I built the 2 channel AB763 circuit using the schematic found on this site with the power tube modulated tremolo ckt. It sounds great but I can't actually get it to go into overdrive at all until you get up to around 9-10 on the volume (either channel) and it is only a really mild od at that volume. My voltages seem in order (listed below) but not sure what else to check. I thought this ckt usually started breaking up around 4-5.
AC to Rectifier 700v
Rectifier output 394v
After choke: 391
After first dropping resistor: 309
After second dropping resistor: 258
Both of my dropping resistors are 10k
v1-1 169.8
v1-6 165
v2-1 182
v2-6 180
v4-1 173
V4-6 174
v5-1 varies with oscillator
v5-6 391
v6-1 175
v6-6 173
v7-3 387
v8-3 384
v7,v8 -4 390
Tubes biased to 24ma
Thank,
Rut
AB763 Question
Moderators: pompeiisneaks, Colossal
Re: AB763 Question
It would be helpful to reference / link to, the schematic used.
Also key info that don't tend to be noted on a schematic, eg OT primary impedance.
The Fender system requires both the amp model (inc reverb status) and the version reference, as different models may have the same ref,eg AB673.
As it stands, your voltages seem low; that may be because a lower than normal voltage power transformer has been used, or a lot of current is being drawn etc.
The settings at which an amp tends to overdrive are affected by lots of factors, eg guitar, playing style, pedals, amp channel, control tapers, tube characteristics, tube operating conditions, transformer characteristics.
Note that on Fender reverb amps, reverb channels have higher gain than normal channel, on non reverb amps, the normal channel has higher gain than the trem channel.
The effect described may simply be due to the pot spec, ie a 10% audio taper, rather than 30%.
Also key info that don't tend to be noted on a schematic, eg OT primary impedance.
The Fender system requires both the amp model (inc reverb status) and the version reference, as different models may have the same ref,eg AB673.
As it stands, your voltages seem low; that may be because a lower than normal voltage power transformer has been used, or a lot of current is being drawn etc.
The settings at which an amp tends to overdrive are affected by lots of factors, eg guitar, playing style, pedals, amp channel, control tapers, tube characteristics, tube operating conditions, transformer characteristics.
Note that on Fender reverb amps, reverb channels have higher gain than normal channel, on non reverb amps, the normal channel has higher gain than the trem channel.
The effect described may simply be due to the pot spec, ie a 10% audio taper, rather than 30%.
My band:- http://www.youtube.com/user/RedwingBand
-
Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: AB763 Question
Set your meter for ac volts and hook it up across the input to the PI tube and play the amp cranked while pounding on the guitar, what do you then read for drive voltage?
I use a test signal input of .160 volts @ 1K and that small voltage is more than enough to drive the amp to its max output at clipping.
I use a test signal input of .160 volts @ 1K and that small voltage is more than enough to drive the amp to its max output at clipping.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: AB763 Question
As mentioned, your voltages seem lowish; what power transformer did you use, and what are the voltages across its primary and secondaries, ie Vac from one end to the other of each winding, in both standby and operate modes?
My band:- http://www.youtube.com/user/RedwingBand
Re: AB763 Question
If you want to make the Fender circuit overdrive more easily, lower the preamp tube plate voltages to 120 - 150 VDC. That will make it a bit browner and spongier.
The other thing you can play with, and you might be surprised at how much difference it makes, is changing the value of the 3M3 resistor on the verb channel (the reverb signal is taken off just before this series resistor). Drop it to 1M and report back!
If you change the preamp bypass caps to .68 mF, then you'll enhance the upper mids. You can also change the midrange resistor that goes from the bass pot to ground. The higher the value (to a point), the more mids are preserved and the less signal is bled to ground. Raise it from 8k to 22k and listen to the difference, then go to 33k or even 47k. Changes like these are all easy to try and are reversible if you don't like them.
The other thing you can play with, and you might be surprised at how much difference it makes, is changing the value of the 3M3 resistor on the verb channel (the reverb signal is taken off just before this series resistor). Drop it to 1M and report back!
If you change the preamp bypass caps to .68 mF, then you'll enhance the upper mids. You can also change the midrange resistor that goes from the bass pot to ground. The higher the value (to a point), the more mids are preserved and the less signal is bled to ground. Raise it from 8k to 22k and listen to the difference, then go to 33k or even 47k. Changes like these are all easy to try and are reversible if you don't like them.
Rich Gordon
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
Re: AB763 Question
The basic thing is that AB763 amps are pretty clean, designed that way.