AB763 Deluxe reverb clone - Ugly break up

[kleuck]

Sounds like an octave-up to me, did you try to put a huge grid resistor on the PI tube ?

"...The much greater problem with the cathodyne occurs when it is itself overdriven. Because it has such a large cathode resistor only a little grid current is required to 'jack up' the cathode voltage. When driven very hard this can cause an inverted copy of the cathode signal to appear at the anode, effectively creating a sort of full-wave rectified or frequency doubled signal at the anode. This is shown in the lower photograph, and it is usually this which causes the ugly 'blatting', 'swirling' or 'grainy' sounds sometimes heard in amps using this kind of phase inverter. Fortunately, the cure is simple. We add a large grid stopper to the cathodyne, to keep this grid current in check. A value of 100k to 1Meg is usually necessary. Before you worry, this will not affect the treble response though, because the cathodyne only has unity gain! Therefore it's input capacitance is extremely low, at about 2*Cga + Cgk, which is only 4.8pF for the ECC83! This is the real 'secret' to obtaining a smooth, consistent sound from the cathodyne, no matter what kind it is. If you are using a cathodyne always give it a nice big grid-stopper. The tonal reward is startling!!! Yes I know Leo Fender didn't use any, but he wasn't designing amps to be overdriven, and this is the 21st century..."

The Valve Wizard

[el_fela]

Well, I allmost forgot about this thread... the thing is that with a new power transformer this amp came alive... i get now that the bass beakup is supposed to be ugly, but the highs are not hurting me anymore, and the mids are now much more clear. I can play My Bloody Valentine -like distorted chords and it sound great now.

I made some other minor changes, like using a separate cathode resistor/cap for V4 (kills noise and signal bleed from reverb circuit ), 5y3 instead of gz34 etc... but the PT replacement had a notable (not huge) impact on the sound. What was it? I don't know, but I'm happy to have a really rocking deluxe reverb. The crunch you get when you crank it, beautiful. The reverb is quite awesome too... I'm glad I didn't give up on it.

To me the amp sounds good. Maybe the guitar is kind of high in output? Did you play the amp in a band setting?

Yes überfuzz, my guitar has unussually high output, why? In a band setting it was muddy and the highs would hurt people's ears (even the drummer!). I realize that the speaker is now softer, yes-

Kleuk, this design uses a LongTailPair phase inverter, not cathdyne.

Hope this helps someone. Thank you guys for the feedback!

Felipe

[barfoden]

wow that power transformer looks like it belongs on a 100 watt amp, not a 22 watt deluxe.

I can only concur on what has been said. The blackface amps has a lot of bottom end and that is what distorts when you have the bass on 10. I have seen a demo of a cranked deluxe reverb reissue and the bass was set around 3 or less. Albert collins ran his quad reverb with the treble 9-10, middle at 10 (10k value, the deluxe comes stock with a 6.8k resistor)), and bass pot pratically off when he dimed the amp.
The fender has large bypass caps (25µf), and 0,1µf coupling caps in the tonestack and in the power stage. Both the ones in the tonestack and power stage can be halved in value to 0.047µf and you will still have a full sounding amp. But first i would set the bass at 2 or 3, middle all the way up (it is still slightly scooped with a 10k value), and the treble half way. Cut the 47pf bright cap if you are using startocasters or telecaster..

Later marshalls had a 0.68µf bypass cap (only mid and treble boost) and a 0.022µf coupling cap in the cathode follower tonestack. The fender tonestack is plate feed.. And therefore respond very differently. (the knobs do a lot compared to a marshall).

Wonder why a vox ac15 with 15 watt slice through a band mix.. The combination of a mid rich speaker (alnico blue), bass light circuit with a 500pf coupling cap from the first gain stage in the top boost channel... It does not waste a lot of energy on the bass frequencies. And the lack of negative feedback lets all the natural amp harmonics through. Perhaps why the vox top boost actually cuts the high mids (800 to 2000 Hz) as these would be far to dominant and you have a cut control to even further cut the highs.

Also the Voxes lack of negative feedback gives less damping around the speakers resonance freq (often around 75-100 Hz) so the amps circuit needs to limit the bass in that area.
The blackface is more of a hifi approach with its negative feedback. Remove it and sometimes you will start to get oscillations and other weird effects.