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Re: Parallel triodes

3/16/1999 11:22 PM
Steve A.

Re: Parallel triodes
    Even without doubling up the triodes you can use a similiar trick to get 3 sounds out of a preamp channel. With a center-off DPDT toggle switch you can mount a ~ 1.0uF Ck cap for one position and a ~ 22uF Ck cap for the opposite position (and with no Ck cap at all in the center position). To eliminate/reduce switching noises, solder small resistors between the switch contacts (like 470k). Remove the stock Ck cap (if used) and use shielded cable to bypass the Rk resistor on the board (with the center conductor going to the cathode end of the resistor and shield going to the grounded end). Or if the tube sockets are chassis mounted remove the stock Ck cap and wire the shielded cable directly to the tube socket.  
    Here's a drawing of the switch:" target="_blank">"TARGET="_top">  
    There is considerably less gain with the 0uF center position, but that can be useful if you're trying to get a clean sound from humbuckers. The 0.68-1.0uF position is great for overdriven leads because you're not wasting so much of the amp's power on the low frequencies (which get drowned out by the bass player anyway). And when you want the full bass response from the 22uF Ck cap, you have that option as well.  
Steve Ahola
3/18/1999 3:33 AM
Steve, you're expanding my preamp horizons considerably. Funny that I'd never thought of doing such things - but now I'm starting to get a whole lot of possibilities. Nice one.
3/18/1999 9:59 AM
Benjamin Fargen
Re: Tone options
The Ck cap switch is a great idea.  
Have you ever tried switching between two different tone stacks? Having one set of controls on the panel and switching the network of parts to achieve two different characteristics.  
In conjunction with your CK cap switch, you could have two different preamps at the flip of a switch........Vox/Fender perhaps!  
Once again you have posted a great idea to get the wheels spinning!  
3/18/1999 9:58 PM
Steve A.

Have you ever tried switching between two different tone stacks?  
    Well, yes and no. Any other questions?  
    To switch the configuration of the tone stacks between 5F6A tweed bassman to Vox to BF would involve more wafers on a rotary switch than I would care to deal with; even if you got the switching logic figured out, to make the switch as noiseless as possible could be a real headache. Besides the 5F6A stack seems to work best after the cathode follower and the BF stack seems to work best after a single gain stage (or maybe two...) So to go from 5F6A to Vox to BF I'd really recommend using separate preamp channels (each fine tuned for their particular tone stack).  
    Just sticking with the 5F6A tone stack design, I worked out a great mode switch for my Pignose G40V that switches between a 330pF treble cap/56k slope resistor (Bruce Collin's recommendation) and the stock "porcine" values of 1000pF treble cap and 33k slope resistor. I find that to be a definite improvement over the typical mid-boost switch that only increases the value of the treble cap (which reduces the effects of the bass and mid controls). By lowering the slope resistance as you increase the treble capacitance, you send a stronger signal to the bass and mid controls, too. While 1000pF/33k works well with my li'l pig, I've been using 720pF/56K on "The Tweed Bassman" mod for my Peavey Classic 50; it is a more subtle boost and sounds nice with clean sounds as well as OD sounds. (Unboosted uses the 330pF treble cap; the boost switch adds in a 390pF boost cap- all dipped mica cap, BTW.)  
    As for the effect of bass and mid cap values, it is really a lot more subtle than changing the treble cap and slope resistor. When I first rebuilt my butchered-up 65 Pro, I had this great idea to put DPDT mini-toggle switches on all of the tone caps (to allow me to select between the various values used by Fender and the amp modders). For the Normal channel I used 2 position toggle switches: treble was 250pF and 500pF, mid was 0.022uF and 0.055uF, and bass was 0.047uF and 0.094uF. For some reason the BFSR values always seemed to sound the best: 250pF/0.022uF/0.094uF.  
    For the post-CF tone stack, a pair of 0.022uF for the bass and mid caps seems to work great in most amps. For the heck of it, if you can measure capacitance, try comparing the readings across the bass and mid caps. At most reasonable settings of the tone controls, I found that I'd get the same reading at each cap, even if the values were different! So when switching from a 0.022uF bass cap to a 0.055uF value, the difference was very subtle, and boosted the lower mid frequencies more than the bass frequencies. (At least that was my subjective opinion.)  
    The very unscientific conclusion I reached after screwing around with different values for the bass and mid caps in both the BF and 5F6A stacks was that once you found the optimium combination (which may vary from amp to amp), just stick with it. You can usually get all of the sounds you'd ever need just by adjusting the controls. If you need to boost the bass response a bit, try a lower value for the slope resistor; if you want to boost the mids then raise the value of the treble cap. But leave them bass and mid caps alone!  
Steve Ahola  
P.S. The Torres book has an interesting design which switches between a Vox preamp and a Marshall preamp (or both). All going to the same tone stack (which switches between a 50pF treble cap for the Vox mod and an added 500pF boost cap in the Marshall or both mode).  
    Details: There is an added gain stage wired up to a switching jack and let's ignore that... Each mode has its own triode, with 220k plate resistors and 300 ohm series resistors feeding each of the grids. The Vox triode uses a 1k Rk resistor bypassed with a 22uF cap, and goes through a 500pF coupling cap and a 1M mixer resistor to the volume control ahead of the dc-coupled pair. The Marshall triode has a 500pF cap bypassing the plate resistor, an Rk of 2k7 bypassed with a 0.68uF Ck cap and goes through a .022uF coupling cap followed by a 1M mixer resistor to the same volume control. The first stage of the dc-coupled pair uses an unbypassed 1k Rk resistor.  
    The added gain stage uses stock 68k series resistors from the two jacks going to both triodes in parallel. The plate resistor is 68k bypassed with a 0.001uF cap. The unbypassed Rk resistor is 680 ohms. The coupling cap is 0.01uF followed by a 330k series resistor to a 500kA pot (the wiper of which is connected to the switching contact of the switching jack).  
    An extra stage is added to the power supply to feed the Vox and Marshall mode triodes; with a B+ of 350 to 400 volts, a 2k2 to 4k7 resistor is added, with a 10uF 450 filter cap. Might as well describe the rest of it... the wiper from the treble cap goes through a 33k series resistor to a 0.005uF coupling cap going into the PI. Other values look like a typical BF, only the feedback resistor is 47k and there is a dual ganged 500kA post-PI MV. (You can skip the 500pF bright caps going to the dual ganged 1 meg "presence" control unless you really need to scrape the wax out of your ears...)
3/18/1999 11:28 AM
Re: Parallel triodes
Can you use two resistors in series (say 100k and 82k) for plate resistors and connect a switch in parallel with the 82k resistor so that when the swith is closed it shorts out the 82k resistor giving a plate resistance of 100k and by opening the switch it would place both resistors in series giving a plate resistance of 182k thereby increasing gain?  
Would that be dangerous because of the high voltage the switch would be sitting at?  
3/18/1999 10:10 PM
Steve A.

    If you ran the B+ through the 100k resistor first, it would lower the voltage (to ground) going to the switch, perhaps to the level that it might be okay to use a mini-toggle (use a DPDT and double up the contacts?). I really haven't done much experimenting with the plate resistors because of the voltages involved... (I'm assuming that the B+ going to the preamp plate resistors would be under 400vdc; anything higher than that would probably require a full-sized toggle switch.)  
    Maybe you can try it out and let us know how it works! (You may get switching noises; perhaps adding a 22pF to 47pF cap between the switch contacts might help that problem, although it would alter the tone a bit when the switch was open.)  
Steve Ahola

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