VVR position into a 5E8A

[pampeo]

hello everybody
i'm a beginer in amp building and a have a few question about VVR install
do you think the layout modification on the photo (layout property of weber)
is good ?

thanks so much

[FYL]

The VVR will see pulsating DC as it's connected between the recto and the first filtering cap. Could work but the amp will be *very* noisy.

Either add a B+ filter cap between stdby and VVR or insert the VVR after C1.

Don't forget to add a suitable heatsink: the mosfet will have to dissipate up to 25 watts or so.

[pampeo]

oky thanks
i just updated the photo
I insert the VVR after the C1 and before the choke !
the Bias layout it's ok ?
ok for the heatsink

[pampeo]

so I don't understand why a should be add the VVR after the fisrt cap ??
i you explain please ?
it's possible to add after the choke ?
thanks

[FYL]

the Bias layout it's ok ?

The bias section is OK. You'll have to fine-tune the 56K dropping R if post-VVR bias voltage isn't acceptable.

[FYL]

so I don't understand why a should be add the VVR after the fisrt cap ??

The recto(s) put out pulsating DC - visualize the chopped top of a sine wave. Adding the VVR here isn't a good idea: noise-induced voltages/currents are *very* high and grounding becomes quite problematic.

Shifting it to after the first cap allows it to work on relatively clean DC, with much less grounding- or noise-related problems - OTOH dissipation will be higher.

it's possible to add after the choke ?

Yes. The 5E8 uses a CLC first section, you may insert the VVR between C1 and L or between L and C2.

[pampeo]

Ok thanks
do you think it's a better performance in insert VVR between C1 and L or between L and C2
If the voltage is twice filtered ,it's better ?

[pampeo]

the Bias layout it's ok ?

The bias section is OK. You'll have to fine-tune the 56K dropping R if post-VVR bias voltage isn't acceptable.

So if i understand i can change the 56R by a 56K bias pot ?

[FYL]

Do you think it's a better performance in insert VVR between C1 and L or between L and C2

Not tested in a Twin. I used a broadly similar VRM in a tweed Bandmaster-like amp and inserted the VRM after C1. Works fine.

If the voltage is twice filtered ,it's better ?

Ripple will be lower, but the pass mosfet won't see a difference.

[FYL]

So if i understand i can change the 56R by a 56K bias pot ?

I'd rather build a voltage divider by keeping a series R with a 25K trimmer connected as a variable resistor to ground. Tap bias at the junction of the R and the pot. Values should be calculated according to the raw bias voltage you've got, factoring in the losses in the VVR bias section.

[pampeo]

So if i understand i can change the 56R by a 56K bias pot ?

I'd rather build a voltage divider by keeping a series R with a 25K trimmer connected as a variable resistor to ground. Tap bias at the junction of the R and the pot. Values should be calculated according to the raw bias voltage you've got, factoring in the losses in the VVR bias section.

yes but the VVR have already a bias trim pot for regulate the bias ?

[FYL]

One allows you to set bias range, the other one actual bias voltage. But you may use a single pre-VRM dropping R as well.