Checking a VVR install in DIY 5E3 build

[pedro]

Hi all,

I just installed a VVR in my recent 5E3 build.

I used a typical VVR circuit but with a 16uF 450v cap and 220K bleeder before it - right after the 5Y3 rectifier. I am scaling the whole amp, so the VVR sits right before the usual 5E3 filter cap section.

I used what I had to hand to build it up from scratch - so it uses a 20 Ohm 7 watt resistor and a 12v Zener. Pot is a 1Meg audio taper CTS pot. I have a 47K in the leg to ground to get volumes really low at lowest setting.

As I was testing it out I heard a loud crack - so shut everything down quickly.

Strange thing is - the VVR and amp seem to work absolutely fine, and I cant find what cracked or shorted out in the amp at all - everything seems to check out OK - all voltages look good etc.

testing again last night with the Paul plugged in, and using my Thiele cab with a Celestion G12-65 it still sounded righteous !

I have been taking voltages measures and found that when the VVR is set really low the voltage appearing at the 16uF cap after the rectifier gets as high as 460 Volts - which worries me a bit.

So....any advice on how I can check the Mosfet, Zener etc in circuit to ensure they are working OK. Also, is it usual for the extra filter cap to carry so much voltage ?
My plate voltages without the VVR in circuit were around 330-340 volts. I am using a 270R cathode resistor for the 6V6's.

Any advice gratefully received !
cheers
Pete

[sliberty]

If the VVR is working correctly, the output voltage from the board will drop as you turn the VVR level lower, and it should be close to full expected voltage when the pot is up full.

That crack begs the question - did you insulate the MOSFET from the chassis using the little insulator sheet (often clear, looks like plastic)?

[pedro]

Hi ,
yep,
I insulated the MOSFET and have continuity tested to ensure there is no short.

I wonder if I should remove the additional pre-VVR filter cap anyway.

I might make up another tagstrip with new MOSFET and Zener and put it in just to be sure.

Still - is there a way to test the MOSFET pins and Zener whilst still in circuit ?

cheers
Pete

[sliberty]

Like I said, if your output voltage is tracking the way it should, then the MOSFET and Zenar are working. when one or both of them die, the voltage will not go up and down with the pot.

[vibratoking]

The rectifier needs to 'see' the proper capacitance according to the data sheet. I installed my VVR between the first cap and the first dropping resistor. Works great and there is no need to add extra caps in the supply. I had to add a heat sink in order to keep from destroying the FET. I blew 2 FETS before I got smart.

[martin manning]

I think definitely keep the reservoir cap between the rectifier and the VVR, otherwise there will be high current pulses going through the MOSFET. It'd be a good idea to replace it with a 500V rated part, too.

[pedro]

OK many thanks guys

I am definitely getting the right voltage range up and down so all looks good there. Very slight hum when pot is fully up but otherwise all OK.

cant find anything that shorted and made the "pop" maybe I had some swarf or small wire offcut hanging around in the chassis that caused that crack noise.

I have replaced the additional post-rectifier cap with a new 22uF 500V rated part with a new 220K 2watt bleeder resistor.

All sounds good so I will leave it on bench test for a few hours and do the old smoke test.

If its OK have some recording planned with this amp so hoping all good now...

Interesting that I could place the VVR between the first 5e3 PS cap and the 4.7K dropping resistor - might try that for size.

Also interested to try scaling just the 6V6s and leaving the preamp untouched later ... I guess I will need to add a dropping resistor to the preamp to compensate for the current which would have been taken by the 6V6s.

Regards
Pete

[vibratoking]

If you scale just the output, then you will probably need a master volume to attenuate the signal coming off the preamp. Not worth the effort IMO.

[pedro]

thanks VK,

I will stick with scaling the whole amp - I am getting some great tones with it.
Cheers
Pete

[Synchu]

I installed a VVR in my first amp build (when I opened it, I had the moment of - shouldn't I redo the whole thing ) few days ago, a Tube amp doctor 5E3 kit, scaling the whole amp. I ditched the Toshiba MOSFET that came with the VVR kit, for an IFR840, since the Toshiba one had some issues.
Works great - the only difference being that it now has much more headroom esp. At bedroom volumes.
There is tone and overdrive behavior change, but overall sounds good and gets the job done.
Niki

[tonewood]

Works great - the only difference being that it now has much more headroom esp. At bedroom volumes.
There is tone and overdrive behavior change, but overall sounds good and gets the job done.
Niki

Hi Niki- Interesting that there is more headroom when you lower the voltages to the tubes of the whole amp.
I lower the voltages to the power amp for the opposite reason. I want less headroom so I can distort PA at lower volumes. I haven't tried lowering the preamp voltages yet but would have guessed that they would either distort earlier or not change much. I'll have to give it a try & see what is going on.

Thanks for the info.

[pedro]

Hi there

I used the IRF840 too.
Having played the amp quite a bit and recorded some stuff with it in the home studio at quite low(ish) volumes , I am pretty pleased with the full amp scaling.
Sure there isn't the bloom you get when its dimed and you can feel the sag...but it can still get great deluxe tones at reasonable volumes. The cleans are still good with judicious use of volumes and guitar volume pot. The lead sound with a the Paul into a thiele cab is bloody great - even at low volumes.

I have used speaker attenuators before with my 50watt and 100watt heads and i think scaling is a more convincing approach to power tube distortion tones ( which I prefer over preamp distortion...even with a dumble clone in the house...)

So far so good. Only issue is that I only have 450v rated caps for the pre-VVR filter and the cap is probably being voltage stressed too much with the max voltage (460v) across it at the moment - I need to change that soon.

Rgds
Pete

[Synchu]

@tonewood - the headroom thingy sounds counter intuitive and I haven't done the calculations or any measurements to "see" what is actually happening there, but sounds good. I recorded some noodling for test purposes, with a strat at almost whisper quiet volume (which obviously eliminates any speaker - an WGS G12C/S - influenced tone changes - whatever minimal they are even at fully cranked volumes since it is quite efficient at almost 100 dB. That is part of the overall greater headroom of this amp, but still with VVR it doesn't seem to overdrive as easy :/)
https://soundcloud.com/synchu/testing-vvr

@Pete - It is a good idea to change the cap only from longevity perspective with this rather small difference. Higher than rated voltages will shorten caps live (within certain range of'course - throwing 450v at 25v rated cap will have quite an instantaneous "blooming" effect over it and can be dangerous).