New wreck/matchless hybrid

[dass101]

After fooling around with designing my amp in 3D with Solidworks I finally finished the chassis project and had it laser cut.
The amp started life as a Lightning clone, for which I ordered the transformers from TDS. I also designed it on paper to have VVR for the power section only. For now I finished it without VVR, that's why there are some loose wires inside, but I think it's needed to fully enjoy this amp at apartment volume.
As I was designing the component layout I decided to try some Zwreck style ideas, since that amp is pretty close to a higher powered Lightning.
I also decided to try a passive insert FX Loop. Other "personal" touches: A switch between 100p/none/180p bright cap, a switch between "Spitfire" (single stage) and "Lightning" (Top Boost)

This was my first "serious" build, I tried to do everything as I saw in a lot of beautiful builds here, sometimes I succeeded, sometimes I didn't, I learned a lot of things and the next one will be much better

Some oppinions and tweaks:

Regarding the long debate about the Lightning Screen power supply - the amp sounds MUCH better without the dropping resistor. I first tried a lower value, 3K3, which dropped around 30V, the amp sounded bright and shrill, I didn't even bother going to 22K. I'm almost positive that the schematic is incorrect, I can't imagine why would anybody want that type of sound.

The mismatched OT - I don't like it. It sounds thiner, a bit more compressed and unfocused. I used the 4Ohm tap with my 8Ohm cab to restore the 8K primary, it's much better. Unlike the first issue, I can see some people liking this, but not my cup of tea.

The amp has a lot of bass and low mids. I was under the impression that all these Vox inspired amps have a leaner bass, this one was almost farty. I decided to use the Wreck 500p coupling cap instead of the 1500p like the Matchless, it helped a lot. I'm not sure if I could make it work with the original value, especially with humbuckers.

Trying to deal with the bass I fooled around with Duncan's tone stack calculator and decided to change one of the 22n caps to 10n, it got rid of the lowest frequencies and made the bass knob more usable across the whole range.

The passive FX loop - it's not worth it. I tested it with an El Capistan, it overdrives it event at low gain settings. This happens with the 220k/100k voltage divider before the PI, and I liked the amp more with the 220K/220K divider like the Zwreck, so it's even less usable. I think I'll take it out.

That's about it, I also used bigger coupling caps in the PI and power section, 0,22u and 0,47u, I replaced the cross line MV with a Lar Mar, which helps, but I still plan to add VVR. I also added a variable line out, with a 20K pot, and it works just like it should. I use it to record and then add impulse speaker sim.

The power tubes run REALLY hot. I think I'll try to add some ventilation.

Really long post, I know, maybe someone will find it useful

[Structo]

Is that galvanized steel?

If you plan on painting the chassis you might want to give it a acid bath.

Vinegar will work.

I had to look up to see the electrical conductivity of galvanized steel.

The plating is Zinc which is actually more conductive than steel.

Aluminum is 26.5 nano ohms/ meter

Steel is 180 n ohms/ meter

and Zinc is 59 n ohms/ meter.

Plan your grounds carefully.

[dass101]

Yes, it's Zinc (I believe it's the same as galvanized steel)

I didn't know about different conductivity, I just chose it because I wanted some protection against corrosion.


I used a grounding scheme as close as possible to a star grounding:
All relevant grounds connect to the stage filter cap, all filter cap grounds are connected, there is only one connection to the chassis at the input jack.
OT ground and PT center tap are connected to the screen and reservoir caps, at the other end of the main ground line.

I wasn't sure what to do with the heater center tap, so I connected it to the chassis, near the IEC ground connection.

The amp is very quiet, I get some 120 Hz hum when I crank the MV, and I located the source of the hum somewhere between the CF and the PI (or in the PI itself). I suspect it has something to do with the unshielded wires that do the switching between "Spitfire" and "Lightning" You can see those as the parallel red and yellow wires that attach to one of the switches.

Any advice on how to better troubleshoot this is more than welcome.

[dass101]

I made some quick demos for the amp:

First is with 500p coupling cap
http://soundcloud.com/dass101/lightwreck-demo

Second with 1000p coupling cap
http://soundcloud.com/dass101/lightwreck-demo-2

I think I like the 1000p better.
This was very low volume, I expect the amp to sound much better when cranked. I get the basic preamp and PI overdrive at this volumes but I'm missing the power tubes spice.

[Reeltarded]

Wow, that is direct?

Second clip about halfway before the dirtier part sounds like a GREAT AC30.

WTG!

[dass101]

Thanks

Yes, this was a direct out, taken from the 8 ohm tap through a 20K pot. Speaker sims applied in computer.

[dass101]

Ok, so I ditched the lower gain (Spitfire) mode, and did a Hot mode, with a tone bypass switch. This is shown in the examples above.
I liked the hot mode but found it was a bit over the top, also the volume jump was too big. So I used a DPDT switch, and removed the V2 cathode bypass cap in the Hot mode. This works great, the hot mode is now much more usable and provides an almost perfect volume boost.

Here is a sample: http://soundcloud.com/dass101/lightwreck-demo-3

I only changed the guitar, I didn't touch the gain or master volume, so you can hear the balance between the overdrive and volume of the two modes.

The even better way to do this would be with a 3PDT switch, and also modify the voltage divider before the PI to better balance the volume and overdrive.

The amp can really function as a two channel amp this way, the hot mode is great and it sounds great on its own, not only as a somehow forced variation of the normal circuit. I'm not sure if I have room to do some relay switching, it would be great to have these modes footswitchable.