Tube Amps / Music Electronics
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|3/27/2005 7:40 AM|
||add vibro to clone champ|
I have built a Champ that works/sounds great and would like to add the oscillator from the AA764 VibroChamp. I have a B+ of 273V and no oscillation. I'm in over my head and any sugestions on how to adapt this circut would be a big help.
|3/27/2005 7:47 AM|
Try the tubeless vibe circuit in the MOSFET Follies section at http://www.geofex.com
|3/27/2005 7:48 PM|
Don thanks for the link nothing against Mosfets I would like to keep this one all tube.
|3/28/2005 10:59 AM|
How accurate is your build to the AA764 3-knob non-vibro Champ already?
|3/28/2005 12:42 PM|
Mike my version is almost identical but for the lower B+ and more filtering. It's been used for recording and is very quiet. The point in the circuit I would like to pull the power is 273V.
|3/29/2005 11:29 AM|
Take a look at these two drawings:
On the second audio triode (*V1b*) of the vibrochamp, there is a nominal 1.5V on the cathode. With different audio 12AX7's this value can change slightly, but if you put in a 12AY7 or any other pin-compatible tube (usually even a 5751), you may lose all or most of your audible trem. Even when the LFO 12AX7 is working perfectly. Because then the cathode voltage of V1b will rise to 2 or 3 volts or more and the faint voltage oscillation coming from the LFO tube is then insignificant. With triodes having much higher current than a bogey 12AX7 in the V1b position, its cathode can then deliver enough current through low enough plate resistance to swamp whatever is coming out of the 68K cathode resistor of the LFO.
Then take a look at how the LFO is wired so that the two triodes are supposed to self-oscillate (at a very slow rate like tremolos do). Sometimes an oscillator needs a little kick in the pants to get it going. Notice how the B+ for the LFO is connected directly to the screen of the 6V6. This ought to get it going if nothing else will
Too bad you do not have the same B+ to drive the LFO that is on the stock VC. If you are already connected to the screen of the 6V6, I guess you could go no higher, especially if you like the way the tone of the basic amp is now. I know I like the sound of 6V6's at less than 300V as well as at the higher voltages normally seen in various champs.
You definitely need a decent 12AX7 for the LFO tube since it depends on the high gain triode for the feedback to be amplified enough to regenerate. Good quality caps for the 0.02 & 0.01 values (the LFO feedback loop) are essential, since this is a fairly high impedance loop. If there is much cap leakage, there can be failure to oscillate, like when crummy ceramic caps have their dipped coating compromised by age & humidity and conduct a little DC around the outside of the dielectric. These caps do need to charge & discharge with sensible time constants relative to the 3Meg adjustable drain pot and 1Meg grid & cathode-connecting resistors. Lowering the value of one or both 0.01's slightly will allow them to charge faster, perhaps changing your maximum rate from 1/4 cycle per second (which you may have now even if it is inaudible) to 3 or 4 cps which would be not bad. Lower the uF too much however, and they will roll off the very low frequency which you are trying to propagate.
So as long as the LFO is wired properly and you are still not getting it to oscillate by itself (disconnect the wire from the 25K intensity pot to the V1b cathode), I would guess the low B+ is then the problem. Maybe there is just not enough swing in the *signal* among the LFO triodes. Then you could try more gain in triode V2a by reducing the 4700ohm cathode resistor while making sure the 25uF bypass cap is in excellent condition or even increase its value to 47 or 100uF.
These ideas may not help if the problem is really difficult, in which case my experiences with an unrelated trem may be even less applicable but here is a little more information:
Hope this helps,
|3/28/2005 7:41 PM|
Understood and understandable, it's just a matter of heater current. If you have the 300mA to spare, go for it.
The MOSFET approach (and this is the last time I'll mention it) only touches your signal with the resistive element of the LDR, so no tone worries.
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