AndyK
Well-known member
HeimBrent":1urdloqu said:
Right!
Right!
H
HeimBrent
New member
Pulling, pulling aaaaand pulled. Eureka! It works!
V
vic
New member
sorry to hear that your 900 is broken but after a few mods mine sounds great
H
HeimBrent
New member
Broken? It was just bad tubes. Next problem is that the pot that controls the loop is kinda iffy, but that's an easy fix. Sounds great though. Long live the 900s.
MississippiMetal
New member
ericsabbath":2wm8ss9c said:vic":2wm8ss9c said:beef it up give it some balls it sounds thin but it does have a really good tone ive found this schematic dunno if its good or not but some people seem to like it
Uploaded with ImageShack.us
there's no reason to replace R1 or to remove C2
the stock 10k on R1 sounds thicker, clearer, less fizzy and more dynamics-friendly than the usual signal sucking 68k resistor that most amps have
leaving C2 may be useful to avoid DC(?) noise
it doesn't suck tone
the rest of the mod basically involves taking the diodes out, which may remove some fizz
but it won't give you more balls at all
I'd replace R4 with 220k and R7 with 100k, remove C18, add a 4.7uf capacitor in parallel to R8 and replace D1 with a 1uf capacitor (instead of bypassing it)
this will thicken up the gain and sustain and give you a much fatter midrange
you can raise the phase inverter coupling caps (C11 and C22 on the power amp board) to 100n if you want more low end
If you're looking for less fizz, having two consecutive stages with 220K plate loads is not gonna help that goal. If anything I'd drop R7 to a 100k.
There is no such thing as tone "sucking". The signal is either being loaded less or more. You could more accurately call it "tone loading".
C2 is most likely there to make up for the smaller 10K grid resistor, since one of the grid resistor's purposes is to act as a VHF low-pass filter in conjunction with the tube's input capacitance to help prevent blocking distortion, oscillation, and picking up of radio frequencies.
a 4.7uF cap bypassing R8, which by the schematic is 1K5, yields a corner frequency of around 22hz - about 60hz below the lowest frequency on a six string guitar. That might as well be full-band pass for this application, if anything it'll simply pass the lowest available frequencies and contribute to muddy and unfocused bass, a trait that will be further enhanced by increasing the coupling capacitors at the PI, as you recommended.
Changing D1 to a 1uF cap will bypass the 3.3K capacitor with a corner frequency of around 48hz. Again you're catching all of the lowest bass that can be passed on, amplified and distorted (muddied).
The trick here is to shelve the lower frequencies that tend to get distorted, and then shelve the highest frequencies that cause your fizz, and end up with a nice, focused spectrum.
I won't go into every single thing I would do to this amp, but for starters I would change R1 to a 68K, jumper C2 and remove the Zener diodes, then I would change R3 to a .47uF. Next, I would change C4 to a 1 nanofarad, change R7 to a 100K, R5 to a 4.7K, change R8 to an 820 ohm, and bypass R7 with a 470pF silver mica or ceramic cap.
V
vic
New member
all mods done and sounds great now thanks guys
S
smitco
New member
MississippiMetal":1f5efb7w said:ericsabbath":1f5efb7w said:vic":1f5efb7w said:beef it up give it some balls it sounds thin but it does have a really good tone ive found this schematic dunno if its good or not but some people seem to like it
Uploaded with ImageShack.us
there's no reason to replace R1 or to remove C2
the stock 10k on R1 sounds thicker, clearer, less fizzy and more dynamics-friendly than the usual signal sucking 68k resistor that most amps have
leaving C2 may be useful to avoid DC(?) noise
it doesn't suck tone
the rest of the mod basically involves taking the diodes out, which may remove some fizz
but it won't give you more balls at all
I'd replace R4 with 220k and R7 with 100k, remove C18, add a 4.7uf capacitor in parallel to R8 and replace D1 with a 1uf capacitor (instead of bypassing it)
this will thicken up the gain and sustain and give you a much fatter midrange
you can raise the phase inverter coupling caps (C11 and C22 on the power amp board) to 100n if you want more low end
If you're looking for less fizz, having two consecutive stages with 220K plate loads is not gonna help that goal. If anything I'd drop R7 to a 100k.
There is no such thing as tone "sucking". The signal is either being loaded less or more. You could more accurately call it "tone loading".
C2 is most likely there to make up for the smaller 10K grid resistor, since one of the grid resistor's purposes is to act as a VHF low-pass filter in conjunction with the tube's input capacitance to help prevent blocking distortion, oscillation, and picking up of radio frequencies.
a 4.7uF cap bypassing R8, which by the schematic is 1K5, yields a corner frequency of around 22hz - about 60hz below the lowest frequency on a six string guitar. That might as well be full-band pass for this application, if anything it'll simply pass the lowest available frequencies and contribute to muddy and unfocused bass, a trait that will be further enhanced by increasing the coupling capacitors at the PI, as you recommended.
Changing D1 to a 1uF cap will bypass the 3.3K capacitor with a corner frequency of around 48hz. Again you're catching all of the lowest bass that can be passed on, amplified and distorted (muddied).
The trick here is to shelve the lower frequencies that tend to get distorted, and then shelve the highest frequencies that cause your fizz, and end up with a nice, focused spectrum.
I won't go into every single thing I would do to this amp, but for starters I would change R1 to a 68K, jumper C2 and remove the Zener diodes, then I would change R3 to a .47uF. Next, I would change C4 to a 1 nanofarad, change R7 to a 100K, R5 to a 4.7K, change R8 to an 820 ohm, and bypass R7 with a 470pF silver mica or ceramic cap.
i just carried out the mods you listed here and now i have very very faint sound coming through my amp lol, power amp side is still fine, i didnt have any 470pf so used 2 250pf in parralel is that a problem? if any one has any ideas on how to sort this please let me know, just wanna make this amp sound as good as possible
D
donho
Member
are these for the 50 watt head only?
also are there 2 r7? i think there are 2 pcb and they each have the same r number. which pcb are the mods for?
one says r7 to 100k and then it says to bypass r7 with a 470pf cap??
i have the 2100 head. will these mods work.
any body have pics of this?
thanks
also are there 2 r7? i think there are 2 pcb and they each have the same r number. which pcb are the mods for?
one says r7 to 100k and then it says to bypass r7 with a 470pf cap??
i have the 2100 head. will these mods work.
any body have pics of this?
thanks
JackBootedThug
MURDERATOR
doesn't Brent gutierrez do a satanic hispanic mod on these? puppies die when you hit an e chord type sound?
Starman22
New member
Thread is necro 5yr.
JackBootedThug
MURDERATOR
Starman22":1ig9hxgh said:
ha! back from the dead bump....
D
donho
Member
hey gang.
yah just looking for some info on these mods.
i realize its an old thread, but would like some confirmation on these mods.
like i stated there are 2 boards inside and i think from last looking at my 2100, they can have the same number resistor.
so anyone have any mods for the 2100.
yah just looking for some info on these mods.
i realize its an old thread, but would like some confirmation on these mods.
like i stated there are 2 boards inside and i think from last looking at my 2100, they can have the same number resistor.
so anyone have any mods for the 2100.