adding parallel resistor to drop speaker impedance - effect on tone?

[mxrshiver]

got a Fender Bassman '59 Reissue that i want to run into a Torpedo Captor X (load box/attenuator/speaker sim) for direct recording. it's a reactive load, and is a pretty excellent, flexible speaker simulation option. the Torpedo is 8 ohms of impedance, while the Bassman has a 2 ohm output. so i was thinking of putting a 2.67 ohm wirewound power resistor in parallel with the Torpedo, to make 2 ohms total.

i've confirmed with the company that makes the Torpedo that this *should* be safe and the math checks out... but don't come crying to them with a warranty claim if anything goes wrong! i'm planning to use a 100W resistor, as it will be handling most of the load from the amp, and this beauty is actually putting out about 68W...

what i'm concerned about is tone coloration and frequency response. how should adding a parallel resistive load like this affect the response of the reactive load? my guess would be a more flat frequency response... from what i've gathered, the effects should be minor at best (not like adding a resistive load in series), but i am concerned specifically because the resistor will be handling so much of the load, that whatever effects incurred will be more prominent.

[pdf64]

It would remove the point / benefit of the reactive load, turns it into a resistive load, near as dammit.

[kagliostro]

I know near nothing about that

only I remembered the existance of this article

https://www.duncanamps.com/technical/dummyload.html

Franco

[acheld]

The more I think about it ( a loaded phrase!), is this really the right solution?

I don't use these devices for recording, so I may be blowing smoke . . . but it seems to me that you can't really do any better than approximate the sound with a cab simulator, and then to add a resistive load on top of that -- you probably won't get what you want.

On the other hand, I do use various guitar plugins to achieve an effect in my DAW, and they work pretty well.  Why not just record from the Bassman and apply some "makeup" in post? 

Or, are the cab simulators that much better??

[mxrshiver]

The more I think about it ( a loaded phrase!), is this really the right solution?

I don't use these devices for recording, so I may be blowing smoke . . . but it seems to me that you can't really do any better than approximate the sound with a cab simulator, and then to add a resistive load on top of that -- you probably won't get what you want.

On the other hand, I do use various guitar plugins to achieve an effect in my DAW, and they work pretty well.  Why not just record from the Bassman and apply some "makeup" in post? 

Or, are the cab simulators that much better??

cab sim packages can be a lot of bang for your buck, and the Torpedo's program has a ton of options for mic choice, placement, room type, f-ing cabinet wood type... it's a very viable option for someone whose mic selection is limited and wallet is tight! delving into cabinet IR's is very worth it at this point in the technology, but there's still no substitute for a well mic'ed up speaker - they're both different, very useful, tools. for a wide array of sounds, i can get what i need out of their IR loader, and most of the features of a guitar speaker that are damn near impossible to accurately emulate - stuff like OT saturation, dampening, and the real feeling of it 'pushing air' - really shine live in a room, whereas there's often not as much room for those sorts of peculiarities in a mix with a band. of course, more and higher quality mics in a room with tempered acoustics can make it sound a lot more natural and give more options to fit it in the mix, but again, i don't have the space or expertise or funds for that 😅

[pdf64]

Unless playing the lowest notes on a bass at full power, guitar amp OTs are very unlikely to saturate.

[mxrshiver]

Unless playing the lowest notes on a bass at full power, guitar amp OTs are very unlikely to saturate.

yes. lol i play 7 string tuned down to G# (52Hz) and 6 string bass, and really, REALLY like OT saturation for certain 'blown out' sounds. also, lots of guitar amps spec their OT's power handling down somewhat to save a buck, don't they? i thought that was the major factor in OT saturation.

[pdf64]

Haha, fair enough 
It’s worth bearing in mind that when the OT saturates, its primary inductance will collapse, so the primary impedance will follow suit. The output valves will then only be loaded by the primary resistance, and their dissipation will shoot up.

[mxrshiver]

Haha, fair enough 
It’s worth bearing in mind that when the OT saturates, its primary inductance will collapse, so the primary impedance will follow suit. The output valves will then only be loaded by the primary resistance, and their dissipation will shoot up.

oh wowwwwww i knew it was running them extra hard, but i didn't know it ran it all the way down to... wait, you mean the load at that point is just the DC resistance?! i guess i'd better measure that and see where the loadline is for it before i go to try and record stuff at that level where i might have to keep it like that for many takes... thank you so much for telling me how that works!!

[HotBluePlates]

Haha, fair enough 
It’s worth bearing in mind that when the OT saturates, its primary inductance will collapse, so the primary impedance will follow suit. The output valves will then only be loaded by the primary resistance, and their dissipation will shoot up.

oh wowwwwww i knew it was running them extra hard, but i didn't know it ran it all the way down to... wait, you mean the load at that point is just the DC resistance?! ...

"Transformer Saturation" ≠ "Distortion"

When the transformer saturate in the way pdf64 is talking about, yes the load impedance drops to the transformer's DC resistance.  But when that happens, power output drops to almost-zero (because there's no load to develop voltage across).

Meanwhile, your output tubes redplate & melt.


If you don't experience an almost complete drop of output power, then you're not really saturating the output transformer.

[mxrshiver]

"Transformer Saturation" ≠ "Distortion"

When the transformer saturate in the way pdf64 is talking about, yes the load impedance drops to the transformer's DC resistance.  But when that happens, power output drops to almost-zero (because there's no load to develop voltage across).

Meanwhile, your output tubes redplate & melt.


If you don't experience an almost complete drop of output power, then you're not really saturating the output transformer.
[/quote]

this makes a lot of sense... i THOUGHT the DC resistance sounded like way too heavy of a load to even have the audio sound like... anything at all haha!

my understanding was that the OT contributes to distortion as it approaches saturation, as it begins to clip the waveform, essentially limiting the voltage swing in order to supply the current demanded. is this accurate?

[HotBluePlates]

... my understanding was that the OT contributes to distortion as it approaches saturation, as it begins to clip the waveform, essentially limiting the voltage swing in order to supply the current demanded. is this accurate?

That's what I thought a long time ago.  Now I'm not as-sure we run a transformer to a point it materially distorts the audio.

Regardless, we tend to learn the wrong terminology from Amp & Pedal controls, where "Drive" and "Saturation" may be labels on controls that allow more/less signal to pass.  And if you turn those up, something else in the circuit winds up distorting.

On the other end of the spectrum, transformer designers say "saturation" to mean that any further increase of voltage or current cannot cause the transformer's magnetization to increase.  Even if we can apply enough/to-little signal to a transformer to encounter distortion in the transformer's transfer-curve, we haven't "saturated" the transformer the way designers mean.

If we do "saturate" a transformer (or inductor), it stops being a transformer.  Inductance drops to zero and the couple-hundred ohms of winding resistance is all we have present at the transformer primary.

     Ohm's Law --->  Voltage = Current x Resistance
                              Power = Voltage x Current
                              Resistance ---> kilohms fall to hundred-ohm = Primary Volts falls very-low
                              Saturation Power ---> Few-Volts x Same-Current = Watts drop to almost-nothing

So when you saturate your output transformer for-real, speaker volume drops to almost-nothing.  Most likely, you heard your amp very-loud when you heard the distortion & just assumed the output transformer was "saturating/distorting" along with the output tubes (and/or preamp).

[mxrshiver]

... my understanding was that the OT contributes to distortion as it approaches saturation, as it begins to clip the waveform, essentially limiting the voltage swing in order to supply the current demanded. is this accurate?

That's what I thought a long time ago.  Now I'm not as-sure we run a transformer to a point it materially distorts the audio.

Regardless, we tend to learn the wrong terminology from Amp & Pedal controls, where "Drive" and "Saturation" may be labels on controls that allow more/less signal to pass.  And if you turn those up, something else in the circuit winds up distorting.

On the other end of the spectrum, transformer designers say "saturation" to mean that any further increase of voltage or current cannot cause the transformer's magnetization to increase.  Even if we can apply enough/to-little signal to a transformer to encounter distortion in the transformer's transfer-curve, we haven't "saturated" the transformer the way designers mean.

If we do "saturate" a transformer (or inductor), it stops being a transformer.  Inductance drops to zero and the couple-hundred ohms of winding resistance is all we have present at the transformer primary.

     Ohm's Law --->  Voltage = Current x Resistance
                              Power = Voltage x Current
                              Resistance ---> kilohms fall to hundred-ohm = Primary Volts falls very-low
                              Saturation Power ---> Few-Volts x Same-Current = Watts drop to almost-nothing

So when you saturate your output transformer for-real, speaker volume drops to almost-nothing.  Most likely, you heard your amp very-loud when you heard the distortion & just assumed the output transformer was "saturating/distorting" along with the output tubes (and/or preamp).

fair enough! OT saturation sure makes a ton more sense to me now. i just figure there's got to be some sort of gradient approaching that point, like the signal isn't just gonna be wholly unaffected by the OT approaching full saturation until it all of a sudden disappears. i'm sure in a lot of cases it's power amp distortion being clipped to such an extent that it sounds like the OT is saturating - can't really speak to whether most guitar amps actually push that limit. but i figure if any amp is capable of it it's this jacked up 59 Bassman Reissue, that is running 6L6GC at quite a comfortable and cool bias point, but is pumping about 68W audio power into a 50W OT... and almost the whole Class B side of the waveform is beyond the max dissipation zone heading up to around 300mA at the max swing because it's heading through a heavy 4k-ish ohm load impedance at about 500V on the plate and the exact same on the screens... so there's just a LOT being sent through this sucker.

this is mostly conjecture and guesses on my part! but the sound i believe i'm hearing that is signifying OT saturation to me isn't added harmonics, not distortion in that sense. it's the opposite of the "pushing air" feeling those powerful 10"s give at high volumes, like it's "stealing air" every time a hard low chord is hit. i've got a master volume modded into this guy, and with the channel volume just past noon, i'll start getting power amp distortion at around 60% on the master, speaker distortion around 60-70%, and the "stealing air" effect sets in around 80%. at max it gets to where there's not much attack at all to chords unless you're playing really soft, it's really slick for a heavy blues rhythm that still has lotsa dynamics. this makes sense to me with the idea that the waveform isn't so much 'distorting' as the OT approaches saturation, as it is leaving gaps when it momentarily cannot react with massive voltage/current demands.

[mxrshiver]

oh, as to my original aim - i think i'm gonna go for the 2.67 ohm power resistor, but will probably spend the most time with 1 of the 8 ohm speakers disconnected to run into the Torpedo, so i can record DI at the same time as a live mic'ed setup. i'll just deal with the noise - it sounds way better from the back anyways 😂

[thetragichero]

having used a resistive load while scoping amps, if you're really going to attach a power resistor to the ot secondary, plan on several times the output wattage.... maybe three 8ohm 50w resistors in parallel to your 8 ohm load box?
why? because they get HOT and eventually the value will drift. you want em to stay cool as a cucumber

[mxrshiver]

having used a resistive load while scoping amps, if you're really going to attach a power resistor to the ot secondary, plan on several times the output wattage.... maybe three 8ohm 50w resistors in parallel to your 8 ohm load box?
why? because they get HOT and eventually the value will drift. you want em to stay cool as a cucumber

excellent tip, thank you!!