kg Date: 2/11/2002 2:50 PM Subject: weekend discoveries a) partition noise in pentodes can be greatly reduced by decreasing cathode current. b) a couple of small ferrite beads work very well in place of grid stoppers. c) regular old 1n400x diodes (reverse biased) work very will in place of rk/ck in the cathode circuit, approximating a fixed bias stage... difference is no time constant in the cathode circuit which translates to quicker recovery from transients. LEDs also work very well and offer greater vf. d) you really have to try inductors in a tone control situation. very sweet tones. e) when you're diy, you can eliminate the "grid leak" resistor from the input stage of your amp, and instead use the circuit inside your guitar to provide the necessary path to ground. that's all for now. more later. ken ----------------------- e) when you're diy, you can eliminate the "grid leak" resistor from the input stage of your amp, and instead use the circuit inside your guitar to provide the necessary path to ground. Hey KG, what, if any effect is there on tone of gain? D.KT88 --------------- d) you really have to try inductors in a tone control situation. very sweet tones." I'd love to. Have you had a chance to dig up your list of the inductances of those little trannies? When you talk about using diodes in the cathode circuit, are you referring to preamp stages or the output stage, or both? In a preamp stage, how do you translate the formulae for calculating gain, output impedance, & frequency response when you're using a diode in the cathode? Do you have a mini-schemo for the diode-in-the-cathode trick? Shea ----------------- Ken,my old boogie MKIIB was using rectifier diodes on the cathodes!But i don't know why,hence the amp was fixed bias.May be like thoses fenders,mix between cathode and fixed? Best regards. Max. -------------- I posted this but it was buried in Mook’s thread. I think the gain, output impedance and frequency response will be the same as for a bypassed cathode resistor with an infinite value cap. i.e as if the cathode were grounded to AC. Dave >>I haven’t tried it myself but you could get maximum gain and eliminate the cathode RC by using a couple of diodes in series with the cathode to bias the 12AX7s. It works for power tubes too with a zener. If your B+ is a bit hot you can knock a few volts off it and get a free bias supply. It can be made adjustable by putting a pot across the zener with the wiper connected to the grid resistors. ------------ mini schemo: b+ | / \ Rp / | | p -------g | k / | \ v gl / | | v 3 x 1n4007, cathode band towards ground. | | gnd v | gnd When you talk about using diodes in the cathode circuit, are you referring to preamp stages or the output stage, or both? In a preamp stage, how do you translate the formulae for calculating gain, output impedance, & frequency response when you're using a diode in the cathode? so far, i've only used it in the preamp section. as dave points out, the diode presents a very low impedance when forward biased--i.e. conducting--yet it retains the ~0.6v drop across it, so you don't need to use a bypass cap to increase gain. for more bias voltage you simply series up more diodes. the diodes are very quiet when operated in this way. the only downside that i can see is that you have eliminated the DC NFB inherent in the rk of your typical cathode biased stage, so that different tubes will have operating points that are more disparate when diode biased... regardless of whatever tube you put it, in will have the same effective -vg1, which may or may not put the vp where you originally designed it. on the other hand, this merely serves to increase the differences between tubes as you swap, which can be cool in its own right. so far, i've used it on an ef86, a 12ax7 and a 6c45pi. ken ---------------- "the only downside that i can see is that you have eliminated the DC NFB inherent in the rk of your typical cathode biased stage, so that different tubes will have operating points that are more disparate when diode biased." That can be compensated for by using a NFB loop from the plate output (after the blocking cap) to the grid, right? That will also let you control the amount of feedback more closely than you can control the feedback from a cathode. IIRC, this type of negative feedback also evens out the differences between tubes. Shea --------------- Cool stuff Ken! B) is used by Carvin feeding their input grid. C) I think this is something that Jack Orman used in the FX community to bias FETs. When I put it in, I thought I heard a more "animated" tone. I need to try this. D) Yep! My old Pignose Crossmix had inductors for the tone controls - especially the mids.... very sweet. E) does anything bad happen when you unplug the guitar and for some reason don't have a good ground contact with the switch in the input jack? Thanks, Aron -------------- E) does anything bad happen when you unplug the guitar and for some reason don't have a good ground contact with the switch in the input jack? actually i use a nonshorting 1/4" jack on the front pannel, and when i disconnect the guitar the tube tends to pinch off its plate current, pretty much silencing the amp. i have seen no ill effects due to this so far. different tubes may react differently to this treatment. i'm using a 6c45pi input tube; if this one doesn't go batty (running at 150vp, 35mA) i don't think any of them will. ken- -------------- d) you really have to try inductors in a tone control situation. very sweet tones. Hi kg, just try an inductance of 1H beside the presence cap with a poti. Regards Ingo