'68 Silverface Bassman AC568

[txstrat]

The amp would work with no screen grid resistors but those help to keep the screen grid under plate potential (limit screen current) and hence save the tube.
If the grid is on the same potential it draws current and if it draws too much current it might blow up.

[akimball442]

As for mods- I have an AB165 Bassman, which is a fantastic amp on it's own. The normal channel is a nice good old Fender clean sound. The Bass channel uses two tubes. Perfect for a 5F6-A preamp conversion. Instead of a middle pot, I just used a 22K resistor, because I normally leave that cranked up anyway. You can use any value from 0 to 25K as a 'mid' control, or stick a pot in the back someplace. It's my main gigging amp. I turned the deep switch into a different sort of deep switch- I used a .68uF cap for the bypass on the first cathode, and the deep switch parallels a 25uF cap with it which gives a big boost on the bottom when you need it (ie low volumes or with single coils). I've enjoyed this amp for a couple of years now.

Also, the 568 (I can't find a schematic at the moment) may have some strange local feedback things going on, 220K resistors in some unorthodox place, I think. Remember that this was supposed to be a bass amp, so they did everything they could to make a 50ish watt amp have as little distortion as possible. It's not a bad amp, but it's not generally as useful for guitar in a conventional sense as some of the blackface models. Definitely read about 'blackfacing' Or just find the right schematic for yours, and compare it with the AB165. You can easily convert it back to a 165, and then mod it from there. You won't regret it.

[The Kirbster]

I use a Bassman Ten, and mine is almost silent between songs. VERY low volume "ssssssssssss" is it. Have to get real close to the speakers to even tell it's on. I run the amp as dead clean as I can, & get any distortion from my pedalboard (Boss GT6). It's a tone-monster.

+1 on the blackface mod. Mine has not only a bias knob, but they added another small screwdriver-type pot to make extra fine adjustments. I had a friend help me as i biased-I played, he twisted the screwdriver. We found a spot where the volume picked up slightly but noticeably, and the overall tone seemed more clear. We tested with the VOMeter, it was a little hot, we backed off a fuzz, lost the sweet spot. Found it again & left it there. Then we started swapping out preamp tubes until we found the combination that sounded best to us. My next mod will be a Presence knob.

So, they CAN operate silently-almost. Keep at it, you'll get there. I bet once you do the blackface mod & get a real bias knob installed, you can probably tweak away a lot of that noise.

[akimball442]

Mine is silent too. The quietest amp I've ever owned.

I added a presence control also, I forgot about that. It's on the back in the ground switch hole. This requires a little tweaking of the NFB circuit, as the Bassmans of the late 60s and moving forward used some strange values, often bypassed by small capacitors. I just looked your pics again- In pic #4, near the upper left there is a 47K resistor with a small cap across it. You can start by removing that cap, it's a tone robbing little monster that might go unnoticed in a bass amp, but you'll notice it with a guitar for sure. It sucks treble. That is your NFB resistor.

Various resistor values were used in the Tweed and early BF years- but your late 50's Tweeds had the 'good' presence control. The common values you will see are 27K, 56K, and 100K(Marshall). The larger the value, the less NFB you will have. You could add sort of a 'damping control' also to modify the NFB on the fly, or just to experiment and see what you like best. Consider using a 27K resistor in series with a 100K pot. From there, your presence control is just a 5K pot that removes highs from the NFB loop, typically through a .1uF cap. Removing the highs from the NFB actually increases them at the speaker. Look at the 5f6-A schematic for details, it's a very simple mod.

[The Kirbster]

Don't suppose you could provide pics or drawings or a REAL DETAILED written description of how to do that mod. I need a little more high end cut, I think a good presence control would do the trick. I can use the ground switch hole too, to avoid drilling the chassis.

I'm not real electronically literate, but I can solder, use a VOMeter, drain a cap, & have sense enough to work on amps while standing on a rubber mat, with one hand in my pocket as much as possible. Most schematics go over my head, though.

Any suggestions as to which resistor would give me the most highs?

[akimball442]

high end CUT on a silverface bassman?

Adding a presence control is going to have the effect of increasing treble.

[The Kirbster]

Sorry-bad choice of descriptors on my part there. I need more treble, I need it to CUT thru the mix more. Basically the Bassman is fine with distortion, for blues or rock. But, when I play clean country, & try to get that Roy Nichols/Don Rich "quack" from my old Esquire, it just lacks a little something.

There I go again with my weird descriptions of sounds. My desired tone-I call it "THE tone" is almost there. I love the deep percussive bass, and the in your face mids, I just need a touch more treble occasionally. Technically, I need more EXTREMELY high-end treble. I'm after the chime, the "glassiness" is all that's missing. It's too close to perfect for me to go changing speakers. I know the speakers CAN reproduce the frequencies I want, as I can tweak the EQ on my pedalboard to get the sound I'm after.

I'm greedy I guess, or too much of a perfectionist. I'd like to be able to get it without the pedal. I'd like to be able to swap back & forth between the Bassman & my Hot Rod Deluxe without having to reset my EQ every time. I really feel like a presence control could get me there.

I demand a lot from this amp. I go from loud bar rock and/or blues gigs to doing a classic country music show, where they maintain a painfully LOW stage volume. It's hard to get a good tone turned down that low, but this amp can pull it off. And on top of that, it's SO quiet when it's idling-have to get within a few inches of the speaker to tell it's turned on.

A dear friend GAVE me this amp. He couldn't get it to overdrive to suit him, said it was to clean. I begged him not to hack it up, to let me buy it from him. Next thing I kow, he's loading it in my car! I have several other amps, but they've gathered a lot of dust since he gave me this one! It's got a lot of sentimental value to me, besides sounding fantastic.

[akimball442]

You don't demand a lot from the amp. They have the potential to be spectacular.

If you have a good schematic (and layout diagram) for the AB 568, send it to me. I used to get them all on SchematicHeaven, but it's been down a while now- a BIG bummer. I wish I had saved all the diagrams I ever use.

Remove the ceramic caps circled. The ones across the 100K plate resistors (one is 500p and the other is .01 if I recall) suck treble. That's their job. The one across the 47K resistor forms the negative feedback loop. You can adjust the resistor probably up to 100K without any major ill effects- The amp is going to get looser as you increase it, ie a bit louder, more distortion, and tighter/darker as you decrease it from the current value. Mine has 56K in it and sounds good. Or you could leave it 47K for now, but ditch the cap. It dumps some more treble (remember this was a bass amp).

The blue caps are your tone control for the bass channel. You can reduce them to .022 to make that channel more useful (and different) from the normal channel.

Hidden by the .01 cap that I circled to remove in the Bass channel is probably a 250p cap, which is the treble cap on the tone stack. You can increase this to 500p to give the channel some more top end and upper mids. You can reduce the 100K capacitor in the tone stack to 56K to get a response more like a Bassman. That resistor is located between 250p treble cap and the first .1 tone stack cap looking at the pic.

The other .1 cap is for the 'deep' switch, which is certainly pretty much worthless for the guitar amp. I used mine to modify the cathode bypass cap on the first stage of that preamp- You can just leave it off, omit it, or choose to find something interesting to do with it at a later date.

See the pics:

[The Kirbster]

Well, I tried to post the layout & schematics for the AB568, but my kung-fu is not strong enough. I found them at: Free Schematics schematic circuit diagrams

Once there, click on the Fender amp button, then do a site search for the AB568.

Wow. Thanks for the pics! I'll try this one step at a time. Clip the ones across the 100k plate resistors and the one across the 47K resistor for the NFB first, see where that gets me.

I really appreciate you going to all this trouble to gather pics for a newbie like me (if one can still be a newbie at 53). I've played almost all my life, semi-professionally (fancy wording for Weekend Warrior) since I was 10, and I've always serviced my own guitars. Nothing in there that would kill me though! Never was around anyone to learn how to work on amps, but knew enough to stay out of `em until I DID learn a little. Thank you so much for nudging me a little further along.

[The Kirbster]

Geez, I just remembered, I do have pics of MY amps innards that I KNOW will post, they are already loaded up in the attachment gadget. Let's see if I can get'`em on here now.....

Well, here's the first 9-I have a few more, I'll put `em in another reply.

[The Kirbster]

And here's the last of `em......

[akimball442]

I got mixed up Kirbster- You have the bassman 10. I thought you were the OP with the 568. They are radically different amplifiers. I will examine the schematic for yours and post my thoughts later on.

[The Kirbster]

I just assumed-correctly-that you were trying to help someone else, too. After I scanned over my pics, it's obviously not the same model amp. I don't even see the caps you had said to remove. My amp has definitely been modd-ed before I got it. Take a look at the shot of the back panel, and the notch chiseled into the top of the speaker cab to make room for a larger tranny. Supposedly it's been blackfaced, and has a REAL bias pot. I thought the "fine tuner" on the bias was a neat idea.

[The Kirbster]

If you look at pic #100-210, you will see what I believe is the resistor for the NFB front & center. In the background, you can see the .001 Sprague orange drop we swapped for the ceramic .47 that was across the volume pot on the Studio channel. Got a hair more treble out of it, but barely noticeable. Not enough.

[The Kirbster]

Folks, I think I've messed up. I've posted about this same subject on another thread a few days ago, named "Fender Bassman Ten tone mods". I got some excellent help there from members Steve Connor & Tubeswell.

Honestly, I never intended to hijack this post. I'm always scanning various forums looking for more info on my amp. I saw something that looked & sounded familiar, & threw a reply at it.

From there, it just seemed to run on it's own. I don't know if I've broken forum protocol or not. If I did, I offer my sincerest apologies, I never intended to take over this thread, but in retrospect it sure looks like I did exactly that. Dumb Newbie!

I'm sorry!
The Kirbster