Hot Rod Deluxe iii with a 16Ohm extension cab?

dandl

Hi,

I just wondered if the good people of The Fretboard could throw some light on this for me please.

Im quite fancying a Hot Rod Deluxe but also want the option to hook up an external 16 ohm cab that i have to run alongside the internal speaker. The usual googling has brought forth a whole host of differing opinions.

I was hoping to use it as a pedal platform so nice loud cleans is a must for me.

What would be the general consensus from the knowledgeable forumites? Will it take an external 16 ohm cab and if so will there be any detrimental effects?

Thanks in advance

Richardj

I believe that the amp is designed to run an external 8ohm cab plugged straight in alongside the internal speaker. It could be that a 16ohm cab might fractionally drop the output, but others here are better qualified to comment on that.

colourofsound

Having done extensive research on this myself, i can tell you the answer is both yes and no.

You can run a 16ohm cab in the short term, but in the long term it could well do damage to your power output stage. You may never see a problem, but you might.

The ideal setup is a cab with 2 16ohm speakers in it or 1 8ohm speaker I'm afraid, plus the internal 8ohm speaker.

olafgarten

I think the hot rod deluxe can have 3 configurations.

1. 8 ohm in the main speaker out and nothing in the other output. 
2. 8 ohm in the main speaker output and a 8 ohm in the other output. 
3. An open Jack in the main output and a 4 ohm load in the other output. 

ICBM

You can run a 16-ohm extension cab indefinitely, at any volume, and you will not do any harm to the amp whatever.

When you use a 16-ohm and an 8-ohm speaker together the total load is 5.33 ohms. The HRD switches to 4 ohms automatically when a plug is in the Ext jack, and 5.33 ohms is so far within the safe mismatch range for 4 ohms that it's barely even worth thinking about it as a mismatch.

Impedance matching is *nowhere near* as critical or even as accurate as you may think - the impedance curves of different speakers could well be more different than between 4-ohm and 5.33-ohm nominal impedance.

Richardj

@ICBM , what's the maths for getting to that figure?

dandl

Thanks everyone for your input and help, that is really useful. I really do love this forum  

So if it will run and not cause a problem to the amp am i right in thinking there should be no reduction in the power going to the additional speakers? Im hoping to run an Orange 4x10 with it.

ICBM

Richardj said:

@ICBM , what's the maths for getting to that figure?

1/Ztotal = 1/Z1 + 1/Z2

But in fact you don't really need to use the maths - there's a simple rule of thumb… if the amp is set to 4 ohms, you can use any combination of any standard speakers safely, because the worst cases are two 4-ohm cabs (2 ohm total) or two 16-ohm cabs (8 ohm total) which are the outer ends of the safe range usually - a 2:1 mismatch in either direction is usually OK for a valve amp. So any other combination is closer to 4 ohms than those and you don't need to know what the actual value is. (Obviously if your amp does have an 8-ohm setting you're best to use that, with two 16-ohm cabs.)

This is also probably why Leo Fender designed most of his amps with a 4-ohm output and no impedance selection. He liked to make things simple and fail-safe musician-proof.

ecc83

 "The usual googling has brought forth a whole host of differing opinions."

No change there then! If you ask a MANUFACTURER they will almost certainly tell you that the amp must ONLY be used with the exact load. This is an A saving exercise because...

1) Handbooks are vetted by lawyers.

2) There will always be some idiot that takes things to extremes such as belting the beans out of a 50 watter tapped to 4 Ohms into a 16 Ohm speaker but even that worse case scenario (apart from NO load of course!) will usually not result in instant destruction and might in fact last indefinitely.

However, in the case of solid state amps the usual minimum load resistance quoted is 4 Ohms. This should be strictly observed. It is often said that sstate amps do not need a load and whilst true in theory I would always put 100R or so across the speaker terms'.

Dave.

ICBM

dandl said:

So if it will run and not cause a problem to the amp am i right in thinking there should be no reduction in the power going to the additional speakers? Im hoping to run an Orange 4x10 with it.

There will be a very small reduction in power due to the mismatch, probably around 10% or so - it's usually only around 20-25% even with a 2:1 mismatch - which will be easily outweighed by the greater efficiency of using extra speakers. 2/3 of the power will still go to the internal speaker, so it may match quite well with the cab in volume terms.

ecc83 said:

 There will always be some idiot that takes things to extremes such as belting the beans out of a 50 watter tapped to 4 Ohms into a 16 Ohm speaker but even that worse case scenario (apart from NO load of course!) will usually not result in instant destruction and might in fact last indefinitely.

When I started repairing amps in the 80s, there was a "secret trick" going around to get "better tone" (ie more distortion, in those days!) at lower volume from your non-MV Marshall - if you set it to 4 ohms with the standard 16-ohm cab.

But this is well outside the safe range, and very often it did give the desired result... for a while, then blew the OT. Especially as the amp was usually cranked right up.

That might be where some of the overly-strict warnings come from.

not_the_dj

I've run mine like this for a while based on similar advice ICBM gave on here a while back...it hasn't exploded on me yet. 

colourofsound

Sorry @ICBM I don't understand the maths here at all. Simply for curiosity's sake, how does that equation work in the particular instance? I've been having trouble understanding impedance and this is just making it worse!

ICBM

Sorry, if it's not obvious how to get the number you want out - it's total impedance = 1/(1/impedance1 + 1/impedance2).

So in this case…

Total impedance is 1/(1/8 + 1/16), which is 1/(3/16), which is 16/3… or 5.333

Does that make sense?

Things are much simpler if the impedances are in series, you just add them up!

colourofsound

Nope, sorry. Are you dividing things there? Or are you adding and timesing fractions together? My knowledge of equations is pretty flawed. And I'm assuming you're combining maths with physics (electricity of course) and the laws of impedance which are also lost on me. This might be a lost cause, i'm sorry in advance.

ICBM

colourofsound said:

Nope, sorry. Are you dividing things there? Or are you adding and timesing fractions together? My knowledge of equations is pretty flawed. And I'm assuming you're combining maths with physics (electricity of course) and the laws of impedance which are also lost on me. This might be a lost cause, i'm sorry in advance.

No problem . I can't really make it amy simpler than that! Yes, it's a mix of maths and physics.

If you're not good at that sort of thing, go by the simple rule of thumb: if you're adding unequal impedances in parallel, the result is always more than half the lower one and less than half the higher one.

And if you know that an amp is safe with between half and double its matching impedance, you should be able to work out what to set it to. And unless you're dealing with something very strange, the answer is always 4 ohms .