Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on December 07, 2011, 07:24:58 pm

Title: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: kagliostro on December 07, 2011, 07:24:58 pm

Is possible to modify the schematic of a VVR to obtain a progressive conduction from a minimum to the max value of B+ ?

I heard about circuits using the charge of a cap to "drive" the conduction of a mosfet, but I didn't find a schematic with values of component as to be practically used

(http://www.audiofaidate.org/uploaded/mrttg/ScreenHunter_01%20Apr.%2029%2018.36.gif)

Thanks

Kagliostro

EDIT: a nice pdf about Brimistors   http://www.domino405.co.uk/documents/Brimistors.pdf (http://www.domino405.co.uk/documents/Brimistors.pdf)

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: kagliostro on December 08, 2011, 06:10:52 am

I've find this, but I'm not sure about it

Someone can confirm that this is usable ?

Thanks

Kagliostro

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: HotBluePlates on December 08, 2011, 07:06:33 pm

That looks essentially right. You should wait for someone knowledgeable to confirm that.

Assuming only the 33k and 22uF determine the timing of the circuit, it looks like you'll rise to full d.c. voltage in ~3.6 seconds.

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: Tiny_Daddy on December 08, 2011, 07:16:25 pm

I would use a higher voltage FET. The old VOX I have worked on were running 525V, important when switching on with SS rectifier.

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: kagliostro on December 09, 2011, 09:43:18 am

Many thanks HotBluePlates & Tiny Daddy

Quote

Assuming only the 33k and 22uF determine the timing of the circuit, it looks like you'll rise to full d.c. voltage in ~3.6 seconds.

As to have an (initial and definitive) fine tuning of the time to rise voltage is better to work with the resistor or with the cap value ? Should a trimmer in series with a lower value resistor do the job the right way ?

Quote

I would use a higher voltage FET. The old VOX I have worked on were running 525V, important when switching on with SS rectifier.

I'll follow your advice

EDIT: will this be a good choice for this circuit and for the VVR circuit ??

http://docs-europe.electrocomponents.com/webdocs/0db2/0900766b80db2783.pdf (http://docs-europe.electrocomponents.com/webdocs/0db2/0900766b80db2783.pdf)

Kagliostro

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: kagliostro on December 12, 2011, 09:50:49 am

I've read about Capacitor Multipliers and I'm a bit confused

isn't the "Mosfet Brimistor sostitute" a Capacitor Multiplier ?

someone can clarify ?

Thanks

Kagliostro

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: HotBluePlates on December 12, 2011, 06:12:45 pm

The time to charge a capacitor to an applied voltage is proportional to the value of the capacitance and the resistance in the circuit.

A cap will charge to about the full applied voltage in 5 time constants; 1 time constant is equal to R (in ohms) * C (in Farads). See RC Time Constant (http://www.tpub.com/neets/book2/3d.htm).

The bigger the R or C, the slower the rise time from 0v to the full applied voltage. Increasing either (or both) makes each time constant bigger, and it still takes 5 of them to reach approximately full voltage.

You capacitance multiplier is essentially making C bigger. Therefore, it makes R*C (time constant) bigger, so the rise to full voltage takes longer. So yes, a capacitance multiplier and "MOSFET brimistor" are two names for the same circuit.

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: kagliostro on December 13, 2011, 10:05:25 am

Thanks for the infos HotBluePlates

I'll read with attention the link you posted

as to obtain the slow B+ increase and a good ripple rejection
(given by the 470k resistor paralleled with the cap in the capacitor multiplier)

do you think that to add a cap between the 470k resistor and ground will give the wanted effect ?

Kagliostro

Title: Re: There is a way to modify a VVR as to obtain something like a Brimistor ?
Post by: HotBluePlates on December 13, 2011, 03:51:09 pm

If you read the text that accompanies the figures you posted, I bet it will say that the 470k resistor was added to solidly define the bias of the MOSFET. If so, I wouldn't add the extra cap.