1st from scratch build - AB763 With sprinkles

[plokij88]

Hello, long time Lurker first time poster. I'm embarking on my first "from scratch" build.

By "from scratch" I mean that I'm buying all the components individually and machining the chassis myself. I am a furniture maker so hopefully I'll manage to make the cabinet.

In terms of the circuit I have just taken Rob Robinette's Blackvibe and thrown a couple of mods at it. It should be

Blackvibe
+Presence control
+Lar-Mar/TW PPIV master volume with double gang pot
+Switchable negative feedback values

There is nothing new, original or revolutionary here, so if you see something that is it's a mistake and please tell me about it.

I've drawn out the schematic, I'm pretty sure I know physically how I should connect things but electronically I'm not so sure. Currently (PUN INTENDED) I have some concerns with it but I'll leave it for other people to point them out before I ask specific questions.

Any and all opinions/advice appreciated.

Also, a guy on here built me a 15w Trainwreck head and shipped it to England from the USA but I've lost all the emails/documentation about it so if that was you say Hi.

[sluckey]

I think your presence circuit should look like this...

[plokij88]

This was definitely on my list of concerns thank you

[plokij88]

blacknick schematic 0.3.png

hopefully easier to look at

[Stevem]

I have some sprinkle suggestions for you.

For the heater string install a hum balance 5 watt pot like used in Fenders.

Get a Londonpower BMK2 bias mod kit.

You might also be interested in one of there power scaling kits since your wiring in the needed master volume anyway.
A proper and compact layout can make a bunch of good parts sound great or make for a noisy oscillating mess and waste all that money spent on those good parts.

[cdemike]

I'd recommend including some safety resistors across the PPIMV so that if you have the wiper on the master volume pot fails open you don't roast your tubes or your OT by running the tubes with zero bias voltage. If you use a dual 1M linear pot, you could put a 300k resistor between the wiper and the lug feeding bias voltage, which would be equivalent to a 230k grid resistor. Doing so will also make the taper of the pot logarithmic like most volume pots.

It's possible it's just personal preference but any reason you went with a bias balance pot with a bias range adjustment pot? My thinking generally is that if you're going to have two pots anyway, just make them two independent bias circuits for each tube. It could make the amp less finicky to bias, since the bias procedure would be to adjust each side independently (depending on how much power supply sag you have) rather than having to go between the two pots if the tubes' balance pot interacts too much with the range pot. That'd involve one more capacitor for the other leg of the bias supply, but if it's a scratch build you can make space. The current design makes it so that bias voltage is zero-sum between tubes, so when one tube's bias gets hotter, the other gets colder. That level of interactivity could make biasing pretty frustrating if you get some mismatched tubes.

Any particular reason you chose 470R screens resistors over 1K? Also, what value reservoir caps were you planning on?

[LOUDthud]

100K mid pot with a 6.8K in series ?

[cdemike]

100K mid pot with a 6.8K in series ?

Meant to post on this but forgot. Forgive me if you've already run the simulation, plokij88, but in case you haven't: https://www.guitarscience.net/tsc/marsh ... pot=Linear

Agree with the questions about the tone stack. If you're going for powerful tone stack with a lot of midrange control, this accomplishes it, but it seems to me that your bass control might function more as like a "deep switch" since it mainly boosts very low frequencies. It also seems to lack authority unlike the treble and middle controls, so the discrepancy between the EQ controls could make for a strange user experience. Changing the 47nF to 22nF and the bass pot's value from 250k to 500k gives a more proportional response between controls and shifts the response to cover both bass and very low mids. Simulation of suggested changes here: https://www.guitarscience.net/tsc/marsh ... pot=Linear

If you're not committed to the traditional Fender/Marshall style TMB setup, Hiwatt-style, James, and Baxandall tone stacks are even more authoritative.

[sluckey]

100K mid pot with a 6.8K in series ?

The 100K pot acts as a standard mid pot for the first part of rotation. Turning up more begins to lift the tone stack, making it less effective. This produces more gain with a bit of hair. It's a popular mod for AB763 amps. I've used a 250K-A pot for a few AB763s. So, 0-5 acts like a MID pot, and 5-10 acts like a RAW pot.

https://sluckeyamps.com/misc/AB_Lite_II.pdf

[Helmholtz]

Schematic has missing ground connections at OT secondary and (stacked) reservoir cap.

Added presence:
The amount of presence boost depends on the difference (more exactly the ratio) between power amp gain with and without global NFB.
The more NFB the stronger the presence effect. No presence without NFB.
The NFB ratio depends on the feedback divider resistors and the forward gain.
As the PPIMV lowers forward gain, presence reduces at lower settings.

[pdf64]

100K mid pot with a 6.8K in series ?

The 100K pot acts as a standard mid pot for the first part of rotation. ...

But a standard mid control CCW is 0ohms, not 6k8.
That seems bonkers. For me, the point of having a mid or any control is to adjust the parameter either side of where it sounds best.
If CW isn't too much / CCW not enough, then things need tweaking.

[pdf64]

I'm a strong advocate for HT fusing and heater elevation.
It's great to make amps more resilient to likely failures, but seems remiss to uprate the screen grid resistors from 1W to 3W without also adding HT fuse.

[Helmholtz]

It's not clear from the schematics if there's a grounded heater CT.

[pdf64]

It's not clear from the schematics if there's a grounded heater CT.

If zoomed in, it looks to me that the heater winding has a grounded CT?
It's not as apparent as the same arrangement on the HT winding.

[plokij88]

blacknick schematic 0.5.png

Wow thank you for all of the input, any question of "why have you used x-value for y-component" the answer is always, that's what is on Rob's schematic. I'm very open to swapping in and out different values later, but I thought it best to just start in 1 place and adjust from there.

For the heater string install a hum balance 5 watt pot like used in Fenders.

Added thanks

I'd recommend including some safety resistors across the PPIMV...

I had this in mind but had not remembered to add them to the schematic, thanks

It's possible it's just personal preference but any reason you went with a bias balance pot with a bias range adjustment pot?

Purely because that is what on Rob's Blackvibe schematic

Changing the 47nF to 22nF and the bass pot's value from 250k to 500k

I will certainly try those variations thank you.

If you're not committed to the traditional Fender/Marshall style TMB setup, Hiwatt-style, James, and Baxandall tone stacks are even more authoritative.

I'm really interested in those tone stacks, but right now I'm specifically trying to make a fender'y amp.

It's not clear from the schematics if there's a grounded heater CT.

It did, but it was very tiny, now that I've added a heater hum pot I have removed the centre tap.

I'm a strong advocate for HT fusing and heater elevation.

Where does the HT fuse go? Is it after the choke, or right after the rectifier tube? Or somewhere else entirely?

Heater elevation. I believe that I replace the ground on my Humdinger/hum pot with a voltage but I don't understand where I take that voltage from