Can somebody explain this circuit to me please. I am not sure if it is between V1 and V2 or V1 and V3. It is between the second half of V1 and the next pre tube that only utilizes one triode. Right between the 0.1uf coupling cap and the grid on that nest AX7, there are two 220k resistors in series and a .001 cap in parallel with the last 220k. What does that little circuit do? It is one of the differences between the AA864 and the AB165.
Thank You
http://www.el34world.com/charts/Schemat ... _AA864.pdf
Bassman AA864
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solderstain
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Re: Bassman AA864
I've got a lot of experience with AA864 and AB165 Bassmans, and I'm not sure what all of your questions are about.
I will say this - there's not much in common between the AA864 and the AB165. The best thing to do is to print out the schematic for both and lay them side-by-side and walk through the circuits. The third gain stage is VERY different between the two designs. So is the power stage.
The 220k resistor network is a voltage divider that drops the the signal going to the third gain stage of the bass channel.
The best Bassman sound I've ever heard is actually a hybrid of those two circuits. The AB165 front end is actually better for a guitarist, but its power section sucks. Take an AB165 Bassman and convert the power stage to the AA864 version. PRESTO! Killer Bassman. That's what I did to my '66 Bassman, and the basis of what I did for my Bassman/Reverb combo.
But I don't understand your "I am not sure if it is between V1 and V2 or V1 and V3. It is between the second half of V1 and the next pre tube that only utilizes one triode." question. What is the "it" that you're asking about???
I will say this - there's not much in common between the AA864 and the AB165. The best thing to do is to print out the schematic for both and lay them side-by-side and walk through the circuits. The third gain stage is VERY different between the two designs. So is the power stage.
The 220k resistor network is a voltage divider that drops the the signal going to the third gain stage of the bass channel.
The best Bassman sound I've ever heard is actually a hybrid of those two circuits. The AB165 front end is actually better for a guitarist, but its power section sucks. Take an AB165 Bassman and convert the power stage to the AA864 version. PRESTO! Killer Bassman. That's what I did to my '66 Bassman, and the basis of what I did for my Bassman/Reverb combo.
But I don't understand your "I am not sure if it is between V1 and V2 or V1 and V3. It is between the second half of V1 and the next pre tube that only utilizes one triode." question. What is the "it" that you're asking about???
Re: Bassman AA864
I am only asking one question.solderstain wrote: and I'm not sure what all of your questions are about.
But I don't understand your "I am not sure if it is between V1 and V2 or V1 and V3. It is between the second half of V1 and the next pre tube that only utilizes one triode." question. What is the "it" that you're asking about???
The "it" refers to the circuit I am asking about......the reason for my post. The tube sockets on the schematic I provided are not labeled, so I do not know if the tube in question is V2 or V3.
Does that make things more clear?
Thank You
Re: Bassman AA864
Are you simply interested in the two 220K resistors and the .001 cap before the third gain stage on the Bass channel? I think that's your question.
I think Solderstain answered. The pair of 220K resistors form a voltage divider. They are the same value resistors, so half the input voltage is shunted to ground and half is send to the input grid of the next tube. The cap is a filter to control what frequencies are passed or filtered out to shape the tone further. Sorry, I'm not good with how caps filter frequencies. Someone else with better technical chops than me needs to explain that bit of it.
I think Solderstain answered. The pair of 220K resistors form a voltage divider. They are the same value resistors, so half the input voltage is shunted to ground and half is send to the input grid of the next tube. The cap is a filter to control what frequencies are passed or filtered out to shape the tone further. Sorry, I'm not good with how caps filter frequencies. Someone else with better technical chops than me needs to explain that bit of it.
Re: Bassman AA864
Phil_S
Yes, that answers my question perfectly. I have a SF Bassman, and I am wondering how much difference there is in the "bass channel" between the AA864 and the AB165. That is one of the areas in question.
Perhaps neither of those channels is real great for guitar, and I should put something different in that first channel. Then I can just utilize the Power section as "solderstain" suggested.
Thank You
Yes, that answers my question perfectly. I have a SF Bassman, and I am wondering how much difference there is in the "bass channel" between the AA864 and the AB165. That is one of the areas in question.
Perhaps neither of those channels is real great for guitar, and I should put something different in that first channel. Then I can just utilize the Power section as "solderstain" suggested.
Thank You
Re: Bassman AA864
Tom
Don't let that smoke out!
Don't let that smoke out!
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solderstain
- Posts: 220
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- Location: Phoenix, AZ
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Re: Bassman AA864
Okay - here's what you need to know:hired hand wrote:Phil_S
Yes, that answers my question perfectly. I have a SF Bassman, and I am wondering how much difference there is in the "bass channel" between the AA864 and the AB165. That is one of the areas in question.
Perhaps neither of those channels is real great for guitar, and I should put something different in that first channel. Then I can just utilize the Power section as "solderstain" suggested.
Thank You
Since you have an AB165 Bassman already, you're all set to do some minor tweaks (at least minor to me) and have a KILLER amp.
There are some TERRIFIC things in the AB165 preamp section for guitarists. Some major architecture departures from the AA864 that make it a WONDERFUL circuit for guitar.
One MAJOR change in the AB165 is that BOTH channels go through the third gain stage, instead of just the bass channel in the earlier design. The normal channel now goes through the extra gain stage, and it didn't before this design.
Speaking of the third gain stage...
The change that makes this amp so killer for guitarists is that they completely reconfigured the third gain stage in the AB165. In this design, the third gain stage was provided with a feedback loop it didn't have before and the voltage divider in front of the stage is gone. The result is that the third stage now compresses and overdrives MUCH sooner - a result that probably WASN'T what they were looking for, but there it is.
The BAD part of the AB165 design is all the weird stuff they did in the output stage. It's just plain odd, and the changes were NOT 'for the better'.
As I said before, reconfigure the output stage to AA864 specs. Because you'll be changing the feedback loop's polarity, you'll also need to reverse the polarity of the leads from the secondary of the OT to the output jacks.
Leave the preamp stage alone except for one thing - clip out the bypass caps on the plate resistors of the first stage of each channel. Otherwise, leave the preamp alone.
Fix the output stage and then go rock. There's a damn-good reason I duplicated that configuration for my project amp.
Seriously, if you haven't yet, put both schemos side-by-side. All that I'm saying will become clear.
EDIT: I forgot I did this illustration so I'm adding it to the post. It shows which preamp tube positions are which in the circuit.
[img:800:600]http://www.doctorvintage.com/amps/ab165 ... atubes.jpg[/img]
Re: Bassman AA864
Check, check, and triple check.....gotcha.solderstain wrote:Okay - here's what you need to know:hired hand wrote:Phil_S
Yes, that answers my question perfectly. I have a SF Bassman, and I am wondering how much difference there is in the "bass channel" between the AA864 and the AB165. That is one of the areas in question.
Perhaps neither of those channels is real great for guitar, and I should put something different in that first channel. Then I can just utilize the Power section as "solderstain" suggested.
Thank You
Since you have an AB165 Bassman already, you're all set to do some minor tweaks (at least minor to me) and have a KILLER amp.
There are some TERRIFIC things in the AB165 preamp section for guitarists. Some major architecture departures from the AA864 that make it a WONDERFUL circuit for guitar.
One MAJOR change in the AB165 is that BOTH channels go through the third gain stage, instead of just the bass channel in the earlier design. The normal channel now goes through the extra gain stage, and it didn't before this design.
Speaking of the third gain stage...
The change that makes this amp so killer for guitarists is that they completely reconfigured the third gain stage in the AB165. In this design, the third gain stage was provided with a feedback loop it didn't have before and the voltage divider in front of the stage is gone. The result is that the third stage now compresses and overdrives MUCH sooner - a result that probably WASN'T what they were looking for, but there it is.
The BAD part of the AB165 design is all the weird stuff they did in the output stage. It's just plain odd, and the changes were NOT 'for the better'.
As I said before, reconfigure the output stage to AA864 specs. Because you'll be changing the feedback loop's polarity, you'll also need to reverse the polarity of the leads from the secondary of the OT to the output jacks.
Leave the preamp stage alone except for one thing - clip out the bypass caps on the plate resistors of the first stage of each channel. Otherwise, leave the preamp alone.
Fix the output stage and then go rock. There's a damn-good reason I duplicated that configuration for my project amp.
Seriously, if you haven't yet, put both schemos side-by-side. All that I'm saying will become clear.
EDIT: I forgot I did this illustration so I'm adding it to the post. It shows which preamp tube positions are which in the circuit.
[img:800:600]http://www.doctorvintage.com/amps/ab165 ... atubes.jpg[/img]
Everything you have said so far is within my level of understanding. In your opinion, does this hold true for the "blonde" (6G6 circuits) as well? That is to say, do you find the blonde bassmans to be problematic (for guitar use) in the preamp, just like the AA864 is? If we throw the 6G6 into the conversation, do you still think the AB165 pre (with your few changes) and the power section of the AA864 is the way to go for guitar?
Thank You
Re: Bassman AA864
Solderstain,
Will you please be specific about where to splice the AB165 into the AA864?
On the AB165, after the 3rd gain stage, there is a .1 cap then the NFB conneciton, then the PI. Where to cut off the AB165? Before or after the .1 cap?
On the AA864, after the third gain stage, there are a pair of 220K mix resistors. Following the mix resistors is a 500p coupling cap to the PI. Am I understanding that you mean to keep that 500p, all the way back to the OT?
Thanks.
Phil
Will you please be specific about where to splice the AB165 into the AA864?
On the AB165, after the 3rd gain stage, there is a .1 cap then the NFB conneciton, then the PI. Where to cut off the AB165? Before or after the .1 cap?
On the AA864, after the third gain stage, there are a pair of 220K mix resistors. Following the mix resistors is a 500p coupling cap to the PI. Am I understanding that you mean to keep that 500p, all the way back to the OT?
Thanks.
Phil
Re: Bassman AA864
I have an AA864 from mid-1964. When I got it in the mid-80's I don't believe the fanaticism about the 864 was in evidence, as the original owner charged me 50 dollars US for it. I liked the Normal Channel well enough (despite all the hoopla, it's the same design as most of the Blackface amps sans-reverb). I HATED the Bass channel for bass or guitar. It's basically set up to produce a loud, muffled, boomy, bassy sound from guitar or bass- in fact there are capacitors in several places in the circuit which attenuate treble frequencies. I modified mine soon after I got it with a 59 Bassman tone stack and a Ken Fischer-suggested bright switch mod that was inspired. Over the years it was my experimentation chassis, and has been a 6G6-A, 6G6-B and 59 Bassman before guilt and public opinion forced me to convert the power amp back to 864 (I kept the bright mod and the Bass channel mods).
Hope this helps, BTF.
Hope this helps, BTF.
Re: Bassman AA864
BTF -
What did you think of the "bass channel" on those 6G6 circuits?
Thanks
What did you think of the "bass channel" on those 6G6 circuits?
Thanks
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solderstain
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Re: Bassman AA864
First of all, I just want to make it clear that I didn't come up with this idea; let's face it - there's really nothing 'new' out there. I read about it somewhere... if I remember where, I'll post a link. BUT... this really is a simple concept, and once I laid the two different schematics side-by-side, it all made sense. So I tried it in my '66, and liked it enough to duplicate the architecture in another chassis. All the recommendations I'm passing on are from another source. I just followed the instructions. The only part that I came up with was the solution of how to add reverb to the design without destroying the original tone. I did that development work on my project amp.
The AB165 puts the Normal channel through the third gain stage, which just happens to break up sooner, and FOR ME, that makes it attractive. I can get some grit at a somewhat lower volume.
For those who don't want the early break-up, then using the AB165 preamp design probably ISN'T for you.
But the short answer is that the place to START the AA864 configuration is WITH the input coupling cap to the PI. If you're not sure what you want, then I'd suggest that you make your first attempt look EXACTLY like an AA864, and then have a little supply of different cap values, and experiment. What you end up with may not be what I ended up with. You can also fiddle with NFB resistor values.
It sort-of goes without saying that you need to include the bias circuit needs to be included in the change from AB165 to AA864.
It takes a little head-scratching and forethought about how to do the layout when you do this. The weird layout of the AB165 output stage results in the fact that the locations of a number of the eyelets are in slightly different places due to the different configuration of the NFB. But if you get the layout PDF of the two different designs and look at how the off-board wires are run under the board to the eyelets, you'll see how to route things.
My opinion is not really important. All the different generations of Bassmans have their fans. My personal feeling about this configuration I'm in love with is based on the fact that it has the Normal channel go through the third gain stage, and that the third gain stage breaks up MUCH earlier in this design than the earlier designs. The AA864 and the various 6G6 circuits don't have the Normal channel going through the third gain stage, and as 'BTF' mentioned, the Bass channel is not really conducive to a guitar player.hired hand wrote:Check, check, and triple check.....gotcha.
Everything you have said so far is within my level of understanding. In your opinion, does this hold true for the "blonde" (6G6 circuits) as well? That is to say, do you find the blonde bassmans to be problematic (for guitar use) in the preamp, just like the AA864 is? If we throw the 6G6 into the conversation, do you still think the AB165 pre (with your few changes) and the power section of the AA864 is the way to go for guitar?
Thank You
The AB165 puts the Normal channel through the third gain stage, which just happens to break up sooner, and FOR ME, that makes it attractive. I can get some grit at a somewhat lower volume.
For those who don't want the early break-up, then using the AB165 preamp design probably ISN'T for you.
Phil - the coupling caps on the PI are a place you can play around and 'tune' a bit. I use a .001uf cap at the input of the PI, and .047uf caps at the output of each leg. These are the tunable areas.Phil_S wrote:Solderstain,
Will you please be specific about where to splice the AB165 into the AA864?
On the AB165, after the 3rd gain stage, there is a .1 cap then the NFB conneciton, then the PI. Where to cut off the AB165? Before or after the .1 cap?
On the AA864, after the third gain stage, there are a pair of 220K mix resistors. Following the mix resistors is a 500p coupling cap to the PI. Am I understanding that you mean to keep that 500p, all the way back to the OT?
Thanks.
Phil
But the short answer is that the place to START the AA864 configuration is WITH the input coupling cap to the PI. If you're not sure what you want, then I'd suggest that you make your first attempt look EXACTLY like an AA864, and then have a little supply of different cap values, and experiment. What you end up with may not be what I ended up with. You can also fiddle with NFB resistor values.
It sort-of goes without saying that you need to include the bias circuit needs to be included in the change from AB165 to AA864.
It takes a little head-scratching and forethought about how to do the layout when you do this. The weird layout of the AB165 output stage results in the fact that the locations of a number of the eyelets are in slightly different places due to the different configuration of the NFB. But if you get the layout PDF of the two different designs and look at how the off-board wires are run under the board to the eyelets, you'll see how to route things.
Re: Bassman AA864
Solderstain's recommendations have become VERY popular for conversions from 65's, at least as far as I can tell from the web.
HH: I converted the power amp and Normal channel to 6G6 for a couple of reasons. First, in my area I worked as a service tech for a medium-sized service center since the 80's. In all that time, I saw about 5 Blonde/Brown amps. And not one Bassman ever came through for a try! I have to say that truthfully (and in my own opinion, of course) the 6G6A circuit was wonderful! It was superbly fat clean and crunched with a rich distortion. Honestly, were it not for the guilt of savaging a real 864, I would have left it. The 6G6B was okay, but I use Strats and Teles and I thought it was a bit bright (there are just a couple of capacitor changes between them, but I thought it made a great deal of difference in the sound and feel). I changed the normal channel and power amp back to 864 earlier this year.
I kept the bass channel as it currently is and has been since '85. I parallel-trioded the first input, fed the tonestack from the plate with a 59 Bassman tonestack, and recovered it with the old-standard 1500 ohm/25uF Fender stage. The bright switch was a take from Ken Fischer's presence mod. The bright switch parallels a 1uF film cap with the 100 ohm offset cap in the PI tail. This way the bright works for both channels and isn't reliant on volume settings. The amp works well. Sure, its value has been diminished, but overall I like it for guitar much better.
Regards, BTF.
HH: I converted the power amp and Normal channel to 6G6 for a couple of reasons. First, in my area I worked as a service tech for a medium-sized service center since the 80's. In all that time, I saw about 5 Blonde/Brown amps. And not one Bassman ever came through for a try! I have to say that truthfully (and in my own opinion, of course) the 6G6A circuit was wonderful! It was superbly fat clean and crunched with a rich distortion. Honestly, were it not for the guilt of savaging a real 864, I would have left it. The 6G6B was okay, but I use Strats and Teles and I thought it was a bit bright (there are just a couple of capacitor changes between them, but I thought it made a great deal of difference in the sound and feel). I changed the normal channel and power amp back to 864 earlier this year.
I kept the bass channel as it currently is and has been since '85. I parallel-trioded the first input, fed the tonestack from the plate with a 59 Bassman tonestack, and recovered it with the old-standard 1500 ohm/25uF Fender stage. The bright switch was a take from Ken Fischer's presence mod. The bright switch parallels a 1uF film cap with the 100 ohm offset cap in the PI tail. This way the bright works for both channels and isn't reliant on volume settings. The amp works well. Sure, its value has been diminished, but overall I like it for guitar much better.
Regards, BTF.
Re: Bassman AA864
BTF and solderstain -
Thanks for you individual opinions. I knew this was not going to be an easy decision. I know that the Bassman/JTM45 is also a popular mod for the first channel of a SF Bassman Head. I may just have to try a few things.....tear them out and rebuild if it is not the best option.
Thanks Again
Thanks for you individual opinions. I knew this was not going to be an easy decision. I know that the Bassman/JTM45 is also a popular mod for the first channel of a SF Bassman Head. I may just have to try a few things.....tear them out and rebuild if it is not the best option.
Thanks Again
Re: Bassman AA864
Hey Solderstain....
I realize Phil already asked you this question, but I am going to ask it again. I probably have this wired wrong at the PI, and it may be hard to see in my pictures because the coupling cap is a black mica that blends into the board.
Both channels from the preamp are feeding into a 500pf cap at the PI. I am feeding that into a 220k resistor before it hits the coupling cap. Should I remove the 220k and just have the preamp go direct to the 500pf coupling cap?
Thank You
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864001.jpg[/img]
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864002.jpg[/img]
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864003.jpg[/img]
http://www.el34world.com/charts/Schemat ... _AB165.pdf
http://www.el34world.com/charts/Schemat ... _AA864.pdf
I realize Phil already asked you this question, but I am going to ask it again. I probably have this wired wrong at the PI, and it may be hard to see in my pictures because the coupling cap is a black mica that blends into the board.
Both channels from the preamp are feeding into a 500pf cap at the PI. I am feeding that into a 220k resistor before it hits the coupling cap. Should I remove the 220k and just have the preamp go direct to the 500pf coupling cap?
Thank You
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864001.jpg[/img]
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864002.jpg[/img]
[IMG:800:600]http://i111.photobucket.com/albums/n143 ... 864003.jpg[/img]
http://www.el34world.com/charts/Schemat ... _AB165.pdf
http://www.el34world.com/charts/Schemat ... _AA864.pdf