'69 SL power supply question

[shakti]

I'm currently building a clone of a particular early '69 Super Lead. I'm basing it on a batch of amps which all share some similar features, and were built in February/March 1969 according to the test date. Hendrix supposedly had some of these amps. One specific feature is that they had WIMA caps for the 0.022s and the 0.68uF cathode caps.

Anyway, I only have a few photos, none high-res, so I'm struggling to work out a couple of details, specifically about the wiring of the HT supply.

I'm using a self-contained bridge rectifier like the one Valvestorm sells, and no snubber caps. But how to route the HT taps from the transformer? I guess there are several ways to do it, but would like some input on what you consider the pros and cons of each method.

1) Route the two halves of the HT AC directly from the transformer to a DPST standby switch. From there to the bridge rectifier. Ground from the rectifier to #5 ground point (Larry grounding). Output from the rectifier to the mains filter caps, then to the HT fuse, and then to the OT CT. Power transformer CT goes between the two mains filter caps. Does this sound correct?

2) Route the two halves of the HT AC directly from the transformer to the bridge rectifier. Rectifier ground to #5. Output from the bridge rectifier to the mains filter caps, and from there to the standby switch (SPST only needed). From the standby switch via HT fuse to OT CT. Power transformer CT between the two mains filter caps.


What confuses me is that in later (1970ish) amps and in the Metro 100W instructions, as well as BillyBatz' 1969SL layout, there's also a connection from a point in the rectifier (this time with four diodes, and not as a self-contained bridge rectifier) that goes between the filter caps along with PT CT. It appears to come from the snubber caps, so I assume I don't need this connection when I'm not using snubber caps, but do I need to add this to my two possible scenarios as outlined above? And what's the disadvantage of not using the snubbers?

[Tazin]

Route the two halves of the HT AC directly from the transformer to the bridge rectifier....This would be the Blue & Black wires. Rectifier ground to a solder lug that's located at the filter capacitor clamp mounting screw....This would be the filter cap located behind the PT. There are two Red wires attached to the output of the bridge rectifier. One of the Red wires goes to the HT fuse, and the output of the HT fuse has a Red wire that goes to the STANDBY switch (SPST only needed). The other solder lug on the STANDBY switch has the OT center tap wire (White) and one of the choke wires (Black) attached to it. Now, the other Red wire from the output of the bridge rectifier goes to the '+' solder lug of the mains filter capacitor located nearest the STANDBY switch. The Yellow PT center tap wire goes to the '+' solder lug on the other mains filter cap. I hope this makes sense.....

[shakti]

You're exactly right, Tazin! You have a great eye for details...I was looking at one of these amps at the Amp Archives and couldn't quite make it out, but now that you say it, that's exactly how it's done. THANKS!

[demonufo]

Snubbers are really only there to reduce switching noise and hash, usually induced by poor quality diodes/bridges. They are one component at risk of failing though, hence why it's best to use good quality film caps and 1000V ratings. Personally, I'd rather just use decent diodes in the first place, but then I'm not the type to go cloning to this degree of accuracy, and my OCD won't allow me to build something that I know is technically inferior, well, at least for myself.
Original vintage amps are something else though where I would always choose authenticity wherever possible, and I certainly appreciate why people go to this level of detail.

[shakti]

This amp has a loud "thud" when flipping the standby switch. I figured it was because the filter caps are wired *before* the standby switch, so they charge up in standby and then deliver full power immediately when the amp is switched from standby to on. I rewired it so that the wire going to the + of the mains filter caps is on the far end of the standby switch, along with the OT CT and a choke wire. The thud goes away - great!

However, now there's another problem...the amp is passing sound in standby! It's faint, gated and distorted, but it's there. What gives? In standby, the rectifier is receiving HV, and puts it out via the HT fuse to the standby switch. The mains filter caps should not be receiving anything, unless something is passed through the PT center tap, which goes between the two mains filter caps?

Is there a way to make this amp quiet in standby, and still remove that annoying thud? The obvious way is of course to change the standby arrangement altogether so that the switch cuts the HV taps (like on a 68/12-series) but I am reluctant to do that since I aimed to make this as close to a specific 69, to the smallest detail, and I think it came out extremely nice.

[Tazin]

I don't think there is....That's why Marshall started to use the snubber caps in the circuit.

[shakti]

But they also went back to using the standby switch to break the HV taps, so essentially cutting them before even the rectifier. In any case, I suppose I can use the amp even if it passes a tiny bit of sound in standby...doesn't happen if the volume is on 0 anyway.