NAD: 18W Plexi build

Johnny-B

I finally got all the parts and finished the 18W Plexi build over Easter. Here are couple of pics.

The extra circuit board and cap on the right hand side of the pic is a VVR (Variable Voltage Regulator). It gives good overdrive tone at low volumes. Magic little device! Circuit board courtesy of Attila. ?

ezietsman

Nice!

Are you scaling the whole amp with the VVR or just the power section? Is this a fixed bias design or does it use the cathode biased 18 watt power circuit?

Johnny-B

ez wrote: Nice!

Are you scaling the whole amp with the VVR or just the power section? Is this a fixed bias design or does it use the cathode biased 18 watt power circuit?

I'm scaling the whole amp. It's the cathode biased (self-biasing) 18W TMB circuit.

ezietsman

OK. That's by far the easiest yeah. My 18 watter is in a head like yours, it has the Lite IIB channel and the 'Superlite' channel, which is like the TMB but more gainy. I've only scaled the Phase inverter and Power section. My Tweed has whole amp VVR and I must be honest, it becomes a bit spongy with the VVR below half where the 18 watter doesn't suffer from that unless you scale it way down.

Have fun! Best amps in the world *

* Don't tell the others lol.

singemonkey

Vera nice. EZ built my amp with only the liteIIB channel with no second channel (power, volume, tone, klaar). It sounds really good with the power under 2, and better than anything else (besides say, a JTM45 running loud enough to blow the windows out) beyond that. Also power amp scaling.

This is the best amp there is today.

Gearhead

Johnny B wrote: I finally got all the parts and finished the 18W Plexi build over Easter. Here are couple of pics.

Good to see this complete and looking so good as well!

ez wrote: My Tweed has whole amp VVR and I must be honest, it becomes a bit spongy with the VVR below half where the 18 watter doesn't suffer from that unless you scale it way down.

This is the part I don't understand. If by spongy you mean that it has a lot of sag, then why does the Tweed circuit have it and the 18W not? I thought that because the capacitors do not scale down with whereas current does, the sag becomes less when scaling down?

Attila

Looking good there .....

ezietsman

Gearhead wrote:
Johnny B wrote: I finally got all the parts and finished the 18W Plexi build over Easter. Here are couple of pics.
Good to see this complete and looking so good as well!

ez wrote: My Tweed has whole amp VVR and I must be honest, it becomes a bit spongy with the VVR below half where the 18 watter doesn't suffer from that unless you scale it way down.
This is the part I don't understand. If by spongy you mean that it has a lot of sag, then why does the Tweed circuit have it and the 18W not? I thought that because the capacitors do not scale down with whereas current does, the sag becomes less when scaling down?

Not sure. I think the valves are all so starved for voltage that the attack goes meh at low VVR. It still sounds like a an amp but it doesn't behave nicely. Not like the 18 at low VVR.

Edit: By valves I mean the preamp valves. The 18 watter's preamp's voltage doesn't get scaled so the preamp still runs like it normally would. This does cause the power tubes to get driven by a bigger (relatively) signal at lower VVR whereas scaling the whole amp doesn't have this problem. The fix is to turn the volume down at lower VVR settings.