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Groove Tubes STP-G II. Tube power amp, reactive load, cab-sim filter. $1400 list.

Groove Tubes STP-G II 35 watt all tube preamp/amp w/ spk emu. Options: Rack case in tweed or brown Tolex, chrome front panel, red chassis, chrome cans. groovetube at aol.com www.groovetubes.com. 2 6V6 power tubes. New features for version II: EL84 capability, Class A or A/B switching, parallel FX loop with balance [blend/mix] control. 24 watts into 8 ohms when Class A. 35 watts into 8 ohms when Class A/B.

On my early production unit at least, the STP-G II has a 35 watt 2 x 6V6 tube power amp. There was no loop to separately play with the amp and the load and speaker simulator, very unfortunately. Actually I should have listened to the speaker outputs to see if they had such overly-cut treble, like the line outs. I owned the original, bad version of the STP-G II. The tone sounded power tubish, but was incredibly dulled. They must have overdone the attempt at trying to use treble roll-off to "emulate" a speaker cabinet. The latter effort is inadequate to capture speaker dynamics and speaker-tube interaction dynamics, I believe, but on top of that, they just plain dulled away all the sound until nothing was left. In the original design. I heard they improved something about the unit later, but I was tired of this 30 watt unit. Too powerful anyway. Why not just use a single EL84, so you can have a nice 3-watt tube power amp to push against a real speaker, with no emulation/attenuator getting in the way?

GT Guitar gear pricing - STP-G II - Studio guitar tube preamp, 35 watt tube amp, soak, and Speaker Emulator to record direct, power a speaker, or drive a monitor amp. Switchable output between Class A & Class A/B $1395.

Soak history and patents

Re: Power Attenuators/Soaks. (From here you can read the entire thread.)

Steve wrote: > Hello Randall Aiken,
> I'm going to pass your designs on to my electronically inclined brother. I myself have little electronic knowledge--but your post impels me to ask a couple of questions:

> What's with the Aspen Pittman Groove Tubes Patent on their inductive load?? Do you know what's patented, what's patentable, and what isn't?

I have a copy of the Pittman patent (Number 4,937,874, June 26, 1990). Here is the abstract in its entirety:

"Circuitry is provided for simulating and emulating the sounds of a loudspeaker driven by a classic rock and roll guitar amplifier in the form of an electrical signal which, when input to a recording console or used to drive high fidelity headphones or a high quality monitor amplifier and monitor loudspeaker, accurately reproduces such sounds. The circuitry is incorporated at the output of a classic tube-type rock and roll guitar amplifier, and consists of three basic modules: a load impdedance module (FIG. 3) designed to offer to the tube amplifier output a load impedance as a function of frequency (FIG. 1) which is modeled after and nearly identical (in both real and imaginary components) to the impedance of a classic rock and roll guitar speaker; a highpass filter circuit (24) which emulates the low frequency resonance and rolloff characteristics (FIG. 7) of a classic rock and roll open-back guitar speaker cabinet; and further filtering circuits (26,30,32,34,36,40) which emulate the mid and high frequency characteristics (FIG. 7) of a classic rock and roll guitar loudspeaker. Electrical outputs (50) are provided for driving a recording mixer, headphones, a speaker, and an external power amplifier."

There are 7 claims, and 3 drawing sheets. The reactive load consists of the following circuit topology:

     |               |     |
     |               C1    L2
     |               |     |
  >-- )--------*-----*------
     |         |
     |         R2
     |         |
      ----L3---*----> To filter input

Essentially, this circuit simulates (to a low order approximation) the impedance characteristics of a speaker. The series resistor R1 is the resistive component of the load, the inductor L1 simulates the rising impedance of the speaker at higher frequencies, the parallel combination of C1 and L2 simulate the impedance peak at resonance. There is no attempt to limit the peak of the impedance rise at either the high frequencies or at the resonant impedance peak.

L3 and R2 provide further lowpass filtering of the signal going to the active filtering and tone control circuitry. This circuitry consists of a 30dB pad, further highpass, lowpass, and bandpass circuitry, and output driver circuitry.

Interestingly enough, the patent states that Butterworth filters are preferred for this circuitry because of their "minimal phase error characteristics". Butterworth filters are actually known for their maximally flat amplitude characteristics, and can have quite poor phase and group delay characteristics at higher orders; Bessel filters (and other linear phase equiripple and transitional Gaussian filters) are known for their minimal phase error and maximally flat group delay characteristics.

The patent covers the use of a reactive load for simulating the impedance load variation with frequency, the combination lowpass/highpass/bandpass conditioning circuitry for simulating the frequency response of a speaker, and use of this circuitry when connected to a speaker, recording mixer, linear amplifier, or headphones. In addition, a claim is made where the highpass filter is a slow rolloff Butterworth filter, and the lowpass filter is a Butterworth filter of at least five poles for a quick rolloff. These items represent the seven claims of the patent.

As prior art, the patent mentions (among other things) the Scholtz Power Soak, claiming it is different because it is only a resistive device.

It is also interesting that the patent refers to the Radiotron Designer's Handbook pages 837-838, which show an impedance plot and almost exact replica of the emulation circuit covered by the patent, including circuit values! (At least the RDH4 circuit included a resistor to limit the resonant peak!) I personally feel that this patent should not have been granted when the information was so obviously available as prior knowledge...what's next, Pittman patenting the pentode? But then again, what do I know about patent law? :)

From what I have heard, Marshall is paying a royalty to Pittman for their Power Brake; obviously Scholz didn't have to; I don't know about the other attenuator manufacturers. I don't know whether or not you could manufacture a replica of the RDH4 circuit without getting sued.

-- Randall Aiken

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