VVR & SAG

[geoff 1965]

6V6_Lite IIb_Conger.gif

i'm planning on using these values for the power & phase section of my 6V6 project including the VVR "preamp excluded" and my question is will the placement of the 5W wirewound in the VVR circuit act as a sag resistor or would i have to add one in series either pre or post in the B+ line?
i'm hoping this lite2 6V6 version will keep the touch response of the 18W but would appreciate any advice/opinion.

[zaphod_phil]

My understanding is that a VVR provides an adjustable fixed voltage reduction, due to the presence of the zener in parallel with that resistor- ie no added sag.

[geoff 1965]

thank's for confirming that,the parallel zener made me uncertain.
that brings me to the next question,is sag most effective for the power tubes or the whole B+ line? i'd only want a small amount "22R" of sag.

[zaphod_phil]

22 Ohms will barely give any noticeable sag. A sag resistor is most effective when placed in series with the whole B+ rail. However, since the preamp and PI draw such a tiny amount of current compared to the power valves, it makes little difference either way. Also don't forget that there is quite a bit of sag inherent in the PT.

[geoff 1965]

i used the 20 watters in the downloads as a guide for the sag having the higher 312-0-312 HT, they have either 22 or 30R with 50-50uf first filtering so i've followed suit.that should give me a tighter/faster response than the lite2/ez81 i've got and now i know i will leave the current sag where it is.just going to focus on one section at a time,i've noticed versions of VVR that only adjust the power/phase section use a diode on the tap to the preamp,is this applicable wether you have diode or tube rectifier?

[zaphod_phil]

If you regulate the preamp rail, you need to be aware that you can get DC appearing on the first preamp grid and getting into your guitar's volume pot. So you would need to put a DC- blocking cap in front of the grid and the 1M resistor to ground, say 0.047 or 0.1uF or so. Solid state or vacuum rectification makes no difference to how a VVR is used.

[geoff 1965]

i appreciate your explanation's ZP! my background is heavy engineering "welding" so this last 3 years or so has been a brand new learning curve for me with valve amps.but i love it and am completely obsessed now with schematics,hand's on experimenting etc.
with VVR's are the value of components set? i.e. the 12V 1W zener,does it have to be a particular type, i have some 12V 5W zeners so could i use one?
also another point of interest is if you look at the schematic at beginning of the post the screen resistors for the 6V6's are 470R and when you watch the guys that repair amps they usually swap those to 1K,is there a good reason for doing this other than dropping a bit more voltage?

[zaphod_phil]

12V 5W zeners will work fine, and might even last longer than 1W.

The 1k screen grid resistors will help prevent the screen grids from burning up.

[geoff 1965]

thanks for your help, going to remove the rectifier board and make one to incorporate the VVR components, also use 1Ks on the screens.

006.JPG

[geoff 1965]

just one last query on VVR, there are a few different versions which if you've not used one can be confusing! the one at the beginning of the post sits between the rectifier and first filter cap whereas the one below from the downloads sits between the first and second filters.which do you think would be best? i'm not regulating the preamp voltage only the power & phase section. the TMB version below has the extra diodes into the vvr and preamp feed so would i need to use these although i will have 4 in series rectifying?

18_watt_tmb_vvr_pa_pi_198.gif

[Daviedawg]

Geoff. I have installed the version inserted after the first filter as shown in your last post (TMB) with the PA and PI voltage managed. It has been in use for about nine years pretty much constantly, with no issues. It is very effective on the 18 watt working down to low voltages whereas on some amps it results in a poor tone.

Dd

[colossal]

Geoff, the extra diodes are for isolation. I would include them.

[Daviedawg]

Yes with added diodes. Sorry I was not clear on that. I built mine to that schematic.

Dd

[geoff 1965]

thanks lads, i'll build the second TMB version with that good feedback.

[geoff 1965]

008.JPG

fireworks! just powered on with the new VVR in place and there were sparks from the potentiometer and the 22R was smoking and red hot! so switched off straight away and time to investigate, double checked circuit and hoping the mosfet isn't damaged.
replaced 22R and switched stanby off so no power to potentiometer/mosfet and i have a constant 457 VDC unloaded at first cap and 22R is cool to touch so it must be the pot or the mosfet at fault.
just realised the voltage rating for that welwyn W22 10R is only 200V! i presumed it being 7watt it would be 500.

[katopan]

It's really hard to tell in the photo (one from directly above would be better) but does the black wire onboard connect to the middle pin2 of the mosfet (which it should) or the outer right pin3?

Is the mosfet fully insulated from the chassis?

[geoff 1965]

yes katopan the black B+ in goes to pin 2 on the mosfet and i used 2mm thick heatsink pad under the mosfet, i will double check i haven't tightened it too much. the sparks looked to come from the pot but the mosfet sits underneath and i switched off straight away so i'm not sure. just waiting for a correct 10R and new mosfet "just in case" to arrive, but i'll start by removing the potentiometer and checking for shorts/resistance.
hard to see from angle of the pic but here's the layout

VVR.jpg

[geoff 1965]

okay i've replaced the 10R with a correct voltage rated resistor and checked the potentiometer and it's good. now there is no fireworks or burning resistors but the B+ voltage is at max with no adjustment from the potentiometer,can i assume this mosfet is fried?

[katopan]

Can you set the pot to 50% and measure the voltage on the three pins of the mosfet? That will tell you if it's fried or not.

[geoff 1965]

set the potentiometer at 50% katopan and i measure 448VDC on all 3 pins of the mosfet.