First...Thanks to all for the good info here...you guys Rock!
I wanted to share the most effective changes to my Bronco (Vibro Champ)
I have a "Bullet Hole" Fender Bronco (SF Vibro Champ) that I picked up a few years ago for small change.
The previous owner must have used it as a test mule because the back side of the chassis was Swiss cheesed when I got it.
Definitely not a collectors piece, It had also been hogged out for a 10 in speaker.
I changed the speaker to front loaded and got more room for big output tubes behind the speaker and a little better tone from the cabinet as well.
The stock grill board was 1/2 in particle board and warped badly so I used 3/8 ply. This also gives room for the front loaded speaker flange.
The stock OT was gone so I installed a 4K 20W SE OT I had laying around.
I also lowered the values of the coupling caps to deal with flabby bass issues.
For a long time I have been chasing an annoying overtone in this amp. It was centered around the 7th fret G string (what ever that freq is?)
I also like my amps quite as far as hum and buzz so I got the soldering station going.
1. Removed "death cap" and installed 3 wire cord with separate earth ground for the cord.
2. Rewiring the fuze and power switch from the neural leg to the hot lead.
I'm told this is SAFER and also seemed to get rid of some buzz coming down the neutral from other stuff on that line in the house.
3. Modified filtering network. I didn't want to change the regulator or stiffen the amp to much.
My B+ string is as follows 5Y3 pin 8 - 20mf atom - 1H 70ma choke - 40ufcap can + 20uf cap can to Plate feed - 2w 1.5K screen dropper -20uf cap can - screen feed - 12k dropper - 20uf atom - preamp voltage selector switch with droppers to get (254V SF "stock" / 120V "tweed" at pin 1 V1).
the amp was much quieter but still humming slightly.
4. Rewired heaters and grounds completely to modern two wire style heaters, (the stock Champ/Bronco uses one side of the 6V winding to ground).
I installed two 1W 100 ohm resistors for a Heater circuit virtual center tap then ran that line to pin 8 on the 6V6.
For the Grounding circuit, I ran a buss wire from the lower 6V6 cathode ground eyelet along the board to the V1 cathode ground eyelet.
The added filter caps, OT ground and power transformer center taps were grounded to the chassis near the power transformer and a line from the preamp ground buss was run to this point as well.
Rewiring is a lot of work, but had the most effect as far as the noise floor and the fuzzy overtone thing.
That is down to the point where it's almost not perceptible.
The amp is so quite now I have to visually check if it is on, and still retains it's original Champness.
I wouldn't ever recommend this for a pristine example of a vintage amp, but if you have a beater or are building new, hope this helps.
Topic: Thanks to all and what I did to quite a Noisy Bronco (Read 2040 times)