers recommendations (16 Ohm throughout) vs setting the JMP to 1/2 the impedance. The last thing I want to do is cook my head. Note that I intend to use an attenuator so I'd appreciate if we can reduce any backlash of using attenuators....

Point 2: JMP has two cab outputs and the 2x12 has a single jack input. Do I need to wire the Celestions in series/parall and if so does anyone have a photo of how this should look?

many thanks everyone.

I've used THD Hotplates for decades on my JMP and JCM Marshalls, as designed and impedance matched - no issues. I have also used the Weber Mass attenuator, which is nice because it has selectable ohm settings. Be aware that just about any attenuator will add some coloration of its own (in my opinion) . I've even tried a chjeap Behringer attenuator and it never hurt anything. I've heard good things about the Rivera Rock Crusher, but at $650, a bit steep in price. I've been pretty happy with the Weber unit mainly because of it's flexibility, and you can get a 100 watt Mass used for about $150 - $200. Nothing wrong with the THD units either. https://musicgoround.com/locations/...Mtwrx9r7CGj6b_NSZw0QwJxekV-bv2qUaAuB8EALw_wcB