Blackfaced Ninja Toaster

[Steve Conner]

Hello all,

I was bored on Sunday so I decided to "blackface" my old Ninja Toaster amp. Everyone always told me it sounded really dark, and since I started using it with an EVM12L, I had to agree. I suspected the weird PI circuit that I used, which needed a big capacitor dumping highs to keep it stable, and I thought maybe a dose of Fender twang would cure it.

I ripped out the PI that was in there, and rewired it according to the Fender AB763 circuit, except that I used 1k and 6.8k feedback resistors instead of 100 and 820 ohms. I put a 0.1uF cap across the 1k for a presence boost, and a 470pF across the 6.8k to help with stability. This measured out as a 3dB presence boost from about 1kHz to 20kHz.

When I was done, I was really surprised with the result. It opened the whole tone up, like taking a blanket off the speaker. I was worried that it might ruin the high-gain tones, but they just sound plain evil now

My only worry is that the voltage might be a bit low. The AB763 print specifies 450V supply to the PI, and I'm only running 340. Would anyone care to comment on what effect that might have? Would swapping the 12AT7 out for a 12AX7 help?

I also need to do something about the cooling fan that sounds like the Ninja Hairdryer.

http://scopeboy.com/toaster.html

[chipaudette]

Howdy,

I've got a Fender 65 Deluxe Reverb Reissue and my PI is fed off the "Y" point in the power supply. The schematic calls out "Y" as being only 311V, so I would think that your 340V value should be just fine.

Chip

[martin manning]

Which AB-763 are you referring to? The Deluxe Reverb AB763 schematic shows 325V (driving two 6V6's), and the Showman runs at 440V (four 6L6's). Maybe the question is do you have enough headroom and voltage swing to drive your output tubes?

MPM

[Steve Conner]

I thought I was referring to the blackface Twin Reverb, which does run four 6L6s at high voltage. The silverface Twin has the same but with 47k plate resistors in the PI.

I'm running KT88s, so yes, voltage swing is my main concern. When I had it on the bench, my signal generator didn't have enough output to fully drive it once the NFB was applied, so I had to put it back together without testing.

[Loogie]

You could try that 12AX7, but, and I know this is heresy, you could also try another type of inverter. Its hard to imagine an AB763 with a paraphase or a cathodyne (and its driver), I guess (though its possible the latter appears somewhere), but I think you can get more gain out of them. At least I can, though I'm sure that's do in large part to my shortcomings as a designer. I've tried the three I know how to make in the same circuit and I get more gas out of the cathodyne and paraphase. I like the way the paraphase sounds as well.

[Steve Conner]

Well, that would be fine except that the PI I ripped out was a paraphase I just fancied trying something different.

I measured the power output and I'm still getting the same 60-70W as before, so it's cool. When I scope the output, the clipping seems to be at the power tubes, the same as it was with the previous PI.

[GibsonLover]

Paraphase

A friend brought a HI-FI book in last week published in 1953. There was a section on the venerable paraphase, advising how inadequate the fixed paraphase was compared to the 'improved' floating paraphase. So I went off and changed my 5C8 clone from fixed to floating. It sounded better, though I cannot scope it until next weekend. I'm guessing the PI is more balanced and probably has 'better' distortion. Blackface was on my mind too...