Plug this pedal into any high wattage power amplifier and experience the Super Valve Technology sound without tubes
This design is based on the original Ampeg SVT but with the 5 frequency bands mid boost / cut rotary switch used in the later SLM MkII model.
I left the bass cut option in the ultra bass switch control out as well just as they did in the MkII
However I modeled the gain and phase relationships of the original amplifier which used the 12WD7 tubes as much as possible. One thing I feel worth mentioning is the back to back clipping diodes used by Ampeg at the output of the preamplifier section. these diodes are no longer easily available but have very similar characteristics to the good old standard 1N4148 devices, and these are what i have used in the pedal version. They have a Vf of 1 volt according to the datasheets but in practice I have found them to conduct at slightly lower voltage. A very interesting thing with the Ampeg design is that these diodes clamp the preamp signal voltage just before the output tubes and so the clipping is predominantly due to these components NOT the output power amplifier section 
I do not recall ever seeing this used in any other tube bass head by other manufacturers. The Ampeg SVT power amplifier stage has about 43dB voltage gain and apart from a slight high and low end rolloff accurately and transparently tracks the preamplifier response and phase characteristic. For this reason I have made no attempt to model the power amplifier section behaviour in this pedal other than duplicating the low end and high end rolloffs in the final buffer stage of this design.Another thing worth mentioning
In the Ampeg SVT, there is a cathode follower buffer stage following the mid range section. In my design I have used a gain stage in this position to raise the signal level enough to clip the back to back diodes at exactly the same level as in the original tube amplifier 
I had to reduce the gain of the three preceeding stages to avoid clipping the op amps and this is why the need for the additional gain stage after the tone controls. Working with 9 volts instead of 300 volts means careful consideration of gain structure is paramount in a design such as this.Another interesting observation - the bright input actualy reduces the low end response of the SVT, and this gives the impression of a brighter sounding amplifier
. instead of having two input sockets, I decided to use a single input socket and incorporate a bright / normal mini toggle switch I changed the mid frequency boost / cut frequencies slightly - 220Hz, 440Hz, 880Hz, 1k8Hz and 3k5Hz - as close as possible to octaves of "A 440 "
Enough waffling - here it is - i will build and verify this in the next few weeks - waiting on some inductors to arrive from China ( I intend to use five of these 10mH coils in a series chain https://www.ebay.com/itm/10pcs-Inductor ... 2749.l2649)
cheers
bajaman
- baja ampeg svt vero cuts.png (7.62 KiB) Viewed 18374 times
