Vox AD60VTH Valvetronix Head

[guitarmanrrg]

I've doing some research in the hope of finding a "good" modeling head.
The Vox AD60VTH Valvetronix Head seems to have mixed reviews. I've heard some clips on Youtube that sounds good. What is the deal with the TUBE in a Solid State amp?

I'm not sold on Vox yet.
Can someone recommend a inexpensive Solid State Head? I'm looking for something light.
I already own the Crate Powerblock.
Thanks.

[teemuk]

In that amp, both halves of the tube are wired as sides of a push-pull stage that is transformer-coupled to "amplified" reflection of the used speaker load. Basically, the tube stage works like a power amp driving a speaker – but just imagine all impedances scaled a lot higher. The output signal of this Valve Reactor stage is simply fed to the power amp. The circuit topology is basically identical to a generic LTP-input tube power amplifier but the LTP is realized with MOSFETs. The tube stage can be configured to run in class-A or class-AB. In those early amps AD60 and AD120 the rail voltage of the tube stage was about 180V (if I remember right) and therefore transformer coupling was done with a real transformer. Later circuit revisions only had a 30V supply rail ("starved plate") and a "virtual transformer" circuit. Sort of "heretic".

Since a single 12AX7 can only produce few milliwatts of power the signal is naturally further amplified with a solid-state power amplifier running in class-AB. Attenuator in between the tube stage and SS power amp controls the amount of output power. Second half of the same attenuator circuit controls the amount of feedback (from the power amp's output) that reflects the speaker load BUT in higher impedance to the secondary side of the coupling transformer. This to keep the amount of reflected impedance unchanged regardless of output power level. A second feedback loop with an implemented tone control is wrapped around the tube stage. This feedback loop can also be disabled for some tonal effects. These analog effects are controlled by the preamp batches with the aid of digitally controlled switches.

There is a headphone jack post the tube Valve Reactor stage that is filtered to mimic the equalising effects of a speaker cabinet. Preamplifier uses DSP.

There are few example schematics at the schematic section of this forum. They are for the lower power never revisions of those amps. The basic topology is nevertheless principally about the same. Just imagine a real transformer in place of the virtual transformer circuit, MOSFETs in place of the BJTS in the LTP and a higher B+ voltage. Yeah, the power amp is also discrete in those early amp models. The new "Black Diamond" has a class-D power amp, which is a very cool idea (pun intented).

[guitarmanrrg]

WOW. That gets really intricate.
Thank you so much for all the info. I've got a ways to go before I understand it all.

When I read that SS and Tubes are combined it make me think of oil and water?

Are they reliable?

[mesaboogieonline]

I've played through several vox ad heads and combos and found that they all sound pretty good for what they are.  They obviously don't do high gain very well but they work well for the in between.