First of all, RIP Billm and we're all much better off through your contributions. When I first added the Billm mods years ago to my Blues Jr, something was always a bit off and I couldn't put my finger on it. I revisited the mods last week and discovered that R30 was the correct resistor designation for the phase inverter input but that only applied to board revisions pre 1999. Post 2001, the phase inverter input resistor, unbeknownst to me, was re-labeled by Fender to R15. So for 10+ years I had this wrong in my amp. Added (or should I say moved) the 100pf cap from the erroneous R30 -> R15 and low and behold, no it works perfectly as intended... my amp is actually sounding like I expected it to sound. I also spaced the fact that when I changed the input jack to a Switchcraft type, I added the Billm bias mod and it also works like a champ setting the bias to lower 2.40V recommended (as measured from the red to brown wire). I've attached the reference information if interested.
Blues Jr III Billm Mods, when is R30 not R30 for the phase inverter?!?
- Thread starter brown2bob
- Start date
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Yeah, Fender issued the amp with four different PC boards (IIRC) and Bill was very specific about which mods worked with specific boards. But it still created confusion.
I did probably 15 or 20 BillM mods on BJrs over the years.
He was a cool cat and is greatly missed.
I did probably 15 or 20 BillM mods on BJrs over the years.
He was a cool cat and is greatly missed.
GotA24Fretter
Tele-Holic
I'm not quite following your post. Which schematic applies to your amp, and what mod were you attempting?
The reference designators are indeed very different across these versions, but I don't see an R15 or R30 on either schematic the can be called a PI input resistor. Either your terminology is off or your mod is still incorrect.
The reference designators are indeed very different across these versions, but I don't see an R15 or R30 on either schematic the can be called a PI input resistor. Either your terminology is off or your mod is still incorrect.
GotA24Fretter
Tele-Holic
That's a grid stopper for the 3rd gain stage.
Bypassing that with a 100pF will affect frequencies ~10x higher than the threshold of human hearing, and about 20x higher than the capabilities of most guitar speakers.
What mod were you attempting?
Bypassing that with a 100pF will affect frequencies ~10x higher than the threshold of human hearing, and about 20x higher than the capabilities of most guitar speakers.
What mod were you attempting?
