THD attenuators and Vintage Cabs

[commachio]

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.

 

[PelliX]

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.

General concensus, common sense and probably the instruction manual. So far, you're good.

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).

Not really. If you crank an amp, it will wear out the output valves faster. Using an attenuator you are more likely to crank the amp. So, comparing an amp running at low volume natively and running at high volume through an attenuator obviously indicates that pushing it harder will wear things out faster. Assuming you're already diming it, there's no difference, *really*.

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).

Negative. While it would probably survive that just fine, you would be reflecting the wrong impedance on the output valves. Certainly not desirable.

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?

Easy. Amp -> Attenuator -> cab. Done.

 

[fitz]

Setup - Marshal 100W JMP and Marshal 2x12 with two 16Ohm Celestions.

Two 16 ohm speakers would be wired parallel for an 8 ohm load.

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.

Now you are saying the cab is 16 ohm?
Does it have two 8 ohm speakers wired in series?
You should confirm what you have in that cab to get a correct answer to your questions.

Do I need to wire the Celestions in series/parall

Two speakers can only be wired either parallel or series.
Perhaps take the back off your cab and get some pics of the speakers and wiring.

 

[PelliX]

@fitz I believe the THD's abstract the load from the amp pretty much entirely, no?

 

[fitz]

@fitz I believe the THD's abstract the load from the amp pretty much entirely, no?

Not sure of the inner workings of the THD, but I'd assume the amp would see a 16 ohm load from the 16 ohm Hotplate.
Might just be a tone issue using an 8 ohm cab, but again, I have no personal experience with the THD gizmos.

If the cab is actually 8 ohm, perhaps an 8 ohm attenuator should be what is used.
OP should confirm two 16 ohm speakers or a 16 ohm cab - it can't be both.

Edit: OP should probably also specify what model (power handling) these two "Celestions" are.

 

[commachio]

Okay…..the 2x12 Cab has two Celestion speakers, each speaker is 16 Ohm. This was a photo I took of the way they were wired prior to taking them out

 

[commachio]

General concensus, common sense and probably the instruction manual. So far, you're good.



Not really. If you crank an amp, it will wear out the output valves faster. Using an attenuator you are more likely to crank the amp. So, comparing an amp running at low volume natively and running at high volume through an attenuator obviously indicates that pushing it harder will wear things out faster. Assuming you're already diming it, there's no difference, *really*.



Negative. While it would probably survive that just fine, you would be reflecting the wrong impedance on the output valves. Certainly not desirable.



Easy. Amp -> Attenuator -> cab. Done.

So two leads from back of Amp into two inputs on the THD and then a single output from the THD into the cab…..

 

[Frodebro]

That's an eight ohm cab. The ideal configuration would be using the eight ohm output on the amp with an eight ohm attenuator.

 

[JohnH]

Have you got the Hotplate yet? There are many more options for attenuators these days, as compared to when Hotplates were the go.

 

[PelliX]

So two leads from back of Amp into two inputs on the THD and then a single output from the THD into the cab…..

No. If you look at any instruction manual of any attenuator, you will see that it is connected with a single cable to the amp as it represents a single "virtual cab", aka load. Without sounding snide, I would recommend that you at least browse through a manual and come back with any questions. Things will make a lot more sense then, I believe.

 

[79 2203]

I use a 16 ohm Hotplate on both my vintage halfstacks.
Stock 70 Superbass or 71 1987 set to 16ohm.
Speaker cable from amp to Hotplate.
Speaker cable from Hotplate to either of a pair of 16 ohm 68 straight cabs.
The Superbass is turned up to 8 and attenuated-12db, and the 1987 is turned up to 7 and attenuated -8db. So not exactly cranked, but running pretty hard.
Both amps are still pretty loud, but not that loud I’m gonna go deaf or blow my expensive Pulsonics.
Been using the Hotplate that way for about 3 years and never had an issue. Tubes seem fine. The Superbass does run hot at those settings, but I’m told that’s something peculiar to that era of Bass spec Marshalls. Modern wall voltages being higher than they were in 1970 also play a part in that apparently.
On hot days I put a small fan behind the Superbass(back panel off)and it keeps the tubes nice and cool.

 

[Fernieite]

Have you got the Hotplate yet? There are many more options for attenuators these days, as compared to when Hotplates were the go.

Pfft, like you'd know...

 

[neikeel]

If he has what he states then he matches amp to attenuator each 16 ohm. That is what the amp ‘sees’ and will be ok. (Remember you can use the Hotplate on load with no cab (I use mine like this for testing and then into my bench 1x12.
Having 1/2 step mismatch to çab is no issue.
I agree with JohnH, however if you use it for mild attenuation(-4 or -8) it is great. Heavy attenuation there is a damping effect that is not as good as more modern devices that have knobs and whistles to compensate. I have an original Alex and Motherload but there are newer’better’ much more expensive bits of kit (and a nice DIY version) if you have the money or alternatively time!

 

[commachio]

I’m

That's an eight ohm cab. The ideal configuration would be using the eight ohm output on the amp with an eight ohm attenuator.

okay…..bit lost and my apologies but never been inside a can to do any sort of rewiring and I’d like to understand this to get this right. Why is it an 8 Ohm speakers setup? Both speakers are 16 ohm which suggests it’s a 16 ohm load…..is it because the speakers are setup in parallel or in series that affects the impedance load? Many thanks

 

[commachio]

No. If you look at any instruction manual of any attenuator, you will see that it is connected with a single cable to the amp as it represents a single "virtual cab", aka load. Without sounding snide, I would recommend that you at least browse through a manual and come back with any questions. Things will make a lot more sense then, I believe.

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?

 

[PelliX]

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.

Sounds about right. A 2:1 mismatch (say a 16 Ohm load on an 8 Ohm tap) is not going to damage most amps. Not recommended, but fair enough.

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.

That's a bit of a roundabout way of saying the same as above. Provided you have, for example, an 8 Ohm tap, 8 Ohm Hotplate (as described above) and a (speaker) load equal to or greater than 8 Ohms, it would be fine. I see how that's a little confusingly worded, maybe.

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).

There are a couple of ways find out;

1) take a multimeter and measure the resistance of the cab. Plug a cable in the back of the cab and check the other end with your meter. You will need to round the number a bit for reasons that are irrelevant right now. Let's say you measure 14 Ohms; that's a 16 Ohm cab. If you read 6 Ohms or 9 Ohms, that's an 8 Ohm cab. The meter will give you a ballpark figure.

2) do the math; two equal resistances in parallel = resistance / number of resistances. So two 16 Ohm speakers in parallel would be (16/2) 8 Ohms. Two resistances in series add up. So if you have two 16 Ohm speakers in series, that's 32 Ohms. Two 8 Ohm speakers in series would be 16 Ohms.

Pics and/or check it with your meter.

 

[Francis Eckersley]

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?

Your speaker cab has 2 x 16 ohm speakers wired in parallel to present an 8 ohm load. Set your amp to 8 ohms output, use an 8 ohm attenuator. If you rewire your 2 speakers in series you will be presenting a 32 ohm load…

 

[december]

If your cab has two 16 ohm speakers, it's an 8 ohm cab (speakers in parallel). So you need an 8 ohm attenuator. Always match impedance: amp 8 ohm output -> 8 ohm attenuator -> 8 ohm cab.

 

[Kossfinger]

I know Marshall amps struggle w volume issues, especially older ones. I’ve never had any luck w attenuators. They all have cooked my tubes into red plate, within ten minutes. I think caution, and preservation are in order. There’s so many ways to go on that one. Lower powered amps perhaps. Silent speaker cabs also are a great way to tame audio levels. If you love the plexi thing and can’t use anything else, perhaps a kit build, in place of the valuable legend? There’s a lot of talk about Marshall being sensitive to attenuation because impeadance isn’t matched accurately. Also, the ot and tubes are stressed more so, as a result.
I know there’s guys like EVH who have done it. I can’t help but think there’s more to it, in terms of reliability. Perhaps the new breed of attenuation addresses that ? Good luck !!

 

[PelliX]

They all have cooked my tubes into red plate, within ten minutes.

Um, with all due respect, that has nothing to do with your attenuator in that sense. If your bias is dropping, you have other issues in the amp. That's just not how it works, sorry.

There’s a lot of talk about Marshall being sensitive to attenuation because impeadance isn’t matched accurately. Also, the ot and tubes are stressed more so, as a result.

This is just more clueless rambling. Impedence is a curve over a spectrum. Let's take a single speaker; it may present 8 Ohms impedence at 1kHz. That value will be different at 500Hz and different again at 2kHz. Impedence matching is always an imprecise balance, because the frequency of the tone changes constantly (or you listen to extremely boring music ). As for the OT and valves being stressed harder, no that's generally not the case. If you dime an amp, it's hard on it. Cab or attenuator or combination thereof doesn't make a *big* difference. There are more variables at play, but to simplify it by saying "that's bad for the amp" is just nonsense spouted and regurgitated by the uninformed. No offense intended whatsoever - just trying to set the record straight here.