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8th June 2011, 05:02 PM  #1 
diyAudio Member
Join Date: Jul 2009

Jolida 502a Snubber Mod
Hi
Has Anyone done this to any tube amps,Are there Caps useds? Thanks for your time Install a "snubber circuit" consisting of a 10ohm  10W resistor (2 resistors actually) at the two wires from the input transformer to the power supply circuit board. The resistors are soldered in line with the two wires before connecting to the power supply circuit board. Resistor values of 3ohm  10ohm can be used, 10W rating is important. You will have to rebias the tubes after this mod. The idea is to reduce AC noise before the rectifiers. A better snubber circuit utilizes an RC circuit. I have not experimented with caps in the snubber circuit yet. 
9th June 2011, 01:50 PM  #2 
diyAudio Member
Join Date: Aug 2008

This is really not the way to do it. This will just create more heat, and lower your voltage a bit. If you want to snub anything worthwhile, put 4 100n/600V caps in parallel with every diode in the diode bridge.Better yet replace the cheap bridge with hexfreds,and snub them too.

9th June 2011, 05:09 PM  #3 
diyAudio Member
Join Date: Nov 2005
Location: SoCal

IMO, RC snubbers are the way to go to dampen power supply ringing, but they need to be tuned properly. Caps across diodes never helped much for me.

9th June 2011, 05:34 PM  #4 
diyAudio Member
Join Date: Aug 2003
Location: Santa Cruz, California

This article gives a good overview on calculating RC snubbers:
www.hagtech.com/pdf/snubber.pdf It's a fair bit of math, but the results are worth it. 
9th June 2011, 06:01 PM  #5  
diyAudio Member
Join Date: Aug 2008

Quote:


10th June 2011, 05:11 AM  #6 
diyAudio Member
Join Date: Nov 2005
Location: SoCal

Here is a method for the mathmatically handicapped (such as myself). Modified from a maxim application note
1. Measure the frequency of the spike resonance. 2. Add a shunt capacitor across the parasitic component and adjust the value of this capacitor until the frequency of the spike resonance is reduced by a factor of two. The value of this resulting capacitor will be three times the value of the parasitic capacitance that is creating the voltage spikes. 3. Because the parasitic capacitance is known, the parasitic inductance can be determined using the formula: L = 1 / [(2 x Pi x F)² x C] where F = resonant frequency and C = parasitic capacitance 4. Now that both the parasitic capacitance and inductance are known, the characteristic impedance of the resonance can be determined using the following formula: Z = SQRT(L/C) where L = parasitic inductance and C = parasitic capacitance 5. The resistor in the RC snubber circuit should be sized to the value of the characteristic impedance, and the capacitor should be sized between four and ten times the parasitic capacitance. The use of larger capacitors slightly reduces the voltage overshoot at the expense of greater power dissipation and less inverter efficiency. 
11th June 2011, 07:03 AM  #7 
diyAudio Member
Join Date: Nov 2007

I agree. I built a regulated supply once and the damped oscillations (caused by diode reverse recovery exciting the parasitic resonant circuit in the power transformer secondary) slipped through the regulator without much attenuation. The right way to solve this problem is lowering Q, and a resistor offers easy control of Q. Caps just lower the frequency of the oscillation and attenuate a bit. A resistor stops the oscillations dead.

11th June 2011, 09:48 AM  #8 
diyAudio Member
Join Date: Feb 2009
Location: Melbourne, Oz

The fundamental reason for the switching transient is the current in the secondary winding inductance, which partially dissipates in to the other windings as well as charging the diode node voltage (ie. including the diode parasitic capacitance). The diode path is sometimes a relatively large loop.
An alternative method of managing the transient is to place capacitance across the secondary winding, with as 'small' a loop inductance as possible  which can be used to divert a significant portion of the energy in the winding leakage inductance. This bypass capacitance can also benefit from close to critical damping. If high frequency noise is causing noticable problems, then this method can benefit from added special inductance in series with each diode  such as amorphous core amobeads. Ciao, Tim 
11th June 2011, 11:05 AM  #9 
diyAudio Member
Join Date: Aug 2008

Very, very interesting, but how to calculate the parallel capacitance on the secondary?

11th June 2011, 12:20 PM  #10 
diyAudio Member
Join Date: Feb 2009
Location: Melbourne, Oz

Everything is a tradeoff. More shunt capacitance = lower transient voltage disturbance and lower ring frequency  but higher mains current.
If you have a problem, then that provides the best benchmark for choosing a value. I happily add 10nf to HT secondaries  but that is just a gut feeling. 
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