Finished Express but have buzz

[Jeph]

Are there any limits to this method? Meaning, if I used 25k and 75k resistors in order to get 1/4 voltage gain, will there be any adverse effects?

[paulruby]

Are there any limits to this method? Meaning, if I used 25k and 75k resistors in order to get 1/4 voltage gain, will there be any adverse effects?

Go for it! It acts just like an MV at that point, so no adverse affects other than the obvious impact of reducing signal level at that point.

FYI, you realize you drew the connection to the PI wrong in that schematic, right? The split load is drawn correctly.

[Jeph]

Cool, thanks Paul! BTW, I just modified the A1b schematic found in the blueguitar pdf in the files section to add the split load, didn't touch the PI area.

[MooreCowbelle]

Jeph, the connection from the split load should go directly to the top .01 uF coupler in the PI. You have it drawn connecting to both sides, as well as the 10K leg...

[MooreCowbelle]

Paul,
while we are on the subject: I noticed that some designs put a 250-500pF cap across that plate resistor. What effect does that have on the signal?

I think I can guess that it peels off some of the higher frequencies that might cause oscillations...

[Jeph]

Yeah, it's incorrect on the A1b schematic as well. I updated my pic a little for future reference (if necessary). I'm still a bit schematically challenged

[paulruby]

Paul,
while we are on the subject: I noticed that some designs put a 250-500pF cap across that plate resistor. What effect does that have on the signal?

I think I can guess that it peels off some of the higher frequencies that might cause oscillations...

This is used to cut the top end in a stage either just before or just after the bright cap is used on a volume pot. It's worth messing with. When a bright cap is added to a volume pot, like 500pf, you get accentuation of the high mids (typical marshall bite) but you also get a huge boost in the really high frequencies which can make the amp harsh. A wreck with a 500pf bright cap played throuh a 4x12 with G12H30/V30 will take your head off. You can take off some of that top ear pierce with a cap on the plate resistor of the stage just prior or just after the volume pot. If you use a DPDT bright switch, you can switch both caps in and out of circuit at once. This is a common JTM45 mod as well.

[rhinson]

hello, on my bright switches i use the 100pf for the chime tone cap and use a .0047mf for the other. turn the amp up halfway, flip the switch to this value, and you get the best ass kicking mid tone you ever heard. i also tweak the presence cap just a tad---i found that .18mf was just right for me. give it a try----you'll dig it! rh

[Legin]

Well... A wreck has no MV and the volume pot is really a gain pot. So, a wreck does tend to go to full volume very early on the volume pot and then just controls the amount of dirt and feedbacm/sustain.

FYI, this is somethin I tweak on mine by either reducing the plate resistor of the 3rd stage or making a split-load plate resistor on the 3rd stage. Simply reducing the plate resistor works very well. There s a slight difference in tone using a split load since that allows the tube plate voltage to run full scale (so you get the 2nd order harmonics) while the output signal is reduced.

In short, you need to hand-tweak the "MV" that is hard-wired into the the plate resistor of the 3rd stage.

Brilliant! thanks again Paul
Nigel

[Legin]

I just pulled the board and added a new turret lug and split the plate resistor on the third stage into 47K and 56K (closest I could get) this is great, I can now get the volume I want and still crank and get the feedback ridden dirty grind. I took the opportunity to clean up while I had the board out and rearrange the ground run so it is even quieter now. Thanks everyone, especially Paul, what a great amp
Nigel

BTW I am running a EH 5751 gold in V1 two NOS 12AX7s and a pair of JJ Blue EL34s, the trannies are Hammonds a 372JX and a 1650G

[gearhead]

Is there any way to do the split load on the power tube end of things?

[paulruby]

You could split the PI plate resistors...

[gearhead]

On the 90 kelly, would those be the 82k and 100k? Wouldn't that just reduce the power to the power tubes (meaning the PTs couldn't be run full bore)?

[paulruby]

On the 90 kelly, would those be the 82k and 100k? Wouldn't that just reduce the power to the power tubes (meaning the PTs couldn't be run full bore)?

Yes. This would act just like a post-PI MV turned down. It would take more signal from the preamp to get the power tubes up to full bore, which is what we're talking about tweaking.

[MooreCowbelle]

Paul,
when are you going to write a book on this stuff?

You have a gift for making things understandable in a clear and concise way. You could go far in this business!