Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 14, 2014, 05:21:58 pm
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(http://www.ozvalveamps.org/playmaster/vibratoneg.gif)
at ozvalveamps.org the image of this unit is followed by this explanations
One half of the twin triode is used as a phase shift oscillator at the vibrato frequency.
The other half is used as a buffer or gain compensation amplifier for the modulated signal attenuator that follows.
This consists of a modulated path with a light dependant resistor (LDR) in one leg and a fixed resistor in the other.
The depth control allows cross-fading between unmodulated and fully modulated signal.
The optional cathode bypass capacitor sets the stage gain - no cap, low gain; small cap, treble boost; large cap, makeup gain (should be about unity overall).
This arrangement would later be used in the Playmaster 116 & 7
The question is
which is the advantage using the modulated path with the LDR in one leg and a fixed resistor in the other
rather than the more usually seen connection of the LDR between signal and ground standing alone ?
Thanks
K
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which is the advantage using the modulated path with the LDR in one leg and a fixed resistor in the other rather than the more usually seen connection of the LDR between signal and ground standing alone ?
This is an add-on tremolo unit, apparently with the goal of maintaining a signal about the same size as the original dry signal.
Look at the 12AX7 between the input and output. The effective plate load for a.c. signals is the parallel combination of the 47kΩ plate load and the 100kΩ & 15kΩ resistors in series. The other 100kΩ and LDR are isolated by the 1MΩ and the LDR's own high off-resistance. So the effective plate load is ~33kΩ.
Given this low effective plate load, the gain of the 12AX7 is 34.8 with a fully bypassed cathode resistor, or more like 17.5 with no cathode bypass cap.
The 100kΩ/15kΩ series resistors to the right of the 1MΩ pot form a voltage divider with a "gain" of 0.13, which is to say it knocks the 12AX7 output down to ~1/8th of what it was initially. With no bypass cap, the dry output is 17.5/8 = ~2 times its original size, which in voltage gain stages is "no-gain."
The LDR has a similar voltage divider setup, so you get smooth variation from full-trem to no trem without changing the size of the original signal.
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Note that in this scheme, the Neon also carries the oscillator's idle current.
So the Neon never goes completely dark.
So the LDR always cuts signal, even on the "loud" side of the tremolo swing.
Also, as said, this is an Add-On. It should give roughly "unity gain".
But what is "unity gain" when the whole idea is for gain to go up-and-down?
Some less-loved Fenders (opto) only cut gain for tremolo.
Others (bias-trem) increase-decrease gain around the non-trem gain.
Since the LDR alone can't increase gain, we throw in a gain-stage. Handy to use a twin-triode. However the oscillator *must* give gain over 29, so we need a high-gain tube. Then the second half used for audio gain has "too much" gain. So we mis-load the audio stage a bit, then waste-off the excess with a divider on the "normal" side.
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MANY THANKS HotBluePlates & PRR
Franco