So to recap, if you have a Marshall, you can use a standby switch (useful for when you unplug a guitar when the volume is cranked) but it would be better to also have the Valve Wizard mod. Then whatever you do, all is good? Also, why do Marshall use two diodes in series on each leg of the rectification? Why not use a bridge rectifier configuration? Great series of articles, thanks for posting!
The big gotcha is how the standby switch is implemented. Where it is in relation to the reservior cap, determines it's level of safety. As for using two diodes in series, it just increases the capability of their reverse leakage voltage. This simply means that if a single diode is close to it's maximum rating, adding a series diode will increase the maximum rating a bit, allowing you to get away with it. The downside is there will be a .6v - .7v drop for each diode in series. So two diodes in series will drop 1.2v - 1.4v of forward volatge. The only major difference between a Bridge Rectifier and a Full Wave Rectifier, is that one does not require a ground reference from teh diode string. Depending on how you run your Bias supply, can change the need for the two topologies. A Bridge Rectifier requires you to have a seperate bias supply on your transformer if I am not mistaken. Conversely, using a simple Full Wave circuit allows you to run your Bias directly from the B+ transformer supply.
Im glad you started to do those kind of videos :) very helpful. Quick question about output transformers. Is it a big deal let say you are using a 50W rated OT for a normally 30Watt. More precisely is a plexi OT would work on a quad EL84 considering it has the right impedance?
It would work just fine. Going with oversized transformers usually means less OT saturation and wider frequency response. The downside is cost and size. The other hard part is finding an ideal primary impedance. I think larger OT's emphasize the primary impedance aspect of the tube interaction with the OT more. Meaning a small change equals a bigger sonic difference. Because the transformer is always working in its linear operation point, the impedance has a more audible effect. You could say you hear the tubes more than the transformer at that point.
How about putting the standby switch in before the choke to remove the HT supply from the OT screens and preamp tubes only ? With a flyback diode around the choke of course.
There are many ways to implement one, but realistically you don’t need one at all. In amps that can benefit from one, you can protect them with simple circuits that don’t utilize a switch and lengths of wire.
you didn't build a good case for solid state rectified amps not having a standby. seems it is only negatives and zero positives. A lot of "well this doesn't happen too often" and "its not too bad" -when in reality none of this stuff happens very often etc.. For tube rectified amps, you successfully made your case.. the upshot: Solid state rectified amps- always use your standby Tube Rectified amps-never use your standby
In my opinion in the Fender Bassman circuit the tiny 0.05µF capacitor behind the rectifier tube is surely not the first reservoir cap. How should it support to charge the two 20µF caps, when the standby switch gets closed?
.05uf is certainly small, but still better than nothing. Objectively, you would want to use a bigger cap there, 20-30uf for instance, but something is better than nothing. I still think it’s better to not use the standby switch at all.
