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FDP Forum / Amp Mods, Repairs, and Projects / Brownface Concert issues

rockman627

New York

Aug 13th, 2019 07:45 AM Edit Profile
I refer you to http://ampwares.com/schematics/concert_6g12a.pdf.
A friend of mine retired from playing and gave me his brownface Fender Concert amp.It wasn't working so I ended up rebuilding the circuit board. I managed to not screw anything up; however, the normal channel was way louder than the vibrato channel. I routed the normal channel to the vibrato circuit and vice versa and used the amp for a couple of years. Then I started getting weak or no sound intermittently. So I put it aside and used something else.
Then I read on Rob Robinette's site that old tube sockets could give you that problem, so I replaced them all and that seems to be working. But I always wanted to find out why the vibrato channel was lower and address that before I put the chassis back in. I think I finally found it.
V1 pin 6 goes straight to a .05 capacitor while V2 pin 6 goes to a 100k resistor, and then a .05 cap.
My initial plan was to switch the 100k resistor with the 10k next to it, but I may be messing with some function I don't know about, so I figured I better check with the experts.If that works what about putting a resistor between V1 pin 6 and the .05(actually .047)cap? The normal channel is very loud and starting to break up at 10 o'clock and I would like to tame that a little.
Thanks for your consideration on this.

pdf64

UK

Aug 14th, 2019 08:57 AM Edit Profile
The vib channel uses split load plate resistors to reduce the signal level at its coupling cap to ~1/10 of the level at its plate.
Yes, that seems quite severe, so maybe tinker with those values a bit.
It will affect the precise operation of the filter freqs of the vib circuit a bit, but if it still sounds ok then it is ok.

The split load idea could also be used to calm down the normal channel gain a bit.
Have a read of Steve Aloha's webpage on the topic (RIP Steve!, your legacy lives on!).
Split load

rockman627

New York

Aug 18th, 2019 10:38 AM Edit Profile
OK this is what happened:
In the vibrato channel, I removed the .003 cap (actually .0022), removed the 10k resistor, moved the 100k to where the 10 K was. This makes the normal preamp and the vibrato preamp literally identical. I did get an increase in volume on the vibrato channel, but the normal was still a bit louder, as opposed to way louder. Then it occurred to me to put a smaller gain tube in V1 so I put a 5751 in there. It tamed the normal channel nicely. The volume of the normal channel at 3 equals the volume of the vibrato channel at 4 1/2. I suppose the remaining difference may be due to the input resistors and/or the tone controls drifting off value. I'm going to keep it this way because I am trying to keep what I can original, although with a new circuit board and new tube sockets it's starting to turn into an amp kit. It does however, have a sweet crunch. Can't wait to take it out to some jams.

BTW the .003 cap does not show up on the layout. I built the circuit board according to the layout and when I was done the vibrato channel had no sound. It took me a while to see the schematic had an extra component. I checked with the old circuit board and figured out where to put the .0022 cap.

pdf64

UK

Aug 20th, 2019 10:30 AM Edit Profile
I can't see why not fitting that cap would stop the channel from working. It just rolls off some high end. Maybe things flip into oscillation without it?

FDP Forum / Amp Mods, Repairs, and Projects / Brownface Concert issues

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