Ultra Fast diode rectifier and PT snubber on guitar amp

[Matteo Masin]

Hi everyone,

I'm new to the forum but a long time reader, I write this post because I would like to see if someone can give me a good advice 🙂

I have a Fender Vibro-King, I love its sound but I'm always disappoint about it's noise floor comparing to my other amps. The noise it's like a 100hz hum, maybe it's part of its design... yes I can live with it (I play it since 2012) but recently I read some posts here regarding UF diode vs standard and the UF seem from what I've read less noisy especially if paired with a snubber circuit.

I read that the best result is using four UF4007 in the full wave rectifier, and before it, on the secondary of the PT the snubber (47ohm resistor and 22nf 1500V cap between secondary).

I would like to try it, but I have a question watching the schematic. Here I attached it...

The best place for the snubber is right before the full wave rectifier before the standby switch? Before the 8200uf caps? Or 2 snubber circuits from both the secondary leads to the Central Tap?

What do you think? And do you think it's a good idea?


Thank you very much to everyone!

 

[giobo91]

uf4007

hi! you can do 2 major things:
1) replace 1n4007 with UF4007 >>> you don't reduce hummm but hssss and the sound will be less harsh (they are fast in recovery time so ideally you will have less "spikes" from recoveries (the snubber was here for spikes) or you can switch to Schottky diodes (for me more musical)

congrats: now you can turn high the presence control and the sound will be brighter but not harsh as before

2) you could try to match and replace the 100ohm resistors (R62 and R63) (use 1W metal film); try a humdinger pot and elevated heater mods..... probably the hummm is picked from the heaters! and amplified!
The Valve Wizard
and rewire the tube heater propery (if it's wired on chassis)

3)if you like modding, you can change C1, C18,C19,C6,C9,C8,C13,C14 with higher grade Sprauge 225p or 6ps 600v...... you will have a newer sound, more beefy but with highs and harmonics.


i built 2 SLO100 2 JMP2203, a plexi 1959 and hybrid 3ch 2203 and they are very silent with 100ohm artificial tap and elevated heater.


if you have trouble i'm here!

Greetings from Padova
Giovanni

 

[Johno]

Hi MM, each secondary winding needs a snubber and each snubber needs to be as close to the transformer as possible.

Snubbers are needed to damp (over damped or critically damped is fine) the oscillation that can occur between the inductance of the transformer secondary and the internal leakage capacitance of the winding. That ringing at many MHz radiates into the amplifier circuit at short bursts twice every mains cycle (ie at 100Hz for 50Hz mains). This sounds like 100Hz hum.
Diode type makes no difference and bypassing diodes with a small capacitor simply couples the noise more readily into your amp and can make the effect worse.

 

[Enzo]

I suggest finding the source of the hum and dealing with that instead of just throwing parts at everything that might have something to do with humming.

 

[rayma]

Right, there's lots of possibilities in there.

 

[JMFahey]

Layout and wiring are awe inspiring. 😱

I would suggest tearing every single wire out and rewiring it properly.

Serious.

 

[Matteo Masin]

hi! you can do 2 major things:
1) replace 1n4007 with UF4007 >>> you don't reduce hummm but hssss and the sound will be less harsh (they are fast in recovery time so ideally you will have less "spikes" from recoveries (the snubber was here for spikes) or you can switch to Schottky diodes (for me more musical)

congrats: now you can turn high the presence control and the sound will be brighter but not harsh as before

2) you could try to match and replace the 100ohm resistors (R62 and R63) (use 1W metal film); try a humdinger pot and elevated heater mods..... probably the hummm is picked from the heaters! and amplified!
The Valve Wizard
and rewire the tube heater propery (if it's wired on chassis)

3)if you like modding, you can change C1, C18,C19,C6,C9,C8,C13,C14 with higher grade Sprauge 225p or 6ps 600v...... you will have a newer sound, more beefy but with highs and harmonics.


i built 2 SLO100 2 JMP2203, a plexi 1959 and hybrid 3ch 2203 and they are very silent with 100ohm artificial tap and elevated heater.


if you have trouble i'm here!

Greetings from Padova
Giovanni

Hi Giovanni,
thank you very much! I already replace the 100ohm of the artificial center tap in the past, with 0,1% tollerance metal film resistors. I will try the heater elevation mod.

Greetings from Treviso 🙂
MM

 

[Matteo Masin]

Hi MM, each secondary winding needs a snubber and each snubber needs to be as close to the transformer as possible.

Snubbers are needed to damp (over damped or critically damped is fine) the oscillation that can occur between the inductance of the transformer secondary and the internal leakage capacitance of the winding. That ringing at many MHz radiates into the amplifier circuit at short bursts twice every mains cycle (ie at 100Hz for 50Hz mains). This sounds like 100Hz hum.
Diode type makes no difference and bypassing diodes with a small capacitor simply couples the noise more readily into your amp and can make the effect worse.

Thank you very much! So you say I need 2 snubber... here and here

 

[Matteo Masin]

Right, there's lots of possibilities in there.

Hi Rayma, this is not the inside of a Fender Vibro-King, the one in your picture look like a heavy modded Vibro-King... this is the inside

 

[Matteo Masin]

I suggest finding the source of the hum and dealing with that instead of just throwing parts at everything that might have something to do with humming.

Hi Enzo, thank you. Probably this is the smartest thing to do but sadly I could't think I can do it by myself 🙂

 

[Printer2]

Layout and wiring are awe inspiring. 😱

I would suggest tearing every single wire out and rewiring it properly.

Serious.

Somebody wanted to finish the spool.

 

[PRR]

Millions of g-amps have been enjoyed without snubbers or ultra rectifiers. What makes this amp so different?

 

[Matteo Masin]

Yes I'm agree with you, also my other amps don't have snubbers and they have tube rectifier, but this amp have more noise, so I'm trying everything for make it quieter. Everything that can reduce noise especially the 100hz that are prominent will be a good upgrade.

 

[giobo91]

uf4007

you need to rewire better the heater wires!.
Wires need to be tightened

this is the best wiring you can do
next step: have you turned the power transformer 90° offset to the power transformer? (deve essere ruotato di 90 gradi rispetto al Trasformatore della 220V )

 

[wg_ski]

The best place for the snubber is right before the full wave rectifier before the standby switch? Before the 8200uf caps? Or 2 snubber circuits from both the secondary leads to the Central Tap?

Those 8200 pF caps are supposed to BE the snubbers. If you’ve changed the rectifiers and replaced them with a fast type, they may now be ineffective. They are really supposed to have resistors in series with them - I typically use 15 ohm. This will make them more effective over a broader range of conditions.

Of course a “hum” might be caused by something else, rectifier “noise” usually sounds more like a “bizzzzzzz” than a “hummmmm”. More high frequency harmonics and less low frequency content.

 

[Matteo Masin]

Thank you very much! This is a really helpful answer 🙂

 

[Rick PA Stadel]

Is the 'hum', by any chance, quieter when the reverb is turned all the way down?

That section has a transformer and 2 coils, one of which has a lot of gain behind it, all of which are inherently susceptible to stray magnetic fields. Perhaps the tank has been moved from its original factory location?

Cheers

 

[Matteo Masin]

Hi thank you for your answer. No the hum doesn't change when I turn down the reverb mix pot, and the tank is far away on the bottom of the cabinet.

I notice this hum is present also when the amp is in standby

 

[Zung]

...
I notice this hum is present also when the amp is in standby

Then it could be mechanical, maybe transformer buzz. How loud?

 

[Printer2]

Does it hum without tubes in?