RR763-based build

[aon]

Hi,

Here's my first amp build. It's mostly a proof of concept in whether I can do this kind of stuff, and based on the Rob Robinette RR763 Blackvibe but as I re-used the chassis, cabinet, transformers, speaker and a few other bits from a Fender Super 60, there are some differences to accommodate for that. Most importantly the rectifier is solid state, and I did mess around with the power supply a bit and "simplified" the bias. In hindsight I should've put in a trimmer...

Depending on the time of day and my state of mind it either sounds fine or a bit too bright and stiff. Maybe I should try increasing R23 to reduce the NFB?

There's also a quite noticeable mains hum even when the volume is turned all the way down. I should probably check the grounding of all the pots and jacks and add star washers - I can't remember which ones (if any) have them, it's been a couple of months since I did the bulk of the work...

I also have been thinking of adding a 6BM8 based reverb using the Super 60 tank. The circuit I've been thinking of is in the second attachment, it's pretty much the one used by TIMBO on his Moody GA-40: https://el34world.com/Forum/index.php?topic=16984.0. I did try to calculate new values for the plate resistors based on the voltages I have available. R_IN and R_OUT are planned to go before and after R10.

What do you think?

[pdf64]

So the NFB is being sampled from the 16ohm secondary output?

Bear in mind that the ‘mid’ resistance affects (apart from low bass) the full bandwidth; think of it as setting the baseline level, that the treble and bass can add their boost on top of. 
So given the mid arrangement and the inclusion of the presence control, unless you’re habitually setting both to minimum, right off the bat it’s already brighter than a real Fender normal channel.

Note that you’ll probably find that reducing the degree of NFB will tend to make the tonality brighter.

[aon]

Thanks for having a look!

So the NFB is being sampled from the 16ohm secondary output?

It's off the 8ohm tap (see the attachment cropped off the Super60 schematic).

Bear in mind that the ‘mid’ resistance affects (apart from low bass) the full bandwidth; think of it as setting the baseline level, that the treble and bass can add their boost on top of. 
So given the mid arrangement and the inclusion of the presence control, unless you’re habitually setting both to minimum, right off the bat it’s already brighter than a real Fender normal channel.

I hadn't really thought of that - thanks! Yesterday I noticed that setting the mids to minimum and compensating with volume does definitely help. Will try to set the presence to minimum too to see what that does.

Note that you’ll probably find that reducing the degree of NFB will tend to make the tonality brighter.

Is this because of the presence control circuit or is it in general the case with NFB? If it's the lack of NFB in general that will emphasize highs, why is it so? Sorry, I'm kind of a dunce with this analog stuff...

One thing I don't like very much is that the onset of distortion is quite sudden. I understand less NFB could help to make the transition area more vague? I play clean but find that the tone is usually the best pretty close to the edge of breakup.

[pdf64]

If operated open loop, the power amp’s freq response will pretty much directly track the speaker’s impedance, ie big treble boost and a massive peak at bass resonance.
But with negative feedback, that will tend to get flatten out.

And yes, reducing the degree of NFB will tend to reduce the sharpness of the transition into distortion. Easiest thing might be to move the NFB sample to be a lower impedance output.

[jordan86]

Lowering B+ may help with both stiffness and top end. I just put my 65 Showman on a variac at 100v. Much, much sweeter. A bit saggy and less edgy. Like a huge Princeton.

A warm speaker could help too.