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Re: 5E3 tone vs tweed champ

12/11/2004 5:54 PM
Bruce /Mission Amps
Re: 5E3 tone vs tweed champ
They are not class A.  
A 5E3 is a class AB amp and or AB1.  
You are right, it is not just about how far a previous stage can drive the power tubes into clipping or shut off.  
Roughly speaking,  
a class A amp circuit is one so designed that the bias is set on the linear portion of the characteristic curve based on current and B+ voltage and in which the input signal amplitude is confined to the linear operating region.  
Class AB1 is an amplifier with slightly higher bias than the Class A and with a larger input signal having amplitudes reaching (but not exceeding) the cutoff AND positive regions of grid-signal swing.  
Class AB2 is an amplifier with higher bias than the Class AB1 and with grid signal excursions extending into the cutoff and positive tube characteristic regions.  
This is tough to do with simple RC component coupled drivers and the reason why some designers like to use a transformer to drive the power tubes.  
In the case of the little 5E3, the cathodyne phase inverter driver has no problem swinging the grids way past their bias of around 19vdc-22vdc.  
And as far as class A goes, the plate voltages are a little high, at 335v to 355v (plate to cathode) to stay idling squarely in class A and still keep the tube at it's class A rating with positive or negative going excursions.  
We are talking about the driver and PA as a class A circuit, and in class A, it cannot be built with plate voltages and bias voltages that allow either power tube to be driven hard enough with it's negative going signal to shut the tube off, or it's postive going swing to clip/overdrive/draw grid current.  
Shutting one tube off by overdriving the bias makes it class AB or AB1, even if it is only shut off for a small portion of the cycle.  
But that is what happens eventually, hence the reason I'm sure it's really class AB or AB1.  
Whether or not the original designers were thinking class A or not, I don't know but I don't think so.  
You're right on this point too, with the higher AC mains and plate voltages we see plus the way we play these amps, all that is down the drain anyway!  
12/10/2004 8:58 PM
Single-ended amps like the Champ don't have the same sag and smooth transition from clean to overdrive that a 5E3 gives.

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