Hey guys!
I have a couple of 6n2p (Russian 12ax7?) and I wanted to add a bias shifting tremolo to a mic preamp based on two 6au6 pentodes that I built out of an old Akai tape recorder.
Im a bit frustrated after hours of messing around with it.. Just cant get that tube to oscillate😕
I tried different feedback caps and resistors for the RC stages, and also experimented with the cathode cap and resistor values and still nothing..
I have 320V for a B+.
Would appreciate any help to get that LFO up and running 🙂
I have a couple of 6n2p (Russian 12ax7?) and I wanted to add a bias shifting tremolo to a mic preamp based on two 6au6 pentodes that I built out of an old Akai tape recorder.
Im a bit frustrated after hours of messing around with it.. Just cant get that tube to oscillate😕
I tried different feedback caps and resistors for the RC stages, and also experimented with the cathode cap and resistor values and still nothing..
I have 320V for a B+.
Would appreciate any help to get that LFO up and running 🙂
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> I tried different feedback caps and resistors
Did you try "plagiarizing" (research) the many-many-many 12AX7 tremolo oscillators in guitar amplifiers?
Did you try "plagiarizing" (research) the many-many-many 12AX7 tremolo oscillators in guitar amplifiers?
6N6P is not anything close to a 12AX7. Stenak called it, the 6N2P is the 9AJ version of a 12AX7.
6N6P is close to an ECC99 though.
6N6P is close to an ECC99 though.
Oh sorry I just saw now that I wrote 6 instead of 6N2P,
What I have is the 6N2P (sorry for the mistake)
Anyway.. I replaced the resistor and capacitor on the cathode with a LED, and it blinks real fast (thats a progress I guess) and I read a small voltage movement on the anode ( varies from 220-240v). strange, right?
Not sure what to do with the other half of the triode..
Any input would be appreciated !
What I have is the 6N2P (sorry for the mistake)
Anyway.. I replaced the resistor and capacitor on the cathode with a LED, and it blinks real fast (thats a progress I guess) and I read a small voltage movement on the anode ( varies from 220-240v). strange, right?
Not sure what to do with the other half of the triode..
Any input would be appreciated !
I would suggest finding a circuit that uses the 12AX7 and basing it off of that. 6AU6 is a pentode after all... Much higher gain etc.
Thread moved to Instruments and AmpsSo it seems I get an oscillation now, Just wondering how and where to inject it in the 6au6 preamp. Any advice on that?
"Bias wiggle tremolo" circuits that I've seen inject the tremolo signal into the (pentode) control grid.
Wiggling the screen grid voltage should also work on paper, but this is a less sensitive electrode, and will need more volts of "wiggle" to do anything.
-Gnobuddy
For Pentode tremolo check out the Gibson GA40 schematic.
I've used that circuit with 6BR7 pentode and a 1/2 12AX7 subbed for the 6SQ7 oscillator.
Should work with 6AU6 OK.
Lusher tremolo that the bias wiggle.
Cheers,
Ian
I've used that circuit with 6BR7 pentode and a 1/2 12AX7 subbed for the 6SQ7 oscillator.
Should work with 6AU6 OK.
Lusher tremolo that the bias wiggle.
Cheers,
Ian
The oscillator's modulating the pentode anode voltage? The electrode least sensitive to voltage changes?
The five-stage high pass filter to block tremolo oscillations, with every stage heavily loaded by a succeeding stage of the same impedance, is another head-scratcher. That filter will probably turn out to have the soggiest frequency response curve ever seen in a passive filter. Fortunately there's over a decade's worth of frequency bandwidth between 7 Hz tremolo and 82 Hz low E on a guitar.
-Gnobuddy
The oscillator's modulating the pentode anode voltage? The electrode least sensitive to voltage changes?
The cathodes are connected together. That's where the magic is injected.
Plates interconnect to semi-cancel plate current variation. Yes, it does seem somebody was over-anxious about LFO getting out of this stage.
The real baas-cut is the 0.005u feeding the wiper of the volume control. Anything less than full up, it sucks.
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I saw they were connected together - but they also appeared to be shorted to ground via the reactance of the 20 uF capacitor, so I thought Gibson engineers were just cost-cutting with a shared cathode bypass cap, like Leonidas did in the 5E3 preamp.
Not quite a short, of course, though a low enough impedance to be treated as a short in most valve circuitry - a 20uF cap has only about 1.1 kilo ohms reactance at 7 Hz tremolo frequency. At the anode, that would be a short to all intents and purposes. However the cathode is a low impedance node, so not quite a short there.
I noticed that, too. I hate everything about that particular bass-ackwards way of wiring a volume control - but adding the soggy high-pass function with variable corner frequency manages to make an already horrible idea even worse. 😱
-Gnobuddy