freestompboxes.org

reputedly based on the vox UL730 and 7120 amps for revolver/sgt. pepper-era beatles fuzz and more.

I can only guess it's based on this bit, (which appears to be a bit like the fuzzface) with the treble and bass pots merged into a mids pot. It's run on 25v in the amp circuit:
the: sustain pot could be yer typical FF Q2 emitter pot, or maybe adjusting the 220k feedback resistor like the CBread katzenkonig..There also appears to be a "gate" trimpot on the picture of the PCB at bugg's place but the bugger hasn't posted the build doc yet, I suppose it could be some sort of biasing/starve thing.

I too am terribly curious about the schemo in the build docs at bugg's place. I'd assumed it would be more likely based on the first two transistor stages with the diode clipping (the footswitchable distortion on the original amps) left in, and including the mid control, but who knows? I guess we'll see eventually...

pacealot wrote: ↑08 Jan 2023, 22:28 I'd assumed it would be more likely based on the first two transistor stages with the diode clipping (the footswitchable distortion on the original amps) left in, and including the mid control

Looking again at the PCB picture, I can see three diodes (plus a protection diode?) - I didn't notice the mids control was directly after that first stage so I suspect you are correct.

If it is related to the diode distortion channel, then I'm very curious about the possibility of combining it with the Pharmacist (his version of the Aclam Dr Robert, which replicates the "normal" or non-distortion channel of a 7-Series) to get the best of both worlds!

I spent a bit of time with this circuit a few years back, doing a lot of simulation in LTSpice as well as creating my own version of the preamp (just the first channel as shown below). The distortion/fuzz is triggered by a footswitch which grounds the diode pair and also activates TR3, TR4, which are set up as switches to affect the gains of TR1 and TR2. My LTSpice sims indicated that TR1 and TR2 are operating at a supply voltage of 9V (starting from 25V and going through a series of resistor voltage drops). Hence, you can easily set this up on a breadboard and experiment with it. Note that the TR5 and TR6 transistor amplifier stage is operating at around 20V, which you can get with a charge pump chip.

The main thing I learned in studying the Vox UL preamp is that, aside from the distortion, it produces basically a big, clean amplified signal which overdrives a tube power amp stage. It's this combination of solid state preamp and tube power amp that produces the crunchy overdriven tone you hear on many Revolver and Sgt. Pepper tracks (e.g. Taxman, Dr. Robert, She Said She Said, Paperback Writer).

pacealot wrote: ↑08 Jan 2023, 22:28 I too am terribly curious about the schemo in the build docs at bugg's place.......... I guess we'll see eventually...

Three months on and still no build doc? Come on bugg!

So I'm just going to breadboard the circuit above (i.e. Q1+2 up to the mds control) but the base-to-ground resistor on Q2 looks odd - I've never seen it be the same value as the emitter resistor, usually much higher - Is this maybe where the. gate trimmer visible on bugg's PCB is situated?

Nocentelli wrote: ↑29 Apr 2023, 20:43 So I'm just going to breadboard the circuit above (i.e. Q1+2 up to the mds control) but the base-to-ground resistor on Q2 looks odd - I've never seen it be the same value as the emitter resistor, usually much higher - Is this maybe where the. gate trimmer visible on bugg's PCB is situated?

That’s a mistake on the schematic. It should be 47K and not 3.3K. I think this was confirmed by others after examining actual 7120 amps.

By the way, if you want to know what these amps sound like, here’s an example from YouTube.

Frank_NH wrote: ↑30 Apr 2023, 01:55 That’s a mistake on the schematic. It should be 47K and not 3.3K.

By the way, if you want to know what these amps sound like, here’s an example from YouTube........

That makes much more sense, thanks. My current guess is that the gate trim is the Q1 emitter resistor. I will have a play around today and see if I can come up with something I like: I'm tempted to try a pot between the clipping diodes and ground, and a 2k dual-gang sustain pot in place of the resistors between emitters and electro-caps-to-ground. Nice demo btw, thanks for the link - I love the fact he has a period-correct guitar to go with the amp too.

Nocentelli wrote: ↑30 Apr 2023, 08:01

Frank_NH wrote: ↑30 Apr 2023, 01:55 That’s a mistake on the schematic. It should be 47K and not 3.3K.

By the way, if you want to know what these amps sound like, here’s an example from YouTube........

That makes much more sense, thanks. My current guess is that the gate trim is the Q1 emitter resistor. I will have a play around today and see if I can come up with something I like: I'm tempted to try a pot between the clipping diodes and ground, and a 2k dual-gang sustain pot in place of the resistors between emitters and electro-caps-to-ground. Nice demo btw, thanks for the link - I love the fact he has a period-correct guitar to go with the amp too.

Sounds good. I should note that the the "Dr. Robert" effect pedal is modeling the normal channel and therefore requires a gain stage (in their case, a mu amp overdrive stage) at the end to get any crunch. That's because the the normal channel (and the vibrato channel with distortion disengaged) basically produces a relatively clean signal (with mid, treble, bass EQ), and the crunch comes from the tube power amp.

If you want to model the distortion, then you simply ground connections going to TR3, TR4, along with the diode connection. That is what the footswitch did. Transistors TR3 and TR4 are simply acting as switches. Also note that the voltage divider R14/R15 just down from the clipping diodes greatly reduces the signal. However, this is made up later by the amplification stage after the volume control. The point here is that you need that amplification to get a reasonable signal out of the preamp. So if I were making a distortion pedal from the preamp, I would include the two transistor amp stage running at 18 or 20V.

One problem is that there is no control over the distortion level. It's either on or off. However, you can control this by putting variable resistors between C3 and ground and C5/R13 and ground. According to my LTSpice models, about 1K gets you more of a crunchy overdrive.

Let us know how your breadboard experiments go!

Frank_NH wrote: ↑01 May 2023, 12:27 .......distortion level. It's either on or off. However, you can control this by putting variable resistors between C3 and ground and C5/R13 and ground. According to my LTSpice models, about 1K gets you more of a crunchy overdrive.

Let us know how your breadboard experiments go!

That's what i meant by using a 2k dual-gang for sustain. Going down to zero ohms between emitter and cap-to-ground is a bit much (but you can easily do the feedback intro to "It's All Too Much") so I've currently just got the 10k emitter resistor on Q1 replaced with a fuzzface-style fuzz pot.

I went ahead and ordered a board from PPCB and traced it, and it looks more or less like we suspected — the first two stages of a 7120 with the distortion circuit hardwired on. A smattering of tweaks, including an extra diode for asymmetrical clipping, useful controls for the gain and biasing at Q2, and missing the resistor in parallel with the 10nF cap in order to keep the overall level high enough to not need the final two transistor stages:
Looking forward to building this one!

Thanks for the schematic. After I initially responded to this post, I went to my breadboard and recreated the original Vox 7120 preamp, including the gain stage and bass/treble control circuit after the volume control. Using a charge pump, I ran the source voltage to about 25V, with resistors to drop the voltage in the first transistor stages. I controlled the "fuzz" in a similar way to the LHF by limiting the cap bypass on the second transistor. Also, I used the symmetric diode clipping of the original.

In the end, my breadboard version sounded OK but not amazing. I think the asymmetric clipping in the LHF will sound better, but it's not like the original. I also think the bass/treble controls add something to the sound. And of course it would be nice to simulate the tube power amp, which can be done in a variety of ways. Still, the demos of the LHF sound good and it should give you the essence of the Beatles Revolver/Sgt. Pepper fuzz tones.