Modern Multi-Watt Amps

[jlapalme]

Hi all,

I noticed that their are many multiple watt amps available on the market.
Some of those amps use use a switch to change the number, the configuring and wiring of the power amps tubes.
However, I don't really get what some of the other amps are doing.
Would anybody know how these models are implementing multi wattage :

Peavy Classic 20 mini head.
Laney CUB-12R.

Cheers,

James

[shooter]

no clue;
this is my all time favorite version of power switching 

[kagliostro]

if this are the correct schematics I don't see how they can change the power 

http://www.filedropper.com/laneycub-12r2010sch

http://www.filedropper.com/peaveyclassic20

Franco

[HotBluePlates]

... Would anybody know how these models are implementing multi wattage :

...
Laney CUB-12R.

if this are the correct schematics I don't see how they can change the power 

http://www.filedropper.com/laneycub-12r2010sch

The Laney has 2 input jacks, one marked "15w" and one marked "<1w."

From Franco's schematic, the "<1w jack" activates a relay that brings in a voltage divider between phase inverter & output tubes.  Drive to the output tubes is ~25% of what it is when the "15w jack" is used.

It's not "changing power" so much as just being a preset post-phase inverter master volume.

__________________________

I haven't seen a schematic for the mini head, and unfortunately the schematic Franco linked is not the correct version.

[jlapalme]

... Would anybody know how these models are implementing multi wattage :

...
Laney CUB-12R.

if this are the correct schematics I don't see how they can change the power 

http://www.filedropper.com/laneycub-12r2010sch

The Laney has 2 input jacks, one marked "15w" and one marked "<1w."

From Franco's schematic, the "<1w jack" activates a relay that brings in a voltage divider between phase inverter & output tubes.  Drive to the output tubes is ~25% of what it is when the "15w jack" is used.

It's not "changing power" so much as just being a preset post-phase inverter master volume.

__________________________

I haven't seen a schematic for the mini head, and unfortunately the schematic Franco linked is not the correct version.

Great. Thanks. So I guess that means that it is impossible to drive the power section ?

[d95err]

It can drive the phase inverter, which typicallly accounts for most of the distortion in a classic non-master volume amp. So the level of distortion when cranked is probably similar.

However, you won’t get the compression or ”sag” as when running at full power, since rhe power supply doesn’t have to provide a lot of current.

[HotBluePlates]

... the phase inverter, which typicallly accounts for most of the distortion in a classic non-master volume amp. ...

Got a source for the analysis that says this?  I've seen it repeated verbatim on other amp forums, as though it is a plainly-obvious fact.

Personally, I disagree, thinking that output tube grid current clamps the output of the phase inverter (meaning output tubes distort first).  But I'm wanting to understand where folks are reading this, as maybe I'm missing something.

[d95err]

I have tried post-PI master volume in several amps and my experience is that the level of distortion stays fairly constant when you turn it down. This happens both with or without negative feedback. To me this indicates that the power tubes contribute less distortion than one might expect.

The power tubes certainly distort to some degree though, no question of that.

The type of power tube make a difference of course. For example, EL84s should be much easier to drive into distortion than EL34s.

[pdf64]

By turning a type 1 or 2 master volume down from max, the degree of distortion might only reduce a few dB, perhaps a negligible amount. But that shouldn’t be taken to indicate that the most significant clipping stage with the master at max is not the power tube control grid - cathode diode.
It has to be, as how else could the amp achieve its max class AB power output?

[PRR]

> output tubes distort first

Simple good engineering. If you pay for 45 Watts of output, but the driver craps-out before that, it is like designing a 450 horsepower race-car but assigning a driver too short to push the pedal all the way down. Yeah, I've had machines that I couldn't reach the pedals well, but we usually try for an ample fit.

But indeed it may be hard to tell when the limit fades-over from power-tube to driver, especially with the long-tail driver.

[HotBluePlates]

I have tried post-PI master volume in several amps and my experience is that the level of distortion stays fairly constant when you turn it down. ... To me this indicates that the power tubes contribute less distortion than one might expect. ...

Thanks for clarifying.

If we're talking traditional non-master volume amps, the output tubes always distort first.  If it's a home-brew design, all bets are off (depending on the builder/designer's capability/intention).  If we're talking a master-volume amp, the stage right before the master distorts first except when the master is full-up (all bets again off for a home-brew design, or any modded amp).

In his Bassman Book Kuehnel shows the output tubes distort for-certain with a 48v peak input (to one side).  However, the phase inverter has an output headroom of (at least) 57v peak; page 112 says, "... an overdriven Fender Bassman 5F6-A is characterized by power amp distortion, not preamp or phase inverter distortion, and that downstream stages are overdriven before upstream stages under all but extreme control settings."

This is reinforced by Kuehnel's Guitar Amplifier System Design, were Chapter 3 (which gets to the nitty gritty of designing the amplifier) proceeds with the power section, then power supply, driver, feedback, and master volume in that order. The preamp is dealt with after all those other sections are done, as the end of the signal chain sets the requirements for whatever is placed before.

I have tried post-PI master volume in several amps and my experience is that the level of distortion stays fairly constant when you turn it down. ... To me this indicates that the power tubes contribute less distortion than one might expect. ...

If you begin with anything other than the master-volume at maximum, the test is already rigged by reducing signal level driving the output tubes.  If the master-volume is not incorporated properly into the circuit it can destroy the headroom of the phase inverter, leading to a self-fulfilling prophecy.

It's also possible to skew the situation with pedals.  A blackface Fender amp has a tone stack & volume control before the 2nd gain stage.  The input stage of these amps can usually handle a ~1v peak signal before serious distortion, which is often 5-10x what is applied by a naked pickup.  It's possible to use a strong enough boost pedal to slam the input stage into clipping while keeping the volume control almost off, so that the 1st gain stage is the first thing to distort.

The exceptions to the above tend to prove the rule, as in Laney's "<1w input" that reduces drive signal to the output tubes so severely the phase inverter is guaranteed to be the first thing to distort.  But it's also so atypical an approach that it confuses people.

The Laney has 2 input jacks, one marked "15w" and one marked "<1w."
...
It's not "changing power" so much as just being a preset post-phase inverter master volume.
...

... So I guess that means that it is impossible to drive the power section ?

Not at all!  If you want the output tubes to distort, you plug into the "15w jack" and crank the amp up beyond 15w of output. Start with this amp's "Volume" control full-up, and slowly advance the "Gain" control until you hear distortion.  It's gonna be loud!

[shooter]

for those taking notes, this is where a scope comes  in real handy 

[jlapalme]

Thanks ............  ..... ......  .......  It's gonna be loud!

I get that... but my comment was regards to the <1 watt input. So should I understand that it is not possible to drive the power tubes into distortion with the <1 watt input and that any distortion heard with the Volume at full is caused by PI distorsion ?

{EDIT} not necessary to quote-back entire large messages. -PRR

[pdf64]

Yes, that’s probably correct.
Note that the actual power output available in that mode will be affected by the output stage gain / plate current / bias setting.

[kagliostro]

As to have distortion at low Watt settings, as far as I can know, one of the best solution is to use a VVR on Power Tubes and, in junction, a PPIMV

this way you can have distortion on the Preamp and Phase Inverters Tube + distortion on the (low powered) Power Tubes, the lost is on OT Core distortion


Different PPIMV will give different tone performance, so you must find the correct one for your taste and your amp

Franco

[jlapalme]

As to have distortion at low Watt settings, as far as I can know, one of the best solution is to use a VVR on Power Tubes and, in junction, a PPIMV

this way you can have distortion on the Preamp and Phase Inverters Tube + distortion on the (low powered) Power Tubes, the lost is on OT Core distortion


Different PPIMV will give different tone performance, so you must find the correct one for your taste and your amp

Franco

What does OT Core distorsion mean ? What does VVR and PPIMV mean ?

[kagliostro]

EDIT (For more clarity I added some explanation links - I hope that now is more clear)

OT core distortion

Is the amount of distortion due to the saturation  of the iron on the Output Transformer

https://www.pearl-hifi.com/06_Lit_Archive/16_Lists/Joe_2018/1802_Distortion_in_Xfrmr_Cores.pdf

(Note that in HiFi OT Core Distortion is unwanted but in guitar amps it may be a pleasant optional)

VVR

Means Variable Voltage Regulator

https://dalmura.com.au/static/VVR%20info.pdf

https://londonpower.com/power-scaling-faq/

https://londonpower.com/power-scaling-vs-variable-power/

(Note that at London Power they call power-scaling a circuit similar to the VVR circuit)

PPIMV

Means Post Phase Inverter Master Volume


Here some explanation about MV (Pre and Post Phase Inverter Models)
https://robrobinette.com/5e3_Modifications.htm#Add_Master_Volume
https://robrobinette.com/images/Guitar/Trainwreck/The_Trainwreck_Pages_searchable.pdf


https://grangeramp.com/product/ppiv-master-volume-kit-2/

Franco

[pdf64]

My understanding is that core saturation is something that’s mentioned far more frequently on guitar forums than it actually occurs in a guitar amp.
It’s generally perceived to sound awful, as when the iron saturates it may as well not be there, so the winding inductance collapses and the impedance drops pretty much to the resistance value. Hence transformer and amp designers to their best to avoid it.
If saturation does occur, it’s when the OT is pushed beyond its rated power at the very lowest frequency of its intended range.
At almost any signal level below saturation, magnetic materials, especially those used with PA grade, vintages type OTs, have non linearities in their magnetisation curve etc, which may add pleasing, low order harmonics to the signal.
Have a google of bh hysteresis curve, eg http://coolmagnetman.com/magfund07.htm