6g15/ab763 single channel 40w head build thread

[Jatchley]

so i'm trying to design an amp that will be as follows. single channel ab763 with vari bias trem voiced similiar to a super reverb but with a trafo compliment like the vibrolux reverb and a 6g15 as the verb. and i guess my problem is i'm not exactly sure how to replace the entire reverb circuit from the ab763. i want a true 6g15 in that position in the circuit. i've seen the vibroking. but that circuit has wayyyy too much gainy tweediness for what im aiming for. i want BF CLEAN but in the vibroking format with bias vari trem. i've also thought should i just build a no trem/no verb ab763 with a weber revibe on the front end. but then again how could i implement that into one single head and circuit? and then im thinking how to account for these changes with the PT if it needs it? would i just replace the ab763s verb circuit with the 6g15 at the .002 cap coming off the preamp? but in the 6g15 that pre tube is a 12at7 and the ab763 its a 12ax7 so i figure ill need to lower gain at that point to get the 6g15 verb circuit to operate correctly?  is the difference between just adding dwell and tone to the ab763 that much worse than a real 6g15? it seems like the easier way but ive heard tone is compromised. so i just want to get it right.

[vampwizzard]

I think the best bet is to start with the vibro king, keep the reverb circuit, but change the components to make the amp sections match the AB763. Do the 6V6 thing, or see if a 6K6 from the 6G15 and those values make sense. Experimentation is the spice of life.

[Jatchley]

something like this? i found these 2 schematics and combined them. one is the 6g15 upfront connected to a princeton? i just cut out the princeton and put a bandmaster i'm still looking up the compensations for cutting out a channel of an ab763. i also wanted to change the trem to the 6g16 and add a mid pot. also should i have kept some of the 68k input resistors to the grid of the 7025?

[sluckey]

This should be just what you ordered...

[Jatchley]

WOW THANKS SLUCKEY! THIS HELPS ME OUT SO MUCH!

[Jatchley]

can i just add a B+ node at the end of the power rail for the 295v line to the 6g15 section? how would i calculate that? i'm sourcing a hammond 290D2X 710VCT with 300ma on the HV and 5A on the filaments and the 6g15 PT is only rated for 25ma so i should be ok on current handling for the extra 6k6.   

[Bieworm]

The 6g15 is designed to run in front of the amp, so keep it like that. When placed after the tone stack it would loose the feel you're after. Sluckey gave you valuable info and it should be a nice challenge to build. That hammond PT should work for your project. Reverb can be the cause of noise in the circuit, so I'd recommend all grounds of the reverb going to a separate star ground as far away from the amp star ground as possible.  And I use shielde wire for all long wires and wires going to every grid.
For the record..  the 6g15 ain't that much more superior to the BF reverb. You can add tone and dwell controls to any BF reverb...

[Jatchley]

The 6g15 is designed to run in front of the amp, so keep it like that. When placed after the tone stack it would loose the feel you're after. Sluckey gave you valuable info and it should be a nice challenge to build. That hammond PT should work for your project. Reverb can be the cause of noise in the circuit, so I'd recommend all grounds of the reverb going to a separate star ground as far away from the amp star ground as possible.  And I use shielde wire for all long wires and wires going to every grid.
For the record..  the 6g15 ain't that much more superior to the BF reverb. You can add tone and dwell controls to any BF reverb...

yeah! i'm totally doing it that way. i'm really stoked to build this one. i know i could've added dwell and tone to a normal ab763 but i wanted something weirder in topology for learning purposes i guess. i'm still going to be modifying the circuit in the future too. i want to add a .5w speaker out and possibly change up the tone stack or other ab763 mods. but first i want whatever this amp is to work around stock voltages for each section involved.

[Jatchley]

so i consolidated and added some stuff to that schematic from sluckey. you all think this will work? i'll be adjusting the dropping resistors for node e & f in the real world to make sure these hit around 300vish and 250v.

out of pure curiosity i'm also wondering what would happen if i replace that cathode follower in the verb circuit and begin the preamp with the b side of the reverb return and have the tube and half gain stage like the vibrato channel and put the mix knob insert into the normal ab763 spot in the circuit? im gonna draw it up in the next day or so and see how it looks.

[PRR]

> i replace that cathode follower in the verb circuit and begin the preamp with the b side of the reverb return and have the tube and half gain stage like the vibrato channel and put the mix knob insert into the normal ab763 spot in the circuit?

huh? 

[Jatchley]

> i replace that cathode follower in the verb circuit and begin the preamp with the b side of the reverb return and have the tube and half gain stage like the vibrato channel and put the mix knob insert into the normal ab763 spot in the circuit?

huh? 

oh i was thinking of how to get the extra gain stage of the vibrato channel. so i was wondering about the dry signal buffer in the 6g15 section and if theres a way around that... the only conclusion i came to was to insert the 6g15 where the verb circuit is on the ab763. but then that kind of defeats the purpose of how the 6g15 was designed?

[sluckey]

If you want some extra gain just remove the components in the shaded area. That'll give you more gain than adding the third triode in the vibrato channel.

[Jatchley]

If you want some extra gain just remove the components in the shaded area. That'll give you more gain than adding the third triode in the vibrato channel.

ohp i had it backwards. for some reason i thought the vibrato channel was cleaner because of the extra stage i now see that isn't true. i want not a ton of gain just cleans mostly. cool ill just leave it as is and get to the diylc part of the journey.

[Jatchley]

diylc will not run on my macbook for some reason. weird because it worked just a few weeks ago. anyway after getting frustrated with that i made a mock up. i'm gonna put that load resistor group sluckey talked about on a "gain" switch. building it in a 4U rack chassis only gives me 1" of clearance between the reverb tank and OT. but i feel like it will be ok as long as my tanks input is on the PT side.

[PRR]

> is a dry buffer necessary in this design?

Are you trying to avoid the cost of a half-tube?

Yes, there may be some consolidation possible when two boxes are combined to one. That is what the *later* amps did real well, however without the Dick Dale level underwater verbverbverb. I wouldn't beat my brain shaving pennies in a DIY one-off.

[Jatchley]

> is a dry buffer necessary in this design?

Are you trying to avoid the cost of a half-tube?

Yes, there may be some consolidation possible when two boxes are combined to one. That is what the *later* amps did real well, however without the Dick Dale level underwater verbverbverb. I wouldn't beat my brain shaving pennies in a DIY one-off.

not trying to avoid anything but high end attenuation of my dry signal before it even gets to ab763 preamp. from most of what i read i see is that its both partially the circuit to blame and the output cable capacitance? so most of me thinks it'll be totally fine since the "cable" in this case is hardwired and short. and you are right when the mix is turned up it's gonna be janky from the tank anyway. and idk im young im just trying to learn why it is or isn't necessary, just more understanding for me to learn. i can see why it would be there if it was standalone...but as part of an amp does it need to be?? could this be something else fun? i know its "historic" and sounds good traditionally. i'm just curious and like to experiment. i came to the conclusion last night im ultimately going to leave it in there because i think i want to add an output jack at the junction where the mix knob goes to the preamp [like the standard 6g15 output jack] and have a wet only/wet-dry switch which is just tapped from the mix pot lugs. i was also going to add an 8ohm 50w safety dummy load for if theres no speaker load. i'm adding this because i'm building this rackmount so having a standalone reverb for other things in a studio environment i know would be fun. the speaker is on the volume pot so as long as a speaker is loaded just turn it down for a traditional fx unit and the dummy load is for if there's not a speaker load and honestly i can tap it for my scope too. both a safety and perk in this case.

[Jatchley]

here is where i'm landing. i can't really think of much else maybe some bias test points for easier bias metering later on. any suggestions?

edit: is it ok to run the Hi/Lo input jack into the 6g15 i want high and low impedance inputs but then i notice it goes through a 68k into the preamp grid and a 1M to ground simulating an input jack. is this ok?

[Jatchley]

Wooo it’s up and running. Still got a few tweaks. The bass voice switch when activated makes all of my pots scratchy. And I get reverb feedback
/oscillation at about 2oclock on the dwell knob But only if I plug into the high impedance Input... on the low impedance input I can crank to my hearts content into dick dale land with No squealing... Still need to install A NFB lift switch. Also my impedance selector switch was too big so I need to find something smaller. I have a bit of hiss and negligible hum but I just pulled out of the used tubes box so if I buy some nos low noise I should be in shape.