i'll check during the day and let you know.. but this 100k, pls remove, and solder directly to the G PIN. this resistor can be also 10k, maybe good would be 1k, carbon composition.
here are missing 2 electrolytic caps, one on the input of VVR ("from rectifier"- where is electrolytic cap), and one at the exit, should be very low value, 1-2,2uF 450V , the best is after 10 ohms .. wires input to VVR (from rectifier) and output from VVR to cap, cannot be long because inductance in those wires will be significant for mosfet operation. the best would be input capacitor /22uf+ and output 2.2uF put duirectly to the VVR place.. depending of the place ofcourse
here is the schematic proposal for the start wiring, zener directly on the pins, output capacitor for stability can be up to 4.7uF, gate capacitor for the ripple reducing from the Ub+ can be 10uF-22uF, but also in some cases can sound too sterile (not in every case, depending what is supplied with this VVR, anodes also or only g2, can be omitted, but i would start with ). added 22k for ripple removing efect maintained still when in max potentiometer position..
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ChopSauce wrote: ↑Fri Dec 17, 2021 9:16 amI'll try that sooner or later.
Here is a bare-bones VVR for your amp. Same schematic as the one in your post above. Note the Zener connects to the down-stream end of the 10Ω resistor to make a current limiter. I often see 12V, but with a 9.1V Zener you should get just under 500 mA max. I'm not sure where your OT CT is running now, but move it to the VVR output as shown.
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Thanks Martin. I've been explained almost the same thing about the role of the pair (5W resistor, zener) but I've been suggested to use a 47R for more/real protection - possibly depending upon the MOSFET being used. This is being "discussed" at the moment on a french forum (Projet G5). I'll report the outcome here.
Anyway, I've already made most of the modifications you suggested but that seems weird to move the center tap of the power transformer to the VVR output...
I think I understand elevating the CT for the heaters to the cathode voltage, but to my limited understanding, your suggestion sounds like elevating something above itself, which definitely puzzles me...
Also, I suppose the 4k7 - from VVR/out to ground - is of unit pF (that would be 4.7 nF) right?
I'll post an updated pic or the corresponding schematic/layout, as soon as I'll get it.
This is what I'm suggesting. For more current limiting you can either increase the series resistor or decrease the Zener voltage. I chose to keep the series resistance low to minimize power loss and heat generation. Limiting comes in at (Vzener - Vgs) / Rseries, where Vgs is about 4.5V. The 4k7 is the screen node dropping resistor, relocated to the VVR terminal strip. You have to move that to put the VVR between the reservoir capacitor and the OT CT, and I believe you can connect the VVR output directly to the OT without adding another filter. At full power the B+ ripple will be the same as it was without the VVR, and it will be reduced as you bring the voltage and current draw down.
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at the exit of VVR / after 10R small electrolytic cap must be placed. it is not good idea to connect kilometers of wire (primary of output transfomer) at the mosfet source and expect that this will work good, and never oscillate.
bepone wrote: ↑Wed Dec 22, 2021 4:42 pmat the exit of VVR / after 10R small electrolytic cap must be placed. it is not good idea to connect kilometers of wire (primary of output transfomer) at the mosfet source and expect that this will work good, and never oscillate.
I've seen a number of schematics that do not use a cap there, so it must be possible to get along without it.
I'd rather say that any VVR schematic I could see shows no cap at all there, but that doesn't mean it is not worth adding and it is not a big deal to add one, anyway...
At least that was important to clarify the role/position of the (10R, zener) pair!
Now, if the MOSFET oscillates, will that occur most when the amp is running at full or reduced voltage?
(because it sounds as perfect as I could expect at reduced voltage)
ChopSauce wrote: ↑Thu Dec 23, 2021 8:39 am
I'd rather say that any VVR schematic I could see shows no cap at all there, but that doesn't mean it is not worth adding and it is not a big deal to add one, anyway... :?
At least that was important to clarify the role/position of the (10R, zener) pair!
Now, if the MOSFET oscillates, will that occur most when the amp is running at full or reduced voltage?
(because it sounds as perfect as I could expect at reduced voltage)
So you have it working now? If so, and it sounds good at any power level, I would not add the cap.
Sorry, I have to do some emergency plumbing repair right now and can't power on the amp or open it...
The VVR works (*) very well but when I turn its potentiometer rightmost. At max voltage it is too noisy. bepone was suggesting some ways to diminish its noise level if I understand it well.
I'll put CF resistors and rework the layout maybe, but I wish I understood more precisely the purpose of his suggested mods : added 22k resistor and 100k (R1) resistor lowered to a few kOhms, plus the 1.5 to 4.7 uF cap on output (right before the following filter cap) ...
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(*) as per this schematic with R2 ~= 50k and an IRF820
