50W Plexi, Dual Rectifier, PPIMV Budget Build

[martin manning]

Well that's a bit more like it! I guess you ought to toss that GZ34. I think a 1/4W resistor will be okay there, but It's going to be close. Measure the voltage across it with the amp on in vacuum mode, then calculate V^2/R. If that's ~0.15W or less (for some margin) it's good.

[Guitarnut]

I have the resistors in place and the numbers are identical to what I posted just above. Tolerances being what they are, the 470K and 220K metered at 149.9K.

I hope I like the tube recto as much as I do the SS now that the voltages and bias are correct.

While I was finishing up the metering, I checked for voltage between eyelets on the board. I didn't see anything other than the meter toggling back and forth between .000mV and .001mV. I did see a little DC leakage on the .022uF and .0022uF coupling caps. They each show .004mV.

I think I'm gonna play guitar for a while.

Thanks again Martin for your guidance and willingness to share your knowledge. It's much appreciated!

Peace,
Mark

[Guitarnut]

Well that's a bit more like it! I guess you ought to toss that GZ34. I think a 1/4W resistor will be okay there, but It's going to be close. Measure the voltage across it with the amp on in vacuum mode, then calculate V^2/R. If that's ~0.15W or less (for some margin) it's good.

I guess I updated my post while your were posting. I went with a 1/2W 220K and 470K in parallel so no worries on the wattage.

Peace,
Mark

[martin manning]

No problem, Mark. Seems like it's pretty well sorted out now. Nice work, and this project is proof that an unconventional layout can work just fine if you pay attention to what's important. Post a gut shot of the finished product when you get time.

[Guitarnut]

I had a little time to play in the studio so I did some sound clips. For some reason my Cubase license wouldn't launch so I had to do these in Audacity...not as clean on the A/D conversion. Used an SM57. No changes to tone controls just switched between rectifiers.

I have to say, for a parts build, it came out pretty sweet. Lots of crunch and nice tone. The clips are full vol on the bright channel, MV at about 30%. Lead II with Carvin M22SD humbucker, single vol, no tone control.

Solid State (Dry)
http://www.crenshawweb.com/plexi50/ss.mp3

Solid State (256 ms Delay)
http://www.crenshawweb.com/plexi50/ss_delay.mp3

Tube (Dry)
http://www.crenshawweb.com/plexi50/tube.mp3

Tube (256ms Delay)
http://www.crenshawweb.com/plexi50/tube_delay.mp3

[martin manning]

Pretty subtle difference between the tube and SS. I think the tube has a little more robust low-end, but it's hard to tell, even using decent in-ear 'phones.

[Reeltarded]

And tubes break and they are expensive to use in building and might not be available on saturday night at 8pm in Pookalookatomperskon WI.

Wait, I have heard me say this before. The tube really starts showing it's stuff at around 7 on the output. Ewww.

Just say noooooo!

[Guitarnut]

And tubes break and they are expensive to use in building and might not be available on saturday night at 8pm in Pookalookatomperskon WI.

Wait, I have heard me say this before. The tube really starts showing it's stuff at around 7 on the output. Ewww.

Just say noooooo!

[Guitarnut]

After playing the amp for a few hours and trying different settings, I'm yearning for a bit more metal tone at lower volumes. A mod that seems to be tailor made for the Plexi is the "one wire" mod that cascades V1a and V1b so they're in series.

I've seen it done by connecting a 1M from the Bright channel vol wiper to the junction of the 68K resistors on the normal channel. The wire that was on the vol goes to ground.

It also appears to be done as a jumper from pin 3 to pin 8 and pin 8 cathode R and bypass cap are removed from the board.

Is this a doable mod for the circuit I built? I would likely use the 1M resistor method to try it out.

Peace,
Mark

[Reeltarded]

Yeah, any of the cascade options could work. Lots of tweaking to do around there!

[Guitarnut]

Yeah, any of the cascade options could work. Lots of tweaking to do around there!

Cool! One of the reasons I started building amps is so I could tweak them.

[Guitarnut]

If I like the mod and want to be able to switch it, would this work? I feel good about it except sending the 1M to ground. Would this take it out of the circuit? Or should I just leave it unconnected when the mod is out?

[Guitarnut]

Just finished hard wiring the one wire mod! That's one hell of a big gain in, well, gain!

Me likey!

[Guitarnut]

Here's a shot of the inside as it was before I added the one wire mod. Per Martin's request.

[martin manning]

I'm sure there are some folks scratching their heads wondering why it sounds good and doesn't hum or oscillate, and also how unlucky they must have been to build an amp that did! I suppose that this isn't a particularly high gain design, and so maybe it's not all that sensitive.