My 66 blackface bassman. tonal questions?

[Hella1hella]

As many of you know I got a 66 bassman not too long ago, and I LOVE it. I am taking it in soon to get a cap job and wanted to see if he could maybe do some other things to it so I wanted to get some suggestions from you guys. someone on here said it has to extra turns to something or another ( I dont know much about wiring amps) and wanted to know if this would add more gain. Basically what I want is less gain from this amp ( crazy huh?) when I get around 4 it breaks up quite a bit and around 9 its just insane, into distortion realms. SO any suggestions? I am not saying I don't want breakup at all, but just less of it. I will also throw on some pictures for you guys to examine the patient. P.S I want to keep it as close to the original circuit as possible, no crazy mods.

[Tom Phillips]

As many of you know I got a 66 bassman not too long ago, and I LOVE it. I am taking it in soon to get a cap job and wanted to see if he could maybe do some other things to it so I wanted to get some suggestions from you guys. someone on here said it has to extra turns to something or another ( I dont know much about wiring amps) and wanted to know if this would add more gain. Basically what I want is less gain from this amp ( crazy huh?) when I get around 4 it breaks up quite a bit and around 9 its just insane, into distortion realms. SO any suggestions? I am not saying I don't want breakup at all, but just less of it. I will also throw on some pictures for you guys to examine the patient. P.S I want to keep it as close to the original circuit as possible, no crazy mods.

As I mentioned in the original thread, your circuit is the AB165. Most people don't like that version and they often talk about modding it back to an AA164 or something in between the two circuits. HOWEVER, you have stated that you love the amp so I wouldn't encourage you to jump into mods. You may eventually find someone who has an earlier 1964 Bassman that you could sound check against yours. It would need to be in good (and un-modded) condition to make a fair comparison. If you double loved the earlier version then you would have some reason to change yours.

As to the "breaks up at 4" that is perfectly normal for the Fender amps. The circuit could be changed so it broke up later but it wouldn't have more power or sound different. The real point is...can you get the sound you want by setting the controls at a given point. The actual number at the arrow isn't important. There have been other discussions about this. I tell my customer's to close their eyes and twiddle the knobs until they get the sound they want. If that is successful then all is well. It is true that you won't often use the higher range on the volume control but it's similar to not using the higher range on the speedometer on many cars. No one takes their car into the shop and says "fix it so I can drive with the speedometer showing 140 MPH but I still want good mileage, quiet operation and I want to stay on the road when I go around sharp corners.

Cheers,
Tom

[Chuck H]

I agree with Tom somewhat on this. Not quite so finite, but I do have a pet peave with novice wannabe techs suggesting things like A10 taper pots or lower gain preamp tubes to get more "headroom". Niether of those things will give you any more headroom. They will only change the volume knob number at which the amp starts to break up, not the actual volume at which the amp starts to break up. That will remain the same. But if that is a concept you can understand then we can make some progress for sure.

If you don't need the amp to be louder and you are just bothered by the fact that it starts to break up at 4... Get over it. It is the nature of the beast with most vintage tube amps. FWIW if your amp IS an AA165 and not an AA864 you shouldn't have any problems with circuit changes. They are virtually the same already but the AA864 is a better circuit IMHO. Since both circuits occupy the same layout and control panel changes are easy. It's worth noting that some changes to the AA165 model were good ones and many leave those changes in place. As Tom also stated.

There are things that will make the amp louder before it starts to break up:

Make sure the amp is biased properly and has a good set of tubes.
Change the feedback loop to the AA864 design. This would also involve swapping the grid leads on the power tubes.

These two things will make the amp "feel" cleaner. As in more positive attack and cleaner lows. but the amp won't really be much louder. If you really want to get louder the best way is:

Get more efficient speakers. I cant say enough about how much this can increase volume. A 3dB increase in speaker efficiency is like doubling your watts. I know you like the speakers that are in the cab now so perhaps you could get a Mojo cab and load it with more effecient speakers for gigging and keep the old cab stock. If just buying a cab whenever you need one isn't in your budget (as it isn't in mine) then you may want to just box up the old speakers for preservation and load more effecient speakers into the existing cab.

I use to have a pdf copy of a 1965 Jensen speaker catalog. I lost it to a computer crash. But IIRC most of the Jensen speakers that Fender used were between 96dB and 98dB efficiency. So a speaker with an efficiency of 101dB will give you notably more volume. Good sounding speakers with 101dB efficiency are common today.

[Hella1hella]

thanks guys that helps alot, its not that I am really bothered that it breaks up at 4, I just have never really realized how much gain these really get, like its nearly in heavy metal realms. I am not really too concerned about volume, 99.9% of the places I play mic the amp anyhow. I just dont have another amp to compare it to so I am not sure if this amount of gain is normal or not.

[Hella1hella]

Also, will changing the circuit over to AA864 devalue my amp? will someone even know if it wasn't original? I got a quote for $160 to change the power cord to 3 prong, and do all the electrolitics. would changing the circuit drastically change this price? ( in most cases )

[Tom Phillips]

To clarify - You have an AB165 circuit which is significantly different from the AA165. The tip off is the terminal strip holding the 220k feedback resistors installed between the phase inverter and output tune plates.

Also, will changing the circuit over to AA864 devalue my amp?

It's hard to predict what someone else will think about that. It it's done right then I say probably not. But that's just me. If it were mine I would convert.

will someone even know if it wasn't original?

Probably not if the work is done well.

I got a quote for $160 to change the power cord to 3 prong, and do all the electrolitics. would changing the circuit drastically change this price? ( in most cases )

It would if it were a full conversion and the work was done to good standards. It takes time and there are 5 extra wire connections under the eyelet board in the AB165. Plus other subtle differences. However, there are subsets of the differences between the two circuits that could be implemented as already discussed. i.e. partial conversions are common. Remember that you said you love it as it is. Also, the mods won't change the breakup at 4. (You could loosen the volume knob set screw and make the breakup at any number you desire )

I have attached a document that is floating around the net with the disclaimer that it is not totally complete or correct IMO but it is food for thought and someone will probably bring it up eventually. Give it a read but remember that you like your amp as is.

Cheers,
Tom

[Hella1hella]

thanks tom I really appreciate it. As far as the break up at 4 deal. I am not saying I do not want this, I just dont want there so be as much gain when I get past that around 8 it gets pretty heavy. I cant find how to get the attachment you attached ( I am sort of new here). Also a little bit of a question, how can you tell the difference between these circuits? ( from a noob point of view) is there just somewhere that I cant look? will the tube chart tell me? I looked up some videos on youtube and it seems like the breakup on the AA864 seems a little bit smoother. what is the most desirable of the circuits?

[Tom Phillips]

Turns out that the document I mentioned is posted here: AA864 Bassman vs AB165 Bassman

You can look up the various Bassman circuits at schematic heaven at Schematic Heaven. Where All Good Amp & Effect Designs Await Resurrection...

Cheers,
Tom

[Hella1hella]

thanks tom, that helps out alot!!! in the article it said that the AA864 lacked some excitement in tonality world. He explains that the AB 165 had more compression and an extra gain stage. I dont know much about modding thats why I wont attempt this myself, saying that the ab165 has an extra gain stage, is this maybe why I am getting more gain? If I converted over to AA864 would I have less gain? ( I am not looking for 0 gain, just less) He is pretty much saying not to fully convert it BUT for me would this possibly be a good idea from what I have told you?

[Chuck H]

Other than the funky NFB loop I think the aa165 is a good circuit. Most players do leave the local feedback loop around the third gain stage on the bass channel because they like the sound. I know I poo poo'd the preamp tube substitute method but there are exceptions IMHO. You can try a 12ay7 in the V1 position (or the V2 position depending on which channel you use). I like what a 12ay7 does to the tone. More tubey if that makes sense. More throughty with bottom that gets under the treble notes and top that adds attack to the lower notes. It will decrease gain a good bit too. Plus it's a non invasive mod that will only cost the price of the tube. If there is an electronics salvage place in your area they may have boxes of pulls. In these places 12ay7's are usually pretty easy to find. Otherwise you'll probably need to order one.

[Tom Phillips]

...in the article it said that the AA864 lacked some excitement in tonality world.

Yes. Many people say that. The normal channel is a pretty basic medium gain Fender circuit. However, if it’s reconfigured as a single channel amp and a 12AX7 is used in the PI then I think it sounds good for guitar. We could write a book about all the things that could be done. I do temporary changes and let the customer listen to make sure I’m on the right path before I make final mods. It’s very difficult to figure out exactly what someone wants from the adjectives used to describe the sound.

He explains that the AB 165 had more compression and an extra gain stage. ... If I converted over to AA864 would I have less gain? ...

The AA864 and the AA165 used three triode gain stages in the Bass Pre-amp Channel and two in the Normal pre-amp channel. For the AB165 the summing point for the two channels was moved such that both channels went through three gain stages. Therefore, you would reduce the gain of the Normal channel if you converted to the AA164. However, that in itself is not the whole picture. Keep in mind that the blackface Fender amps with reverb have three gain stages before the PI for the “vibrato/reverb” channel and only two in the normal channel. Just about everyone I know prefers the sound of the “vibrato/reverb” channel even if they are not using any vibrato or reverb. Note that the voicing is different in the Bassman vs. the reverb amps. This makes it hard to decide what will work best for you unless you play through various different amps. They you need a competent, trustworthy tech to do your set up or have the knowledge to do it yourself.

One recommendation I have, since you love the sound of the amp, is to just keep playing it for a while as is. You won’t be missing out on your chance to mod the amp. You will just be putting it off for a while. In the meantime, you will run across people with other amps that you can audition. That will give you first hand knowledge with which to make a final decision. Meanwhile you will be playing a cool amp that you “love” and you will continue to grow as a guitar player. In addition you won’t have any regrets that you jumped into a mod too soon. If you aren’t doing the work yourself it can get pricey if you keep going back to the shop for changes.

Tom

[Chuck H]

What Tom said. +++