New build : Fender Blackface Super Reverb

[ilfungo]

Hi
I've built a Fender Blackface Super Reverb Amp.
I've built it with an Upgraded transformer with 4-8-16 ohm taps.
The original use 2 ohm output transformer with 820 feedback resistor.
Wich value I need to use to have the same ammount of feedback on 8 ohm tap?
Schematic attached...

 

[dmp]

Well, first notice that fender used 820 ohm across the lineup, pro, twin, vibrolux, etc which had different speaker ohms. So starting with 820 and tuning it to taste wouldn't be a bad method.
The feedback amount starts at the tube plates into the transformer primary, say 6k ohm. The signal voltage steps down sqrt(6000/2) then is reduced by the resistor voltage divider 100/(100+820).

 

[ilfungo]

Thanks dmp!
But I don't know how calculate...
I want to start with same ammount of Feedback and I don't know how...
The primary is not 6k but 4k2.
Thanks!

 

[dmp]

OK, so the transformer you are using is 4k2:8

CJ recently looked at a super reverb OPT and it was 4k:2

So using those two formulas you would use a 1k7 to get the same feedback voltage with your transformer
The reason you use a higher resistor is the 8 ohm tap gives you a higher voltage (sqrt 4k2/8) than in the original (sqrt 4k/2)

 

[ilfungo]

THANKS!!!

 

[CJ]

Try it without feedback first. You might like it. Vox used no feedback. You can also use a 10k pot and season to taste. Or even leave the pot on the back panel. That way you can dial the amp in for the room you are playing, which unfortunately for me is usually a bowling alley with bad nachos.
Do they make an eq that will cut through the sound of pins crashing into each other?

 

[danfrank]

There's 2 ways you can do this with the 8 ohm tap of your new output transformer. The 8 ohm tap has twice the voltage as the 2 ohm tap per given power output. The PI doesn't care about power output it only cares about NFB voltage injected back into it for the OT...
So there's two ways to get the same feedback as the original transformer. You can either double the 820 ohm resistor (1640 ohms) OR half the 100 ohm resister value (47 ohms). One or the other but don't change both.
Hope this helps...

 

[Whoops]

which unfortunately for me is usually a bowling alley with bad nachos.
Do they make an eq that will cut through the sound of pins crashing into each other?

 

[ilfungo]

Thanks to all!!!
The Beast is alive and sound marvelous!!!

 

[Whoops]

Looks great, congratulations

 

[CJ]

FYI , just got done tearing apart a 65 super reverb main board.

I measured every wire so I would not waste that expensive cloth wire on the next build, anyway , here are the total lengths of wire

Yellow: 17 ft.
Brown: 4 ft.
Red: 3 ft.
Blue: 5 ft.
White: 3 ft.
Black: 2 ft.

I left out the green heater wire but 5 ft ought to be ok.


There are about 65 pieces of wire, so if you add 2 inches to every one so as not to come up short and have to rip the board out, you would burn through about 11 feet of extra wire. At 60 cents a foot, you could save enough money to buy a hit of crack cocaine.

 

[Whoops]

you could save enough money to buy a hit of crack cocaine.

That always ends up very well, it's the start of a bright future

 

[Brian Roth]

That always ends up very well, it's the start of a bright future

Bri

 

[ilfungo]

Just to share...
Hand made
The Black one JTM45
The Red one Jtm/Plexi hybryd