Tightening bass in Fender blackface

[SixStringBender]

I decided to try some things. I tried the amp through a Blues Junior speaker. This is an unknown Eminence speaker, probably a Legend, and it is bright. The bass response was nearly tight enough, but the treble was totally ice pick with the bright cap. I removed it and I just could not get the Tin Pan Alley tone from 1985 Montreux show. It was still an ice pick to the tone, but lacked that nice bloom into treble from SRV's mid 80's tone. I tried a Cannabis Rex that came with the amp when I bought it and it is just too dark.

So we put the 120pF SM bright cap back in and reduced the V2A cathode bypass cap to 1uF to tighten the amp for use with the 4x12 cab. The bass is good now, but I need more mids and I need more headroom since I cut R12 and removed and jumpered R35.

So a few questions. Should I reinstall a 220K in R35 or install 330K grid resistors in place of the 1M resistors on the phase inverter. I am in need of a bit more clean headroom.

I can install a mid pot in place of the speed or intensity pot. Will a 25K linear pot wired as a variable resistor in place of R21 do the trick? In other words, replace the 6.8K resistor with a variable resistor?

Also, how do I calculate what cathode resistors to use with 5uF bypass caps? I have a 2.7K in V2A with the 1uF cap and a 2.7K in V2B with the .68uF bypass cap. I may leave it like this, or I may do the Dumble SSS method and use 5uF bypass caps every where to see how that sounds.

[stratoblaster80]

I know this is almost a 10 year old thread, but thanks to it (and schematics) I recently was able to resto-mod my ‘68 Super Reverb to Diaz specs and it’s incredible. The amp had already been tore up from the floor up when I got it so I didn’t have any reservations about doing the work…and in all reality it’s actually in light years better shape now than it was before.

I thought I’d list the mods I did (along with a recap, replacing any and all out of spec components, moving PT-mounted grounds to chassis, etc.) for anyone also interested in diving in:

-220uf series reservoir caps w/ 100K 2W balance resistors (I think this is the first mod anyone trying to get that Texas Blues/SRV tone needs to do. Tightens up the low end but allows the mids and top end to break up which is the key)
-Normal channel disconnected (pulled out 220K mix resistor at PI)
-Trem cut switch
-NOS 1W 100K plate load resistors on V2 and V4
-.047uf mids coupling cap in place of the stock .022uf
-22uf electrolytic cap added to bias pot for noise reduction (courtesy of Lyle @ Psionic Audio YT channel)
-100ohm heater balance resistors in place of the PT filament center tap (also courtesy of Psionic Audio)
-4 JBL E110s w/3/4” baffle

Only thing I didn’t do was the cathode bypass adjustments to V2A & B - I wanted the extended bass (as SixStringBender recommended). The amp already had Orange Drop 715Ps in it when I bought it.

The thing is Stevie’s Super Reverb reincarnate. I was really shooting for his tone from Live at El Mocambo and this amp nails it in spades. With treble at 7, mids at 6, bass at 3 and cranked up to about 7 or 8 on the volume pot it’s that perfect blend of huge, tight lows, crispy highs and that clean-but-dirty breakup that mostly shines through in the upper registers and on chords/rakes/double stops. Just a bit of compression and crazy touch-sensitive.

Anyhow, can’t thank everyone enough for their contributions to this thread - this was my first ever amp project and without this thread I would’ve had no idea where to start.

Cheers!
Chris

[martin manning]

Orange Drop 715Ps in it when I bought it.

Nice work, glad you were able to reach your goal with this one, and thanks for the summary of mods.

To be clear, you left the 715P in place? Are they all the way through the signal path? They are PP (polypropylene), and have a reputation for sounding bright, but they are more stable than PET (polyester). No judgement, just seeking clarification.

[stratoblaster80]

Thank you Martin!

Yes - 715Ps throughout - I have a pic I’ll attach. I actually prefer the sound of them for this application as Stevie’s early tone was crisp and bright. I haven’t tested them against any other caps but do find the amp has a very clear almost hi-fi quality to it. It’s very balanced and responsive across the spectrum.

My next project is going to be a Vibroclone (also Diaz-spec) from a ‘68 Bandmaster - it has the original blue Ajax caps and I’m contemplating keeping them in place to examine the difference. I have a full set of 716Ps as well so I might do some sound comparisons!