Fender 5F2-A and VVR - terrible oscillation

[gitarzysta]

Hello,

I have just built my 5F2-A replica.
I followed the original schematic, including 5Y3GT valve rectifier:
http://www.bustedgear.com/images/schematics/Fender_Princeton_5F2-A.pdf

The schematic calls for two 16uF caps connected in parallel as the first reservoir cap.
I used just one 33uF one which for me makes perfect sense (electrically) - unless there is some voodoo behind and I really should stick to 2x16uF???

I also added a VVR (http://revolutiondeux.blogspot.com/2012/06/voltage-scaling-in-amplifiers-power.html).
I added it right after the rectifier. So there is no filtering (cap) before the VVR and the 33uF cap is right after VVR output.

Now I get pretty awful oscillation as I adjust the voltage - so I am planning to add ANOTHER filter cap before the VVR.

My idea is to add a 20uF cap between B+ and ground before the VVR - see the attachment.



Would it work? Wouldn't it put too much strain on the rectifier valve?

Regards,
Gitarzysta

[sluckey]

Should be fine.

[tubeswell]

The schematic calls for two 16uF caps connected in parallel as the first reservoir cap.
I used just one 33uF

... Would it work? Wouldn't it put too much strain on the rectifier valve?

I suspect the max reservoir cap ratings on data sheets were conservative so that tube manufacturers could avoid too many problems with warranty returns. The reality is that tubes are fairly tough (up to a point), and a 5Y3GT can handle 40uF reservoir capacitance without exploding like a firecracker. Having said that, the higher the reservoir capacitance is, the harder* the tube will work (per unit of time) to pull electrons out of the +ve pole of the reservoir cap (at the requisite voltage during the charging-current part of the cycle), so 40uF will make the tube wear-out faster than 20uF.

* More current per unit of time = more power needing to be dissipated at the rectifier tube plates.

[gitarzysta]

OK, I am going to try the extra cap in a couple of days (I had to order it online), I guess I will change the 33uF one to 16uF as well, so adding extra 16uF one before VVR will be safer for the tube - I'll report back. Thanks for your input.

Regards
Gitarzysta

[PRR]

> stick to 2x16uF???

In 5F2 days, 16uFd was very popular, 32uFd may have cost more than two 16uFd.

Or maybe Leo found a big box of 16uFd at a Real Good Price, and used them everywhere, doubled as needed.

> there is no filtering (cap) before the VVR and the 33uF cap is right after VVR output.

That's not right. The VVR MOSFET can't eat the BIG ripple of rectified UN-filtered "DC", it needs some smoothing.

Yes, the 20uFd before the VVR seems a reasonable thing to try.

[JB]

I recently built a 5E3 Deluxe with VVR.  I didn't add extra filtering, just put it after the first 16uF cap so it feeds the OT and everything downstream.  All seems well.

[gitarzysta]

I recently built a 5E3 Deluxe with VVR.  I didn't add extra filtering, just put it after the first 16uF cap so it feeds the OT and everything downstream.  All seems well.

This is what I started to look at as well - I'll see what is easier due to the way I wired it all up. Cool idea - thanks!

Gitarzysta

[gitarzysta]

So I installed another cap (22uF) before VVR (see the schematic above) and now the oscillation is even worse, high-pitched, very loud - terrible.
The B+ cable from 5Y3 to the board, as I touch/move it with a chopstick seems to affect how much oscillation I get - but it is never quiet.

Any ideas why? Is it the type of MOSFET I use (STW12NK90Z)? It regulates the voltage OK, it is the noise/oscillation that bothers me.

Thanks
Gitarzysta

[plumcrazyfx]

Did this build ever work?  I'm wondering if you added the VVR to a working amp or if you built it from scratch with the VVR and are getting the loud noise.  I've built a half dozen champs and I have one that would squeal until I reversed the primaries.  Strangely it was the 771 - most worked fine but one wanted them reversed. 

[gitarzysta]

Once I removed the vvr and replaced it with a master volume all is quiet.
It was quiet before adding the vvr as well.

[kagliostro]

Did you installed the mosfet on the PCB or you used wires to connect mosfet pins to the PCB ?

Franco

[gitarzysta]

Directly, but the pot is attached with some wires (around 2 inches). See attached pictures.

[kagliostro]

What else is connected to that ground bolt ?

Franco

[gitarzysta]

PT center tap, power filtering caps (except preamp filer cap), 2x100Ohm heaters' virtual center tap resistors (from pilot lamp) and VVR

[gitarzysta]

I rebuilt the vvr with a different mosfet and still get the oscillation and sqeal.
At the moment I installed a master volume and reduced B+ voltage by a series of zener diodes (I needed the vvr not only to act as the master volume but mainly to reduce too high plate voltage: I get 410v vs 320-330 as per Fender 5f1 schematic).
But I don't get why I can't make the vvr work properly 

[kagliostro]

 

Try to connect the VVR to ground at different places, using a gator wire

also

try to move the whole VVR circuit in a different location (during the tests you can also don't connect/bolt it in a definitive way each time)

Franco

[92Volts]

Is the control "signal" of the VVR (I think applied to the gate of the MOSFET) filtered in any way? I'm not familiar with how VVRs are usually set up but if you're trying to give it a steady voltage reference, a cap to ground may be helpful.

Not sure if I missed something in one of your posts but if you haven't included it, a schematic of the actual VVR circuitry could help too.

[mresistor]

I think he should try a 10K 2 watt mf resistor after the VVR that is between the caps. It would help filtration and also drop the voltage to his plates.

[dude]

Try loosening the the small nut on the VVR to chassis, sometimes if it's too tight, leakage can occur.
Also, did you use the small plastic gasket that goes on the small screw before the nut, usually comes with the VVR, not the insulating pad ,I see that..

I've clear up noise problems many times by just loosening that nut holding the VVR to chassis.
A one minute procedure.


al

[gitarzysta]

Yes, I tried to loosen it. And yes I use a plastic washer. But thank you for pointing it.

92volts, Initially I had no filtering before VVR, and then I installed a cap to ground (20uF), which made the sqeal even worse. The schematic of the vvr was linked in my first post:


There's also a drawing of how I implemented it in my amp circuit.

mresistor, thanks, I may try the resistor trick and see how it helps.

kagliostro, I will try your suggestion and post back.

Thank you all for your feedback!

[kagliostro]

Rotating the pot the voltage changes ?

Franco

[gitarzysta]

Rotating the pot the voltage changes ?

Yes, this works well and there are only some spots/pot positions where it sqeals. Usually somewhere in the middle of the pot travel.

[kagliostro]

Recently I've read of an oscillation problem on a SS amp

the problem was due to the isolators used fixing the amp that introduced a capacitance (installed mica instead of silicone)

this thing is completely different from your VVR but .... give a try and disconnect the VVR mecanically from the amp

put on a small heatsink and perform some test

Franco

[doghode]

Hi folks,

Did you ever resolve this? I too am experiencing the same issue with a VVR. Built using exactly the same schematic as posted above, but using a STW21N90K5 MOSFET. I've already built and use a perfectly-working VVR from the same schematic, but with a STW6N95K5 FET. The main difference I can see between the two is Rds(on), which is 0.25 Ohms for the STW21 and 1 Ohm for the STW6.

Like the OP, I find there are good spots on the VVR pot's rotation (e.g. at 11 o'clock on the dial it works great, but at 2 o'clock on the dial I see oscillation at approximately 29kHz using my scope, and at full clockwise rotation it's fine again).

The oscillation only occurs when I have the power tubes installed (this is a 5e3-derived amp using a 5AR4 rectifier and 6L6s). If I omit the 6L6s then everything works perfectly and the VVR is effective and free from oscillation throughout its rotation.

I have tried a few things:

Putting a 1uF/630V film cap across the FET's drain and GND (right on the drain pin and tied direct to the first reservoir cap's negative terminal).
Putting a 1uF/630V film cap across the FET's gate and GND (again, tight against the pin).
Putting a 15uF/500V across the drain.
Putting a 22uF/500V across the drain.
Putting a 22uF/500V tight against the 5AR4's output pin and GND.

I've tried combinations of the above: e.g. 15uF drain cap with 1uF gate cap. There's no point in using a bigger gate cap, it makes turn-on time too slow. I also tried splitting the 100k gate stopper (which is tight against the gate pin) into 100k + 1k with a 1uF cap across the junction of those resistors and GND.

None of these things fixes the issue. In fact the most stable configuration appears to be using no caps at all!!

I was wondering if the OP's problem was ever resolved and, if so, how!

Thanks,
-dogo

[gitarzysta]

No, I gave up - but your post brought some more light and some hope :)

[doghode]

Darn it, I was hoping you'd solved it already!

My first step is going to be replacing the STW21 with a STW6, like I have in the amp with the working VVR (it's the exact same amp except for the different FET). In the working one I don't use caps on the drain or gate. The VVR's input (FET drain) is connected directly to the 5AR4.  The VVR's output (via the 10R on the source pin) connects to the first filter cap.

Another thing I've noticed: it's not just the VVR pot that affects the oscillation. If I rotate the tone control to its full extent in either direction it causes the oscillation. Leaving tone in the middle seems stable. The volume knobs have a similar effect: if I turn up the volume past 11 o'clock-ish the oscillation starts up again. The problem does NOT occur when the power tubes are removed.

Should know more in a couple of days once the new FETs are in place.

[doghode]

Finally I solved it. The solution was actually simple: the 100k gate stopper on the FET was 1" of wire away from the FET's gate pin, which was too far and led to oscillation (technically the resistor is a current limiting resistor for the zener, but in this case its function is also the gate stopper for the FET).

I attached the resistor directly to the FET's gate pin as close to the body of the FET as possible so that there was maybe only a couple of mm between resistor and gate.

Job done. If any of you have similar issues, try getting your gate resistor as close as humanly possible to the FET.

Now I just need to figure out how to do heatsink calculations...

[gitarzysta]

Thank you so much for getting back to us! Much appreciated

[dude]

Now I just need to figure out how to do heatsink calculations...

I use an old PC processor heat sink, one with the 2” fan screwed on top. The sink is 2” square mounted on chassis outside right under mosfet. To run the fan, l use diode rectification and 220uf filter off the 6.3v filament supply. When l turn the power on fan will spin but about only 5 or 6 volts dc get to the 12v fan, this slows the speed but enough to keep the mosfet from getting hot and blowing. If you're interested, l can post a schematic of the fan PS.

[doghode]

Thank you so much for getting back to us! Much appreciated

You're very welcome :) I hope one day this saves someone a headache.

[doghode]

Now I just need to figure out how to do heatsink calculations...

I use an old PC processor heat sink, one with the 2” fan screwed on top. The sink is 2” square mounted on chassis outside right under mosfet. To run the fan, l use diode rectification and 220uf filter off the 6.3v filament supply. When l turn the power on fan will spin but about only 5 or 6 volts dc get to the 12v fan, this slows the speed but enough to keep the mosfet from getting hot and blowing. If you're interested, l can post a schematic of the fan PS.

Thanks :)

I've mounted the FET on a heatsink taken from an old plasma TV. I had a meat temperature probe jammed into the fins of the heatsink near the FET (STW6N95K5 https://www.mouser.com/ProductDetail/511-STW6N95K5) and it stayed under 150°F for a couple of hours under test. I believe that's fine, but I'm receptive to better-informed opinion!

My fan is a 5VDC one, mounted on the side of the chassis to blow on the transformer (I'm using a 5AR4 with 6L6s and a Mojotone Heyboer 5E3 transformer). The airflow also hits the FET 's heatsink, which can't hurt.

The power supply for the fan is a Vigortronix 5V 2A SMPS (https://www.newark.com/vigortronix/vtx-214-010-205/ac-dc-pcb-mount-power-supply-class/dp/40Y2700) mounted on the Vigortronix mounting kit (https://www.newark.com/vigortronix/vtx-214-pcb1/power-supply-accessory-mounting/dp/40Y2667?MER=sy-me-pd-mi-acce).  I'm using it to power some 5V logic and relays, so I figured it would be good for the fan, too.

So far so good!

[gitarzysta]

As a side note: I installed pentode/triode switch (through 1K screen resistor) - and in triode mode I get loud, low freq. hum - in full power mode it is just dead quiet.
Not sure why this is so - tried different valves and moving cable around - but no luck.
Not that I desperately need triode mode in this amp, but it intrigues me anyway....

[PRR]

Draw out the circuit, power OT and tube.

Pentode is high plate impedance. Most (90%)of the supply ripple is dropped across the tube, little (10%) across the OT.

Triode is low plate impedance. Less (33%) of the supply ripple is dropped across the tube, more (66%) across the OT.

Assuming those numbers there is 6 times more speaker buzz with triode. This is part of why pentodes pushed-out triodes in domestic ratios. Cheaper filtering.

[gitarzysta]

This is how I did it (I added 1k screen grid current limiting resistor as well and I use it in triode mode, too - I tested w/o the resistor and the nasty hum was still there, though)

[thetragichero]

maybe it's too much of a redesign, but a pi filter (clc) before the ot would provide much cleaner dc. aes used to have a great little choke for about ten bucks back before magnetic components stopped making their guitar iron