Texture Control On Peavey ValveKing VK 112

[TomCarlos]

MEF:

I am helping a friend with his Peavey ValveKing 112 combo amp. Unlike other posts, I start with a working amp!!

There are two modifications planned. The first is to update the Bias Control by replacing R205 (33K) with a pot. I have found other posts for that info so we are good on this mod:

VK112 Bias Pot Mod

Peavey VK100 Valveking Bias Mod

The second mod replaces the Output Transformer with one that will match to an 8 ohm load. An initial test proved successful; we are waiting for the permanent replacement OT. This is being done to accommodate his collection of preferred speakers and extension cabinets.

And now, the "Texture" control. According to the info from Peavey -

"This feature is used to fine-tune the power sensitivity, response and “break-up” of the power amp section of your ValveKing™ amplifier. Normal, full-power, Class A/B operation results when the TEXTURE control is set at its fully clockwise (wide open) position and should be used as a starting point when setting this control. As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage. This setting still allows the unused power tube(s) to draw idle current, thus retaining the efficiency of the standard Class A/B topology. In this mode, power output is also reduced by as much as 60% versus maximum rated power."

As I look at this, it appears this circuitry is fed from the output and loops back to the Phase Inverter. I did find another post on this topic:

ValveKing 112

In that post, Enzo explains that "The texture control is nothing more than a level control for ONE SIDE of the phase splitter." Ok, simple enough.

My question is - will changing the load from 16 ohms to 8 ohms affect how this feature works? It's not clear to me if the control is driven by output level, frequency, or load impedance (or a combination of 1,2, and/or 3?

Thanks, Tom

[J M Fahey]

In principle nothing will change.
Power tubes have NO CLUE nor care about seconday/speaker impedance, they *always* see, say, 4500 ohms or whatever plate to plate impedance is, it´s the job of the transformer to show that to them.
Secondary windings change depending on actual speaker load , precisely to always show the same at the primary side.

[TomCarlos]

Thanks Juan...

[g1]

You were probably wondering about the NFB loop being affected.
Agree with Juan, the texture control has nothing to do with that, just for some reason it's mounted on that same jack board.
However, I believe the NFB and resonance control will be affected by the OT secondary impedance change.
Some local math whiz can hopefully guide you as to what value to tweak.
If not, it may not be a noticeable change anyway.

[eschertron]

From a less-than-whiz look at your schematic, I can say that the new 8R OT will give you .707 of the voltage the original 16R OT had. My guess is that adjusting R165 lower will improve the NFB ratio towards what you want. The conspiracy theorist in me wants to suggest that the reason for R165 is so that it can be replaced by a jumper to make an 8R OT work with no other changes. But this is not backed up by any math.

I'll also echo that the texture control connects to ground (not signal) at the OT, so no worries there.