1972 BASSMAN 100

[matt h]

https://www.youtube.com/watch?v=XzmrZWe_9eg

[Leo_Gnardo]

I am sorry for posting my this thread. I do not have a truck. And CBS circuits are terrible. I just wanted to share what was happening to my amp. If one have any thoughts feel free to talk about it, if it not constructive keep it to one self. Thank you kindly.Richard salazar

I wasn't too thrilled with CBS Fenders either, ever since I first ran across 'em late 60's, HOWEVER I've worked on plenty of CBS Fenders, and plenty pre-CBS. Even the current models and reissues. If they sound lousy, I make 'em sound good. Thousands of 'em. No exaggeration. All customers happy. No complaints. Don't even have to advertise. Satisfied customers send their friends. I get 'em fixed right. And so can you. I think this is constructive to any reader. Except maybe one.

I even have a customer named - Rick Salazar. And he's happy as a clam with his CBS-Fender Vibrolux Reverb.

Please feel free to send all your bad awful unacceptable nasty horrible worthless CBS Fenders right here. I'll admit, at one time I was prejudiced against them. No more. Not for a long time. When you make them work right, they can be excellent amps. Fender even released reissue silver face amps. If they were really that awful the first time around, why would they do that?

[Alan0354]

I wasn't too thrilled with CBS Fenders either, ever since I first ran across 'em late 60's, HOWEVER I've worked on plenty of CBS Fenders, and plenty pre-CBS. Even the current models and reissues. If they sound lousy, I make 'em sound good. Thousands of 'em. No exaggeration. All customers happy. No complaints. Don't even have to advertise. Satisfied customers send their friends. I get 'em fixed right.

What do you have to do to make the SF amps sound better?

[Leo_Gnardo]

What do you have to do to make the SF amps sound better?

Hi Alan, first the "blackface" operation - has mostly to do with the output drive. Just a few parts, really, not the multi-hundred dollar 'serious mojo' operation some techs try to tell their customers. 330K grid leak resistors? put in 1M. Something other than 470R in the cathode? You know what to do... 22K in the "long tail" position. Pair of 47K plate resistors, nope, 100K & 82K. Way big cap at input to phase inverter, bust it back to 0.001 - here I use a Mallory M150. If the FB R's are other than 820R & 100R, make them so. Amp should be sounding cleaner, brighter and clip evenly by now. A nice option is to sub in Mallory M150 or your fave caps for any "blue turds" or other caps that may be in there.

Big amps with ultralinear circuit - taps on primary of OT fed to screen grids - you have the options of inserting screen grid limiting R's, say 470R or 1K , or forming a new branch off the hi voltage supply to serve those screen grids. And just dead-end those taps. If you have deep pockets, swap in a regular Twin OT. For a screen grid supply, so far I just use a 1K resistor and 47 uF 500V cap branched off the main B+, but if you feel like substituting/adding a choke, live it up.

For amps that have one, optional to leave master volume in place. If you need it, leave it. For a "true to spec" tone, sorry it's gotta go. I find a pinch or two of hi frequency brilliance is lost with the MV in place, and it also tends to pick up a little hum since it's jam up to the power transformer. It IS useful however, dialed down just a bit to say 7, to cut hiss noise. But dialed down to say 3, when you try to use the amp in an overdrive/distortion situation, somehow Fenders don't sound good this way, to my ear anyway. Some people make it work for them nonetheless. That's why I make it optional.

[martin manning]

Leo, to be clear, for dealing with the UL circuits you are saying the options are:

1) Insert screen grid limiting R's, say 470R or 1K and connect those directly to the plate node.
2) Form a new branch off the high voltage supply for the screen grids.

And, either way just dead-end the screen taps? Do you use screen grid R's on the sockets if you make a new branch?

[Leo_Gnardo]

Leo, to be clear, for dealing with the UL circuits you are saying the options are:

1) Insert screen grid limiting R's, say 470R or 1K and connect those directly to the plate node.
2) Form a new branch off the high voltage supply for the screen grids.

And, either way just dead-end the screen taps? Do you use screen grid R's on the sockets if you make a new branch?

Thanks for asking Martin.

1) If you like the ultralinear "sound" it's still wise to current-limit the screen grids. Of course those who wish, experiment with other R values. I keep a pile of 470R and 1K in stock, so them's the choices I've tried.

2) Correct, branch as Ampeg did. In the larger amps Fender didn't have a convenient place for modifiers to pick off a screen grid supply. For those who wish to, no objection to inserting a 2nd level LC hi voltage filter in series with the rest as in earlier Fenders. Extra points . . . so far I've done the lazy way, just R and C and bobs your uncle. Customers like the sound and saving some $$ by leaving out the choke. A well-filtered screen grid supply is essential to keeping hum/buzz to a minimum, so I've read and experienced. So those who wish to go the extra effort & cost, not a bad thing at all.

Those who wish to have some "sag" at high volumes can use a larger value R in the SG branch. I've used up to 6K 10W in a Traynor YBA-1A (OK it's not a Fender but similar enough), and its owner loves it. Granted it dings the power figure some, but the amp sounds plenty powerful and "squashes" just right for his purposes. I expect the output tubes, KT88 in this case, will last longer by not stressing the SG's too badly. But now we're a little off the map for back-to-"blackface" mods.

[martin manning]

So there are three options?
1) Keep the UL, and add some screen grid R's
2) Disconnect the UL taps, and connect the screens to the plate node through resistors
3) Make a new RC branch from the plate node, or insert an LC after the plate node, and possibly use socket mounted SG R's too.

Is that right, or would you not recommend option 2) due to hum problems?

[Leo_Gnardo]

So there are three options?
1) Keep the UL, and add some screen grid R's
2) Disconnect the UL taps, and connect the screens to the plate node through resistors
3) Make a new RC branch from the plate node, or insert an LC after the plate node, and possibly use socket mounted SG R's too.

Is that right, or would you not recommend option 2) due to hum problems?

I'd agree your 2) is problematic due to expected hum. Unless there's a killer filter on the B+ but then, that's off the "authentic" BF map. A pair of 220 uF in series for 110 uF, that's as far as I go. Those who want to load up a chassis with "beer can" caps, live it up, I'm sure that can be good too.

FWIW I've seen the hi fi builders/modders use local filters at each screen grid connection. Haven't tried it, nor have I tried LED's or other SG supply options. None are BF "authentic" but may be worthy of experimentation, something to look forward to. So many options...

[martin manning]

Great, Leo, thanks for the clarification.

[Alan0354]

What do you have to do to make the SF amps sound better?

Hi Alan, first the "blackface" operation - has mostly to do with the output drive. Just a few parts, really, not the multi-hundred dollar 'serious mojo' operation some techs try to tell their customers. 330K grid leak resistors? put in 1M. Something other than 470R in the cathode? You know what to do... 22K in the "long tail" position. Pair of 47K plate resistors, nope, 100K & 82K. Way big cap at input to phase inverter, bust it back to 0.001 - here I use a Mallory M150. If the FB R's are other than 820R & 100R, make them so. Amp should be sounding cleaner, brighter and clip evenly by now. A nice option is to sub in Mallory M150 or your fave caps for any "blue turds" or other caps that may be in there.

Big amps with ultralinear circuit - taps on primary of OT fed to screen grids - you have the options of inserting screen grid limiting R's, say 470R or 1K , or forming a new branch off the hi voltage supply to serve those screen grids. And just dead-end those taps. If you have deep pockets, swap in a regular Twin OT. For a screen grid supply, so far I just use a 1K resistor and 47 uF 500V cap branched off the main B+, but if you feel like substituting/adding a choke, live it up.

For amps that have one, optional to leave master volume in place. If you need it, leave it. For a "true to spec" tone, sorry it's gotta go. I find a pinch or two of hi frequency brilliance is lost with the MV in place, and it also tends to pick up a little hum since it's jam up to the power transformer. It IS useful however, dialed down just a bit to say 7, to cut hiss noise. But dialed down to say 3, when you try to use the amp in an overdrive/distortion situation, somehow Fenders don't sound good this way, to my ear anyway. Some people make it work for them nonetheless. That's why I make it optional.

Thanks for the detail reply. Yes, I always wonder that MV pot. It had long coax running to the pot and then to the PI in my 74 Twin, it got to drown some highs.

I have to try lower the input cap of the PI, never thought of that!!!

Ha ha, I avoid the ultra linear version all together.

Do you remove the 2000p cap on the plates of the power tube? I like the sound without it.

I have a related question about the choke. I am building an amp with power scaling that I vary the voltage of the power tubes and PI and keep the rest constant. Is it correct to put the choke for the screen grid and PI? I am not going to put two chokes, too cheap for that!!!! I am going to just rely on RC filter for the preamp because I think the current to the preamp is lower and I can easily use a 5K to 10K resistor from the +B and 20uF to form a good RC filter. I just feel the screen grid draws current when clipping and it's better to use the choke to keep the voltage stiffer.

[Leo_Gnardo]

Thanks for the detail reply. Yes, I always wonder that MV pot. It had long coax running to the pot and then to the PI in my 74 Twin, it got to drown some highs.

I have to try lower the input cap of the PI, never thought of that!!!

Ha ha, I avoid the ultra linear version all together.

Do you remove the 2000p cap on the plates of the power tube? I like the sound without it.

To try and "open up" the low frequency response, I tried the bigger cap idea and found it to be not so good. 0.0022 uF = 2.2 nF = 2200 pF at most. Turns out it's smart to filter out some super lows. Old audio engineer maxim : "the wider you open the (bandwidth) window, the more s#!t flies in."

You can experiment with leaving the MV and its wiring in place, while bypassing it on the circus board. Takes but a minute if the amp's already out of its box. If you hear extra high frequency response and find it pleasing, well that MV will have to stay bypassed.

At the output tube grids, whether to keep the 2000 pF aka .002, sometimes Fender used .001, I listen and also watch on the scope for ultrasonic noise riding the output. Depending on what's needed I'll remove 'em or not. Bass amp used for bass, you can leave 'em in, no objection. If ultrasonic noise shows up on the output you can choose to leave 'em in, or select a lower value that's just enough to make the ultrasonic racket stop, or leave 'em out and install a cap across the drive tube plates typically 47 pF up to 470 pF to quench ultrasonics. Fender applied this solution in many of their amps.

NOTE you have to drive speakers not a load resistor to see what happens in real life with ultrasonic noise. Lower frequencies say 50 to 300 Hz tend to show you what's going on, so you better have speakers that will take the wattage, and neighbors at a distance. Lucky for me, I have both. Sweep the LF range while scope-squinting as some frequencies may show noise riding the waveform and others not.

[Alan0354]

Thanks Leo.

[Firestorm]

I've modded a lot of Fender 135s, but always left the screen taps as they were, because it worked with the sound I was going for. They don't have chokes, so won't quite get blackface sound unless you add one. But they do have a resistive voltage stabilizer which could be a decoupled screen supply node with a few added parts.