Remove some AB763 parts and keep the good voltages

[bluescaster]

Hi everybody, I want to build a single channel AB763 (2x6L6) amp with reverb. So i need to remove normal channel and vibrato unit. If i do that and want to keep all the voltages ok, i need to put 2 resistors to simulate the current that this units use. (i think it's the simple way ).

I use the schematic we can find here : http://vintagefenderamprepair.com/wp-co ... ematic.jpg

For the normal channel, each tube use (410-270)/100000=1.4ma , so i need a 410/0.0028=150k/2w resistor from D to ground.

For the vibrato channel:

The first stage use (460-280)/220000=0.8ma.
The second stage use (460-390)/10000000=0.07ma and i don't know how many current there is across the OPT (call it Iopt).

So i need a 460/(0.0008+0.00007+Iopt) resistor from B to ground.

Who knows the voltage on the 100k between the OPT and B to calculate Iopt ?

Are you ok with me ?

Kind regards

[Firestorm]

You are overthinking this. The presence or absence of a couple of 12AX7s will not make a noticeable difference to your supply voltages; the tolerances of dropping resistors will have a bigger impact.

What you do want to think about is the attenuation of the passive mixer coupling the two channels: each channel passes through a voltage divider formed by the 220K mix resistors and the plate resistor of the opposite channel. Think of it as a 540K master volume turned down to "6."

Also consider that eliminating the trem (and its 50K "load" pot) will also have a big impact.

If you omit either of these, you will have pretty much no clean sound. Not necessarily a bad thing, if that's what you were trying to do.

[Structo]

I would probably manipulate the value of the 1K resistor the C and B nodes and the 4K7 resistor for the D node.

You may have to tack on one at the end of the change if you can't get the B+ for the preamp down where it needs to be.

I know you would like to use math and figure out the exact value but really, it will be trial and error because all tubes draw different amounts of current and that will change the voltages.

I would look at a similar amp that doesn't have a tremolo and look at it's power supply for ideas.

Like the Blues Deluxe for example.

http://www.el34world.com/charts/Schemat ... DELUXE.pdf

[bluescaster]

Thank you. So i can put a 47K to simmulate the intensity pot and a 470K master pot to replace the 220k ?

[jjman]

Both the input to the grid of the 3rd stage and the output from the plate of the 3rd stage have alot going on. Using the master and 47k (to ground) would be faithful to the loading of the stock scheme but I don't know if the 3rd stage will be capable of overdrive as is. So that master may not be “useful.” I wouldn't bother with the 47k to ground. Many people switch it out when not using trem. Eliminating or reducing the value of the 3.3meg to the grid would reduce the attenuation of the dry signal into the 3rd stage thus increasing the chance of overdriving it. Then a master before the PI would be more useful. And the reverb signal can be increased by reducing the value of the 470k on its pot’s wiper. Can go post PI MV instead. Many options. All fun.

[Smitty]

I just whipped this up in 'paint'. Not pretty but you'll get the idea. I erased everything and chucked in a 1M Master Volume pot in place of the 220K mixer.

You can tack in a 25K pot where the 4K7 dropping resistor goes and dial in your preamp voltage. My preference is 200V. YMMV.

1/2 watt carbon comps everywhere except preamp tube plate resistors where I would put Dale RN65 resistors for low noise.

Coax off the input and volume control wouldn't hurt. I'd throw in a soul control but that's just me.

Follow Leo's grounding scheme.

Good Luck!

Please distribute this schematic freely.

[steeve_a]

IMO the Normal channel kicks more butt than the Vibrato channel in the Blackface amps because the signal is not run through the 10pF/3.3M reverb splitter. You can reduce the 3.3M resistor to 2.2M and use a mica cap to improve the reverb splitter, but I still think it sucks tone. I remember when everybody thought it was great to have reverb on both channels, but in doing so they were giving up one of the finer features of the BF design. At least in my opinion... YMMV

With the high quality analog and digital reverb pedals and processors available today my own preference would be to skip the BF reverb circuit and add a buffered effects loop. The Lee Jackson Mr. Springgy pedal is pricey but being 100% analog I think it is the most musical reverb pedal on the market- the pedal doesn't just add reverb but it seems to make your guitar sound better even though it is very transparent and doesn't change your tone. I guess you could say that it enhances the natural sound of your guitar. It works great ahead of your amp or in the FX loop.

Just my unsolicited and usually irrelevant thoughts on the issue...

Steve Ahola

P.S. For an exact copy of a BF amp I'd be looking more at the iron than the minor details of the schematic.

[ChrisM]

Why did they choose such a huge mixing resistor?
I understand why the cap is there but why a 3.3M resistor to mix the reverb across?

[steeve_a]

Why did they choose such a huge mixing resistor?
I understand why the cap is there but why a 3.3M resistor to mix the reverb across?

To isolate the reverb signal coming back from the reverb circuit from the signal going into the reverb circuit, otherwise you could have a positive feedback loop going on. Bypassing the 3.3M resistor with the 10pF cap adds some brightness to the signal.

The reverb splitter is a problem mainly if you are trying to pass a high gain high distortion signal through it- either via a stomp box on a stock amp or from hot rod mods to the BF design. On one of my projects I ended up with the Normal channel wired up like the Clean channel on an ODS and the Abnormal channel wired up like a Marshall 2204. I had the Normal channel hooked up to reverb but not the Abnormal channel. I could not get the Abnormal channel to sound right until I wired it up like the Normal channel sans reverb. Here is the schematic for that amp- a little Halloween present for 1999... Son of ProManiac!

Steve Ahola

[bluescaster]

steeve_a - "add a buffered effects loop" : this would be cool a parralel effects loop ! I could use my holy grail reverb.

Smitty - On your schematic, why change Bias supply ?

[Smitty]

I like to bias hotter than old Fender amps are able to. The upper end of the voltage range on the standard Fender design is useless for me. Additional capacitance helps quiet the bias supply.