ab763 first channel ideas

[cminor9]

So I am planning a new project, an ab763 with a 6V6 power section. Everyone needs a deluxe reverb, right? What I don't think I need is the non-reverb channel. It seems pretty pointless to me. I mean, just switch off the reverb and tremolo if you don't want it, right?

I guess I could leave the first channel off. It'd certainly cut costs. Since I am going to be drilling my own chassis and making my own board, I certainly have that flexibility.

Or, I could do something interesting with the second channel. Trouble is, what? I was thinking maybe a tweed-ish front end, but with the LTP and the high voltage power amp it sure won't sound very tweedy. But I'd like something sufficiently different from the other channel. I could just trial and error my way there, but that could also be a colossal WOMBAT.

I have been looking at schematics trying to find something that might work with the rest of the amp's topology but no dice. Any suggestions from the kind folks out there?

[Chuck H]

Well, if it were my project I'd just leave it a single channel amp 'cause that's how I roll. If you want a second channel, what would you ideally want it to do? You should do what you want. If I were going to do a second channel I would probably just use the other tube for a high gain circuit and add switching. But that's me.

[mooreamps]

Well, I can tell you want I tend to do with my Jazz amplifiers... Both preamp channels are clean channels. The Lead Channel is 12ax7, and the Rhythm Channel tends to be an octal channel using the 6SL7's.. It's not the "cost effective" way to go, but it's the way I have chosen to do it...

-g

[m-fine]

I have been playing around with that myself. First, the two channels are not exactly the same. The reverb channel has the third gain stage which more than makes up for the extra losses so there is a touch more gain. Also, pull out the normal channel and you get even more gain (or less loss) where they combine.

Right now my normal channel is stock except I changed the bright switch to lift the ground from the mid pot and basically bypass the tone stack. You get way more gain and it actually sounds pretty sweet, but I could also just lift the ground on the other channel and still have the same effect so that leaves me with an unused normal again.

My top candidates are a 5E8 (tweed twin) or more likely Marshall Plexi or JTM45/bassman. Both would require another tube to do it right since you need two triodes for the cathode follower driving the tone stack in these designs. I never use my tremolo/vibrato so I am thinking of stealing tube 5 for this purpose. The LTPI is close enough to the bassman and marshall design and I would also be freeing up 2 pots on the front panel, one of which could go to a presence knob. I think this would sound different enough to justify the effort.

Chucks idea of using V1 as an extra (or 2 extra) gain stage for the main channel is also something I toyed with. Like a built in tube overdrive. That will let you keep the trem, but I decided not to do that since the AB763 pre-amp is not the place I would start for a high gain circuit. If I do the marshall thing, I have an extra half 12ax7 that I could use for a switchable high gain, and I think it will sound better with the cathode follower and marshall tone stack than with the AB763.

[cminor9]

Well, if it were my project I'd just leave it a single channel amp 'cause that's how I roll. If you want a second channel, what would you ideally want it to do? You should do what you want. If I were going to do a second channel I would probably just use the other tube for a high gain circuit and add switching. But that's me.

yeah, I am with you: a single channel amp is cool. Less to carry, less to build, and less to buy. But before I go too far in planning, I just was interested in what others might have done for some inspiration. My idea was to find a tweed circuit with a similar topology, best I could do was the 5G9 since it uses an LTP. But the bias is all different so it'd probably sound more like a 6G3 than anything else. Then there are the voltages in the AB763; way too high for a tweed amp. But it sure would be awesome to have a tweed/blackface deluxe in one package! Trouble is, I don't want to compromise either.

[cminor9]

Well, I can tell you want I tend to do with my Jazz amplifiers... Both preamp channels are clean channels. The Lead Channel is 12ax7, and the Rhythm Channel tends to be an octal channel using the 6SL7's.. It's not the "cost effective" way to go, but it's the way I have chosen to do it...

-g

Oh, now that's interesting! Octal never even occurred to me. Seems like it would be more geared toward a tweed amp, but I think I will look into that a bit. Thanks for the suggestion.

[cminor9]

My top candidates are a 5E8 (tweed twin) or more likely Marshall Plexi or JTM45/bassman. Both would require another tube to do it right since you need two triodes for the cathode follower driving the tone stack in these designs. I never use my tremolo/vibrato so I am thinking of stealing tube 5 for this purpose. The LTPI is close enough to the bassman and marshall design and I would also be freeing up 2 pots on the front panel, one of which could go to a presence knob. I think this would sound different enough to justify the effort.

haha, we're opposites. I want the trem. A lot. blackface trem isn't as nice as tweed or brownface trem, but it definitely has a certain distinctive quality that I want to keep.

I built a 6G3 and put a presence knob on there. Borrowed it from the 6G4, didn't like it and tweaked the hell out of it to get it right. Ended up removing it because it always just sounded like crap, no matter where I left the dial. Totally changed the character of the amp in a bad way. Wouldn't even consider one for this ab763. maybe you'll like it, different strokes for different folks and all.

If you are going with a blackface 6L6 power section you definitely have more options, but even then you are dealing with different voltages which will totally change the character of the amp from that of a tweed. My problem is that I love the character of the tweeds, generally prefer them to a blackface. (then why am I taking on ab763? because I don't have anything that gets me that tone, and if you're going to have one blackface amp I think a deluxe reverb is the one to have) A compromised tweed would just piss me off Again, YMMV.

Chucks idea of using V1 as an extra (or 2 extra) gain stage for the main channel is also something I toyed with. Like a built in tube overdrive. That will let you keep the trem, but I decided not to do that since the AB763 pre-amp is not the place I would start for a high gain circuit. If I do the marshall thing, I have an extra half 12ax7 that I could use for a switchable high gain, and I think it will sound better with the cathode follower and marshall tone stack than with the AB763.

agreed. Don't want a high gain amp. I want a deluxe reverb And if I can figure out something to wedge into that 1st channel to give it a little extra something, even better.

[Chuck H]

I want the trem. A lot. blackface trem isn't as nice as tweed or brownface trem

So keep it a single channel. Also keep the extra tube in the circuit, use it to build the better sounding bias vary trem and ditch the roach style trem.

[Albert Kreuzer]

A few weeks ago I had a custom Black/Tweed Deluxe on my bench. Normal channel converted to tweed specs (vol/vol/tone), reverb channel BF with bias trem, switchable fixed/cathode bias, reverb dwell control in the ext. spkr jack and a "wild" switch (both channels in series).
Nice amp, didn`t have time to draw the schem though. Just needed some new tubes and out of the door it was.

[cminor9]

A few weeks ago I had a custom Black/Tweed Deluxe on my bench. Normal channel converted to tweed specs (vol/vol/tone), reverb channel BF with bias trem, switchable fixed/cathode bias, reverb dwell control in the ext. spkr jack and a "wild" switch (both channels in series).
Nice amp, didn`t have time to draw the schem though. Just needed some new tubes and out of the door it was.

heh, yeah, that might be a bit more work than I want to do with it. Sounds awesome though. I wonder if the 5E3 actually sounds like a 5E3?

[cminor9]

So keep it a single channel. Also keep the extra tube in the circuit, use it to build the better sounding bias vary trem and ditch the roach style trem.

Yeah, leaning that way though I am still thinking about an octal channel. The bias vary and harmonic trem circuits are awesome, but the blackface trem is great for some tones. I really want to keep the character of the deluxe reverb, so I'll use the roach. If I find it to be unusable, I can always mod it.

You know how it is, planning is the part where you tend to go back and forth with ideas. I don't have the cash in pocket now anyhow and just some of the parts, so I have time to mull it over.

Thinking about getting a weber neomag to put in there. A deluxe reverb with a big fat JBL, now that's fun! Except the neomags are *only* about 8lbs.

[m-fine]

haha, we're opposites. I want the trem. A lot. blackface trem isn't as nice as tweed or brownface trem, but it definitely has a certain distinctive quality that I want to keep.

It sounds like you are scratch building, so you have a lot more flexibility that I do. You can build it with hopwever many tubes you need. I would need to get creative to add another preamp tube.

Ended up removing it because it always just sounded like crap, no matter where I left the dial.

I was just toying with the idea since I will have the pots just sitting there if I borrow the trem tube for the marshall channel. If you say it did not sound good, I will be sure to give it a good listen while still on the bench and while the iron is still hot.

If you are going with a blackface 6L6 power section you definitely have more options, but even then you are dealing with different voltages which will totally change the character of the amp from that of a tweed. My problem is that I love the character of the tweeds, generally prefer them to a blackface. (then why am I taking on ab763? because I don't have anything that gets me that tone, and if you're going to have one blackface amp I think a deluxe reverb is the one to have) A compromised tweed would just piss me off Again, YMMV.

My power section is currently 2 or 4 6l6's on fixed bias, basically the standard BF setup except I have no need for 80+ watts so I run it with 2 tubes pulled and double the impedance on the speaker load. This is causing awful mental anguish since I have two open sockets begging for a mod. I was thinking about how I could use them for a pair of 6v6, either fixed bias or cathode biased, or perhaps el34's or... Ever see an Egnator Rebel 20 with the 6v6-el84 blend pot? I wish I hadn't! Anyway, I have pushed that to the back burner for now because I have not come up with anything that I think would add more than it subtracts.

As for the voltage being too high, that is not a difficult problem to solve. Just put an extra resitor and filter cap in the B+ chain after the trem chanel to drop the voltage for the "normal" channels preamp tube(s). Scaliong the B+ for the power section is pretty easy too. I just put in a Vari-Watt FX100 kit so I can also control the B+ going to the power section independantly with a pot that now sits where the death cap switch used to be. I highly recomend this kit if you want to play one of these beasts dirty at lower volumes. I can now get power tube overdrive while my kids are sleeping. With the amp stock, I couldn't overdrive the powertubes without wearing ear plugs with a set of earmuffs on top! You can also get great clean tones at lower volumes without losing all the tone a typical volume/master volume setup can steal.

Don't want a high gain amp. I want a deluxe reverb And if I can figure out something to wedge into that 1st channel to give it a little extra something, even better.

They key is (IMHO) what other amps would you also like to have? Which ones would still sound good with the LTPI and fixed bias 6v6 power section? For my situation, a Bassman/Marshall JTM 45 or early Plexi fits the bill. Something different, something I would like to have, and compatible with the power section. Worst case I don't like the marshal sound with the 6l6's and use the open octals for el34's or 6v6 and either swap tubes for different sounds or put in a 6l6/el34 switch. I would like a tweed deluxe and or a tweed twin as well (I am good at having desires) but I managed to convince myslf that those would be done best in a separate chassis. I worry too much of the tweed sound would be lost with the LTPI and solid state rectifier with enough juice for a 100 watt power section. There would be no sag at all without getting into some tricks that would compromise the blackface sound on the primary channel. That is another key criteria for me, the mod can not detract from the BF channel, only add to it. Obviously the exception to that is giving up a trem I have used a handful of times in the 2+ decades I have owned the amp.

If you can't come up with something you would want tone wise and that would be compatible, perhaps single channel would be best. No sense in building it in if you wont use it.

[m-fine]

Yeah, leaning that way though I am still thinking about an octal channel. The bias vary and harmonic trem circuits are awesome, but the blackface trem is great for some tones. I really want to keep the character of the deluxe reverb, so I'll use the roach. If I find it to be unusable, I can always mod it.

Well, you could build two trem channels, one with each trem. I guess you could also build both trems and put in a switch.

[cminor9]

They key is (IMHO) what other amps would you also like to have? Which ones would still sound good with the LTPI and fixed bias 6v6 power section? For my situation, a Bassman/Marshall JTM 45 or early Plexi fits the bill. Something different, something I would like to have, and compatible with the power section. Worst case I don't like the marshal sound with the 6l6's and use the open octals for el34's or 6v6 and either swap tubes for different sounds or put in a 6l6/el34 switch. I would like a tweed deluxe and or a tweed twin as well (I am good at having desires) but I managed to convince myslf that those would be done best in a separate chassis. I worry too much of the tweed sound would be lost with the LTPI and solid state rectifier with enough juice for a 100 watt power section. There would be no sag at all without getting into some tricks that would compromise the blackface sound on the primary channel. That is another key criteria for me, the mod can not detract from the BF channel, only add to it. Obviously the exception to that is giving up a trem I have used a handful of times in the 2+ decades I have owned the amp.

If you can't come up with something you would want tone wise and that would be compatible, perhaps single channel would be best. No sense in building it in if you wont use it.

good advice there, for sure. The other stuff I wanna build is just so different. I'd like a 5F4 or a 5E5, and maybe some kind of old kt66 plexi type thing. Just trying to cover a few bases tone-wise since I play lots of different styles of music. Already have a 5E3, a Spitfire, a 5F2, and a Fender DeVille I will probably sell at some point (once you start playing your own, playing commercial offerings seems kinda lame).

But beyond that, I dunno. I really don't even need that many amps; I mean you can only play one (or maybe two) at a time. And the 5E3 was actually life changing for me, at least in terms of my playing. I am afraid it's all been downhill from there. I just like building the things so much. On the other hand, that time could be spent on the opposite end of the spectrum...practicing and gigging.

I was thinking single channel and just thought I'd throw it out there to see what ideas other would come up with. Seemed like it'd be kinda fun to see what kind of suggestions I'd get because I kept coming up empty.

Kind of like you are saying, when you try and cram two things into one chassis, you end up compromising too much. A JTM45 and a 5F6a could co-exist nicely though.

If you are going to stick el34s or 6v6s into those extra sockets, that seems like it would take a lot of playing with voltages to get them right, and making sure your PT has enough juice for the el34s, and then making sure your OT can deal with a range of tubes. Not for the faint of heart, for sure.

[cminor9]

well, I just built it as a single channel. Just finished it up tonight.

I was absolutely meticulous about everything... I think I may have checked the layout against the layout diagram about a dozen times. Measured every resistor. Checked for continuity everywhere I could to make sure all the leads went where they were supposed to. Then double, then triple checked. It sure is easier to do that than debug. So guess what? Fired up perfectly the first time.

The B+ was a little high, so I had to remedy that with a zener or two. Just gotta silkscreen the front and back fascias, and load it up into the cab. When I get the money, I'll get me one of those Weber neo speakers (like a JBL) and it'll be twang heaven! I really took my time with this build, and the thing sounds amazing!

Thanks for advice guys.