Tweet
Hi there. I have an early silver face Bassman which I thought had pretty much horrible tone. I was going to do the 'black facing' mods, but that wasn't necessary, because it was an AB-165 circuit (an actual AB165, not just the label). What an awful amp.
This is long, but please read
I am going to give a thorough explanation of what I've done to this amp, trying not to leave anything out so that hopefully when I get to my real question it will be fairly easy for someone more skilled than me to answer.
Anyway, I used it a long time with just the tone stack modded until over the summer, I finally decided to turn this into something useful.
I have turned the former 'Bass' channel into a straight up Tweed (5F6-A) Bassman/Marshall plexi hybrid. Basically the only 'mod' to the Fender circuit is the 2k7/.68 cathode resistor and bypass cap on the first gain stage, and I used the 'deep' switch to switch in a 22uF bypass cap. So when you select 'Deep' the bass response increases. I have a 5751 RCA black plate tube for the first gain stage. This stage is followed by a volume control. The next stage/cathode follower is a 12AX7, which feeds the tone stack, then the signal travels through a bypass cap and into the PI via a 220K resistor.
The PI is a 12AT7, and the output tubes are EL34s. Feedback resistor is a 47K instead of 100K, because I thought the 100K was a little bit 'much.'
The second channel (formerly the 'normal' channel) is a very basic 'tweed deluxe' style preamp circuit. This is the one that's not working *quite* as I'd like.
This is a very basic volume/tone control between two halves of a 12AX7 tube, fed to the PI through a .02 bypass cap and a second 220K resistor.
This circuit has WAY too much gain. It starts to break up at around 2 on the volume control. The volume and tone controls are 500K audio pots, I thought using 500K instead of 1M would actually reduce the gain. It fattened up the tone some, (in a good way) but did not appreciably reduce the gain.
The second thing I did in an attempt to reduce the gain is switch to a 12AU7, and the gain is still too much.
I reduced the gain further by increasing the cathode caps from 1500 to 2700, reducing the bypass cap on the first stage down to the .68 from a 25uF, and completely eliminating the bypass cap on the second stage.
It's still a little too 'wild.'
I must also say that it sounds REALLY good- I'd just like a little more clean headroom.
Now excuse my terminology if I goof here- but the original PI on the tweed deluxe was a split load (?) which used 1/2 of a 12ax7 for the PI, vs this one which I think you call a long tail pair. Could this be the cause of too much gain?
Finally, I removed the second gain stage altogether, and fed directly into the PI... and this is way to LITTLE gain.
The first channel has a little more gain than I'd like too, but it's not nearly as obnoxious as the second channel.
Hopefully someone can make some suggestions. I've cobbed together the schematic (a lot of power supply info is missing, but the circuit should be clear, and all of the voltages are +/- 5V of what the original amplifier(s) called for.
If the schematic doesnt post well, let me know and I will email it to anyone who would like a full sized copy.
Thanks a lot,
Aaron
This is long, but please read
I am going to give a thorough explanation of what I've done to this amp, trying not to leave anything out so that hopefully when I get to my real question it will be fairly easy for someone more skilled than me to answer. Anyway, I used it a long time with just the tone stack modded until over the summer, I finally decided to turn this into something useful.
I have turned the former 'Bass' channel into a straight up Tweed (5F6-A) Bassman/Marshall plexi hybrid. Basically the only 'mod' to the Fender circuit is the 2k7/.68 cathode resistor and bypass cap on the first gain stage, and I used the 'deep' switch to switch in a 22uF bypass cap. So when you select 'Deep' the bass response increases. I have a 5751 RCA black plate tube for the first gain stage. This stage is followed by a volume control. The next stage/cathode follower is a 12AX7, which feeds the tone stack, then the signal travels through a bypass cap and into the PI via a 220K resistor.
The PI is a 12AT7, and the output tubes are EL34s. Feedback resistor is a 47K instead of 100K, because I thought the 100K was a little bit 'much.'
The second channel (formerly the 'normal' channel) is a very basic 'tweed deluxe' style preamp circuit. This is the one that's not working *quite* as I'd like.
This is a very basic volume/tone control between two halves of a 12AX7 tube, fed to the PI through a .02 bypass cap and a second 220K resistor.
This circuit has WAY too much gain. It starts to break up at around 2 on the volume control. The volume and tone controls are 500K audio pots, I thought using 500K instead of 1M would actually reduce the gain. It fattened up the tone some, (in a good way) but did not appreciably reduce the gain.
The second thing I did in an attempt to reduce the gain is switch to a 12AU7, and the gain is still too much.
I reduced the gain further by increasing the cathode caps from 1500 to 2700, reducing the bypass cap on the first stage down to the .68 from a 25uF, and completely eliminating the bypass cap on the second stage.
It's still a little too 'wild.'
I must also say that it sounds REALLY good- I'd just like a little more clean headroom.
Now excuse my terminology if I goof here- but the original PI on the tweed deluxe was a split load (?) which used 1/2 of a 12ax7 for the PI, vs this one which I think you call a long tail pair. Could this be the cause of too much gain?
Finally, I removed the second gain stage altogether, and fed directly into the PI... and this is way to LITTLE gain.
The first channel has a little more gain than I'd like too, but it's not nearly as obnoxious as the second channel.
Hopefully someone can make some suggestions. I've cobbed together the schematic (a lot of power supply info is missing, but the circuit should be clear, and all of the voltages are +/- 5V of what the original amplifier(s) called for.
If the schematic doesnt post well, let me know and I will email it to anyone who would like a full sized copy.
Thanks a lot,
Aaron
Comment