Troubleshooting request for 15 watt LP PCB build

[RJ Guitars]

Scott,

Congrats, you've been conquered by the beast and it will only get better... we fool ourselves if we think we have them in submission to us. The battle is not getting the amp to what what you want it to, but the amp getting us to do what it needs us to do.

Thanks for working through the presence circuit, I'm rewriting the schematic and BOM for that amp so it reflects the same way that you built it.

rj

[RCGPNY1]

Hmmmm...this is a real "dome scratcher".... I'm wondering...was there any difference in the parts RJ sent to Blue Parrott, than the ones I received.
My amp ,in all positions of the bright switch, never gets harsh. The high notes have a nice thickness even in the .005pf position. I dont recall the value of my presence pot, but it has fantastic range...I seldom use it above around 1:00(ecept when I'm going for a little more "twang" with humbuckers...yes my Lerpoole does distorto twang very nicely, thank you). My amp is breaking in...Ive tried many different tubes in the front end...everything sounded good...just slight variations...and it seems to like JJ EL84's just fine. I recently hooked it up to a single Vintgage 30(British made)...with great results.....I wonder what the max power of the Lerpoole is...as it is pretty loud.

[RCGPNY1]

Ok...I've got a 25K presence pot...no additional resisitor. Am I correct that the general consensus is that I'll be happier with a 5K pot?. In what way will it change?

[jelle]

It will become much more aggressive and trainwreck correct. Simple try, right?

[Blue Parrot]

RJ - Yes, this amp has certainly beat me into shape a bit - it is also clear I need to become a getter guitarist as it is very articulate.

Jelle - thanks for the explanation.

RCGPNY1- I recall seeing your build photos and if I remember correctly, there was only one difference in the components that jumped out at me. This was C15 bypass cap. I researched the gold cap RJ sent and convinced myself this was a high quality audio cap (that was fairly expensive too) so I used this. I actually made three substitutions from the components RJ sent, usually because manufacturer labeling was inadequate for me to verify the specs . First sub was for C1 and C7. The light blue foil-wrapped electrolytic caps received were 22 uF that seemed to be rated at 15 volts but I couldn’t really tell the manufacturer or voltage rating for sure. I had 22uF 25 volt Spragues on hand so I substituted. Secondly, I placed a 50 pF (500 V) silver mica in C2 as per Liverpool specs – think my kit came with a 500 pF as per Express specs. The third substitution was for the bleed resistors (R30 positions) where I substituted 1M resistors rated for high voltages. Not sure if any of these subs (except C2) were really necessary; however, I wanted to post these differences so my build photos don’t cause any further confusion.

My understanding is that the 25K pot along with the higher resistance from pins 3/8 on V3 to ground was causing voltage in excess of the intended values for the cathodes on the PI. If you decide to order an alpha 5K linear replacement pot get the metric as the 3/8 in US bushing size won’t fit RJ’s chassis (for example, I got my pot from Antique Electronic Supply part number R-VA5KL).

Scott

[RJ Guitars]

Yeah the potentiometer holes usually start life as clearance for the 8mm pots, unless someone asks for PEC or something that needs the 3/8" hole. One of the best investments an amp builder can make it's a good step drill to work up chassis with. They not only do a nice job of drilling but also are good for deburring holes too.