Vibrolux Reverb Maintenance and Mod Log
Date |
|
Notes |
11.12.04 |
Convert bias to a level adjust circuit per Hoffman article. |
Left in 3.3k bias smoothing resistor between bias board and pot. Removed 100k resistor between bias pot and main board. Removed 100k resistor on main board; replaced with two 220k resistors |
11.13.04 |
Replace filter caps |
Sprague Atoms 16@475 |
11.14.04 |
Replaced all (6) bypass caps. |
New Sprague 25mfd @25v electrolytics. All but 1 cap were 25mfd @25v – one was 5mfd @50v; replaced oddball 5 @50v bypass cap with 25 @25v. |
11.19.04 |
Replaced two power tube coupling caps |
Two .01 caps on the left side of main board - with orange drops 715 |
11.19.04 |
AA964 Blueprinting |
- Replaced (2) 47k phase inverter plate load resistors to 100k on the left, 82k on the right. - Replaced (2)330k phase inverter cathode resistors with 1 meg metal film (AA270 SF schematic calls for 1 meg – but there were 330k in there!) - Removed .022 caps from power tube grids. - Changed phase inverter coupling cap from .01 to .001. <very much added treble/bright to amp> - Removed .002 cap from reverb input jack. - Removed 3.3k bias smoothing resistor. - Replaced bias range resistor with 470 ohm 3w carbon comp. - Bias circuit as described above and below. Left 15k resistor on bias pot (BF=27k) |
11.20.04 |
Bias cap |
Removed the (2) 70mfd bias caps and replaced them with one 100mfd @ 100v. |
11.20.04 |
Coupling caps |
.1 Mallory 150, (2) .022 |
11.21.04 |
Tone stack mod – both channels |
Replaced .1 & .047 tone caps to .022 |
11.21.04 |
Bright balance cap |
Replace with silver mica 10 pf – the mica feels like it gives a rounder treble note. Maybe more musical than ceramic, but maybe not as organic. |
11.21.04 |
Bright switch |
Vibrato channel to silver mica 100 pf (from 120) This is a good mod. |
11.29.04 |
Change phase inverter input cap |
Back to silverface .01 value – better – got some of the beef back. Girthier. |
12.10.04 |
Change phase inverter input cap |
To .01 mfd Mallory 150, from Sprague Orange Drop. Excellent – surprisingly noticeable! Kept the beef, but tightened it up for sure. The .001 would probably be good with humbuckers |
12.11.04 |
Tone stack caps |
To Mallory 150, from Sprague Orange Drop. Went back and forth a few times. The ODs sound excellent, maybe a little looser in the mids/lows, but in a (mostly) good way. Could go with either one but the Mallory’s are a little more well-rounded. |
12.11.04 |
Reverb area caps |
Replace the .022 and .01 OD caps with Mallory 150. Noticeable tightening and woodifying to the mids – excellent. |
Amp sounds killer. Loud and roasty, clean enough in the lows but still very beefy and big. Highs are excellent. Have all Mallorys except output coupling caps, 1 OD tremolo cap, OD normal channel tone stack (unused), and Mica bright caps. Still to do: - Possibly – swap orange drop output coupling caps with Mallory 150s - Swap out power supply (on filter caps) resistor(s) to BF values |
||
12.18.04 |
Screen grid resistors |
Replaced on power tubes with 470 ohm 3w metal film |
12.18.04 |
Phase inverter cathode resistors |
Replace current 1 meg metal film to 1 meg carbon comp |
Jan 2005 |
Tubes |
Tried the JJ 6L6 – good, but a bit strident, compared to the Winged C, which sounded fat and brown – went with the Winged C – very different bias values. |