Author Topic: THD Hot Plates changing impedance? (Read 8223 times)

simonp · « **on:** July 10, 2007, 02:25:44 PM »

Anyone know how hard it would be to convert a THD hotplate to a different impedance? I'm guessing that there is only a couple of components different between the different ones. At the moment I need an 8Ohm one, but may in the future get a 2nd cab so would need a 4Ohm attenuator.
Not wishing to buy 2 at the cost they are converting, or even better switching would be handy.

Twinfan · « **Reply #1 on:** July 10, 2007, 02:50:58 PM »

I always wondered if you could swap them over easily, so I'll watch this thread with interest...

LazyNinja · « **Reply #2 on:** July 10, 2007, 03:49:59 PM »

An ebay seller was trying to tell me that you can use the 16 Ohm one for lower impedances without causing any damage. Not sure how true that is but he said he'd been using his 16 Ohm on a 4 Ohm combo for ages and it was fine. Wasn't totaly convinced so I didn't bid.

simonp · « **Reply #3 on:** July 10, 2007, 03:54:14 PM »

Quote from: LazyNinja

An ebay seller was trying to tell me that you can use the 16 Ohm one for lower impedances without causing any damage. Not sure how true that is but he said he'd been using his 16 Ohm on a 4 Ohm combo for ages and it was fine. Wasn't totaly convinced so I didn't bid.

You'd certainly want the impedance to match the output impedance of your amp. I'm not sure of the makeup of these but it is possible taht the speaker impedeance is less critical.

LazyNinja · « **Reply #4 on:** July 10, 2007, 04:02:23 PM »

Isn't the impedance decided by the speaker rather than the hotplate? It's not like the hotplate has 16ohms of resistance by itself (unless you set it to load mode) so I can see where he's coming from. I think the difference between the models are that they are EQ'd slightly differently, and the impedance when set to load mode.

simonp · « **Reply #5 on:** July 10, 2007, 04:05:58 PM »

Quote from: LazyNinja

Isn't the impedance decided by the speaker rather than the hotplate? It's not like the hotplate has 16ohms of resistance by itself (unless you set it to load mode) so I can see where he's coming from. I think the difference between the models are that they are EQ'd slightly differently, and the impedance when set to load mode.

Ah OK, I'd assumed that the Hot plate presented the impedance to the amp.

Twinfan · « **Reply #6 on:** July 10, 2007, 05:34:40 PM »

It must do, surely?

Then you only need a 16ohm Hot Plate, set your amp to 16ohms, and run any cab you want? Would that work?

Hell Hound · « **Reply #7 on:** July 10, 2007, 08:39:39 PM »

The hotplate do present the impedance to the amp. It's basicaly a big resistor network, the cab is part of that net so changing the impedance of the cab totaly screwup the impedance presented by the hot plate.
Bad idea to mismatch IMO.

andymac · « **Reply #8 on:** July 10, 2007, 09:06:29 PM »

Quote from: Twinfan

It must do, surely?

Then you only need a 16ohm Hot Plate, set your amp to 16ohms, and run any cab you want? Would that work?

I have read on the Les Paul forum that this is the case (THD say so), but never had the bottle to try it.

Andy

simonp · « **Reply #9 on:** July 10, 2007, 11:07:34 PM »

Just found this quote:

"When discussing this with THD, in fact you can use different impedance models, but the frequency compensation e.q.’ing is not as effective when a perfect match isn’t made."

Here: http://www.legendarytones.com/thdhotplate.html

Tellboy · « **Reply #10 on:** July 11, 2007, 02:27:09 PM »

I got a reply from Mr THD hotplate himself - Eddie DeGenaro - following a request for using my 8ohm hotplate with a 4ohm speaker load.

My query:-
"I am about to get an 8 ohm hotplate which I will be using with a Marshall JCM900 combo (1 x 12) and a Fender Champ 12. The Fender has an 8 ohm speaker so will be OK with the HP. The Marshall has a single 8 ohm speaker - however, I have an extension cabinet with this when I gig which is also 8 ohms and therefore requires the amp impedence to be set to 4 ohms (which is altered on the back panel). Can I use the 8 ohm hotplate with the extension cabinet connected - I assume I would still set the amp to the 4 ohm setting and connect the two speaker leads to the 2 outputs of the HP - hopefully this only means that the tone cicuitry of the HP is not optimised as the impedance is lower, but will not affect/harm the Marshall. I would like to be able to use both cabinets with the HP as the extension cabinet has a closed back and greatly expands the sound of the open backed combo."

Eddie's reply
"Dang, I wished all my e-mail were this easy to answer.
You're correct on all counts."

Note: Only possible if HP is higher impedance than speaker load.

No longer have the hotplate but it seemed to work OK with the 4 ohm load.

Twinfan · « **Reply #11 on:** July 11, 2007, 03:50:44 PM »

So you still have to match the speaker/amp load combination then?

What if you have:

* Amp set to 16 ohm
* 16 ohm Hot Plate
* 4 ohm cabinet

???

Tellboy · « **Reply #12 on:** July 12, 2007, 10:43:28 AM »

Quote from: Twinfan

So you still have to match the speaker/amp load combination then?

What if you have:

* Amp set to 16 ohm
* 16 ohm Hot Plate
* 4 ohm cabinet

???

Dang, I believe that the amp and speaker ohms must be set to match as normal. As long as the Hot Plate is not lower ohmage than this setting it will work BUT I did read somewhere that this is only recommended for the 'next step' i.e. 8 ohm HP OK with 4 ohm load, 16 ohm HP OK with 8ohm load. I would guess that using a 16 ohm HP with a 4 ohm load would not harm the amp (as long as it too was set to 4 ohms) but the HP 'optimisation' would be too far out and possibly the sound quality would start to suffer.