Wednesday, 6 March 2013

Runoffgroove Professor Tweed

A request. Info from ROG:
One of the often overlooked Fender amps is the tweed-era Princeton. They are excellent guitar amps, especially for blues playing. The major drawback was the 4.5 watt output wasn't enough to keep up with a band. Again, using the technique pioneered by Doug Hammond, we decided to try to capture some of the sound of this little amp in pedal form so it can be used with a band.
The circuit we chose is the 5F2-A Princeton, one of the so-called "narrow panel" amps produced from 1955 to 1960. It was a very simple amp, using one 12AX7, one 6V6 power tube and a 5Y3 rectifier. Since we used 9V DC power, we left out the 5Y3. We used JFETs to replace the two halves of the 12AX7 and the 6V6 power stage.

We took the 5F2-A schematic and copied it part for part using MPF102 JFETs in place of each tube stage. Each tube Grid was replaced by a JFETs Gate. The tube Plates were analogous to a JFET Drain. Finally, a tube Cathode was replaced with a JFETs Source. We used a 100k trimmer for the plate resistors on the schematic due the fickle nature of JFETs and the much lower power supply involved.

Our "artistic license" is in the choice of the last JFETs output cap and the setting of the dual Low Pass Filters. Our goal was to produce the sound of a Jensen speaker. These are typically a little broader in frequency response than something like a Celestion.
Using all MPF102 JFETs, there is a great range of sounds available. No, you won't be playing "South of Heaven" with this pedal, but some classic blues, country, surf and rock sounds live here. The cleaner sounds are compressed and have an edge to them. As you wind up the Volume knob, you'll hear the dirt level increase. At full Volume, there is plenty of nice overdrive that never loses its dynamic feel. Even at full tilt, the circuit can clean up very well.

Possible mods:
Try using a J201 in the first stage. You'll notice the overall available gain increase and the sound will darken a bit. We preferred the sparkle of all MPF102s, but if you use sockets (as is always suggested) for the transistors, you can experiment until you find the combination you like best.
Socket the cap in the feedback loop. It is the 1uF cap connected to the output cap on one end and the 22k feedback resistor on the other. In this socket, try any value from 2n2 to 1uF. The sound will "open up" most around 10n and will "close up" as you get closer to 1uF. Try different values here to find the sound you like most.

Another possible area for modding would be the Tone control, specifically using different values for the 4n7 and 470p caps. The Tone control works well in its stock form, but you may find something interesting by substituting different values here.





Video of Geiri's build:


20 comments:

  1. looking forward to verification from the group. This was one of my first target builds before azabache came out.

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  2. I'd rather start it knowing it's supposed to work! I would be willing to bet that it does, though.

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  3. I have no doubt it'll work, but I wonder how it measures up against other tweed buolds on here. I'm willing to bet a few boutique designs are based on this ROG circuit anyway.

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  4. I might make it today if I manage to finish two others in time. Probably!

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  5. You are probably right, Jon. It's no doubt the same with a lot of their designs. They do all the heavy lifting and others benefit. That would be great Geiri - with one of your videos, perhaps? Thanks in advance

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  6. The only clips I've heard of this circuit were from ROG and from a guy trying the circuit out as a preamp to a DIY harp amp he built with a SS setup. He seemed to love it, though he was recording in 320 on youtube.

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  7. I have built this and it works but I did not have any 5 x 5 mil presets only 5 x 10 mil ones so I had to widen the layout by 4 columns. The trimmers are very big in value at 100k mine, measured 1.1 to 1.9 k when the circuit was balanced. I don't know how average my MPF102's are but I think that the trimmers could be half the present value or less and that would make it far less fiddly to bias. If I do this again I'll just put sockets where the trimmers are and balance each MPF102 with an external pot and then put in the appropriate resistor. Maybe just put 1.5k in each slot and try it first!
    I tried it with one, then two then all three J201s,. but although there was a slight increase in gain there was an even bigger increase in hiss! In addition the clarity of the circuit was adversely affected. The trimmer positions were a bit different to the MFP102's but I'm afraid that I did not measure the resistances.
    I reduced the value of the (1uf) feedback cap but it did not make the effect more loose, the reverse if anything. The 1uf is just fine I think.
    This is basically a Clean Sound effect that can be overdriven a bit rather than an Overdrive that can be cleaned up a bit. For a feathery Fendery sound the Briggs Face or better still the Zendrive are better options in my view.

    Norwichbadger

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    1. Seems like we forgot to tag this one :) Tagged now.
      +m

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  9. Build this today, it works! My only negative point would be the tone control. so i upped the cap from 4.7nf to 22nf. It helped a little.

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  10. can i put this pedal after a dirtbox and before a cab sim?

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    1. Can't see any reason why not.
      +m

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  11. Just built this with a Condor cab sim after it. Regardless the Professor Tweed stand alone sound amazing. It totally sounds like what they claim. Beautiful. Thanks for the great vero.

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    1. Sounds like something to try!
      Does the condor use xlr balanced output? Or do I still need to run it into a DI box for direct recording?

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  13. I've built this layout twice and both times it works but there is static present. I used all 2n5457s and biased the drains to 4.5v but both builds are giving me static that only the master volume effects. Has anyone else experienced this with this circuit?

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    1. Old post, I know, but for anyone with the same problem: First: make sure to test your transistors on other circuits/pedals (that you know works for sure). Second: In my experience, some transistors just don't work well on some circuits - maybe these trannies are out-of-spec for that circuit in particular or something - No matter if the drains are adjusted to the "stock" 4.5V. So, get your transistors from different sources/batches.

      I had the following problem with my Thor build: I used "verified" trannies in it (got them from a pedal that worked just OK) but they simply wouldn't work on the Thor - I'd get static when touching one of them though, and by turning the gain pot up, a loud, nasty, noisy gain would emerge from it, although all the voltage readings looked OK. Some time later, I simply replaced that trannie that seemed to be the source of the problem with one from another seller and it made the Thor work, easy like that (no other mods were done). Later, that "defectful" trannie went back to it's pedal of origin and kept working wonderfully in it.

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    2. Old post, I know, but for anyone with the same problem: First: make sure to test your transistors on other circuits/pedals (that you know works for sure). Second: In my experience, some transistors just don't work well on some circuits - maybe these trannies are out-of-spec for that circuit in particular or something - No matter if the drains are adjusted to the "stock" 4.5V. So, get your transistors from different sources/batches.

      I had the following problem with my Thor build: I used "verified" trannies in it (got them from a pedal that worked just OK) but they simply wouldn't work on the Thor - I'd get static when touching one of them though, and by turning the gain pot up, a loud, nasty, noisy gain would emerge from it, although all the voltage readings looked OK. Some time later, I simply replaced that trannie that seemed to be the source of the problem with one from another seller and it made the Thor work, easy like that (no other mods were done). Later, that "defectful" trannie went back to it's pedal of origin and kept working wonderfully in it.

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  14. Built from the layout by Miro. I didn't have high expectations while finishing this one because during the build I started to find somewhat poor comments about the circuit, like being too thin sounding or low gainy... to my surprise and delight, the build surpassed all the expectations, since it's definitely not thin neither a low gain effect.

    With the gain all the way up, it has a lot of (amp-like) compression/sag and an amount of dirt that is really true to that of a low wattage tweed amp. Tone control is not extreme but it's useable. Couldn't help but play Cinnamon Girl with it. ;)

    BTW, I used Q1- J201 and 2 MPF102's in mine; all drains to 4.5V. My source voltages on Q2-3 all read lower than what ROG specifies, maybe that has something to do with it. I haven't tried all MPF's or other options, but I would imagine 3 MPF'S would sound clearer/low gainier. Oh yes, I have also built different versions of the Briggs' Face OD/booster and the Prof. Tweed just kills all of them.

    Big thanks for sharing!

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  15. I built this and it produces a brighter, twangier sound than dry. But the volume level with all the knobs all the way up is barely equal in volume to the pedal switched off. From what i'm reading here, it should be pretty loud and gainy at that point. The sample video no longer loads, so I can't compare with that.

    The drain voltages are as close to 4.5 as I could get 'em (between 4.2-4.7) - do they have to be exactly 4.5? I've xactoed all the grooves in the stripboard. Any suggestions what else to try? Could someone who's successfully built it provide other voltage points to check?

    Thanks.

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