Vibro champ tweaks for tight bass and control mids

[wade]

Hey, I built a mojo vibro champ. has flubby bass. By-passing the tone stack increased headroom and tightened the bass but mid-range is heavy and too sharp. Tried moving tone stack after pre-amp with little change in flubb. Any mods for smoothing mids and keeping tight bass?  Thanks,

[sluckey]

Try replacing the two 25µF cathode bypass caps for V1 with 2.2µF or 4.5µF caps.

[wade]

Thanks I'll try that. I have 4.5µF caps on hand.
Also, I'm not too keen on the tremolo so would be willing to sack it to use the V2 for extra mods.
Especially if this could help get some control for smooth breakup or tone tweaking.

[shooter]

before you gut V2, try a 47uF 50vdc cap for the 6V6 cathode.  add a grid R ~ 1k to the 6V6 and take out the 330pF grid to cathode, 10 minutes, and 15 more playing to evaluate, then 10 more minutes if you don't like it 

[wade]

slucky, I swapped the cathode caps in V1 for the 4.7 µF 16 volt caps I had. Definitely smoothed the tone to my ears. Still responsive to guitar control and pedals as well.
shooter, I'll need to get more swap supplies to try your mod, although, I don't see 330pf in the VChamp.
So now I just have the V1, no tone stack, and one volume. I'll be looking for a way to use the remaining knobs for something..
If the tremolo were a bit more "frequency shifty" or organic in some way, it would be more useful to me.
Thanks for the tips!!

[shooter]

this is the schematic I found, but guessing there are 12 flavors 
https://el34world.com/charts/Schematics/files/Fender/Fender_vibrochamp_ab764.pdf

increasing the cathode cap will tighter up bass response, seems to help some in "fixing flabby" sorta thing,
you roll in different speakers, they can contribute maybe 1/2 the tonal "value" in SE type amps

[jjasilli]

This is what I did. 

[Tony Bones]

this is the schematic I found, but guessing there are 12 flavors 
https://el34world.com/charts/Schematics/files/Fender/Fender_vibrochamp_ab764.pdf

Kind of off topic, but I think the BF & SF Champ/Vibrochamp is the only circuit that didn't get changed around a lot. I think the only changes CBS made where to add the 330p cap on the 6V6 (presumably to suppress oscillation) and, for some insane reason, they increased the HT volts A LOT.

Take a look at the schematic you posted. They show 24V across the 470R cathode resistor. That's 50mA. Figure 390V plate to cathode and the poor 6V6 is dissipating 20W from the factory!

I had an (unmodified) early 80's Champ that had 465V on the plate and 28V on the cathode when plugged into 125VAC wall power. Absolutely crazy abuse of a 6V6GT, but the thing sounded awesome. 

[dude]

This is what I did.

.                                                     Wow jj, a totally different amp, 5u4, choke, 6L6, first stage completely changed and on and on. I assume you have a 10” spk at least. I’m sure you upgraded that 5 watt OT and with the 5u4, maybe a bigger PT?  I was thinking about making a SE Princeton along the lines of your what use to be a vibro champ, bet it puts out 12 watts or so. What other type of SE amp does it sound like?

[wade]

JJ got me curious. What program should I use to view your file?
Tony, The schematic that came with the mojo kit had some changes from the one shooter posted.
BTW I made my own cab for this build using a 10" WGS Veteran 20 watt speaker.

[jjasilli]

WADE:  Google Express Schematics.  Free download.   This program is used a lot on this Forum.


DUDE: Many of these mods are suggested by Dan Torres.  Stock the amp could barely reach overdife.  This little amp now sounds big.  The PT is stock.  Thougj the RAW control seems useful, I just leave it at one setting that I like.