trinityamps.com

Hey Guys! Well, I've completed my Plexi Mk II scratch build and things turned out pretty decent with the exception of a couple of kinks I need to get worked out. I was hoping maybe you guys could help me out. I listed my voltages below and as you can see, I do have a few areas of concern. B+ is sitting right around 348 but then right of the bat the voltage drops around 25 volts lower across the 5 watt 1.5k resistor. Thats about 15 volts more of a drop than indicated on the layout. I triple checked that resistor and it's pretty much dead on. In fact, I've measured all resistors 3 times and they are all well within spec. The most noticeable voltage issue is pin 8 of the cathode follower is close to 25 volts off. With that being said, the thing sounds freakin' fantastic! It is so LOUD! Much louder than I had anticipated. I play at the edge of breakup 80% of the time and both channels do this EXTREMELY well. The straight cleans are pretty good too. I don't play at full bore distortion often but man does this thing have it in spades! The normal channel is surprisingly crunchy as well. I do get a fair amount of hiss that becomes much more prevalant once the master is full on and the gain gets around 1/2 way up. I'm not terribly sure what would be considered normal hiss-wise though. There's also a bit of 60 cycle hum I can hear in there as well but it's only present when my guitar is plugged in and the volume on the amp is cranked. Anyway, what do you guys think is causing the voltages to be off in certain areas? Thanks for any help in advance!

Voltages:
v1:
Pin 1-149
Pin 2-0
Pin 3-.9
Pin 4-3.2AC
Pin 5-3.2AC
Pin 6-149
Pin 7-0
Pin 8-1
Pin 9-3.2AC

v2:
Pin 1-155
Pin 2-0
Pin 3-.9
Pin 4-3.2AC
Pin 5-3.2AC
Pin 6-260
Pin 7-150
Pin 8-155
Pin 9-3.2AC

v3:
Pin 1-196
Pin 2-47
Pin 3-76
Pin 4-3.2AC
Pin 5-3.2AC
Pin 6-215
Pin 7-50
Pin 8-74
Pin 9-3.2AC

v4:
Pin 1-
Pin 2-0
Pin 3-12.5
Pin 4-3.2AC
Pin 5-3.2AC
Pin 6-
Pin 7-340
Pin 8-
Pin 9-321

v5:
Pin 1-
Pin 2-0
Pin 3-12.5
Pin 4-3.2AC
Pin 5-3.2AC
Pin 6-
Pin 7-340
Pin 8-
Pin 9-321

v6:
Pin 1-295
Pin 2-
Pin 3-348
Pin 4-
Pin 5-
Pin 6-
Pin 7-295
Pin 8-
Pin 9-

I wouldnt worry too much about the voltage being off at the cathode follower. I suspect the power transformer doesnt have quite enough capacity for the amp which is why it is dropping more voltage.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Is there any significant risk to my PT if that's the case? Here's a link to the spec sheet of the PT I'm using...

http://www.mojotone.com/transformers/BRITISH-18W-PT.pdf

Well, see if it gets hot. That's about it. It should hold up the voltage better though. Ours can deliver 350 or more.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The PT gets a little warm but nothing alarming at all. And that's after a couple of hours worth of use so I should be good. I'll keep an eye out either way. The specs on the Mojo Heyboer look similar to the PT you guys use. I wonder what accounts for the difference in the capacity to hold up the voltage?

Thanks for the help Stephen! You've been extremely gracious in helping me even though this is a scratch build and it's much appreciated sir!

If all the components, design and PT values are the same as Trinity it is probably the PT voltage regulation capabilities as Stephen suggests.

Nice looking build

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http://pdfelectronics.com

Thanks kurtlives! I've come a LONG way in my building skills, which are still meager in comparison to you guys but I'm learning! I went by the values in the layout/schematic for the Mk II pretty much dead on. The only difference is that I used a 500pf in the tone stack instead of 250pf and I didn't include the boost switch.

Is there anything I can do in the circuit to compensate for the weaker voltage regulation? The amp sounds pretty darn good now as it is so I don't want to muck around with it too much but if there's a way that I can make it more efficient then I'm all for it.

Thanks again for the input fellas!

That's a very nice looking amp you have. You even used pink wiring for the controls.

That 250pF vs. 500pF might make a tiny bit of difference actually. If you haven't before, download Duncan's Tone Stack Calculator. Its a simple interactive program that lets you play around with classic amp's tonestacks and change component values to see the affect on the frequency response. Even if you don't know how to read frequency response (Bode Plot) graphs you can at least change values and see if the graph changes visually in any appreciable manner.

I wouldn't worry about the voltages. Tubes are not super picky and have wide tolerances themselves. If you put in different tubes you would probably see your voltages change some more. If you want to play with voltages think about a VRM.

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http://pdfelectronics.com