Sure Vocal Master for guitar amp

[Ritchie200]

Talked to a guy today who is using an old Sure Vocal Master (amp and cabinets) for a guitar amp and he SWEARS by it.  He said he uses the Anti-Feedback filter switches to give the "amp" completely different tones and character which he says are amazing.  Being solid state he is using it as a clean boost for a Line 6 that he is running into it.  Yes, I know, I was thinking the same thing you all are...  However, he has been playing for over 30 years and also has an old Marshall stack, old Ampeg, old Fender.... (saw the pictures so I have no reason to doubt his story) so he has some nice tube amps.  My dad did something similar to an old mono 6V6 phono amp he "converted" for me for guitar back in the 60's - basically some notch and high pass filters (in series with the existing treble and bass pots) and it sounds amazing as well.  Looking at the Vocal Master schematic it shows the Anti-Feedback circuit as a series of linked RC filters, however they also incorporate a 900mH choke?  Can someone who knows what the heck we are looking at take a look and esplain it in plain English to me?

Again, I would not have given it a second thought but this guy was acting like he found the Holy Grail and due to my experience with something similar certainly peaked my interest.  My little 6v6 amp can sound like everything from Fender clank to full blown Tony Iommi Sabbath - without a pedal.

Jim

http://shurevocalmaster.com/documents/Shure_Vocal_Master_pro_va300%20schematic.pdf

[HotBluePlates]

... the Anti-Feedback circuit as a series of linked RC filters, however they also incorporate a 900mH choke?  Can someone who knows what the heck we are looking at take a look and esplain it in plain English to me? ...

The A, B and C anti-feedback filters are series RL resonant circuits. The L has a rising reactance/impedance for rising frequency, and the C has a rising reactance/impedance for lowering frequency. There is a resonant frequency where their reactance/impedance is the same, and is a minimum overall for the series circuit.

You see these filters A, B and C ultimately go from collector to ground, so they are there to reduce the strength of some band of frequency. You calculate the resonant frequency with the equation on the bottom of the attached picture.

For resonance, I calculate A as centered on ~4.3kHz, B on ~2kHz, and C on 923Hz (likely intended to be ~1kHz). To switch these out, a 220kΩ is in series with each filter to isolate them from the collector output. When they are switched in, the switch shorts the 220kΩ resistor and places a 10kΩ resistor in series between the collector output & filter to ground.

Filter D is a parallel resonant filter, made of 25uF and a 900mH inductor. For parallel resonance, the circuit impedance is at a maximum at resonance and equal to the resistance in parallel with the L & C (10kΩ in this case). Because this filter is in the emitter circuit, the increase of emitter resistance to 10kΩ plus the fixed 470Ω notches out gain. For 25uF & 900mH, this happens at ~33Hz, and is probably a pretty broad notch. When switch out, the switch shorts a 33kΩ to bring 6x (!) 50uF caps in parallel with the rest of the circuit. This lowers the resonance to ~9Hz, well below audio range.

[Ritchie200]

Well those are all within the normal speech/singing range.  I wonder how that would translate to a guitar?  He talked like he punched in several of the filters at once some times.  That would be a big chunk scooped out, would it not?

Jim

[HotBluePlates]

... He talked like he punched in several of the filters at once some times.  That would be a big chunk scooped out, would it not?

Well, I'm not inclined to attempt to figure Q (bandwidth) for the filters, especially since we don't know enough about the inductor. Since the filters are spaced an octave apart starting ~1kHz, we can guess there is pretty narrow bandwidth of the notches.

1kHz is above pretty much all guitar fundamental notes, so you're talking about adjusting harmonics somewhat, which is probably most audible with distortion.

[PRR]

> he found the Holy Grail

Everybody finds their own Grail.

They may be like snowflakes (no two alike; though that has not been proven.)

> esplain it in plain English to me?

No. It is complicated, very dated, and not easily re-created.

HBP gave the basics. A 1990 ROSS graphic EQ pedal used cut-only will do most of this, and more too. (That may be the problem with grafic EQs.)

I would suggest that band "D" may be the 1uFd cap, not the 50. This puts the anti-ring at 167Hz. (The 50 puts it sub-sonic, "out".) 167Hz is a much more likely area for limited-bass (6x9s) speakers in boomy rooms to need a little smack-down.

The 920Hz notch has Q of about 2 (Zl is 5.1K and there's maybe 10K-14K of dead resistance) so almost two octaves nominal bandwidth. This may be near the Fender mid-notch. However the notch depth is 3dB or 6dB, so not as deep as a full-dip Fender. Also flat-bottom where Fender has a narrow dip bottom.

Many matchbooks or much CPU could be expended on curve-plotting. But where do you get 900MH inductors, shielded (not speaker crossover), at a price you can like four of them? Also you need to duplicate Shure's amp stage impedances, and Q29 is kicking my butt. But clearly awful low for tube-work.

The 220K are just pop reducers for the caps. The switches could be SPST, but I remember those switches very clearly. They were cheap and very cheesy. They needed 2 rows for lateral stability, and both end-contacts for good feel both ways. The contacts were just stamped metal strip, there's a machine would spit 1,000/hour. The cardboard backplates were always 6-slot even on SPST versions; they always used the 6-knife stamp. So the DPDT cost little to no more than any other. And Shure probably did need DPDT other places. So they just standardized on the one part.

You should also note that the VocalMaster's power output stage is unusual in that it has both voltage feedback and current feedback (T2). Damping control rather than infinite damping as in hi-fi or slight damping as in many classic Fenders. This tweak has to be finagled for the specific speakers to be used.