Texture Knob

[Andy Le Blanc]

Whats the deal with the peavey texture knob. On the dash it swing from
"class A" to "class AB", but has no bearing at all on the operating point of
the circuit. I seems very misleading to associate "class" with this feature.

[Tavda3172]

Peavey's "Texture" knob is similar to Kevin O'Connors "Body" control. It dials out one of the output tubes in a push-pull pair, allowing a single ended tone due to predominantly even-order distortion. If there is just one tube conducting during the audio cycle and one tube contributing to the audio signal at all times (if it actually works out that way in this circuit) that should satisfy the definition for class A. Therefore, turning the knob all the way up/down gives you Push-Pull, class AB, and the other way gives you (at least something similar to) Single Ended, class A. How it works in practice is for someone else to explain. Hope that helps.
-Kevin

[Andy Le Blanc]

Thank you..... The peavey system is on the cathode side of a cathodyne inverter.
It goes to a cap, and then the wiper of a 1M pot with a 82k on the ground side, then a second cap out.
You create an extreme imbalance of the AC signal. It seems a very simple
and safe way to drastically alter the performance of a push pull circuit.

But class is determined by conduction angle, the data use to find the
conduction angle it the difference of currents between zero and max signal
conditions. If your not changing this you not changing "class", no matter how
closely the circuit appears to resemble somtheing approximating single ended.

Even many single ended affairs aren't strictly true class A.

Anyway..... It is a functioning tone tweaker.

The one other thing I've noticed in some peavey designs, is that g3 of the
power tubes is incorporated into the bias system (thru a 10k res.) instead
of the usual cathode connection. I have seen some alternate grid connections
for a el34, I guess you can sub a 6l6 but with pin one absent, it seems to
be specifically for el34.

[Wayne]

I thought one of the builders here had incorporated a control like this in one of their amps, but called it something else. The "dead" tube (the one not receiving drive) still conducts bias current to keep the OPT from saturating in SE mode.

The G3 thing I read about at Tone Lizard. Negative voltage on G3 is supposed to keep G2 current down on peaks. Traynor did it in the 70s in response to increasing B+ with decreasing tube quality. As far as I know, this is compatible with 6L6 because that pin isn't used.

W

[mhuss]

RE varying drive to one side of a push-pull amp -- Mark Durham (RIP) had done this several years ago on his Vajra amp design published on 18watt. I don't know if he thought it up or cribbed it from elsewhere, but that was the first place I saw this idea implemented.

If you make G3 (on a true pentode) negative with respect to the cathode, it will limit current flow in the case of bias failure, hopefully keeping the tube from burning up. I don't think it significantly alters normal operation, but it must slightly alter the characteristic curve.

--mark

[Merlinb]

But class is determined by conduction angle, the data use to find the
conduction angle it the difference of currents between zero and max signal
conditions. If your not changing this you not changing "class", no matter how
closely the circuit appears to resemble somtheing approximating single ended.

If one of the valves really doesn't get any input signal at all, then the remaining valve does operate single ended (albeit rather cold biased for SE), and so it is fair to say the amp runs in class A.
(Single ended amps are class A by definition, unless they are class C, which is unlikely in a guitar amp)

[Andy Le Blanc]

Its tough to arbitrarily apply a definition, even SE amps sometimes fall short,
the math has short comings too. but the control is one half of a post PI
master volume, your not shutting down half the P-P circuit, your reducing signal, but its still a P-P circuit.