Guitar amp fizz causes and the Paul Ruby mod

Have you ever wondered what causes the infamous 18-watt fizz? Well, we are going to try and close the book on it, and show a couple fixes as well. I hope this answers some questions at the very least.

[@ShredEngineerPhD]

Hey man! I just discovered your channel and I like your work! Increasing grid stoppers helps a lot indeed. Learned that the hard way building my own amp... ^^

[@theguitaramptech]

Great explanation and fix for minimizing the grid block effect.

[@markbruno9956]

I really like your explanations and you amazing knowledge on the subject. I have watched years and years of many different videos and just found you today.... and I just had to say thank you for posting these, and am your newest subscriber.

[@atech9020]

Wow, thank you!

[@sylvainleclerc3785]

Gonzo tinker here. Thank's. I fix my amps. Guitar amps. Tinkering with a Epiphone Valve Junior, and this is going to help me to take some of the fiz that I get, as soon as I can desifer all this info! Thank's...

[@XmasShoes]

Man, excellent video! Subscribed!

[@d.a.t.4699]

cool vid, but from a guitarist, would love to hear a before and after guitar play thru...cool to listen to your knowledge tho...cheers.

[@KingArthur-os4id]

Great video ! Thanks ! I've done it on my marshall 1974 18w kit by Tube amp doctor and it fixed all the fizz/buzz sound I was getting by turning all the way up the amp ! The only différence I can tell is that my standy switch seems to "pop" when I turn it off... Is that ok ? I've only put the 100k resistor

[@atech9020]

It can pop, it can take out the cap, it can take out the Rectifier tube. I woudl hardwaire the standby switch to be on, or stop using it. The standby switch is not needed on amps witha rectifier tube.

[@Mark70609]

The Paul Ruby mod can conduct voltages that are negative with respect to earth. The P.R. mod has no effect on the amp clipping. The amp clipping occurs as the signal exceeds the bias voltage. The negative voltage occurs with the valve forward conducting and providing a path for the cap to charge up.

[@RAfiote2]

Can those mods could work on a fixed bias circuit lets say marshall 1987 or something similar?

[@atech9020]

No. The issue is that the Cathodes in fixed bias amps are tied directly to ground, so there is no resistor to place to put the diodes across. And because the negative bias source can only go up ( less negative ) under load, you can't make it shift downwards with doides in the circuit like this.

[@iannapier4321]

What value of zener did you place on the cathode resistor?

[@atech9020]

It ended up being a 13v zener. Generally you shoot for a zener voltage a couple volts above the voltage drop across your cathode resistor. Because there is little current flow, sometimes need a smaller zener to achieve your desired drop. In my case I needed a 13v to lock it to a 2 volt change.

[@Mark70609]

@@atech9020 the resistor has a voltage drop across it and the bypass cap blocks DC voltage and passes ac voltages. I don’t really see the point of the zener across the cathode resistor apart from clamping the bias.

[@atech9020]

@@Mark70609 That is entirely the idea. That voltage drop is bias shift. It shifts in the cold direction, making the amp run towards class B ( not good in our case ). The zener reduces that potential, keeping the amp in class A/B. All that’s left to deal with after that is blocking distortion, which is reduced inherently by the lack of bias shift.

[@sting1111]

what is up with the annoying hand gesturing? wow