They aren't even close. They will not work without a major redesign of the circuit. They might work a little better in the stock circuit if the LEDs were replaced with resistors, but you'd be moving backwards to use an inferior tube.

The 6C8G is not a substitute for a 6F8G/12AU7.

The Speedball will increase current delivery to the load at the output, which will increase available power. The stock circuit has a ton of voltage swing; that's rarely an issue.

Most IEC power cords will be stranded copper with a PVC jacket.

Solid core copper. If it appears to be silver, it is tinned to prevent the copper from oxidation, which is not at all abnormal and will allow the wire to be reworked easily at a later point. Stranded wire should not be used for these purposes.

Encouragement would be saying something like "go ahead and do it". The statement that there are many mistakes in the clones is some evidence that there isn't approval on that end. I don't see any encouragement in anything you've posted from that statement. The statement also talks about...

I don't see any words of encouragement there, that seems to be your addition to the conversation.

I sure hope the winding isn't 15VA, the Crack amp draws more than 30VA from that winding; the SEX amp draws more. AC and DC current are not the same. The transformer only cares about AC current.

The Crack transformer is also in the SEX amp, and that's the clue to the capacity of the HV winding. From the original SEX amp and Crack 1.0, that indicates a high voltage winding capacity of 72VA and a LV winding capacity of 22VA, so I guess it's only 94VA. This is to keep the transformer...

The stock transformer is spec'd at about 105VA and runs under 50VA.

For those who may consider a larger/cooler running transformer to be some kind of improvement, it's worth remembering that underloading a transformer leads to poor regulation, and poor regulation can leave your 6.3V heater winding putting out far more than 6.3V, which can reduce tube lifetime...

The large Speedball board heatsinks dissipate the same amount of heat as the 3K resistors do, but both the large Speedball board and the small Speedball board have some additional current flowing through them to permit the current sources to operate, so the overall heat dissipated inside the...

The heat dissipated under the chassis goes up when the Speedball is installed. The localized heating of the chassis plate right where those 10W resistors sit will decrease when the Speedball is installed. (Always remember to install the rubber feet too)

If your amp passed the voltage checks, then it is running as hot as any other working Crack. The kit has been out for over a decade with no reports of failing wood finished from heat, and consequently I would suggesting using whatever you like.

No amount of parts swapping on a stock Crack will bring it up to the performance level of the Crack-two-a. Granted there's a lot of enjoyment in parts swapping.

In a Crack with no Speedball, you can run it with no 6080 and plug these tubes in. The 12AU7 will produce the expected plate voltage that's around 75V or so. A 12AX7 is going to show very, very high plate voltage. With or without the Speedball, it's going to be up close to whatever your B+...

One could certainly put feedback around the Crack circuit and get the output impedance super low, but it does indeed come with some negative side effects. As Gwyn pointed out above, there are many objective benefits of global negative feedback, and it is present in nearly all currently...

This will put tubes in the signal path, but it won't add the "tube" sound IMO. Part of the tube sound comes from the way tube amps interact (or don't interact) with the load your headphones present vs. a solid state amp.

Gift horse, etc.

What would you hope to learn from this?

This sounds like a good idea until you run the amp with the switch on 12V and a 6V tube installed. Either you destroy the tube or the power transformer.

The 6SL7 is absolutely not at all acceptable for use in the Crack. With the Speedball, the plate voltage on the 6SL7 will not drop much below the B+, so the 6080 will have absolutely no voltage to work with and the current sink under the 6080 cathodes would end up dissipating way too much heat...

In the output stage, the output tube sees a certain amount of bias voltage at its grids (from the first stage), the B+ voltage, and the load under its cathodes. When you use a 5998, the magnitude of the grid to cathode voltage drops a bit, but that change is inconsequential for proper amplifier...

They are in the same family and they do have the same heater voltage, heater current, and pinout. They are otherwise completely different devices and will need very different component values and operating points to work properly.

1. Too much gain. The 12AU7 is already pretty hot, but the 12AT7 and 12AX7 have significantly more gain that is absolutely not desired. 2. Available B+ voltage. The 12AU7 is perfectly fine with 80V on the plate, but a 12AX7 with 80V on the plate will either draw grid current or draw...