Custom AB165 Bassman - how to use unused gain stage?

[Ibanezgrind]

Hi Everyone,

This is offically my first post and glad to be here. I hope this is the area I need to post this.
I saw a video on YouTube of a awesome sounding AB165 Blackface Bassman. Loads of gain and crunch - sounded wicked awesome! Long story short, I asked the channel owner who did the mods if he had any layouts available to cut the guess work out. Unfortunately he did not so I had to watch his video over and over again and did my best to take notes on everything he did. I am at the point now where I can start populating my eyelit board but I want to make sure what I have is correct. I sent him my pdf of the layout but he is igorning me. Anyways, I will have my layout below, it is not a complete layout so don't pay attention to the pots/control names. I do believe I have enough to show the mods and using the extra gain stage.

What I think he did was doing a cascading mod? It seems like that anyways. Correct me if I am wrong but he has a 1M resistor simulating a Bright Channel volume pot on a marshall or Tweed Bassman. I put some notes down in my layout to what I think the components are for in the circuit. I would be very greatful if I got some feedback on it! Am I heading in the right direciton? I am open for suggestions to make it better since I have no clue if this will work.

My other question is, since this is an AB165 circuit, should I convert the phase inverter section to an AA864 with 820/100 feedback resistors? 22K tail resistor? Power tube couping caps to .1uf instead of .022uf? PI coupling cap to .001? The standard blackface fixed bias or cathode bias? Anything else at all to help make this as efficent as possible?

My ideal goal is to mod the normal channel as it is in the layout and leave the bass channel stock in case I want to plug my bass in or jumper them together if I feel like it.

Thanks a bunch for taking the time to read this and I look forward to starting this project!

Best regards,
Miguel

[sluckey]

Layouts, especially incomplete ones, are not very useful in understanding amp circuits. Schematics are the way to go.

[Ibanezgrind]

Hi Sluckey,

Unfortunately I don't have a schematic for this unless I make one up myself. I will gladly do that but it will take me some time. In the mean time I will complete the layout of what I intend to do. Thanks for letting me know.

[sluckey]

How do you complete a layout without a schematic?

[Ibanezgrind]

My layout came from viewing a youtube video over and over again and visually seeing what modifications were done. I took  notes from the video and transcribed everything to this layout. I was only guessing that it would be enough information to go off or if what my layout had made sense to anyone. That's ok if it doesn't, I'll do some more work to make it more straight forward. I just wanted to have a starting point to share with everyone.

[sluckey]

I started looking at your layout and comparing to the original schematic. I got as far as V1 pin 1 and stopped. Pin 1 is connected to the wrong side of that 100K resistor.

You can find the AB165 schematic including layout by following the link to the Tube Amps Schematic Library at the bottom of this page. I highly recommend using the originals as a starting point.

[Ibanezgrind]

I appreciate your attempt!
I was hoping you would be the first to give me feedback since I have seen you help so many other people out. Now I know what I have to do. I will check out that link and try to get something soon, schematic and COMPLETED layout based on ab165.

Thanks again!
Miguel

[Papa Jim]

I agree with Sluckey. Seems like the place to start with the schematics on here and the layout is already done too. Modify them both.
https://el34world.com/charts/Schematics/files/Fender/Fender_Schematics.htm

[sluckey]

I'm not trying to bust your chops or discourage you with this project. I'm just trying to steer you in a good direction. If you can't read or write schematics right now, don't worry. We were all at that point at one time. This is a good project to learn. And this forum is a lot better classroom than watching a video over and over. Most of us are very familiar with Fender circuits. If you have questions, just ask. No one's gonna laugh out loud, well, except maybe Jim Bob.

BTW, post the link to the video that has you so juiced. Maybe we can help you unravel some of the secrets.

[Ibanezgrind]

Oh no, I don't take any offense at all! I have only been building tube amps for the last year and a half. I have successfully built AB763 amps and do a pretty good job following the stock schematics and layouts. This one is a different beast but up for the challenge. I do apprecaite your direction thus far and it makes sense. There is still so much I need to learn but glad you and everyone else are here to help.

Here is the video:

He summarizes the mods right before the guitar demo around the 23rd minute and thats where I got my notes from.

I see where I made the wrong connection on V1 pin1. Stupid me. In any case, I will do my best to get a complete layout down within the next few days, permitting my free time.

Thanks again Sluckey and appreciate the message.

Miguel

[Papa Jim]

First thing I saw was that the amp is an AA165 instead of an AB165, unless later on in the video he changes it.

[Ibanezgrind]

Yes, I noticed that too. He brings up a few different schematics in the video. Aren't they almost the same with a few minor component tweaks?

[Ibanezgrind]

Upon doing a side by side comparison, I now see they are quite different, especially the PI section. I wanted to use the standard PI section like all the other blackface amps anyways!

[Papa Jim]

Either way probably best to use the correct schematic. Don't put the horse before the cart. Get a stock AA165 built and working before ever doing any mods. Then only perform 1 mod at a time.

[SILVERGUN]

I'm hoping you don't get put off by trying to provide a schematic.
I like to use Paint (included in Windows) to rough together schematic ideas on the fly because it just takes a few minutes to cut and paste ideas together and see if they will fly.
Sluckey doesn't need a perfect looking schematic, he just needs an accurate one.

It's a shame that the YouTube poster doesn't show us an amended schematic when he is done because by the time I got to the end of the video his verbal gymnastics had me confused. (besides the fact that I jumped around the video)

Even though I'm pretty sure I understand what he did it just isn't 100% clear without the schematic.

I included a screenshot to give you an idea of how you can use Paint to alter an existing schematic.
This is not my rendition of an accurate schematic based off of the video. I'd have to watch it 5 or 6 more times to get past his mistakes and long winded explanations.

[Ibanezgrind]

Thanks for your suggestion Silvergun. I appreciate it! I am glad I wasn't the only one that didn't get his mods the first time through the video. I will more than likely copy the PI section of that schematic and paste it to something I will come up with. I am pretty sure he did some sort of cascade modificaiton based on how he used the extra caps and resistors coming off of V3. Either way I will know for sure soon enough, thanks to you guys.

The head cabinet will come in today from Mojotone    I just need the transformers! I probably won't get them for another couple of weeks due to lack of funds. I will have ample time to understand these mods and what will work the best.

[SILVERGUN]

To "cascade" is just a fancy term to say that you are running one gain stage into another.
Most times when it's used as a modification term it would be to imply an additional gain stage that wasn't there before the modification...i.e.- using the unused triode he pointed out as an additional gain stage.

So, the gain stage I added in my pretend schematic is "cascaded", due to the fact that it is being fed from a prior gain stage.

I just didn't want terminology to get in your way.

[Ibanezgrind]

Ok, that is clear now. It's more of a general term. Learning so much! Thanks again.

[Ibanezgrind]

Hi Guys,

I have been doing a lot of thinking since and I am not sure I want to do the inital mod I saw in the video anymore. I have since been really busy and put this as my last priority but getting quite antsy as the days pass. The layout is about 75% done and I haven't even taken a good look at a schematic yet and how to get them to match without confusing anyone. Yeah, that amp sounds killer in the video but I already have a Marshall Superlead I built so why make it sound like something similar to what I already have.

Maybe I should take a much more simple approach. Unless you all think I should stick to my plan, I don't see why I can't make the standard AB165, AA165 or AA864 sound good with some more straight forward mods - suggested by you all of course! I did see this schematic that was edited from fenderguru.com and how they used the AB165 and mixed it with the output section of AA864 specs. Im sure you have all seen this in their "BF/SF Bassman" section but I will include the schematic anyways. Let me know if you think this is something worth pursuing and further tweaking it to make it better.

I am pretty much open for anything at this point, I just want to get started on something good. I would love to be able to use the bass channel for a bass guitar if it all possible - maybe even connecting a jumper to the normal if I go the AB165 route to add some bottom-end to the sound. I don't know if that is worth doing... I am really indecisive as you can tell but please help me choose a direction! I am an IC layout designer here in AZ for a semi-conductor company so I can read schematics pretty well if you want to describe the modification (to make it easier on you all) and  I can write it down on the schematic.

Let me know what you all think, thanks for your time!

Miguel

[sluckey]

You may be interested in an amp I built in '76...

     http://sluckeyamps.com/misc/76_Amp_Project.pdf

[Ibanezgrind]

Thanks Sluckey for this link!
Nice webpage by the way. You do nice work! Really liked the high school photo of your band, which one are you?

I was looking over the schematic for the AA864 preamp and noticed you adjusted the values of the components to be optimized for a bass guitar, correct? Then over to your Twin Reverb side, you made your own tweaks to be better for guitar. There is a lot of great ideas here for me to start with. I did notice the PCB layout you drew up, some of the values differ from the schematic. If I were to go with these ideas, which one should I reference?

How does a guitar sound plugged in to the bass channel? Does the normal channel sound good cranked?

Thanks!

[PRR]

> make the standard AB165, AA165 or AA864 sound good with some more straight forward mods

I didn't know they didn't sound good.

Tastes differ and fashions change. But you won't go wrong with building and playing the Classics *before* you think about mods.

[sluckey]

I'm the geek with the red Strat. That pic was taken in '67.

Any component value differences you see in that amp were because that's what I had in my parts bins. A .082µF is the same as a .1µF, right? And if a .1µF was a good coupling value for the output tubes, then .5µF should really be good for a bass.  There was no "tone tweaking" going on. If you should go with these ideas I recommend you start with the original Fender values.

IMO any guitar plugged into the bass channel of an AA864 Bass amp sounds like shit. But plug into the normal channel and use a guitar speaker cab instead of a bass cab and you have the typical blackface tone. There is no normal channel in that amp. It has the vibrato channel from a Twin reverb. And it sounds LOUD and CLEAN, especially with the 2x15 bass cab I had at the time. That cab would not do the screaming treble that a guitar cab could do though.

[uki]

I'm the geek with the red Strat. That pic was taken in '67.

I knew it !!  The other dude with guitar doesn't looks like a geek type !

oops

By the way @Ibanezgrind Gimp is a very good free open source software for image editing and easy to use.

[Ibanezgrind]

Thats a cool pic, I always like seeing those photos of high school bands from the 60s!

Thanks for the clarification Sluckey and I know now more than ever what I want to do and the direction I want to take. I appreciate your wisdom and will look to your advice if I have any further questions.

Miguel

[TheKT88KilledJFK]

You could just run the output of the "guitar" preamp into the "Bass" preamp and have enough controls already to season to taste. 

[punkykatt]

Sears Silvertone Twin 12, Back in the 70`s I had two of them that I double stacked. I made up an adapter chord so I could play both channels of each amp at the same time and kept the volumes wide open all the time.

[Ibanezgrind]

Hi Sluckey,

I did some browsing last night and found these schematics which look interesting. Can you please look them over and let me know if any one of these are worth pursuing? One of them is a little hard to read but the modded stuff stands out.

I noticed that they both modify the bass channel. I am guessing that most players don't use it for a bass guitar and leave the normal channel alone. It's not what I wanted to do but I guess I could always get a bass amp later on or something if it benefits the sound more or is more versatile.

Thanks for your time!
Miguel

[sluckey]

The reason people like to modify the bass channel is because the topology resembles a Marshall. Easy to modify into your favorite Marshall style preamp. They do it for guitar, not bass.

That schematic is very typical Marshall style, gain + gain + cathode follower + tone stack.

If you don't really want to modify the bass channel, then don't. There are plenty of circuits you can build around a single tube normal preamp. If you want a bass amp that's also a guitar amp, well, you already have that. Many people like to convert an AB165 to AA864 circuit.

[Ibanezgrind]

Thanks for your reply Sluckey, with each explanation from you I feel I am understanding more and more about these circuits. It sounds like the AB165 is the direction I am heading, converting it to an AA864. With each channel getting 3 gain stages and jumping the channels together, I really can't go wrong with that.Thanks again!

[plumcrazyfx]

I have an AB165 that was ghosting.  I recapped it, then added a pre filtering stage and removed the 220k resistors and converted to the earlier bias arrangement The thing is a monster just like that with no other mods.  Like a very articulate Marshall with a 4x12 and a huge tight bottom end. I use the bass channel as it was intended with an Ampeg 1x15 cab.  Don't want to do anything else to mess up that sound.  The Dual Showman I have sounds very Fendery compared to the 165 which has it's own great thing going on.