Amp Output with Impedance.

tekbow

How does it vary?

Just finished writing this and came back to say possibly TLDR, but theres a bit to unpack which probably has a simple answer.

Couple of things recently made me wonder about this.

Up until just before christmas, I ran my amp into a reactive load at 16 ohms, 100w MV at about 6.

Then I switched it to 8 ohms. Shortly after, the load started going into its thermal overload state where its overheats, bumps over to a safety load and the sound cuts out.

You stop playing at this point and turn the volume on the guitar down.

It comes out of the protection mode after 30 seconds or so once the fan has done its job.

I got in touch with tech support, thinking there was an issue, and after a bunch of back and forth I mentioned the change from 16 to 8 ohms and the support guy said that was most likely the issue as the amp outputs less power at 16 ohm than 8

Sure enough, I changed it back to 16 ohms and no further issues.

I did some digging and found this.

https://support.two-notes.com/knowledgebase.php?article=198

Now, I remember Two Notes stating somewhere in the literature that even at max output from Reload, it has around -3db attenuation, which as it happens is about the db difference between a 100w amp and a 50w amp given point on the volume control I guess assuming same amp model and exactly the same pots.

I've got the amp and Reload set for 16 ohm, the cab is 8 ohm, so the cab, according to the table in the link, is seeing 50w or a volume drop of -3db. But when the load is set for 16 ohm into a 16 ohm cab the power is 25w and a drop of -6db.

Given tech support said that the amp is out putting less at 16 ohms than 8 ohms, does it follow this relationship from the table? Admittedly I'm unsure what the relationship is.

Basically will an amp set for 16 ohm, into a 16 ohm cab be 3db quieter than an amp set for 8 ohm into an 8 ohm cab. And does this mean that the 16ohm cab is seeing half the power of an 8ohm cab?

ICBM

If it’s a valve amp and the transformer ratios are correctly designed then it will put out exactly the same amount of power into 8 or 16 ohms, if the loads are also the correct impedance.

If one or more of those things aren’t true - which it sounds like they aren’t - then all bets are off.

tekbow

Hmm. Interesting.

Dont know what to make of that then.

Jonathanthomas83

Interesting stuff, how long does it take for that to kick in and the sound to cut out? I've never heard of that happening before.

Reason I ask is because I've been doing a few sessions with the JCM 800 (on 8 ohms), MV at around 5-6, into the Suhr recently, as you know, and not experienced anything like that. I know that doesn't help you though...sorry!

ICBM

What amp is it? It could also be that the negative feedback is connected to the speaker output and not to a particular transformer tap, so it varies with the impedance setting. Some early Marshalls were wired like that - technically wrong. It may change the power output a bit, and even if it’s not much it could be enough to go from just under to just over where the protection kicks in.

John_A

Dont know if it could be something to do with the way reactive loads work?  The power station has a lower input power handling at 8 ohms than it does at 4 or 16

tekbow

@Jonathanthomas83, the Suhr RL, which I also have is a bit different.

Two Notes are quite conservative about their power ratings and encourage you not to push the amp past max volume (as opposed to saturation).

The Reload is rated for 150w RMs and 200w peak.

Theres no such rating on the RL past it being suitable for 100w amps. However John Suhr has said it can handle a dimed Plexi. It also doesnt have a thermal protection mode that I'm aware of.

This is why I didnt buy a captor which is quoted as handling 100w, with no peak value quoted.

@ICBM, it's the SLO. This is what tech support reckoned, that at 16 ohms it's output was just under where protection kicks in. That was without making reference to where the negative feedback is connected.

That being said, what you said flagged something in my head and I went and found this. My amp has a depth knob.

ICBM

tekbow said:

@ICBM, it's the SLO. This is what tech support reckoned, that at 16 ohms it's output was just under where protection kicks in. That was without making reference to where the negative feedback is connected.

That being said, what you said flagged something in my head and I went and found this. My amp has a depth knob.

If it's connected to the common terminal on the selector or the speaker output that would explain it - that's a NFB control for the low-end, and if it's working at 16 ohms there will be more feedback than at 8, which will reduce the power of the amp slightly.

According to the schematic the NFB (on the original version with only a presence control) should be connected to the 4-ohm tap - although in fact that wouldn't solve your problem since that would give even less. There's no reason you couldn't connect the depth control to the 16-ohm tap if you wanted though.

tekbow

ICBM said:

tekbow said:

@ICBM, it's the SLO. This is what tech support reckoned, that at 16 ohms it's output was just under where protection kicks in. That was without making reference to where the negative feedback is connected.

That being said, what you said flagged something in my head and I went and found this. My amp has a depth knob.

If it's connected to the common terminal on the selector or the speaker output that would explain it - that's a NFB control for the low-end, and if it's working at 16 ohms there will be more feedback than at 8, which will reduce the power of the amp slightly.

According to the schematic the NFB (on the original version with only a presence control) should be connected to the 4-ohm tap - although in fact that wouldn't solve your problem since that would give even less. There's no reason you couldn't connect the depth control to the 16-ohm tap if you wanted though.

It actually doesn't bother me too much, I'm happy as long as it doesn't go into thermal shutdown when playing at higher MV. If 16 ohms work, that's fine with me.

The Reload was actually fine with my JCM800 1959 as well. That's jumpered with Volumes set at 7-8 for the bright input and 4-5 for the normal, 16 ohms again.

ecc83

Tekbow, do you mean you changed the load box AND the amp to 8 Ohms?

In any event amplifier 'impedance' is not, as some would have you believe, cast in stone. The load published for a valve is the 'optimum' impedance, that is one found to give a good power output balanced with lowish distortion.

Loading a 16 R tapping with 8 R could give more power but at higher distortion. OR it might give LESS power and less distortion.

Then change the HT by 50V or/and the bias current and it all goes to H in a handcart!

tekbow

@ecc83, yes the amp and loadbox always had matching impedances.

So the Load apparently saw less power when amp and load were both set to 16ohm vs both set to 8ohm.

The cab is 8 ohm, but the good thing about the Reload is that any cab impedance can be used as long as the Reload and amp impedance match. Probably the same with other similar loads.

ICBM

tekbow said:

The cab is 8 ohm, but the good thing about the Reload is that any cab impedance can be used as long as the Reload and amp impedance match. Probably the same with other similar loads.

It's always true with any attenuator set below -3dB - at that point the attenuator is taking half the power, so below that the attenuator is the majority of the load, not the cab. Below -6dB the cab impedance is more or less irrelevant.

tekbow

Or in the case of the Load + Poweramp type, the cab is isolated from the amp completely at any level.

John_A

John_A said:

Dont know if it could be something to do with the way reactive loads work?  The power station has a lower input power handling at 8 ohms than it does at 4 or 16

Did you see my comment above?   Funny that the PS has different handing ratings dependent on impedance

ICBM

John_A said:

Did you see my comment above?   Funny that the PS has different handing ratings dependent on impedance

I think it's to do with the arrangement of the load resistors - they'll most likely be combined in series or parallel to give different loads. If it was a purely resistive load there could be two 100W 8-ohm resistors which are combined in parallel or series to give 4 or 16 ohms at 200W each, but only one can be used to give 8 ohms.

John_A

ICBM said:

John_A said:

Did you see my comment above?   Funny that the PS has different handing ratings dependent on impedance

I think it's to do with the arrangement of the load resistors - they'll most likely be combined in series or parallel to give different loads. If it was a purely resistive load there could be two 100W 8-ohm resistors which are combined in parallel or series to give 4 or 16 ohms at 200W each, but only one can be used to give 8 ohms.

So quite possible that the torpedo is the same therefore a lot closer to its limits at 8 ohms rather than 16?  

Two notes are definitely vague to the point of misleading with some of their literature 

ICBM

John_A said:

So quite possible that the torpedo is the same therefore a lot closer to its limits at 8 ohms rather than 16?

Yes, that's also possible.

tekbow

Well, I ran the masters at 8 for a good few months with no issues @ 16ohms.

Pointless on the OD channel, but thickened the crunch up nicely.

It had a hard time handling 6 on the MV @ 8 ohm

tekbow

@ICBM, sorry to necro this but WRT to all the above I just had a thought.

The amp is running a quad of 6L6GC's rather than the usual Sov 5881's.

Could the amp be putting out a bit more power, all else being equal, with the GC's vs the Sovteks?

As in at least as much as the difference between 16 and 8 ohm?

ICBM

tekbow said:

@ICBM, sorry to necro this but WRT to all the above I just had a thought.

The amp is running a quad of 6L6GC's rather than the usual Sov 5881's.

Could the amp be putting out a bit more power, all else being equal, with the GC's vs the Sovteks?

As in at least as much as the difference between 16 and 8 ohm?

Possibly, although I would expect it to be fairly marginal. What brand are the 6L6GCs?

tekbow

ICBM said:

tekbow said:

@ICBM, sorry to necro this but WRT to all the above I just had a thought.

The amp is running a quad of 6L6GC's rather than the usual Sov 5881's.

Could the amp be putting out a bit more power, all else being equal, with the GC's vs the Sovteks?

As in at least as much as the difference between 16 and 8 ohm?

Possibly, although I would expect it to be fairly marginal. What brand are the 6L6GCs?

JJ's

Would be great if I could drop the output a hair.

I've an old email from Bill at Soldano saying GC's would put out a touch more power, but he was talking about the US made tubes they could still get when Soldano first started.