6n6p for tremolo

alex0terry

Member
2020-02-08 8:50 pm
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:confused:
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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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 !
 

Gnobuddy

Member
2016-03-01 4:10 pm
...pentode tremolo...Gibson GA40...
The oscillator's modulating the pentode anode voltage? The electrode least sensitive to voltage changes? :scratch:

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
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
The oscillator's modulating the pentode anode voltage? The electrode least sensitive to voltage changes? :scratch:....

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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Gnobuddy

Member
2016-03-01 4:10 pm
The cathodes are connected together. That's where the magic is injected.
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.
The real baas-cut is the 0.005u feeding the wiper of the volume control. Anything less than full up, it sucks.
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. :eek:

-Gnobuddy