Tremolo circuits: Princeton single triode style

[topbrent]

Instead of hijacking psychonoodler's post, I figured I would start fresh.

Is it possible to re-purpose the Princeton style, single triode tremolo, to wiggle the cathode bias of the preamp tube(V1-B) instead of the power tubes bias? 

Kind of a similar implementation as the VibroChamp trem? (...which uses 2 triodes to oscillate)

If this is possible, could someone illustrate how to disassociate the trem circuit from the existing princeton style power tube bias arrangement. 

If this is would work, you could have a nice trem, and use what ever power tube bias arrangement you wish. 
Fixed bias Hot or Cold, and/or Cathode bias.

[tubeswell]

Can't see why not. 5E9A did a similar thing, albeit with an LTP inverter cathode wiggle and a 2 stage LFO-CF. You just have to make sure that the voltage swing coming off the LFO is sized right for the bias voltage of a pre-amp tube. There is a schematic in the Jack Darr Book Ch 2 Fig 2-7 which shows single stage 6SQ7 LFO set up for a pre-amp 5879 pentode cathode wiggle (if you have that handy - it is too big to upload here)

[topbrent]

Here is a slight modified schem showing the points of interest on the Princeton schematic.

What to do with points A & B...to connect with point C.

[tubeswell]

Well if you are staying with fixed bias, then B still has to be connected to the output tube grid load resistors. But assuming that you'd do that, then B would be disconnected from the intensity pot that goes to A, and A would be connected to C. But then you would want to reduce the voltage swing coming from the general direction of A, possibly either by lowering the LFO plate resistor to something like 100k and/or by increasing the 1M 'bleed out' resistor from the plate of the LFO to something like 2M or some such (all pure guesswork on my part). You need to muck around with it and see.

[topbrent]

Well if you are staying with fixed bias, then B still has to be connected to the output tube grid load resistors.

Thanks for your comments, Tubeswell. 
Not sure I understand why fixed bias or cathode bias matters at this point, as the goal is to wiggle a preamp tube, not poweramp tubes.

[LooseChange]

I did something a long time ago with the cathode of the last gain stage in the preamp.
I lifted the grounded side of the cathode CAP and placed a jFet in there. Connected the intensity pot output to the gate of the jFet and it worked great.

[sluckey]

Is it possible to re-purpose the Princeton style, single triode tremolo, to wiggle the cathode bias of the preamp tube(V1-B) instead of the power tubes bias? 

Kind of a similar implementation as the VibroChamp trem? (...which uses 2 triodes to oscillate)

The Princeton single triode trem oscillator probably can't drive the low impedance cathode circuit of a preamp tube without a buffer of some kind.

[Fresh_Start]

First off, you'd need to re-wire the Intensity pot like a Vibrochamp's.  That answers the questions about A, B & C.  

BTW, "B" represents an AC ground for the tremolo signal (the "top" of the 25uf cap is an AC ground since any signal is going right through that cap).  That's how the Intensity pot works as a voltage divider for the tremolo.  [Thanks to sluckey for explaining that to me a long time ago.]

You could use one tube triode and a MOSFET as a DC-coupled cathode follower and copy the entire Vibro Champ tremolo circuit.  http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm  There's one important detail in the text there that's not shown on the drawing:  a 220 ohm "gate stopper" resistor between the tube triode's plate and the MOSFET's gate.

I have not tried this yet and know I mentioned it in another thread a day or two ago.  Sorry if you'd already seen it - I don't mean to beat a dead horse.  However, your guitar signal is NOT passing through the evil semiconductor device - just the jiggling voltage.  It would solve the need for a buffer which sluckey points out above.

Cheers,

Chip

[tubesornothing]

You could use one tube triode and a MOSFET as a DC-coupled cathode follower...
I have not tried this yet....

I have tried the mosfet CF, in a few different places and it works just great.

[Fresh_Start]

What started out as a "simple" task turned out to be frustrating... the only SCH file I could find for the AA764 Vibro Champ had a LOT of errors in it.  I've attached and SCH file plus two images of the original circuit and the tremolo portion only with a MOSFET substituted for the buffer triode.

PLEASE check for errors.  Again, I haven't built this but it should work fine for an AA1164-type circuit simply by connecting the "V" to the cathode of the triode where the NFB is returned.

Good luck,

Chip

[tubeswell]

Hi Chip

I am building a source follower in a BFPR clone with a IRF820, and I found out from R.G. Keen that you need a 12V zener between the source and the Gate (cathode pointing toward the Gate) 2CW (Not sure why tho' - I know even less about silicooties than I do about tubes)

[Fresh_Start]

Hi Chip

I am building a source follower in a BFPR clone with a IRF820, and I found out from R.G. Keen that you need a 12V zener between the source and the Gate (cathode pointing toward the Gate) 2CW (Not sure why tho' - I know even less about silicooties than I do about tubes)

In the PR circuit, are you using the MOSFET in place of a DC-coupled cathode follower?  Is in the signal path or are you working on the tremolo circuit?  If the tremolo, I'm VERY interested in your results.  That's on my list of "things I must experiment with sometime soon" list

[ignorance on] What is the "2CW" spec reference?  just a 2 watt zener?[/ignorance off]

Thanks,

Chip

[tubeswell]

In the PR circuit, are you using the MOSFET in place of a DC-coupled cathode follower?

Yup

Is in the signal path or are you working on the tremolo circuit?  If the tremolo, I'm VERY interested in your results.

See schematic (attached). I'll post results in a few days - when I get the circuit finished

[ignorance on] What is the "2CW" spec reference?  just a 2 watt zener?[/ignorance off]

2 Cents Worth. A 1W Zener will work fine

[sluckey]

Tubeswell, your schematic shows the bias circuit connected to the 5VAC winding. Did you really mean to connect it to the HT winding? And through a 100K resistor? The bias filter cap is missing a ground also.

[tubesornothing]

Here is the trem with MOSFET and little blinky LED, that I use with great satisfaction....

[tubeswell]

Tubeswell, your schematic shows the bias circuit connected to the 5VAC winding. Did you really mean to connect it to the HT winding? And through a 100K resistor? The bias filter cap is missing a ground also.

Oops~! fixed. Thanks

[PRR]

> you need a 12V zener between the source and the Gate ... Not sure why tho'

The Gate of a MOSFET is the thinnest possible layer of "glass", just a couple molecules thick. It will break-down at 50V-100V. The spec-sheet says 20V max.

In normal operation you never need more than 10V, often more like 5V.

But what happens when the MOSFET is used as a "cathode follower" from a tube plate at TURN-ON? The tube is cold, pulls no current, so the gate goes up toward +350V. Meanwhile the MOSFET's Source is pulled down toward zero. Ignoring MOSFET action, you could have 350V across a 50V gate-glass layer, ZAP!

In many cases, the MOSFET Source will "follow" its Gate up and up, never exceed its breakdown voltage. However it can be hard to anticipate ALL voltage jerks and jolts. And the gate-breakdown is "instant". And a blown-Gate can be hard to diagnose, 'specially if you know even less about silicooties.

So blow the dime, use the Zener.

[tubeswell]

Thanks PRR - much obliged

Now I know a wee bit more 

[Fresh_Start]

Thanks PRR - much obliged

Now I know a wee bit more 

+ 1 on the THANKS!

Chip